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688 WWW.CROPS.ORG CROP SCIENCE, VOL. 48, MARCH–APRIL 2008

RESEARCH

The main product of the sunfl ower (Helianthus annuus L.) crop is the oil accumulated in the grains. Some hybrids are

widely sown in several countries because they present good agro-nomic traits (e.g., high yield, resistance to diseases) despite their low potential oil concentration. Hybrids that potentially produce high oil concentration usually have kernels covered by a black hull while those with low oil concentration have a striped hull. As most of the oil is stored in the kernel and only 3 to 5% is located in the hull (Connor and Sadras, 1992), the lower proportion of ker-nel in the grain (w/w) in striped hull hybrids accounts for its lower oil concentration compared to black hull hybrids. Through the years, genetic improvement produced an important increase in grain oil concentration by increasing the kernel proportion in the grain or the oil concentration in the kernel (e.g., López Pereira et al., 2000). Most of the increase was obtained by increasing kernel proportion in the grain (e.g., 67% of the gain, Gundaev, 1966 for Russian varieties; A. Vazquez, Sunfl ower Breeder, NIDERA Semillas S.A., Baigorrita, Buenos Aires, Argentina, personal communication, 2004). The eff ect of environmental conditions

Weight per Grain, Oil Concentration, and Solar Radiation Intercepted during Grain Filling in

Black Hull and Striped Hull Sunfl ower Hybrids

Natalia Gabriela Izquierdo, Guillermo Aníbal Adrián Dosio, Marcelo Cantarero, Jorge Luján, and Luis Adolfo Nazareno Aguirrezábal*

ABSTRACT

The effect of intercepted photosynthetically

active radiation (PAR) on weight per grain and oil

concentration and their dynamics was investi-

gated in several sunfl ower (Helianthus annuus L.)

striped and black hull hybrids grown in contrast-

ing environments. The study included the pre-

viously investigated hybrids Dekalb G-100 and

NKT, and two near isohybrids, which differed

in the character of the type of hull. The effect

of intercepted PAR lower than those studied in

previous works was also investigated. Oil con-

centration was affected by intercepted radiation

but striped hull hybrids reached the maximum

oil concentration at lower levels of intercepted

PAR than black hull hybrids. A decrease in inter-

cepted radiation decreased fi nal oil concentra-

tion mainly due to a lower duration of the phase

during which oil concentration linearly increases.

Despite the hull type, changes in grain oil con-

centration that were originated by changes

in intercepted radiation largely depended on

kernel oil concentration. No differences were

observed in the response of weight per grain to

changes in intercepted radiation among black

hull and striped hull hybrids. Low sensitivity of

oil concentration to changes in intercepted PAR

must be considered for modeling grain quality.

Striped hull hybrids could perform as black hull

hybrids in environments with low intercepted

radiation during grain fi lling.

N.G. Izquierdo, G.A. Dosio, and L.A.N. Aguirrezábal, Unidad Integrada

Facultad de Ciencias Agrarias (UNMdP), Estación Experimental Agro-

pecuaria INTA Balcarce, C.C. 276, 7620 Balcarce, Argentina; N.G.

Izquierdo, L.A.N. Aguirrezábal, Consejo Nacional de Investigaciones

Científi cas y Técnicas (CONICET, Argentina); M. Cantarero and J.

Luján, Facultad de Ciencias Agropecuarias, Universidad Nacional de

Córdoba C.C. 509, 5000 Córdoba, Argentina. Received 15 June 2007.

*Corresponding author (laguirre@mdp.edu.ar).

Abbreviations: °C daf, °C d after fl owering; NMR, nuclear magnetic

resonance; PAR, photosynthetically active radiation.

Published in Crop Sci. 48:688–699 (2008).doi: 10.2135/cropsci2007.06.0339© Crop Science Society of America677 S. Segoe Rd., Madison, WI 53711 USA

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

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CROP SCIENCE, VOL. 48, MARCH–APRIL 2008 WWW.CROPS.ORG 689

on oil concentration could be as important as the eff ect of genotype. For instance, diff erences in grain oil con-centration of the same hybrid grown at diff erent locations of yield trials (10 percentage points) were as high as the highest average diff erence among hybrids (Aguirrezábal and Pereyra, 1998).

Photosynthetically active radiation (PAR) intercepted by plants during the grain fi lling period (R6 to R9, Sch-neiter and Miller, 1981) largely determined oil concentra-tion and weight per grain in the black hull hybrid Dekalb G-100 grown under adequate water and nutrient condi-tions (Andrade and Ferreiro, 1996; Dosio et al., 2000). Moreover, intercepted PAR from 250 to 450 degree days (°C d) after fl owering (near the middle of the grain fi ll-ing period, base temperature: 6°C) mainly accounted for weight per grain and oil concentration in the same black hull hybrid (Aguirrezábal et al., 2003). Merging their own experimental data with published and unpublished data from other agro-ecological regions, Ruiz and Maddonni (2006) found that normalized weight per grain follow a linear-plateau response with normalized postfl ower-ing source–sink ratios (calculated as the ratio between intercepted PAR and grain number per plant or leaf area duration and grain number per plant). In contrast, oil con-centration was not related to changes in source–sink ratio in their work. Diff erences in the hybrid hull type were not considered in their analysis. The response of oil concen-tration to intercepted PAR during the whole grain fi ll-ing period was, however, diff erent for a black hull hybrid (Dekalb G-100) and a striped hull one (NKT). While the fi rst increased oil concentration with intercepted PAR, the other remained unaff ected for a range from 13.2 to 104.7 MJ per plant (Dosio et al., 2000). It is unknown whether such diff erent responses are specifi c to the tested geno-types or if they are similar to other black hull or striped hull hybrids. The lack of response of the oil concentra-tion when PAR intercepted by the striped hull hybrid was reduced (Dosio et al., 2000) would be a consequence of the lower level of intercepted PAR investigated in this work (the decrease of oil concentration when intercepted solar radiation decreased could be triggered by a lower level of intercepted PAR in striped hull hybrids than in black hull ones).

Investigating the dynamics of oil yield components dur-ing grain fi lling (e.g., Johnson and Tanner, 1972 for maize [Zea mays L.] weight per grain) could help to identify the underlying mechanism of variations in the fi nal value of these yield components. Recent research characterized the dynamics of accumulation of oil weight and weight per grain in three genotypes with 30, 45, or 58% of oil poten-tial concentration (Mantese et al., 2006) and under diff erent source–sink relationships (estimated as leaf area duration per grain, Ruiz and Maddonni, 2006). In the black hull hybrid Dekalb G-100, the rate of accumulation of weight per grain

decreased when intercepted PAR decreased (Andrade and Ferreiro, 1996). In the same hybrid, changes in grain oil concentration were linked to changes in the duration of the period when oil concentration linearly increased (Dosio et al., 1997). The eff ect of intercepted PAR on the dynamics of accumulation of weight per grain and oil concentration was never investigated in other hybrids. Growth rate of grains inserted on the periphery of the head tends to decrease fol-lowing a 80% reduction in intercepted radiation during the early post anthesis period (R5.0 to R5.0+10 d) while the duration of the grain fi lling period showed the opposite trend (Lindström et al., 2006). Such type of compensation between rate and duration could explain the lack of response of oil concentration of the striped hull hybrid NKT when inter-cepted radiation was decreased (Dosio et al., 2000).

The objectives of this work were (i) to investigate the eff ect of intercepted PAR during grain fi lling on fi nal weight per grain and grain oil concentration in several black hull and striped hull hybrids for a larger range of intercepted PAR per plant than that previously reported, and (ii) to analyze the variations in the dynamics of weight per grain and oil concentration under diff erent intercepted PAR during grain fi lling. Special attention was paid to oil concentration responses to intercepted PAR since there is less information about it, as compared to weight per grain.

MATERIALS AND METHODS

ExperimentsSix fi eld experiments were conducted at the Instituto Nacional

de Tecnología Agropecuaria (INTA)-Balcarce Experimental

Station, Argentina (37°45′ S, 58°18′ W), and two at the Uni-

versidad Nacional de Córdoba, Argentina (31°19′ S, 64°13′ W).

Soils at the experimental sites were Typical Argiudol and Entic

Haplustol at Balcarce and Córdoba, respectively (USDA Soil

Taxonomy). Soil fertility was enough in all experiments to attain

maximum yields of the sunfl ower crop grown under nonlimiting

water conditions (yield > 5000 kg ha–1; Sosa et al., 1999; Andrade

et al., 2000). Rainfall was complemented with irrigation to avoid

water defi cit. Pests and diseases were adequately controlled.

Experiments were designed to cover a wide range of (i) gen-

otypes with black hull and striped hull grains, and (ii) intercepted

PAR per plant during grain fi lling. Seven black hull hybrids and

fi ve striped hull ones, as well as an intercepted PAR ranging from

3.5 to 104.7 MJ per plant, were evaluated (Table 1).

Four black hull hybrids and three striped hull ones were

sown at Balcarce (Exp. A1), and at Córdoba (Exp. A2) during

the 1997–1998 growing season. Black hull hybrids were Deka-

sol 3881 (Dekalb-Monsanto S.A.), Orion (Sursem S.A.), black

Contifl or 3 (Advanta S.A.) and Paraíso 6 (Nidera S.A.), and the

striped hull ones were Morgan 734 (Mycoyen S.A.), striped Con-

tifl or 3 (Advanta S.A.), and ACA 884 (Asociación de Cooperati-

vas Argentinas). The only diff erence between black Contifl or 3

and striped Contifl or 3 was the quantitative trait loci of around

8.1 cM conferring the hull type trait in one of their parent lines

(Leon et al., 1996). Thus, both hybrids diff er in potential oil

concentration. The black hull hybrid Dekalb G-100 (Dekalb

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690 WWW.CROPS.ORG CROP SCIENCE, VOL. 48, MARCH–APRIL 2008

C2, respectively). Treatments to vary intercepted PAR per plant

were applied from 250 to 450°C d after fl owering (base tem-

perature: 6°C), the developmental period during which fi nal

weight per grain and oil concentration were best accounted for

by intercepted PAR per plant (Aguirrezábal et al., 2003). Treat-

ments consisted of (i) 80% shaded and (ii) untreated control.

The black hull hybrid Dekalb G-100, and the striped hull

one Northrup King Tordillo were sown at Balcarce during the

1993–1994 (Exp. D) and 1995–1996 (Exp. E) growing seasons.

Treatments to vary intercepted PAR per plant during the whole

grain fi lling period consisted of (i) 50% shaded, (ii) thinning

to 25% (Exp. D) or 50% (Exp. E) of the original plant density,

(iii) 50% shaded + thinning, and (iv) untreated control. Further

details about Exp. D and E are given in Dosio et al. (2000).

All experiments excluding B1 and B2 were designed as

split plots with main plots completely randomized and three

replicates (except for Exp. D with four replicates). Hybrids were

assigned to main plots and treatments to subplots. Experiments

Monsanto S.A.), and the striped hull one Northrup King Tor-

dillo (Eneka S.A.) used by Dosio et al. (2000) were also sown

in Exp. A1 at Balcarce. Tested hybrids were released between

1983 and 2002 (Table 1). Treatments to vary intercepted PAR

per plant during the whole grain fi lling period consisted of (i)

50% shaded, (ii) thinning to 50% of the original plant density,

and (iii) untreated control. In A2, hybrid Paraíso suff ered severe

plant damage during a storm and was discarded.

The black hull hybrid Dekasol 3881 and the striped hull

one ACA 884 were sown at Balcarce (Exp. B1) and at Cór-

doba (Exp. B2) during the 1998–1999 and 1999–2000 growing

seasons, respectively. Treatments to vary intercepted PAR per

plant during the whole grain fi lling period consisted of (i) 80%

shaded, (ii) 50% shaded, (iii) thinning to 50% of the original

plant density, and (iv) untreated control.

The black hull hybrid Payé (KWS) and the striped hull

one Paraíso 30 (Nidera S.A.) were sown at Balcarce during the

2002–2003 and the 2003–2004 growing seasons (Exp. C1 and

Table 1. Location, sowing date, plant density, hybrids, and cumulative intercepted photosynthetically active radiation (PAR) per

plant (mean of each hybrid) of each experiment.

Experiment LocationSowing

dateDensity

Hull type

Hybrids

Cumulative intercepted radiation

80% shaded

50% shaded

Control50%-shaded-

thinnedThinned

plants ha–1 ————————————— MJ plant–1 —————————————

A1 Balcarce 17 Nov. 1997 60900 Black Dekasol 3881

(1993†)

– 18.9 38.7 – 76.2

G-100 (1984) – 25.3 42.5 – 85.7

Orion (1993) – 19.3 35.8 – 83.1

Paraíso 6 (1993) – 19.5 31.7 – 75.3

Black Contifl or 3

(1997)

– 21.8 45.1 – 90.1

Striped ACA 884 (1991) – 20.0 37.2 – 72.0

NKT (1988) – 24.2 43.0 – 89.2

Striped Contifl or 3

(1983)

– 22.3 40.2 – 86.2

Morgan 734 (1989) – 22.0 38.9 – 75.9

A2 Cordoba 28 Oct. 1997 52000 Black Dekasol 3881 – 10.9 33.5 – 52.5

Orion – Not measured Not measured – Not measured

Black Contifl or 3 – 10.2 26.7 – 48.2

Striped ACA 884 – 15.4 33.3 – 59.2

Striped Contifl or 3 – 8.6 25.4 – 44.3

Morgan 734 – Not measured Not measured – Not measured

B1 Balcarce 3 Nov. 1998 67100 Black Dekasol 3881 7.2 15.7 32.7 – 53.0

Striped ACA 884 7.8 17.6 36.3 – 60.1

B2 Cordoba 24 Nov. 1999 52000 Black Dekasol 3881 3.5 10.6 28.1 – 57.2

Striped ACA 884 5.4 15.5 35.1 – 49.5

C1 Balcarce 22 Nov. 2002 72000 Black Payé (2002) 16.3‡ – 35.6 – –

Striped Paraiso 30 (1999) 23.3 36.9 –

C2 Balcarce 30 Oct. 2003 72000 Black Payé 17.2‡ – 37.5 – –

Striped Paraiso 30 19.4 – 37.4 –

D§ Balcarce 30 Nov. 1993 72000 Black G-100 – 12.3 22.9 34.2 74.3

Striped NKT 14.1 27.3 34.0 78.8

E§ Balcarce 14 Nov. 1995 72000 Black G-100 – 29.7 67.5 61.9 107.0

Striped NKT 37.6 68.9 61.6 102.4

†Year of release of the hybrid.

‡Shaded only during the 250 to 450°C d period after fl owering (base temperature = 6°C).

§Further details in Dosio et al. (2000).

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CROP SCIENCE, VOL. 48, MARCH–APRIL 2008 WWW.CROPS.ORG 691

B1 and B2 were designed as randomized complete blocks with

four and three replicates, respectively. In all cases experimental

units consisted of four rows 0.7 m apart and 6 m long.

Flowering of a plant was defi ned by the appearance of sta-

mens in all fl orets from the outer ring of the capitulum (R5.1

stage, Schneiter and Miller, 1981). Flowering of a plot was reg-

istered when 95% of plants had fl owered. Flowering occurred

between 13 and 20 February in Exp. A1, between 18 and 23

January in Exp. A2, on 28 January in Exp. B1, on 26 January

in Exp. B2, between 22 January and 1 February in Exp. C1,

between 15 and 20 January in Exp. C2, on 10 February in Exp.

D, and on 20 January in Exp. E.

Treatments were applied when 95% of the plants had their

inner fl orets fertilized (after R6, Schneiter and Miller, 1981) in all

experiments except for Exp. C1 and C2. Treatments were applied

at 250°C d after fl owering (R7) in Exp. C1 and C2. Shading treat-

ments were achieved by using a uniform, black, synthetic, and

neutral mesh cloth. Physiological maturity was taken as the day in

which individual weight per grain did not increase compared to

the previous sample (Exp. C, D, and E), or estimated visually (Exp.

A and B), from the hard yellow color of the capitulum back face

and from the brown color of its bracts (Farizo et al., 1982). Treat-

ments ended at physiological maturity except in Exp. C1 and C2

in which treatments ended at 450°C d after fl owering.

MeasurementsGlobal daily incident radiation was measured with pyranometers

(LI-200SB, LI-COR, Lincoln, NE) located 400 m (Balcarce)

or 2000 m (Córdoba) from the experiments. The proportion of

PAR intercepted by the crop as well as daily intercepted PAR

per plant was measured as described by Dosio et al. (2000), in

all hybrids in A1, B1, B2, C1, C2, D and E, and only in Dekasol

3881 and ACA 884 in A2.

Air temperature was measured with copper-constantan ther-

mocouples (Thermoquar, Buenos Aires) in Balcarce experiments,

and with thermistors (Stow Away XTI, ONSET, Computer Cor-

poration, Pocasset, MA) in Córdoba experiments (except in Exp.

A2). The sensors were cross-checked before the beginning of the

experiments. Measurements were registered every 60 s, data were

averaged every 3600 s and recorded with data loggers (Data Log-

ger, LI-COR 1000; Delta-T DL2e Logger, Delta-T Devices Ltd.,

Cambridge, UK; or ONSET, Computer Corporation, Pocasset,

MA). In Exp. A2 air temperature was obtained from a meteo-

rological station placed ≤8000 m away from the crops. Thermal

time after fl owering (0°C d after fl owering) was calculated from

air temperature using a base temperature of 6°C.

Weight per grain and grain oil concentration were peri-

odically measured in Exp. B1 (hybrids Dekasol 3881 and ACA

884), Exp. D, and Exp. E (hybrids G-100 and NKT). Three to

fi ve capitula were sampled, oven-dried (air circulating at 60°C)

up to constant weight, and weighed. Only nonempty grains

(kernel occupying at least 20% of total space in the grain) were

recovered. In all the experiments fi nal nonempty grains were

counted and weighed. Weight per grain was calculated as the

quotient between the weight of all nonempty grains in each

capitulum and its number. Oil concentration was measured by

nuclear magnetic resonance (NMR) technique. Samples were

dried in an oven at 60°C for 18 h and placed in dessicators with

CaO. When they cooled to 23°C (Robertson and Morrison,

1979) their oil concentrations were determined with a NMR

Analyzer Magnet Type 10 (Newport Oxford Instruments,

Buckinghamshire, UK) in duplicate and averaged.

Kernel proportion in the grain and kernel oil concentra-

tion was determined in Exp. A1, A2, B1, and B2. Samples were

prepared as a homogeneous grain mixture of the plants in each

experimental unit. After mechanical dehulling, kernel oil con-

centration was measured by NMR as described for grain oil

concentration. Kernel proportion in the grain was calculated in

triplicate as Wk/(Wh + Wk), where Wk and Wh represented

kernel and hull weight, respectively, considered in a dry base.

Data AnalysisData were processed by analysis of variance procedures (Gen-

eral Linear Model Procedure; SAS Institute, 1988). When sta-

tistical diff erences were detected in more than one experiment,

the highest P value is shown. Diff erences among means of treat-

ment were evaluated with the Tukey test (P < 0.05). When

treatment × hybrid interaction is not statistically signifi cant (P

> 0.05) the group means are presented with their signifi cance;

otherwise, the values for each hybrid and treatment are pre-

sented with their signifi cance.

Two-step conditional models were used to determine the rate

and duration of the linear increase phase for weight per grain and

oil concentration from Exp. B1, D and E. The fi rst step represented

the weight per grain or grain oil concentration during its phase

of linear increase: weight per grain (or oil concentration) = a + b

× °C d after fl owering (°C daf), for °C daf < c, and the second one

the weight per grain (or oil concentration) during the stabilization

phase: weight per grain or oil concentration = a + b × c, for °C

daf > c, where a and b represented the intercept and the rate of the

linear increase phase, and c was the unknown °C d after fl ower-

ing where maximum weight per grain or oil concentration was

reached. Duration of the oil concentration linear increase phase

was considered as c − duration of the “lag phase” (an initial phase

during which oil concentration slowly increased).

To study quantitatively the response of weight per grain

and oil concentration to intercepted PAR during grain fi lling

and to source–sink ratio data from diff erent experiments were

normalized as described by Ruiz and Maddonni (2006). Briefl y,

for each data set corresponding to a same hybrid in the same

experiment, weight per grain, oil concentration, intercepted

PAR during grain fi lling (the source), and intercepted PAR per

grain (the source–sink ratio) were expressed in relative terms as:

[(Xi − X

m)/X

m] where X

i is the individual value of each vari-

able, Xm is the average of the variable in that hybrid across the

diff erent radiation treatments. Normalized data were adjusted

to linear and linear-plateau (two-step conditional) models.

Grain oil concentration, kernel proportion in the grain and

kernel oil concentration were expressed as a proportion of the

maximum value. Linear regressions were performed on these

normalized data.

RESULTS

Final Oil Yield ComponentsGrain number, weight per grain, oil concentration and oil yield per plant were statistically diff erent (P ≤ 0.01) among hybrids in A1 and A2. In general, striped hull hybrids

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692 WWW.CROPS.ORG CROP SCIENCE, VOL. 48, MARCH–APRIL 2008

presented a higher weight per grain than black hull ones (52.2 ± 9.4 mg vs. 42.6 ± 7.8 mg, average of control treat-ments) and no diff erences in the grain number between groups were observed (Table 2). Oil yield per plant ranged from 6.5 to 36.3 g plant–1 among treatments, hybrids, and experiments. In Exp. A1, the hybrid × treatment interac-tion was statistically signifi cant (P < 0.0001). Oil yield per plant in all the hybrids was diff erent between shaded and thinned treatments (Table 2). In A2, the hybrid × treatment interaction was not statistically signifi cant (P = 0.116). Shaded treatment presented a lower oil yield per plant than the control, and the thinned treatment showed a higher value than the control (Table 2).

In A1, diff erences in oil concentration among treatments were not statistically signifi cant for striped hull hybrids, while black hull hybrids increased grain oil concentration when intercepted PAR per plant increased (Table 2). The black hull hybrid Dekalb G-100 and the striped hull hybrid NKT (included in experiments from Dosio et al., 2000) fol-lowed a similar behavior as other hybrids with black hull or striped hull, respectively. The black hull hybrid Contifl or 3 reduced oil concentration in 4.6 percentage points when intercepted PAR per plant was reduced, in accordance with other black hull hybrids. Its near-isohybrid striped Conti-fl or 3 showed constant oil concentration despite the varia-tion in intercepted PAR, as did other striped hull hybrids. In this experiment, shaded treatments presented a lower grain number with respect to the control, and thinned treatments showed the highest one (Table 2). Grains from thinned treat-ments were heavier than grains from the other treatments (Table 2). Mean incident PAR during grain fi lling was 9.1 ± 2.0 MJ m–2 d–1 and the range of intercepted PAR explored among treatments varied from 21.3 ± 1.8 (average of shaded treatments) to 81.5 ± 6.4 (average of thinned treatments) MJ plant–1 (Table 1).

In A2, all hybrids, black or striped hull, reduced grain oil concentration when intercepted PAR was reduced by shading, and no diff erences were observed between thinned treatment and the control (Table 2). Highest dif-ferences among treatments were observed in black hull hybrids (mean diff erence between extreme values was 5.3 and 8.1 oil percentage points for striped hull and black hull hybrids, respectively). Grain number was mostly unaf-fected for variation in intercepted PAR in both black hull and striped hull hybrids, although in some cases shaded treatment showed a lower grain number than the control (Table 2). Most of the hybrids presented signifi cant dif-ferences for weight per grain only between shaded and thinned treatments. In this experiment, mean incident PAR during grain fi lling was 8.1 ± 3.7 MJ m–2 d–1.

Low incident PAR during 10 consecutive days was registered in Exp. A2. Such a period started 13 d after the beginning of treatment application (data not shown). Mean incident PAR during this period was 4.2 ± 1.1, almost

50% lower than in the rest of the grain fi lling period. Such a cloudy period began at 240 and ended at 360°C day after fl owering and partly overlapped the developmental inter-val 250 to 450°C d after fl owering.

In Exp. C1 and C2, an 80% reduction of intercepted PAR during the period 250 to 450°C d after fl owering highly reduced grain oil concentration in the black hull hybrid Payé and the striped hull hybrid Paraíso 30. Grains from control treatments were 20 and 13% heavier than shaded one in the black hull and the striped hull hybrid, respectively, in Exp. C1, and 24% and 26%, respectively, in Exp. C2 (Table 2). The grain number was aff ected by shading in the black hull hybrid, but not in the striped hull one (Table 2).

In B1 and B2, 80% shaded treatments registered lower values of intercepted PAR per plant during grain fi lling than those registered in A1, A2, and in previous works (Table 1). Even the black hull hybrid Dekasol 3881 or the striped hull one ACA 884 reduced grain oil concentration when intercepted PAR per plant was reduced (Table 2). Statistical diff erences were observed between thinned or control treatments and 80% shaded treatment (P < 0.0001), in both hybrids and experiments. In Exp. B2, diff erences were also signifi cant between thinned or control treat-ments and 50% shaded one (Table 2). Maximum diff erence for this variable was observed in the black hull hybrid (17 percentage points, Exp. B). Although both hybrids were aff ected by intercepted PAR, the maximum grain oil con-centration was reached at a lower level of intercepted PAR in the striped hull hybrid (around 20 MJ plant–1) than in the black hull one (around 40 MJ plant–1).

The treatment × hybrid interaction was not statisti-cally signifi cant for grain number, weight per grain, and oil yield per plant (P ≥ 0.13) in Exp. B1 and B2. Increasing intercepted PAR per plant increased grain number from 825 to 1125 in B1 and from 748 to 1090 in B2 (P ≤ 0.002). Weight per grain increased from 39.8 to 52.5 mg in B1 and from 52.2 to 79.6 mg in B2 (P ≤ 0.002). In these experi-ments, variations up to 26 g of oil yield per plant were observed between thinned and 80% shaded treatment.

Reducing intercepted PAR during grain fi lling reduced oil yield per area in all the hybrids. However, striped hull hybrids tend to present higher oil yield per area than black hull ones even for the control (e.g., 1510 vs. 1281 kg ha–1 for striped and black hull hybrids, respec-tively, Exp. A1) or reduced levels of intercepted PAR dur-ing grain fi lling (e.g., 1128 vs. 806 kg ha–1 for striped and black hull hybrids, respectively, Exp. A1).

Relationships between Weight per Grain and Oil Concentration and Source and Source Sink Ratio for Black and Striped Hull HybridsWhen normalized data from all experiments were pooled, relative weight per grain and relative oil concentration showed a linear-plateau response to relative intercepted

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CROP SCIENCE, VOL. 48, MARCH–APRIL 2008 WWW.CROPS.ORG 693

PAR during grain fi lling (r2 = 0.77 and r2 = 0.65, for weight per grain and oil concentration, respectively). The adjustments were not improved by linear functions compared to linear-plateau functions (r2 = 0.73 vs.0.77 and r2 = 0.53 vs. 0.65, for weight per grain and oil concentration, respec-tively). The relative source–sink ratio (intercepted PAR per grain) did not accounted for better than the source neither for relative weight per grain nor for oil concentration (r2 = 0.74 vs. 0.77 and r2 = 0.62 vs. 0.65, respectively).

The variability in relative weight per grain and in relative oil concentra-tion were also better accounted for by the variability in the relative source than by the variability in the relative source–sink ratio when separate functions were adjusted to data corresponding to black hull and striped hull hybrids (data not shown). Relative weight per grain of both black hull and striped hull hybrids increased with a similar rate (0.30 ± 0.04 vs. 0.27 ± 0.04, respectively) up to rela-tive intercepted PAR = 0.24 ± 0.10 and 0.33 ± 0.14 for black hull and striped hull hybrids (Fig. 1). Relative oil concentra-tion increased with a higher rate (0.26 ± 0.05 vs. 0.20 ± 0.06) up to a higher rela-tive source value (0.21 ± 0.13 vs. –0.03 ± 0.13) in black hull than in striped hull hybrids (Fig. 1).

Evolution of Weight per Grain and Grain Oil ConcentrationEvolution of weight per grain and oil concentration of black hull and striped hull hybrids was expressed as a function of thermal time after fl owering (base temperature = 6°C). Data from weight per grain were adjusted, in black hull and striped hull hybrids, to a two-step condi-tional function which described a phase of linear increase and a plateau where the maximum weight was attained. The weight per grain growth rate and dura-tion tended to increase from shaded to thinned treatments in all hybrids and experiments (Table 3). Rates and dura-tions for weight per grain evolution tended to be higher in striped hull than in black hull hybrids (Fig. 2).

Table 2. Oil yield components in striped hull and black hull hybrids in response to the

different radiation treatments. Data correspond to Exp. A1, A2, B1, B2, C1, and C2.

ExperimentsHull type

Hybrid TreatmentGrain no.

Weight per grain

Oil concentration

Oil yield

mg % g

A1 Black Dekasol 3881 50% shaded 570 40.9b† 45.3c 10.7c

Control 869 51.6a 49.7b 22.5b

Thinned 1141 58.6a 53.7a 36.3a

Orion 50% shaded 494 39.6a 47.2b 9.3c

Control 840 42.2a 52.4a 18.7b

Thinned 1153 46.3a 55.3a 29.9a

B-Contifl or 3 50% shaded 719 47.5b 42.4b 14.6b

Control 881 51.1ab 45.9ab 21.1ab

Thinned 1117 55.1a 47.0a 29.3a

Paraíso 6 50% shaded 984 38.5b 46.7b 18.1b

Control 1070 39.8b 48.9ab 21.0b

Thinned 1440 48.2a 51.5a 36.3a

G-100 50% shaded 694 42.0b 45.4b 13.5b

Control 892 47.9ab 50.7a 21.9a

Thinned 1000 49.4a 52.8a 26.2a

Striped Morgan 734 50% shaded 694 58.8b 43.0a 17.9b

Control 851 63.0b 44.1a 23.8ab

Thinned 1058 70.6a 42.4a 31.7a

S-Contifl or 3 50% shaded 714 53.0b 39.7a 15.2c

Control 827 54.7ab 39.5a 18.1b

Thinned 1209 61.5a 40.4a 30.0a

ACA 884 50% shaded 858 51.0b 43.5a 19.2b

Control 1110 58.2ab 45.5a 29.4a

Thinned 1187 65.5a 44.8a 35.1a

NKT 50% shaded 752 65.3a 44.1a 21.8b

Control 957 63.7a 45.5a 27.9ab

Thinned 1225 65.2a 42.8a 34.1a

50% shaded 720c

Mean Control 922b

Thinned 1170a

A2 Black Dekasol 3881 50% shaded 1193a 31.0b 34.1b 13.0

Control 1062a 42.2a 42.3a 18.9

Thinned 1174a 44.5a 44.1a 22.7

Orion 50% shaded 1082b 22.7a 36.5b 9.1

Control 1444a 22.6a 42.5a 15.8

Thinned 1407a 27.6a 43.7a 17.2

B-Contifl or 3 50% shaded 947b 31.0b 35.7b 10.9

Control 1302a 40.4ab 42.7a 22.4

Thinned 1332a 42.2a 42.8a 24.3

Striped Morgan 734 50% shaded 574a 31.8b 35.1b 6.5

Control 786a 43.0a 40.4a 13.8

Thinned 770a 34.7ab 38.7a 10.7

S-Contifl or 3 50% shaded 1213b 34.4a 32.3b 13.7

Control 1395ab 43.3a 38.6a 23.5

Thinned 1595a 42.5a 38.5a 26.0

ACA 884 50% shaded 1100a 33.0b 36.4b 13.3

Control 1392a 39.3ab 41.2a 22.5

Thinned 1262a 45.6a 40.7a 23.6

50% shaded 11.1c

Mean Control 19.5b

Thinned 20.8a

B1 Black Dekasol 3881 80% shaded 880b 39.4c 38.5b 13.5c

50% shaded 892ab 43.7b 44.5ab 17.4b

Control 900ab 48.8ab 47.9a 21.0ab

Thinned 1156a 51.3a 51.1a 30.9a

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Oil concentration slowly increased during approxi-mately the fi rst 200°C d after fl owering, and then it grew faster up to a plateau (Fig. 3). Oil concentration during the

phase of slow increase was considered constant (5 mg g–1) and its duration was calculated for each hybrid and experi-ment as the thermal time when such oil concentration was

attained. This duration ranged from 162 to 208°C d among hybrids and experi-ments. Rates of increase in oil concen-tration were not aff ected by intercepted PAR (Fig. 3, Table 4). Duration increased with the increase in intercepted radia-tion during grain fi lling in black hull hybrids. In the striped hull hybrid NKT, duration was not aff ected in Exp. E. In the striped hull hybrid ACA 884, which decreased grain oil concentration in the shaded treatment in Exp. B1 (Table 2), duration was decreased.

In the three experiments oil concen-tration accumulated faster in black hull hybrids than in striped hull ones (more than 10% faster in Exp. B1 and E). How-ever, neither rates nor duration of the phase during which oil concentration rapidly increased showed statistical dif-ferences among hybrids (P > 0.05).

Kernel Proportion in the Grain and Kernel Oil ConcentrationThe higher grain oil concentration of black hull hybrids with respect to

striped hull ones was mainly due to a higher kernel proportion in the grain (Table 5, Exp. A1 and A2). Despite their lower kernel pro-portion, striped hull hybrids presented a higher weight per grain because they produce grains 13% heavier than black hull hybrids. Increasing intercepted PAR per plant increased kernel oil concentration even in black hull or striped hull hybrids (Table 6). No eff ect of radiation treat-ments was observed on the kernel proportion in the grain, except for Morgan 734 and NKT in Exp A1 (Table 5).

Grain oil concentration was linearly related to kernel oil concentration when both were expressed as a proportion of their maximum value (P = 0.0001, Fig. 4). This relationship was observed either in the black hull hybrid Dekasol 3881 (r2 = 0.92, P = 0.0001, n = 44) or the striped hull one ACA 884 (r2 = 0.84, P = 0.0001, n = 46). When the same variable was related to kernel proportion in the grain the relationship was not statistically signifi cant (P = 0.25) for the striped hull hybrids. This relationship was statistically signifi cant for the black hull hybrids (P = 0.018, r2 = 0.13, n =

Figure 1. Relative weight per grain (a, b) and relative oil concentration (c, d) as a

function of relative intercepted radiation during grain fi lling for black hull (a, c) and

striped hull (b, d) hybrids.

ExperimentsHull type

Hybrid TreatmentGrain no.

Weight per grain

Oil concentration

Oil yield

Striped ACA 884 80% shaded 770b 40.3c 38.7b 12.1c

50% shaded 936ab 48.6b 44.3ab 20.9b

Control 1041ab 51.4ab 44.9a 25.1ab

Thinned 1068a 53.7a 46.6a 26.2a

B2 Black Dekasol 3881 80% shaded 874b 58.2b 29.8c 14.9c

50% shaded 948a 56.6b 35.7b 19.2b

Control 1013a 76.6a 46.6a 35.8a

Thinned 1095a 80.9a 46.8a 41.7a

Striped ACA 884 80% shaded 622b 46.2b 30.6c 8.8c

50% shaded 966a 56.4b 37.1b 20.2b

Control 964a 75.1a 40.9a 29.6a

Thinned 1085a 78.3a 40.1a 34.1a

C1 Black Payé S80 partial 1006b 35.4b 38.4b 13.7b

Control 1308a 44.1a 48.2a 27.8a

Striped Paraíso 30 S80 partial 1069a 38.1b 45.5b 18.5b

Control 1040a 42.7a 49.2a 21.9a

C2 Black Payé S80 partial 999b 37.9b 36.5b 13.8b

Control 1449a 49.8a 48.7a 35.1a

Striped Paraíso 30 S80 partial 974a 39.8b 39.8b 15.4b

Control 1159a 54.0a 48.2a 30.2a

†Different letters within a column indicate signifi cant differences (P < 0.05) among radiation treatments for each

cultivar in each experiment.

Table 2. Continued.

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44), but with a coeffi cient of determina-tion largely lower than that of the rela-tionship using kernel oil concentration as independent variable. A similar trend was observed in the other hybrids (Tables 5 and 6), indicating that changes in grain oil concentration by eff ects of intercepted PAR are originated by changes in kernel oil concentration even in striped hull and black hull hybrids.

DISCUSSIONOil concentration in striped hull hybrids could be aff ected by a decrease in inter-cepted radiation as in black hull hybrids. They are, however, less sensitive to inter-cepted radiation than black hull hybrids since their oil concentration reaches the maximum value at a lower level of intercepted PAR per plant. In a previous work, the oil concentration of the black hull hybrid, Dekalb G-100, decreased when intercepted PAR was low while the striped hull one, NKT, remained unaff ected for a range from 13.2 to 104.7 MJ per plant (Dosio et al., 2000). In agreement with these results, oil con-centration was decreased by 50% shad-ing in black hull hybrids while it was not aff ected in striped hull hybrids in Exp. A1. Oil concentration was, however, decreased in striped hull hybrids when intercepted PAR was lower because of a lower incident solar radiation, obtained through a higher shading intensity (80% shading, Exp. B1, B2, C1, and C2) or a cloudy period during a part of grain fi ll-ing (50% shading, Exp. A2). Despite a high value of intercepted PAR accumulated during the whole grain fi lling, such reduction in oil concentration of striped hull hybrids was still detected in Exp. C1 and C2 when 80% shading was applied only during the period 250 to 450°C d after fl owering. This was the period during which weight per grain and oil concentration of the black hull hybrid Dekalb G-100 were most aff ected by a reduction in intercepted radiation (Aguirrezábal et al., 2003). It could be suggested that such a critical period near the middle of grain fi lling could also apply for other black hull and striped hull hybrids. Finally, when data from all experiments were pooled and normalized in a similar way as Ruiz and Maddonni

(2006), oil concentration attained its maximum value at a lower intercepted radiation in striped hull hybrids compared to black hull hybrids.

Figure 2. Weight per grain as a function of thermal time after fl owering in the black hull

hybrids G-100 (Exp. D and E) and Dekasol 3881 (Exp. B1), and striped hull hybrids NKT

(Exp. D and E) and ACA 884 (Exp. B1). Arrows indicate the time of treatment application

(TA). Sh., shaded, Th., thinned, °Cd, degree days.

Table 3. Effect of radiation treatments on weight per grain growth rate (mg

°C d–1), and duration of the fi lling period (°C d) average for all black hull (G-100

and DK 3881), and striped hull (NKT and ACA 884) hybrids.

Experiments 80% Shaded 50% Shaded Control50% Shaded-

thinnedThinned

Rate B1 0.081a† 0.090a 0.091a – 0.092a

D – 0.094b 0.101ab 0.105a 0.112a

E – 0.115a 0.116a 0.117a 0.119a

Duration B1 544b 568ab 584ab – 616a

D – 490a 512a 509a 527a

E – 521a 574a 556a 584a

†Different letters within a row indicate signifi cant differences (P < 0.05) among radiation treatments.

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The level of intercepted PAR at which striped hull hybrids reach the maximum grain oil concentration is low compared to the typical values of intercepted PAR during grain fi lling in sunfl ower crops grown under potential conditions. These results could help to explain why oil con-centration in striped hull hybrids is often considered as more stable to environmen-tal changes than in black hull hybrids (V. Pereyra, President Biosol Semillas, Bal-carce, Buenos Aires, Argentina, personal communication, 2007). The level of inci-dent radiation needed to obtain a response in striped hull hybrids in experiments presented here rarely occurs in nature. It would be improbable that variations in incident solar radiation decrease oil con-centration in striped hull sunfl ower crops grown under potential conditions. How-ever, reductions in oil concentration in striped hull crops could be still possible under fi eld conditions because of a low leaf area index during grain fi lling. Leaf area index during most of grain fi lling is often lower than the value needed to obtain a 95% interception of the incident solar radiation (2.5 to 3.0, Aguirrezábal et al., 1996, 2003) and other factors as hail, pests, and diseases could reduce it (e.g., Schneiter et al., 1987; Muro et al., 2001). Keeping a higher leaf area index during grain fi lling through crop management could help to obtain the maximum oil concentration with striped hull hybrids.

Genetic improvement in Argentina has directly or indirectly modifi ed sev-

eral traits related to yield, yield compo-nents and grain quality (López Pereira et al., 1999a, 1999b, 2000), including the increase in grain oil concentration (de la Vega et al., 2007), mainly by increas-ing kernel proportion in the grain (A. Vazquez, Sunfl ower Breeder, NIDERA Semillas S.A., Baigorrita, Buenos Aires, Argentina, personal communication, 2004). Results obtained in this work con-fi rm this increase in grain oil concentra-tion even in striped hull hybrids (e.g., the oil concentration of the hybrid Paraiso 30 in Exp. C1 and C2 attained values higher than 48%). Genetic improve-ment, however, does not seem to have aff ected the response of oil concentration

Figure 3. Grain oil concentration as a function of thermal time after fl owering in the black

hull hybrid G-100 (Exp. D and E) and Dekasol 3881 (Exp. B1), and striped hull hybrids

NKT (Exp. D and E) and ACA 884 (Exp. B1). Arrows indicate the time of treatment

application (TA). Sh., shaded; Th., thinned; °Cd, degree days.

Table 4. Effect of radiation treatments on grain oil accumulation rate (mg. g. °C d–1),

and duration (°C d) of the linear phase of oil increasing average of all black hull

(G-100 and DK 3881) and striped hull (NKT and ACA 884) hybrids.

Experiments Hybrid80%

Shaded50%

ShadedControl

50% Shaded-thinned

Thinned

Rate B1 Mean 0.142a† 0.141a 0.143a – 0.139a

D Mean – 0.213ab 0.195bc 0.231a 0.185c

E G-100 – 0.216a 0.198a 0.193a 0.177ab

NKT – 0.185ab 0.144b 0.178ab 0.182ab

Duration B1 Mean 238c 275b 294ab – 312a

D G-100 – 166c 199ab 167c 219a

NKT – 183bc 187bc 174c 214a

E G-100 – 186c 232abc 194bc 254ab

NKT – 209abc 268a 208abc 203bc

†Data correspond to Exp. B1, D, and E. Different letters within a row indicate signifi cant differences (P < 0.05)

among radiation treatments.

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to changes in intercepted radiation in striped hull and black hull genotypes. In our experi-ments, all black hull and striped hull hybrids showed the response described for each type of hybrid, independent of the year in which they where released to the market.

Both the rate and the duration of grain fi ll-ing were aff ected by intercepted PAR. How-ever, the rate of increase of oil concentration was markedly stable despite wide variations in intercepted PAR. In both black hull and striped hull hybrids, a decrease in intercepted radiation decreased fi nal oil concentration mainly due to a reduction in the duration of the phase during which oil concentration linearly increases. Final oil concentration results from the relationships between the dynamics of weight per grain and oil weight per grain. A lower intercepted radia-tion decreased fi nal oil concentration through an early detention of the phase during which oil is accumulated in a higher proportion than the rest of the chemical components of the grain. Such developmental process seems to be modulated by a decrease in intercepted radiation near the middle of grain fi lling (i.e., 250–450°C d after fl owering, Aguirrezábal et al. [2003] and Exp. C1 and C2 in this paper). Interestingly, changes in duration of the phase during which oil con-centration linearly increases also explained most of the diff erences in fi nal oil concentration among black hull and striped hull hybrid (e.g., near 5 percentage points between oil concen-tration values of thinning treatment, Exp. B1). This agrees with Mantese et al. (2006) who observed simi-lar results when studying the dynamics of accumulation of oil in three genotypes with 30, 45, or 58% of oil potential.

No compensation between rate and duration was observed neither for weight per grain nor for grain oil concentration. Compensation between rate and dura-tion of leaf expansion has been identifi ed as a common response in Arabidopsis thaliana (L.) Heynh. to diff erent environmental stresses such as light, soil water defi cit, and temperature (see discussion in Aguirrezábal et al., 2006). It has been also detected in growth of sunfl ower grains following an 80% reduction in intercepted radiation dur-ing the early post anthesis period (R5.0 to R5.0+10 d, Lindström et al., 2006). Based on these results, it could be suggested that compensation between variables underly-ing the dynamics of weight per grain could only be trig-gered by environmental stresses aff ecting the crop during initial stages of the evolution.

The kernel proportion in the grain was aff ected by treatments in both striped hull and black hull hybrids. These results disagree with our previous fi ndings in

one striped hull and one black hull hybrid (Santalla et al., 2002) and with assumptions that the kernel pro-portion in the grain is stable to environmental changes (e.g., Villalobos et al., 1996). The relationship between grain oil concentration and the kernel proportion in the grain was weak but signifi cant for the black hull hybrid Dekasol 3881. However, oil concentration in the kernel was aff ected by intercepted radiation in most of cases. Oil concentration in the kernel was the only variable to account for variations in grain oil concentration in the hybrid ACA 884 while it largely explained most of the changes in the grain oil concentration (the proportion of variation accounted for by kernel oil concentration and kernel proportion in the grain were 92 and 13%, respec-tively) in hybrid Dekasol 3881. Although it is shown in this work that the kernel proportion in the grain is not insensitive to environmental changes its variation was very low. Changes in grain oil concentration follow-ing changes in intercepted radiation during grain fi lling largely depend on its eff ect on kernel oil concentration of both striped hull and black hull hybrids.

Table 5. Kernel proportion in the grain for striped hull and black hull hybrids

grown under different intercepted photosynthetically active radiation (PAR)

treatments during the grain fi lling period.

Hull HybridA1 A2

Shaded Control Thinned Shaded Control Thinned

Striped Morgan 734 0.72a† 0.70a 0.65b 0.74a 0.74a 0.74a

S. Contifl or 3 0.64a 0.64a 0.64a 0.66a 0.64a 0.65a

ACA 884 0.72a 0.71a 0.68a 0.70a 0.69a 0.68a

NKT 0.73a 0.72a 0.67b – – –

Black Dekasol 3881 0.75a 0.76a 0.77a 0.75a 0.78b 0.77ab

Orion 0.77a 0.77a 0.77a 0.81a 0.78a 0.79a

Paraíso 6 0.74a 0.74a 0.75a – – –

B. Contifl or 3 0.71a 0.72a 0.72a 0.72a 0.71a 0.71a

G-100 0.76a 0.77a 0.77a – – –

†Data correspond to Exp. A1 and A2. For each experiment, different letters within a row indicate signifi -

cant differences (P < 0.05) among radiation treatments.

Table 6. Kernel oil concentration (%) for striped hull and black hull hybrids

grown under different intercepted photosynthetically active radiation (PAR)

treatments during the grain fi lling period.

Hull HybridA1 A2

Shaded Control Thinned Shaded Control Thinned

Striped Morgan 734 58.5a† 60.8b 61.8b 50.3a 53.3a 51.6a

S. Contifl or 3 60.7a 60.4b 60.3b 48.6a 54.7b 55.5b

ACA 884 59.8a 61.1b 62.7b 49.3a 55.4b 54.9b

NKT 57.5a 60.8b 61.9b – – –

Black Dekasol 3881 56.4a 63.2b 65.8b 44.1a 52.4b 54.2b

Orion – – – 45.1a 50.9b 51.6b

Paraíso 6 59.3a 62.4b 64.5b – – –

B. Contifl or 3 57.3a 61.8b 62.7b 48.9a 55.8b 55.2b

G-100 54.4a 62.8b 64.1b – – –

†Data correspond to Exp. A1 and A2. For each experiment, different letters within a row indicate signifi -

cant differences (P < 0.05) among radiation treatments.

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Weight per grain and oil concentration followed lin-ear-plateau relationships with the source– sink relation-ship during grain fi lling. These results agree with results from Ruiz and Maddonni (2006). In this work, the source during grain fi lling was, however, a better explanatory variable than source–sink relationship of both weight per grain and oil concentration. Diff erences between our results and those from Ruiz and Maddonni (2006) prob-ably arise from the main origin of source–sink variation during grain fi lling (the grain number and the leaf area per plant in the work of Ruiz and Maddonni, 2006, and the incident radiation in our work). Our results help us to better understand the validity domain of the source–sink relationships during grain fi lling as an explanatory variable of weight per grain and oil concentration in sun-fl ower. The source is a better predictor than source–sink relationships when the crops are exposed to lower incident radiation during grain fi lling. In the land, such conditions are frequent in several regions in which sunfl owers are grown (e.g., the cloudy period at Córdoba in Exp. A2 in this work). Interestingly, Ruiz and Maddonni (2006) did not fi nd a relationship between source–sink and oil con-centration. In this work, a relationship was found when all the data were pooled. Moreover, a clear diff erence in sen-sitivity was detected when normalized oil concentration

of striped and black hull were separately related to both normalized intercepted variation and normalized source–sink relationships during grain fi lling. Diff er-ences between our results and those from these authors could be partly explained by the fact that we discriminated data by hull color. It is also probable that normal-ization applied by Ruiz and Maddonni (2006) could not account for slight diff er-ences among protocols of oil concentra-tion measurement among diff erent works (in our work all oil concentration data were measured in the same laboratory and by using the same protocol).

Contrary to the data described for grain oil concentration, in our experi-ments no clear diff erences were identi-fi ed in the response of weight per grain to changes in intercepted radiation among black hull and striped hull hybrids. Criti-cal periods for weight per grain and oil concentration were similar (Aguirrezábal et al., 2003) and both oil yield components showed similar responses to changes in source–sink relationships in the work of Ruiz and Maddonni (2006). These results suggest that the mechanisms involved in the determination of weight per grain and

oil accumulation are, at least to some extent, unlinked. The behavior of grain number per plant was neither dif-ferent among striped and black hull hybrids. Due to the similar behavior of grain number and weight per grain and the lower sensitivity of oil concentration to changes on intercepted PAR, oil yield per area tended to be similar or higher in striped hull hybrids than in black hull ones at low levels of intercepted PAR per plant suggesting that sowing striped hull hybrids could be considered when low radiation levels are expected.

CONCLUSIONSGrain oil concentration of both striped hull and black hull hybrids could be aff ected by intercepted PAR dur-ing grain fi lling but in striped hull hybrids the maximum oil concentration is reached at a lower level of intercepted PAR per plant. These results were consistently repeated for both kinds of hybrids irrespective of the environment where the experiments were performed or the striped hull and black hull hybrid studied. Accordingly, when all the data were pooled, oil concentration attained its maximum value at a lower normalized intercepted radiation in striped hull hybrids. Such genotypic diff erences in the response of oil concentration to intercepted PAR must be consid-ered in the modeling of sunfl ower grain quality. The low

Figure 4. Grain oil concentration as a function of oil concentration in the kernel (a, c),

or the proportion of kernel in the grain (b, d) for the Dekasol 3881 (black hull, a and b)

and ACA 884 (striped hull, c and d). All results are expressed as the proportion of the

maximum value of each variable. Maximum values for oil concentration in the grain

are 54.2 and 46.9% for Dekasol 3881 and ACA 884, respectively. Maximum values

for oil concentration in the kernel are 67.0 and 69.6% for Dekasol 3881 and ACA 884,

respectively. Maximum kernel proportions in the grain are 0.79 and 0.73 for Dekasol

3881 and ACA 884, respectively.

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sensitivity of oil concentration to environmental changes in striped hull hybrids helps to explain the higher oil yield stability usually shown by striped hull hybrids, especially under nonoptimal conditions. Despite diff erences in the response of grain oil concentration to changes in inter-cepted PAR, no clear diff erences were found for weight per grain, grain number, or oil yield per plant, suggesting that striped hull hybrids could perform as well as black hull hybrids when a low intercepted radiation during grain fi lling is expected (e.g., late sowing dates).

AcknowledgmentsThis work was supported by Universidad Nacional de Mar

del Plata and Instituto Nacional de Tecnología Agropecuaria.

Natalia Izquierdo and Luis Aguirrezábal are members of the

Consejo Nacional de Investigaciones Científi cas y Técnicas

(CONICET, Argentina). Natalia Izquierdo was a fellow of the

Consejo Nacional de Investigaciones Científi cas y Técnicas

(CONICET, Argentina) and the Comisión de Investigaciones

Científi cas de la Provincia de Buenos Aires (CIC, Argentina).

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Aguirrezábal, L.A., Y. Lavaud, G.A. Dosio, N.G. Izquierdo, F.H.

Andrade, and L.M. Gonzalez. 2003. Weight per seed and

oil concentration in a sunfl ower hybrid are accounted for by

intercepted solar radiation during a defi nite period of seed

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