ELSEVIER Field Crops Research 55 (1998) 209-223

F i e l d C r o p s R e s e a r c h

Chlorophyll fluorescence as a selection criterion for grain yield in durum wheat under Mediterranean conditions

J.L. Araus a,*, T. Amaro a, j. Voltas b, H. Nakkoul c, M.M. Nachit c

a Unitat de Fisiologia Vegetal, Facultat de Biologia, Universitat de Barcelona, Avda. Diagonal 645, 08028-Barcelona, Spain b Area de Conreus Extensius, Centre R + D UdL-IRTA, Lleida, Spain

c International Center for Agricultural Research in the Dry Areas, P. 0. Box 5466, Aleppo, Syria

Received 25 March 1997; revised 11 July 1997; accepted 11 July 1997

Abstract

The relationships between different fast (non-modulated) chlorophyll fluorescence parameters measured in the field and yield were studied in durum wheat (Trit icum durum Desf.) grown in a Mediterranean region (Northwest Syria) under three different water regimes. A set of 144 genotypes were cultivated in two rainfed trials (hereafter referred to as environments), 125 of which were also grown under supplementary irrigation. Fluorescence measurements were made on attached flag leaf blades about 3 weeks after anthesis in the three environments, plus a further set of measurements, two weeks after, in the irrigation environment. The photochemical capacity of Photosystem (PS) II was measured by means of the ratio of variable to maximum chlorophyll fluorescence ( F v / F m ) . In addition, the changes in variable ( F v = F m - F 0) fluorescence, the absolute values F 0, F m, and the half-time of the increase from F 0 to F m ( t l /2) were also determined. Growing environment was found to have a strong effect on yield and all the fluorescence parameters. Except for F v / F m, genotype effect was also significant (P < 0.001) within environments. Interaction between genotype and environment was only significant for grain yield. F 0 showed the highest broad-sense heritability (h 2 = 0.745), followed very closely by F m (h 2 = 0.729), t l /2 (h 2= 0.721) and F v (h 2 =0.679), whereas F v / F m showed much lower heritability (h 2 = 0.143) and grain yield an intermediate value (h 2 = 0.506). The pattern of changes in chlorophyll fluorescence parameters between the irrigation and the driest environments paralleled those associated with ageing in the former. Thus, mean values of F v / F m, F m, and t l /2 decreased and F 0 increased with ageing or in the driest environment. Across genotypes fluorescence parameters changed in a coordinated manner different from that caused by environment or ontogeny. High-yielding genotypes showed lower t l /~ ,

accompanied by higher F o and F m. The parameter which showed the best genetic correlation with grain yield was t l /2

( r = -0 .92) , followed by F 0 ( r = 0.88), F m ( r = 0.74) and F v ( r = 0.71), whereas F v / F m ( r = 0.34) was the parameter least correlated. The coordinated pattern of changes in fluorescence parameters across genotypes suggest that the more productive genotypes are those which can avoid or escape the development of drought stress during grain filling. The observed correlations of t l /2 and F 0 with phenological parameters (days from planting to heading or maturity) or indicators of water status of the crop (carbon isotope discrimination of kernels) support this conclusion. © 1998 Elsevier Science B.V.

Keywords: Durum wheat; Yield; Drought; Chlorophyll fluorescence; Carbon isotope discrimination

* Corresponding author. E-mail: josel@porthos.bio.ub.es.

0378-4290/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PH S0378-4290(97)00079-8

210 J.L. Araus et aL / Field Crops Research 55 (1998) 209-223

1. Introduction

While breeding programs in self-pollinating crops rely on selection for yield in advanced generations, few, if any, alternatives to direct selection for yield have been adopted, though many have been sug- gested (Austin, 1993). Indeed, the lack of identifica- tion of appropriate morphophysiological traits has been one major factor preventing improvement of yield in water-limited environments, as well as the reason why plant breeders have not adopted more analytical approaches to selection. Moreover, any desirable physiological trait should be simple, rapid and more economical than yield to assess (Austin, 1993). Therefore such parameters should be easy to evaluate in large numbers of plants (e.g. in early generations) in a relatively short time, and preferably in the target environment.

Drought associated with high temperature and irradiance are the stresses which at present most limit the cereal yield under Mediterranean conditions because they usually occur together during the repro- ductive stages of the crop. During grain filling the combination of high irradiance and water stress and/or high temperatures may have a synergistic effect on the development of photoinhibition (Powles, 1984; Bolhar-Nordenkampf et al., 1991). Thus, the absorption of excessive light may be a cause of damage in water-stressed plants, when stomatal clo- sure reduces CO 2 availability and consequently af- fects electron transport by limiting consumption of assimilatory power (Boyer et al., 1987). In this re- gard, discrimination against the naturally occurring stable isotope 13C (,4) in kernels of C 3 cereals provides an integrated (in time) measure of the ratio of intercellular to atmospheric partial pressure ( P i / P a ) of CO 2 during grain filling (Farquhar and Richards, 1984; Condon et al., 1992), and thus an indication of CO 2 availability in the mesophyll tis- sues during this period. Stomatal closure also leads to an increase in leaf temperature which eventually can exacerbate photoinhibitory processes.

In such a context, chlorophyll fluorescence is a fast, non-destructive and relatively simple technique for detecting the energetic/metabolic imbalance of photosynthesis due to water or heat stress, or both together (Havaux et al., 1988; Moffatt et al., 1990; Flagella et al., 1994; Ali-Dib et al., 1994; Araus and

Hogan, 1994; Nogurs et al., 1994). Photosystem II (PSII) is particularly susceptible to inhibition (Powles, 1984). When determined in the dark-adapted state, the ratio of variable to maximal fluorescence ( F v / F m) attained during the fast induction kinetics of the Kautsky curve, is a measure of the potential (or maximal) quantum yield of PSII. Thus, Fv/F m, typically in the range of 0.75-0.85 for non-stressed plants (Bolhar-Nordenkampf et al., 1989), is closely correlated with the quantum yield of net photo- synthesis of intact leaves with various levels of photoinhibition (Demmig and Bjtirkman, 1987; Schreiber and Bilger, 1993). Photoinhibition caused by excessive photosynthetic photon flux density (PPFD) is a main cause of reduction of Fv/F m under natural conditions (Long et al., 1994). A decline in Fv/F m is a good indicator of photoinhibitory impair- ment when plants are subjected to a wide range of environmental stresses, including drought and heat (Araus and Hogan, 1994; Angelopoulos et al., 1996; Scarasciamugnozza et al., 1996; Yang et al., 1996), associated with high irradiance. A reduction of F v / F m may represent either a reversible photoprotec- tive down-regulation or an irreversible inactivation of PSII (see references in Araus and Hogan, 1994; Baker and Bowyer, 1994; Long et al., 1994). In examining decreases in Fv/F m it is important to distinguish increases in F 0 from decreases in F v. Thus, for example, an increase in F 0, the fluores- cence emission when all reaction centers are open and the photochemical quenching is minimal, is characteristic of destruction of PSII reaction centers, or the impairment of transfer of excitation energy from antenna to the reaction centers. But a decline in F v (caused by a decrease in F m) may indicate an increase in non-photochemical quenching (Bolhar- Nordenkampf et al., 1989). Photoinhibition leads to both of these changes (Baker and Horton, 1987). However, the latter should be termed photoprotection (Osmond et al., 1987) because it is thought to pro- vide a harmless means of disposal of excess excita- tion. In addition, the area in the curve between F 0 and F m is proportional to the pool size of the electron acceptors on the reducing side of PSII, a simple indication being provided by the parameter tt/2, which is the time required for fluorescence to achieve half of the rise from F 0 to F m (Bolhar- Nordenkampf et al., 1989).

J.L. Araus et al. / Field Crops Research 55 (1998) 209-223 211

Even though chlorophyll fluorescence has often been proposed as a useful tool for screening durum and bread wheat for drought (Havaux and Lannoye, 1985; Havaux et al., 1988; Pastore et al., 1989; Ali-Dib et al., 1994; Flagella et al., 1994, 1995) and high-temperature tolerance (Moffatt et al., 1990), it has not yet been really adopted in breeding pro- grams. Indeed frequently the same authors conclude that this technique should be used with other criteria that might indicate resistance to stress (Moffatt et al., 1990; Flagella et al., 1994). Several constraints limit the usefulness of the technique. Studies have usually been performed with few 'representative' genotypes, usually in controlled environments, and even using excised leaves submitted to fast dehydration in the dark. Rapid dehydration is different from drought at the mechanistic level (Ogren, 1990); and the ranking of genotypes grown under controlled conditions may be very different from under field conditions (Mof- fatt et al., 1990). Hence, Moffatt et al. (1990) con- cluded for wheat that, because of the difficulty of simulating field conditions in a controlled environ- ment, screening is best done in the field. So far, measurements of chlorophyll fluorescence (in intact leaves) in the field to determine the actual response of the photosynthetic apparatus under natural condi- tions have been rather limited (see references in Araus and Hogan, 1994; Bilger et al., 1995), mainly due to the weight and bulkiness of the available equipment (Bolhar-Nordenkampf et al., 1989). Moreover, when this technique is used in the field, it is not usually applied to the measurement of geno- typic differences during reproductive growth, when injury is especially critical in temperate cereals under Mediterranean conditions (Moffatt et al., 1990). Moreover, fluorescence indicators in durum wheat exhibited intrinsic variation after heading (Pastore et al., 1989) or flowering (Flagella et al., 1994), which suggests a relationship with plant senescence. Never- theless, the optimum phenological moment to mea- sure fluorescence is still a matter for discussion: some researchers on durum wheat conclude that for maximum reliability the fluorescence test must be applied at the vegetative phase (Flagella et al., 1994). Further, the predictive capability of fluorescence pa- rameters has often been assessed not just using yield itself, but by comparison with a yield-derived stress index such as the drought susceptibility index of

Fischer and Maurer (see Ali-Dib et al., 1994; Flag- ella et al., 1994). However, this and other such indices have a relatively high genotype × environment interaction and different indices may give different rankings at the same site (Acevedo et al., 1991).

The aim of the present work is to evaluate the performance of fast-chlorophyll fluorescence kinet- ics, measured under field conditions, in assessing genotype differences in yield under Mediterranean conditions. Thus, the relationships of grain yield with either F v / F m, the initial (F0), variable (F v) and maximum fluorescence (Fro), and the half-time of the increase from F 0 to F m (tl/2) measured in flag leaves, were evaluated during grain filling, in a large set of durum wheat genotypes cultivated under dif- ferent water regimes. Since yield is the cumulative result of many different processes acting in concert over time, attempts to ascertain a relationship be- tween chlorophyll fluorescence and yield should also consider the change in fluorescence over time (Mof- fatt et al., 1990). Therefore the ontogenetic changes in chlorophyll fluorescence during grain filling and their relationship with grain yield were evaluated in the least stressed environment.

2. Material and methods

2.1. Plant material and growth conditions

A set of 144 genotypes of durum wheat (Triticum

durum Desf.) were planted on 5 and 20 December 1994, in Breda and Tel Hadya (the headquarters of ICARDA, the International Center for Agricultural Research in the Dry Areas), respectively, and then cultivated under rainfed conditions. Both sites are located in Northwest Syria and have consistent dif- ferences in rainfall and evaporative demand (Table 1). Of the genotypes planted, 125 corresponded to the Durum Core Collection assembled at ICARDA, and the other 19 were crosses with wild relatives, including Triticum monococcum L., Triticum dicoc-

coides K6rn. and Triticum carthlicum Nevski. In a third trial the 125 genotypes of the Durum Core Collection were planted on 25 November 1994 at Tel Hadya, in a field which had lain fallow during the previous year and then cultivated under support irri- gation conditions, with 50 mm of water provided by

212 J.L. Araus et al. / FieM Crops Research 55 (1998) 209-223

Table 1 Monthly averages of the maximum (MaxT), minimum (MinT) and mean (MeanT) daily temperatures, monthly rainfall and pan evap- oration during the (1994-1995) growing season at Tel Hadya and Breda

Location Nov. Dec. Jan. Feb. Mar. Apr. May.

Tel Hadya Maximum(°C) 17.8 9.6 12.2 16.5 Minimum (°C) 7.5 1.6 3.7 2.7 Mean (°C) 12.7 5.6 8.0 9.6 Rainfall(mm) 102.4 45.3 41.5 16.8 Pan-evap(mm) 75.4 37.6 34.6 56.1

Breda Maximum (°C) 16.0 8.0 9.1 14.0 Minimum (°C) 6.7 1.6 2.7 2.1 Mean (°C) i 1.4 4.8 6.0 8.1 Rainfall(mm) 59.8 48.0 35.2 18.2 Pan-evap(mm) 91.9 44.8 41.8 66.3

18.9 23.1 31.6 3.4 6.9 11.5

11.2 15.0 21.6 23.3 44.8 18.0 98.1 158.0 275.3

17.7 22.1 30.4 3.2 7.2 12.0

10.5 14.7 21.2 14.8 31.8 15.2

135.5 198.8 336.1

sprinkler irrigation at the beginning of stem elonga- tion. In the two rainfed trials genotypes were planted in flat, 2.5 m long, 6-row plots with 30 cm spacing between rows. The experimental design for the rain- fed trials was a 12 × 12 lattice design with two replicates per genotype. For the supplementary irri- gation trial, genotypes were planted in 1.25 m long, 2-row plots with 30 cm between rows, 60 cm be- tween plots and only one plot per genotype. Soil type was Calcixerollic Xerochrept at both sites. Nitrogen in the form of ammonium nitrate (60 kg ha-1 for both trials at Tel Hadya and 40 kg ha -~ at Breda) and phosphorus (40 kg ha-1 for the three trials) was incorporated before planting as super triple phos- phate. Weeds were eliminated by applying Duplosan Super 600 and Illoxan 36 EC. To avoid border effects, all sampling and field measurements in the rainfed trials were performed on the four central rows of each plot. Grain yield was recorded at maturity. Date of heading was recorded when about half the culms showed emerging spikes and date of anthesis was recorded when about half the spikes showed yellow anthers. Physiological maturity was recorded when spike peduncles changed colour.

2.2. Chlorophyll content, specific leaf weight and relative water content

Field measurements and sampling were under- taken in the penultimate leaf around heading (mid-

April, 1995), and in the flag leaf about 3 weeks after anthesis (mid-May). Leaf area, the specific leaf dry weight (SLDW, leaf dry weight per unit leaf area) and the chlorophyll content were recorded in the penultimate and flag leaves. In addition, relative water content (RWC) was measured in the flag leaf. Total chlorophyll content was measured in intact leaves in the field using a portable meter [SPAD-502, Soil-Plant Analysis Development (SPAD) Section, Minolta Camera, Osaka, Japan] on two consecutive sunny days, between 9 and 12 h (solar time). For each kind of leaf at least 6 recently fully-expanded blades (with the ligule present) were measured per plot. The leaf blades used in the SPAD measure- ments were subsequently sampled. Penultimate leaves were placed in wet bags inside a portable ice box, and transported to the laboratory and their area was measured using a leaf meter (LI-3000, Li-Cor, Lin- coln, NE, USA). Leaves were then dried in a forced oven at 60°C for at least 48 h, and then weighed to calculate SLDW. Flag leaves were placed in a pre- weighed plastic tube (containing around 10 ml dis- tilled water: empty tube) with their cut ends dipped in the water. The tube was tightly sealed and then placed inside a portable ice box, transported to the laboratory for determination of the tube weight (filled tube), and further stored in a refrigerator (4°C) for at least 12 h (overnight) to achieve saturation of leaves. Leaves were removed from the tube, the saturated fresh weight (SFW) was determined and they were then dried in a forced-air oven at 60 °C for at least 48 h, and afterwards weighed to determine the dry weight (DW). Relative water content (RWC) was then determined as the ratio (filled t u b e - empty t u b e - D W ) / ( S F W - DW). RWC values were ex- pressed as percentages. For calculating SLDW, the blade area of the flag leaf samples used in RWC determinations was measured after recording SFW. Further SPAD values were expressed on dry matter basis by dividing SPAD by SLDW.

2.3. Leaf chlorophyll fluorescence

Measurements were done on attached flag leaf blades in the field, using a portable fluorimeter (PSM, BioMonitor; t3quist and Wass, 1988; Bolhar-

J.L Araus et al./ Field Crops Research 55 (1998) 209-223 213

Nordenkampf et al., 1989). The potential quantum yield of Photosystem II (PSII) was determined by means of the ratio of variable to maximum chloro- phyll fluorescence (Fv /Fm) , which can be defined as the efficiency of excitation capture by open PS II centers. In addition, the changes in variable ( F v = F m - F 0) fluorescence, the absolute values F 0, Frn and the half-time of the increase from F 0 to F m ( t l / 2 )

were determined. Measurements were made about 3 weeks after anthesis in the three trials plus a further set of measurements in the irrigation trial about two weeks later. For the rainfed trials, measurements took place on two consecutive sunny days, between 9 and 12 h (solar time). Three measurements were done per plot, and one plot for every genotype of the trial was assayed each day. In the irrigation trial measurements followed the same pattern but, be- cause there was only one replication per genotype, they took only one day.

The light level (photon flux density: PFD), run- time, and dark adaptation period for all the measure- ments were 400 /zmol m -2 s -1, 5 s and 20 min, respectively. All the measurements were done on the adaxial side of leaves by mounting the leaf-holders for dark adaptation in the proper way. Former mea- surements for the above conditions did not differ significantly in the F v / F m values when the run-time was either 2 s or 5 s (Fehrero, 1994). Also, F v / F m measured at 400 /xmol m - 2 S -1 did not differ significantly from the value obtained at 300 /zmol m -z s-~, indicating that a further increase in the PFD of the measuring light would not change F m.

2.4. Carbon-isotope analysis

then calculated. The value of 813C for the air was assumed to be -8.00%0. Total organic nitrogen con- tent in kernels was measured using near-infrared reflectance spectroscopy calibrated against the Kjel- dahl method.

2.5. Statistical analysis

Data were analyzed using NCSS, version 5.03 9 /91 (Dr. J.L. Hintze, Kaysville, Utah, USA) and SAS, version 6.10 (SAS Institute, 1987, Cary, NC, USA) software. Linear correlation analysis was used to determine the relationships between the traits measured. Step forward correlation between physio- logical traits measured in the different environments and yield at the same sites was also used to calculate coefficients of determination and variation. For the set of chlorophyll fluorescence parameters studied, differences between either genotypes or environ- ments and the occurrence of genotype × environment interaction was tested using two-way ANOVA. Analysis of covariance was used to correct values of fluorescence parameters, using the number of days from planting to heading as the covariate. Broad- sense heritabilities of yield, F v / F m, F v, F m, F o and t l /2 were determined by calculation of variance components using the VARCOMP procedure. Ge- netic correlations between these parameters were calculated for the combined variance-covariance matrix obtained from a MANOVA analysis (GLM procedure, SAS Institute, 1987). For these calcula- tions the first set of chlorophyll measurements in Tel Hadya Irrigation was discarded.

Leaves used in the SLDW determination were subsequently ground; and the 13C/12C ratios were determined by mass spectrometry at Isotope Ser- vices, (Los Alamos, NM 87544, USA). The 13C//12C ratios of ground mature kernels were also determined by mass spectrometric analysis at the 'Institut de Biotechnologie des Plantes' (CNRS URA 1128, 91405 Orsay Cedex, France). Results were expressed as ~13C values (Condon et al., 1992). A secondary standard calibrated against Peedee belemnite (PDB) carbonate was used for comparison. Samples of 5 -10 mg were used. Replicate samples differed by less than 0.10%o. Carbon isotope discrimination (A) was

3 . R e s u l t s a n d d i s c u s s i o n

3.1. Effect o f growing conditions on yield, chloro- phyll content and f luorescence

The three trials (hereafter referred to as environ- ments) showed large differences in yield. Mean value across genotypes increased about 70% and almost 4 times when Breda was compared with Tel Hadya Rainfed and Tel Hadya Irrigation, respectively (Ta- ble 2). There were no significant differences in total chlorophyll content on a leaf area basis (SPAD reading) of the penultimate leaf between Breda and

214 J.L. Araus et al. / FieM Crops Research 55 (1998) 209-223

Table 2 Grain yield, relative water content (RWC) of the flag leaf around 3 weeks after anthesis, carbon isotope discrimination (A) of the penultimate leaf at heading and physiological mature kernels, and total protein content in kernels in durum wheat. Plants were cultivated in three different environments of Northwest Syria: two under rainfed conditions (Breda and Tel Hadya Rainfed) and the third under support irrigation (Tel Hadya Irrigation). Values presented are means + SD of the 144 genotypes assayed in the rainfed trials, or the 125 genotypes of the cultivated under support irrigation. Range of genotype values within each trial is also included

Breda Tel Hadya Rainfed Tel Hadya Irrigation

Mean SD Min Max Mean SD Min Max Mean SD Min Max

Grain yield (t ha- 1 ) 1.52 A 0.33 0.63 2.22 2.59 B 0.43 1.60 3.67 6.04 c 1.47 2.33 9.51 RWC (%) 72.4 A 6.0 55.5 87.7 . . . . 86.6 B 3.9 70.6 94.0 Protein in kernels (%) 12.2 B 1.2 7.2 14.4 10.0 A 1.1 7.6 13.3 12.9 B 1.3 10.4 15.5 A--Leaf (%o) 16.5 A 0.4 15.6 17.6 18.5 B 0.5 17.1 19.7 20.5 c 0.6 19.2 22.1 A--Kernels (%o) 14.0 A 0.4 13.0 15.2 15.3 B 0.4 13.8 16.2 16.7 c 0.5 15.2 17.7

Tel Hadya Irrigation (Table 3). However, when ex- pressed on a dry matter basis (SPAD/SLDW), total chlorophyll content of the penultimate leaf increased over 10% from Breda to Tel Hadya Rainfed and almost 55% from Breda to Tel Hadya Irrigation. Although less clear, the pattern for the flag leaf was

comparable. Thus, whereas SPAD value from Breda to Tel Hadya Irrigation increased 10%, SPAD/SLDW ratio increased 20% from Breda to Tel Hadya Irrigation. Therefore both leaves dis- played a pattern of decrease in chlorophyll concen- tration from the wettest to the driest environment.

Table 3 Leaf chlorophyll content and fast chlorophyll fluorescence parameters measured under field conditions in durum wheat leaves cultivated in the three different environments of Northwest Syria studied. Total chlorophyll content was determined in the penultimate leaf (at heading) and in the flag leaf blade (around 3 weeks after anthesis) by a portable photometer (SPAD). Chlorophyll content was also expressed on dry weight basis, dividing the SPAD values by the specific leaf dry weight (SLDW) of the same blades. Fast chlorophyll fluorescence parameters were measured only in flag leaf blades around 3 weeks after anthesis. A further set of measurements was taken in the irrigation trial around 5 weeks after anthesis. Values presented are means ___ SD of the 144 genotypes assayed in the rainfed trials, or the 125 genotypes cultivated under support irrigation. Range of genotype values within each trial is also included

Breda Tel Hadya Rainfed Tel Hadya Irrigation

Mean SD Min Max Mean SD Min Max Mean SD Min Max

SPAD Penultimate 50.8 a 3.1 41.7 57.6 44.6 A 2.8 37.1 54.4 50.2 B 3.3 41.1 57.6 SPAD/SLDW 0.728 a 0.089 0.589 0.989 0.819 B 0.066 0.660 1.080 1.117 c 0.111 0.832 1.406 Penultimate SPAD Flag 47.5 A 3.5 37.5 60.4 - 52.3 B 3.8 40.5 61.5 SPAD/SLDW 0.786 A 0.069 0.665 0.992 - 0.935 B 0.085 0.581 1.122 Flag F v / F m 0.691A 0.039 0.581 0.770 0.737 B 0.022 0.665 0.785 0.751 c 0.031 0.621 0.807

0.702 0.040 0.607 0.770 a F 0 0.322 B 0.042 0.238 0.492 0.343 c 0.044 0.257 0.450 0.300 a 0.028 0.243 0.420

0.316 0.078 0.187 0.557 a F m 1.083 A 0.180 0.686 1 . 6 9 5 1.334 c 0.213 0.870 1 . 8 8 5 1.247 B 0.222 0.777 1.907

1.114 0.361 0.560 2.277 a F v 0.760 A 0.154 0.433 1.275 0.992 B 0.174 0.597 1.447 0.947 B 0.200 0.497 1.480

0.797 0.288 0.350 1.717 a ts/z (ms) 11813 24 69 178 103 A 20 60 168 128 c 21 69 185

115 40 46 226

aMeasured around 5 weeks after anthesis.

J.L. A raus et al. / Field Crops Research 55 (1998) 209-223 215

Differences in the water availability in the differ- ent environments during grain filling were assessed by measuring the relative water content (RWC) of the flag leaf 2 to 3 weeks after anthesis, as well as the carbon isotope discrimination (A) of mature kernels (Table 2). Differences between environments in water availability prior to heading were assessed from the A values of the penultimate leaf. Mean value of RWC increased over 15% from Breda to Tel Hadya Irrigation. Mean A of the penultimate leaf and mature kernels increased (in absolute terms) 4.0%o and 2.7%o, respectively, from Breda to Tel Hadya Irrigation. In addition, A decreased between 2.5%0 (Breda) and 3.8%0 (Tel Hadya Irrigation) when A of the penultimate leaf was compared with that of mature kernels (Table 2).

Differences in water status may explain the above differences between environments in yield and chlorophyll. Indeed, a progressive improvement in water availability from Breda to Tel Hadya Irrigation was evident from d and RWC data. In addition, the decrease in A values from the penultimate leaf to the grains indicates that water stress increased progres- sively during the last part of crop development in the three environments. The crop suffered terminal water stress even in the wettest environment. Thus, in Tel Hadya Irrigation, which (based on A of the leaf) did not experience water stress before heading, values of kernel d ( m e a n = 16.7%o) were below those re- ported for durum wheat (about 17.5%o) as typical of full irrigation under Mediterranean conditions (Araus et al., 1993; Araus et al., 1997). Such a decrease in d may be attributed to stomatal closure in response to declining soil water and /or increasing vapour- pressure deficit (Condon and Richards, 1992; Con- don et al., 1992; Araus et al., 1993). However Di Marco et al. (1988) point out that, in spite of stom- atal closure, the primary effect of drought stress is impairment of carbon assimilation. In our study A of kernels does not support this conclusion. Thus the decrease of A from the best to the worst environ- ment suggests that Pi/Pa is decreasing and therefore the stomatal limitation on photosynthesis increases in the drier environments.

Growing conditions also affected the fast-chloro- phyll fluorescence parameters studied in the flag leaf (Table 3). Thus, the driest environment (Breda) showed the lowest values of F v / F m, F m and Fv and

the highest of F o. This pattern of changes in fluores- cence supports the pattern reported as caused by drought (Araus and Hogan, 1994; Long et al., 1994). However, the only fluorescence parameter that showed a progressive change (decrease) from the wettest to the driest environment was F v / F m. For the other four parameters, Tel Hadya Ralnfed was the environment showing either the highest ( F 0, Fm, and F v) or the lowest value (tl/2). The decrease in variable fluorescence ( F v / F m) from Tel Hadya Irri- gation to Breda was associated more with significant decreases in F m than with increases in F 0. Thus, the depression of F v / F m indicates increased photopro- tection. When leaves are submitted to heat stress conditions, there is a marked increase in the dark level of F 0, an effect which presumably reflects the physical dissociation of the PSII reaction centers from the light harvesting system. In contrast to heat stress, water stress does not usually modify the F 0 level significantly (see reference in Havaux et al., 1988). However, the slight decrease in F v / F m from the irrigation to the rainfed environment at Tel Hadya was associated with a rise in F 0 values, which suggests the occurrence of photodamage rather than photoprotection in Tel Hadya Rainfed. In fact Tel Hadya Rainfed showed some symptoms of N defi- ciency at the end of the crop cycle, in that the total protein content in kernels was about 20% lower than at the two other sites (Table 2). In this regard the extent of photoinhibition is a balance between fac- tors that induce photoinhibition and those that deter- mine recovery (Osmond et al., 1987). Recovery re- quires chloroplast-directed protein synthesis and is slower in nitrogen-limited conditions, which can ren- der leaves more susceptible to photoinhibition (Hen- ley et al., 1991).

3.2. Effect o f ontogeny on chlorophyll fluorescence

The effect of ageing on fast-chlorophyll fluores- cence response of the flag leaf was evaluated in the trial with the best growing conditions (Tel Hadya Irrigation). Two sets of measurements spaced about 2 weeks apart during grain filling were compared (Table 3). The ratio F v / F m decreased almost 7%, which was related to a concomitant decrease of 10% and 16% in F m and F v, respectively. F 0 increased slightly (5%), whereas tl/2 decreased 10%. An in-

216 J.L. Araus et al. / Field Crops Research 55 (1998) 209-223

crease in F 0 associated with a decrease in F m could indicate that the energy distribution within the light harvesting complex and PSII chlorophyll is affected and that the light absorption capacity of all chloro- phyll, including the reaction centres, decreases (Flagella et al., 1994). In addition, an increase in F 0 combined with shorter tl/2 is to be expected in case of a reduced turnover of the D1 protein and /or the occurrence of inactive reaction centers of PSII (Cle- land et al., 1986; Melis et al., 1988). Thus, during ageing, the increase in F 0 together with the decrease in tl/2 may indicate photodestruction and /or a de- crease in the chloroplast density of the mesophyll cells, in addition to a more shade-adapted chloroplast structure (Anderson et al., 1988). Indeed for wheat under Mediterranean conditions the senescence of the flag leaf is associated with a progressive decrease in chloroplast density, accompanied by a slow de- cline in the chlorophyll a / b ratio (Araus and Labrafia, 1991).

The pattern of changes in chlorophyll fluores- cence parameters between Tel Hadya Irrigation and Breda (the two extreme environments) paralleled those associated with ageing in Tel Hadya Irrigation. They are related to faster leaf ageing in Breda than Tel Hadya Irrigation, probably because conditions at the former site were more severe. In fact, grain filling in Tel Hadya Irrigation lasted almost two weeks longer than in the other two trials, probably due to its better water status (see RWC and A in Table 2). Senescence symptoms of wheat are delayed until physiological maturity when plants are grown below 20°C, but degradative processes accelerate and photosynthetic activities decline as the temperature rises (A1-Khatib and Paulsen, 1994). Therefore, even slightly higher temperatures during reproductive growth are injurious in this respect and may seri- ously reduce economic yield. Mean maximum air

1.00

0.80

0.60

0.40

0.20

0 0 0

EZ] broGd sense heritability

I r 2, genetic corre4ation with OY

I Ill Fv/F m F o F v F m t l / 2 GY

F l u o r e s c e n c e p a r a r n e ~ e r s

Fig. 1. Broad-sense heritability (h 2, open bars) and coefficient of

determination of the genetic correlation with grain yield ( r 2, filled

bars) of the various fluorescence parameters studied, h 2 of grain

yield (GY) is also included for comparison.

temperatures during grain filling were near 30°C (Table 1). Hence, high temperature accelerated the thylakoid component breakdown in wheat, which led to a decrease in PSII activity, an effect similar to normal senescence patterns (Harding et al., 1990).

3.3. Genotype effect on yield and chlorophyll fluo- rescence

Pooled analysis of variance indicated that, except for Fv/F m, not only environment but also genotype had a strong, significant ( P < 0.001) influence on grain yield and fluorescence parameters (Table 4). Genotype X environment interaction was not signifi- cant for the fluorescence parameters, whereas it was for grain yield. Therefore, the three growing environ- ments assayed did not have a differential effect for the fluorescence parameters on the set of genotypes tested. Broad-sense heritability (h 2, Fig. 1) of F 0

Table 4

Degrees of f reedom and mean sum of squares for grain yield (GY) and the set of chlorophyll fluorescence parameters displayed in Table 3

and measured in a common set of 125 genotypes cultivated under the three different environments differing in level of drought

Source of variance df Fv /F m F o F m Fv q/2 GY

Env i ronmen t (E) 2 0.211887" * * 7.096 X 10 -2* * * 3.313284* * * 3.132708* * * 23082.94* * * 1.0387 x 109* * *

Genotype (G) 124 1.673 X 10 -3 4.528 X 10 - 3 . * * 0.120297" * * 8.315 x 10 2 . * * 1522.83" * * 2 0 1 3 2 3 5 " * *

E x G 248 1.433 x 10 -3 1.170 x 10 3 3.241 x 10 - 2 2.690 x 10 -2 424.53 993 800" * *

• P < 0 . 0 5 ; * * P < 0 . 0 1 ; * * * P < 0 . 0 0 1 .

J.L. Araus et al./ Field Crops Research 55 (1998) 209-223 217

150

145

140

135

130

125

u~

@ ,d

<3

>.,

o

o

x S

5 0

18

100 150 200

1 7

16

15

14

1,3 5O

m [] n ° [] []

D ~ ~ a E3 E

{) O <3

o o % ~ , o TH,~ ..... ,_

1 O0 150 200

t l / 2 f l a g l e a f ( m s }

Fig. 2. Relationship across genotypes between the half-time of the increase from F 0 to F m 01/2) measured in the flag leaf and (a) the number of days from planting to maturity or (b) the carbon isotope discrimination (A) of mature kernels. At the upper figure values of all the genotypes in the two rainfed environments were plotted together. Fitted equations of the lower figure were as follows: Breda, y = 14.45-0.0035x; Tel Hadya Rainfed (THR), y= 15.97-0.0064x; Tel Hadya Irrigation (THI), y= 17.56- 0.0078 x.

showed the highest values (h 2 = 0.745), followed very closely by F m (h 2 = 0.729), ti/2 (h 2 = 0.721) and F v (h 2 = 0.679), whereas for F v / F m it was much lower ( h 2 = 0.143). Grain yield showed an intermediate value (h 2 = 0.506).

Differences among genotypes in fluorescence pa- rameters could be just due to differences in phenol- ogy. Within each trial, dates of heading (DH) and maturity (DM) differed more than 2 weeks between extreme genotypes. This can lead to measurements of fluorescence on genotypes differing in their onto- genical stage. Indeed DH and DM correlated ( P < 0.001) negatively with F o, Fm, and F v and posi- tively with t~/2 of the rainfed environments (see Fig. 2a for DM/t~/2 relationship). To rule out this possi- bility, all the fluorescence parameters (except for F v / F m) for the same 125 genotypes cultivated in the

three environments, were analyzed using the number of days from planting to heading as a covariate. Even when the covariate was significant for all of the parameters evaluated, indicating that these traits were systematically associated with phenology, strong sig- nificant differences ( P < 0.001) between genotypes were still found for all these traits when corrected for heading time. Therefore, there is still significant genotypic variability in the chlorophyll fluorescence parameters which is not explained by differences in phenology.

Relationships across genotypes between the dif- ferent fluorescence chlorophyll parameters were fur- ther studied (Table 5). F v / F m correlated progres- sively better with F 0 (positively) and 6/2 (nega- tively) from the most (Breda) to the least (Tel Hadya Irrigation) stressed environment. Within each envi- ronment F 0 was strongly negatively correlated with tl/2 and positively with F m (Table 5). Correlations even improved when data from all the environments were combined (Fig. 3). The set of strong relation- ships between F 0 and either tl/2 or F m indicates that these fluorescence parameters change in a coordinate fashion.

3.4. Relationships between grain yield and chloro- phyll fluorescence parameters

The relationships between grain yield (GY) and the different fast-chlorophyll fluorescence parame- ters measured in the flag leaf were evaluated for the set of genotypes within each environment. Very weak correlations were found between GY and the ratio F v / F m, although for Tel Hadya Rainfed and Irrigation they were significant ( P < 0.05) due to the large number of genotypes evaluated (Table 5). Thus, F v / F m explained (by means of r 2, the coefficient of determination) less than 4% and 10% of variability in GY throughout genotypes within Tel Hadya Rain- fed and Irrigation. In contrast, the other fluorescence parameters, and particularly F o and 6/2, were con- sistently much better correlated ( P < 0 . 0 0 1 ) with GY in both the rainfed trials. For Tel Hadya Irriga- tion, the least stressed environment, these parameters also correlated much better with GY than F v / F m when correlations were performed using the second set of measurements (about 5 weeks after anthesis). F o and tl/2 usually explained between 20 and 30%

218 J.L. Araus et al. // Field Crops Research 55 (1998) 2 0 9 -2 2 3

~e

o

0

o

=~

,.7

-x.

I t l

I

I I

I I I I

~ c 5 VI

W

J.L. Araus et al. / Field Crops Research 55 (1998) 209 -223 2 1 9

Table 6 Correlation coefficients of the relationships between grain yield (GY) and various nonmodulated chlorophyll fluorescence parameters measured in the flag leaves of the set of the Durum Core Collection genotypes cultivated in three different environments in Northwest Syria. Measurements took place around 3 weeks after anthesis in the three environments plus a further set of measurements in the irrigation trial about 2 weeks later. For more details see Section 2

Environments, number of genotypes F v / F m F o F m F v tl /2

B r e d a , n = 1 4 4 0 . 0 0 7 0 . 4 8 7 * * * 0 . 3 7 0 * * * 0 . 3 0 0 * * * - 0 . 4 8 9 * * *

Tel Hadya R a i n f e d , 0 . 1 7 1 * 0 . 4 5 5 * * * 0 . 4 4 3 * * * 0 . 4 1 1 * * * - 0 . 4 0 7 * * *

n = 1 4 4

Tel Hadya Irrigation 0 . 3 0 0 * * * 0 . 2 3 7 * * 0 . 3 2 2 * * * 0 . 3 2 2 * * * - 0 . 3 0 3 * * *

1st, n = 1 2 4

Tel Hadya Irrigation 0 . 2 2 8 * 0 . 5 4 1 * * * 0 . 4 8 9 * * * 0 . 4 6 4 * * * - 0 . 5 4 4 * * *

2 n d , n = 1 2 4

* P < 0 . 0 5 ; * * P < 0 . 0 1 ; * * * P < 0 . 0 0 1 .

of GY variability within each environment, whereas F m explained roughly 15-25% and F v less. tl/2 was the only parameter negatively correlated with GY

20

E 1.5

0

E

CN

1.0

0,5 0.

250

200

150

100

r2=0"66 £

o o v c o v o o

, i

0.2 0.3 0.4 0.5 0.6

50 _ rZ=069 u R

40 ' , 0.1 0.2 0.3 0.4 0.5 0.6

F o

Fig. 3. Relationship across genotypes between values of the fluorescence emission when all the reaction centers are open and the photochemical quenching is minimal (F0), and (a) the maxi- mum chlorophyll fluorescence ( F m) or (b) q/2. Values of all the genotypes at the three trials were plotted together.

within each environment (Table 6) and the slope of the relationship becomes more negative in the best environment (Fig. 4). The other parameters were positively correlated with GY within all the environ- ments. The parameter which showed the best genetic correlation with grain yield was tl/2 ( r = -0 .92 ) ,

I ©

Oh

S[} ©

>

8 0 0 0

6 0 0 0

400,9

2000

0 50

[] E] THI r2~0.30 A THR rZ=0 17

~ ~Eh %[Z] O Breda r2=024

, I , I , I ,

I O0 150 200

F i g . 4 . Relationship across genotypes (i.e. within environments) between tl/2 and grain yield. Genotype values are means of either two ( r a i n f e d t r i a l s ) or one (irrigation trial) plot a n d 3 flag leaves per plot. The environments were as follows: Breda (O) , Tel Hadya Rainfed ( T H R , z x ) , and Tel Hadya Irrigation ( T H I , [ ] ) .

Fitted equations were as follows: B r e d a , y = 2 2 9 1 - 6 . 5 8 x ; T H R ,

y = 3 4 5 6 - 8 . 4 9 x ; T H I , y = 8 3 5 1 - 2 0 . 0 6 x .

220 J.L. Araus et a l . / Field Crops Research 55 (1998) 209-223

f o l l o w e d b y F 0 ( r = 0 . 8 8 ) , F m ( r = 0 . 7 4 ) a n d F v

( r = 0 . 7 1 ) , w h e r e a s F v / F m ( r = 0 . 3 4 ) w a s t h e l e a s t

correlated parameter (Fig. 1). As a further evaluation of the performance of

these traits in selection for yield, GY was analyzed within each environment in two subsets of 15 geno- types each, representing the highest and lowest val- ues of either F v / F m, F o, F m, F v, or t l / 2 (Table 7). The subset having the highest values of either F 0, F m or F v, or the lowest values of tl/2, always showed significantly higher GY. In contrast F v / F m was the least reliable selecting trait. Moffatt et al. (1990)

reported in wheat that F v and grain yield were negatively correlated in controlled environments, but positively and not significantly correlated in the field. Nevertheless, genotypes in the high F v group were also in the high yield group, which agrees with our results. Significant differences in GY were also at- tained in both rainfed environments when selection was based on the ranking of traits measured in Tel Hadya Irrigation (Table 7). All the fluorescence traits measured in Tel Hadya Irrigation discriminated bet- ter GY within Breda than the same traits measured in this rainfed environment. Results also suggest that

Table 7

Variation and significance of the difference between the average value of grain yield (GY, the dependent variable) for the two extreme

groups of 15 genotypes ranked according the independent variable. Independent variable was any of the five chlorophyll fluorescence traits

measured either in the same environment as GY (A) or in Tel Hadya Irrigation, the best environment (B). GY measured at Tel Hadya

Irrigation was also used as a complementary trait in the second option

Independent trait Environment A B

Increase in GY Level of significance Increase in GY

1st a 2nd

Level of significance

F v / F m Breda - 1.7% n.s. T H R b 6.1% n.s.

T H I c 1st 24.1% *

T H I 2nd 10.2% n.s.

F m Breda 29.1% * * *

T H R 22.2% * * *

T H I 1st 30.7% * *

T H I 2nd 38.5% * * *

F v Breda 20.9 *

T H R 16.9% * *

T H I 1st 35.8% * * *

THI 2nd 41.3% * * *

F o Breda 38.0% * * *

T H R 30.6% * * *

THI 1st 14.7% n.s.

THI 2nd 43.0% * * *

t l/2 Breda 32.6% * * *

T H R 23.5% * *

T H I 1st 21.2% * T H I 2nd 56.3% * * *

GY Breda

T H R

THI

49.5% 25.7% . . . . .

13.8% 6.2% * n.s.

45.1% 55.5% . . . . . .

5.8% 14.5% n.s. *

45.9% 57.4% . . . . . .

7.3% 15.0% n.s.

30.8% 44.3% . . . . . .

- 0 . 2 % 20.5% n.s. * *

33.9% 59.4% . . . . . .

2.2% 9.4% n.s. n.s.

39.5%

24.5%

aMeasured around 3 ( l s t ) and 5 (2nd) weeks after anthesis. bTel Hadya Rainfed.

CTel Hadya Irrigation.

* P < 0 . 0 5 ; **P_<0 .01 ; * * * P ~ < 0 . 0 0 1 .

J.L. Araus et al. / Field Crops Research 55 (1998) 209-223 221

indirect selection for GY under good growing condi- tions (i.e. in the best environment) based on some chlorophyll fluorescence traits may be as efficient as direct evaluation of the yield to improve GY in more stressed environments.

Because of the coordinate change in fluorescence parameters across genotypes (Fig. 3), genotypes showing higher yield have lower tl/2 values, along with higher F 0 and F m. The resulting lack of geno- type variability in F v / F m could be the basis of the lack of relationship between F v / F m and GY. These sets of coordinate changes in fluorescence parame- ters across genotypes (within environments) seem, at first, different to those changes due to the environ- ment (across environments). Thus, whereas mean value of F 0 was higher, tl/2 and F m were lower in the least productive environment (Breda) compared with the most productive environment (Tel Hadya Irrigation).

To explain the lack of relationship between F v / F m and GY, it has been suggested that, in leaves suffer- ing from drought stress, the slow, but not the rapid phase of fluorescence induction is altered (Toivonen and Vidaver, 1988; Di Marco et al., 1988; t)gren, 1990). Thus, the variable fluorescence yield F v / F m ratio would not provide any information on drought stress (Di Marco et al., 1988; t)gren, 1990). For example, in durum wheat F v / F m showed no differ- ences between treatments or between cultivars (Di Marco et al., 1988). In our results, in addition to the weak relationships between F v / F m and yield (Table 6) or chlorophyll content (SPAD values), no correla- tion was found between this ratio and A of kernels within any of the environments assayed (data not shown).

As pointed out above, lower values of F m may be associated with increased non-photochemical quenching. Additionally, a diminished electron flow from the water-splitting complex could cause impair- ment on the donor side, probably lowering F m and prolonging tl/2 (Demmig-Adams et al., 1989). As an alternative to photo destruction, greater F 0 may also be characteristic of a lower frequency of PSII reaction centers due to a more shade-adapted chloro- plast and /o r occurrence of a lower chloroplast den- sity in the mesophyll cells (see Araus and Hogan, 1994). In this way, significant increases in F 0, F m and F v have been reported with decreasing PPFD or

exposure time (Dambrosio et al., 1996). Lower val- ues of tl/2 may also be related to more shade-adapted chloroplasts, due to a larger antenna size and a smaller plastoquinone pool (Anderson et al., 1988). On the other hand, an increase in F 0 and a decrease in tl/2 is to be expected if the number of heat- deactivating centers (Krause, 1988) is decreased or if energy distribution to PSI is diminished (s ta te l - state2). The latter mechanism enhances the absorbing cross-section of PSII (Fork and Satoh, 1986). The coordinate pattern of changes in fluorescence param- eters across genotypes suggests that the more pro- ductive genotypes are actually those in which flag leaves show the lowest adaptation to sunny-dry conditions: lower non-photochemical quenching, fewer heat-deactivating centers, more shade-adapted chloroplast structure, diminished energy distribution to PSI, etc. This could be because these genotypes can avoid the development of terminal stress while maintaining better water status. In this regard, within each environment A of mature kernels was posi- tively correlated with F 0 and negatively with t~/e (Table 5), which suggests that genotypes with higher F 0 and lower tl/2 maintain better water status during grain filling (see reference in Araus et al., 1997). Whereas this seems particularly evident in the better environments (Fig. 2b), in the rainfed environments (especially Breda) other mechanisms could also ac- count for it. Differences in phenology may affect fluorescence responses as well as yield. Indeed DH and DM were correlated ( P < 0.001) not only with F 0 (negatively) or q/2 (positively) in the rainfed environments (Fig. 2a), but also (negatively) with GY in these environments (e.g. D H / G Y Breda, r = - 0.58; Tel Hadya Rainfed, r = - 0.39). Earlier genotypes tended to show flag leaves with lower tl/e and higher F 0, not only due to improved water status, but probably also because those leaves were more advanced in their ontogeny (more senescent) at the time of measurement. This would also agree with the weak correlations found in Breda between SPAD/SLDW of the flag leaf and t l / 2 ( r = 0.28) and F 0 ( r = - 0.31). Therefore, phenological adjust- ment (i.e. earliness) could explain, at least in part, the correlation between these fluorescence parame- ters and yield in the rainfed environments. For Tel Hadya Irrigation this may not be the case as the slope of the correlation between DH and GY shifts

222 J.L. Araus et aL / Field Crops Research 55 (1998) 209-223

to positive, whereas no changes in the slope were found for the relationship of DH with F o or t~/2.

4. Conclusions

The study provides evidence of the usefulness of fast-chlorophyll fluorescence measurements during grain filling to evaluate yield performance under Mediterranean conditions, t l /2 , F o and even F m can be used as better selection criteria than F v / F m The coordinate pattern of changes in fluorescence param- eters across genotypes suggests that the most produc- tive genotypes are in fact those which could avoid, or escape, the development of terminal stress during grain filling by phenological adjustment.

Acknowledgements

The participation of J.L. Araus and T. Amaro in this work was supported by a grant CICYT AGF95- 1008-C05-03 (Spain). A fellowship DGICYT PR95- 006 (Central Government, Spain) and another 1996BEAI400409 from the Generalitat de Catalunya to J.L. Araus is also acknowledged. We are grateful to Mr. R. Rycroft for correcting the English.

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