Effect of elevated [CO2] on foliar defense chemistry of Triticum aestivum and incidence foliar...

IRJMST Volume 5 Issue 4 [Year 2014] Online ISSN 2250 - 1959

International Research Journal of Management Science & Technology http://www.irjmst.com Page 69

Effect of elevated [CO2] on foliar defense chemistry of Triticum

aestivum and incidence foliar diseases

Author names: Yogesh Yadava,,S.D Singhb and Rupam Kapoora,

Affiliations:

aDepartment of Botany, University of Delhi, Delhi – 110 007, India

bDivision of Environmental Science,

Indian Agricultural Research Institute, Delhi –110012, India

Corresponding author:

Yogesh Yadav

Department of Botany

University of Delhi

Delhi 110 007

India

E-mail: [email protected]

Tel.: +91-9466886914

ABSTRACT

Atmospheric CO2 concentrations are predicted to double within the next century. Despite

this trend, the extent and mechanisms through which elevated [CO2] affect plant diseases

remain uncertain. Triticum aestivum (var. PBW 343) plants were exposed to two different

CO2 concentrations (390/550 ppm) in free-air CO2 enrichment technology (FACE).

Survey of Triticum aestivum for natural disease incidence revealed that elevated [CO2]

increased yellow rust incidence. To assess the mechanisms underlying these changes, we

conducted leaf structural, physiological and chemical analyses. Reduced disease severity

under elevated [CO2] was likely due to increased epicuticular waxes and altered leaf

chemistry. Elevated [CO2] led to increased leaf sugar concentration however there were



no changes in protein concentration. Elevated [CO2] reduced leaf N concentration and

increased the C/N ratio by 10.2%, total phenolics by 16.39%, and PAL activity by

32.01%. HPLC of extract of leaf revealed changes in the concentration and profile of

sugars. Sugars were increased under elevated [CO2] however, composition of sugars

varied.

Keywords:

Free-air CO2 Enrichment (FACE) ,Natural incidence ,Yellow rust ,Sugars

1. Introduction

Carbon dioxide concentration has risen by 31% since pre-industrial times and

carbon cycle models project concentration of 500 ppm – 1000 ppm by 2100 (IPCC,

2007). There is extensive literature on effects of high CO2 on plant anatomy,

morphology, phenology and physiology (Ainsworth and Long, 2005). By contrast, there

have been fewer studies on the potential impact of enhanced [CO2] on plant diseases and

their subsequent impact on plant yield – although the link between climate and plant

disease is well known. Pathogen impacts are yet to be integrated with crop models that

project the fate of agriculture under changing climate. The recent commentaries on food

security advocating priorities for climate change adaptation continue to ignore impacts of

pests and diseases on agriculture production and quality (Chakraborty et al., 2008).

Influence of elevated [CO2] on pathogens and diseases suggests that higher

carbohydrate concentration within host tissue promotes the development of some

pathogens (Manning and Tiedmann, 1995; Kobayashi et al., 2006) but inhibit others

(McElrone et al., 2005; Eastburn et al., 2010). This implies that predicting effects for

unstudied pathosystems will be quite challenging. Significant impacts of elevated CO2

are manifested through changes in host physiology and resistance (Garrett et al., 2006).

Most field studies on the effects of elevated CO2 have been done in Open Top Chambers

(OTCs), OTCs have been known to modify the environment, interfere with the dispersal

of natural inoculums and alter the plant’s susceptibility to a given pathogen (Long et al.,

2004). The controlled environment often does not coincide with the processes that occur



in the field under a more complex environment. The Free-air CO2 enrichment (FACE)

technique allows to expose study plants to altered atmospheric concentration in

agricultural ecosystem with minimal impact on microclimate, and without limiting the

movement of pathogens (Kobayashi et al., 2006). Realistic assessment of climate change

impacts on host pathogen interactions is scarce and there are only handfuls of FACE

studies (Chakraborty et al., 2008). The nature of plant pathogen interactions is complex

and the paucity of knowledge has to be overcome before any generalization. To better

predict how important agricultural pathosystems will respond to future climatic

conditions, additional studies are needed in the natural conditions of FACE facility

(Eastburn et al., 2010).

Among agricultural crops most of the studies on effect of climate change have

been on cereals and legumes under artificial pathogenic stress but there have been no

report on natural incidence of diseases. One cereal crop of particular importance

throughout the world is Wheat. Wheat is the second largest staple crop produced in the

world after rice. India is one of the largest wheat growing country and ranks fourth (after

China, European Union and Canada) in the world in the production of rapeseed. Bread

wheat (Triticum aestivum L.) is vulnerable to a number of diseases. Among all these,

diseases such as rusts play an important role in reducing the yield of wheat. Wheat is

much sensitive to climatic variables and hence climate change could have significant

effect on its production. Currently no research exists on the effect of [CO2] on the

development of natural incidence of yellow diseases. The objective of this study was to

assess effect of elevated [CO2] concentration on disease incidence and relate it to

structural, physiological and chemical responses of wheat plants.

2. Materials and Methods

2.1. Experimental field site and FACE facility

The experimental site (Mid-FACE) is located at the Division of Environmental

Science, Indian Agricultural Research Institute (stands at 28.08 °N and 77.12 °E). The

mid-FACE ring is an octagonal shaped structure 8 m in diameter (Miglietta et al., 1997).



The plenum is made of flexible irrigation pipes 20 cm in diameter with small vents

through which CO2 air mixture is released into the ring. Elevated [CO2] at 550 ppm is

maintained in the FACE ring during daylight hours throughout the growing period of the

plant and an infrared gas analyzer (IRGA) (Licor, 6200) is used to monitor the

concentration of CO2 in the ring. The valves of all eight nodes of the octagon are

independently controlled with a computer based system controller. CO2 is injected from a

CO2 air mixing cylinder which is supplied CO2 from 25 gas cylinder storage having

manifold valves and flow meters. The fumigation of gas from the plenum is directed at

the centre of the field 10 – 15 cm above the crop canopy level. This is done to reduce

CO2 gradient with depth and maintain a uniform concentration of gas throughout the ring.

Height of the plenum is adjusted to the height of the canopy with the help of an

adjustable stand. Wind direction and velocity are monitored, and the flow of CO2 is

released upwind of the plots and regulated according to an algorithm using CO2

concentration and wind-speed as parameters.

2.2. Plant material

Triticum aestivum var. PBW 343 seeds were sown in rows with spacing of 40 cm

within the rows and 20 cm within the plants (Clay loam soil; pH 7.0; N 10 g m-2

P 30 g

m-2

K 6 g m-2

). Seed rate was kept uniform and thinning was done after one month. The

plants were allowed to grow and no fertilizer or pesticide was added to the soil during the

course of the experiment. Weeding was done mechanically at regular intervals and plots

were irrigated with tap water. The plants were exposed to CO2 treatment after one month

of sowing. CO2 concentration was 390 ppm (ambient) and 550 ppm (elevated) in order to

determine the effects of elevated [CO2] on wheat plants.

2.3. Assessment of disease incidence

Natural disease incidence and severity in wheat plants was recorded. During our

study Triticum aestivum was found to be infected by two foliar disease yellow rust and

brown rust

Yellow rust is caused by the biotrophic pathogen Puccinia striiformis It is an

important concerned disease of north india. The pathogen can infect aboveground leafy



part of the plant, producing characteristic stripes of uredospores. Severe infection

culminates in whole leafy shoot which is the main cause of yield loss in susceptible

cultivars. Brown rust is caused by the biotrophic pathogen Puccinia triticina; the disease

initially appears as irregular yellow patches on the leaves. These lesions later turn tan to

light-brown. During cool and humid weather conditions, the fungus develops brown

uredospores over whole leafy area of leaves. Heavily infested leaves have a blighted

appearance as a result of numerous infection sites. This leads to reduction in

photosynthetic area, defoliation and accelerated senescence.

Incidence and infection status in mustard plants was determined through visual

inspection. Forty group of tillers were selected randomly, labeled and disease incidence

of two rusts were recorded using 0 5 scale described by Sangeetha and Siddaramaiah,

(2007).

The percentage disease index (PDI) was calculated using the formula (Wheeler,

1969)

PDI = Sum of numerical grading X 100

No. of leaves examined X Max. disease grade

2.4. Leaf epidermal characteristics

2.4.1. Extraction and estimation of epicuticular wax

The epicuticular wax concentration was determined using the method of Barnes et

al., (1996) with slight modifications. Leaf discs of 1 cm diameter were immersed for 2-3

s in HPLC grade chloroform at room temperature with gentle agitation. The chloroform

extract was filtered through Whatman filter paper no.1 in evaporating flask. The filtrate

was evaporated under vacuum. The wax was collected and weight per unit leaf area

calculated.

2.5. Estimation of carbohydrates

The concentration of total carbohydrates was determined in leaves according to

Yemm and Willis (1954), using anthrone reagent. The absorbance of the solution was



determined using UV-visible spectrophotometer (Beckman Coulter DU ®730) at 630 nm.

The concentration of sugar was determined from a standard curve of glucose and

calculated on a fresh weight basis.

2.6. Estimation of total proteins

Freshly harvested leaves (1 g) were ground in liquid nitrogen and homogenized in

0.1 M phosphate buffer, pH 7.0. The homogenate was centrifuged at 12,000 g for 10 min

at 4 °C. Total soluble protein concentration was determined by the method of Bradford

(1976), using bovine serum albumin (Sigma) as standard.

2.7. Determination of carbon, nitrogen, and sulphur

The carbon, nitrogen and sulphur were determined in a CHNS analyzer (VARIO

Elementar III). Oven dried leaf samples were oxidized in the combustion tubes in the

presence of oxygen at higher temperature at 1150 °C using tungsten (IV) oxide as

catalyst. Helium gas was used to swap out the combustion product out of chamber and

passed over high purity Cu to remove any oxygen not consumed in the initial combustion

and to convert any oxides of these elements into their gaseous forms. All elements were

separated by gas chromatography and were detected by thermal conductivity detectors.

Sulphanilic acid was used as standard.

2.8. Determination of Phenylalanine ammonia lyase activity

Phenylalanine ammonia lyase (PAL) (E.C .4.1.1.5) activity was assayed

according to Dunn et al., (1998) with slight modifications in three months old leaf when

specks of disease were visible. The absorbance of trans-cinnamic acid was read at 290

nm using UV-visible spectrophotometer and the activity was expressed as trans-

cinnamic acid µg ml-1

h -1

mg-1

protein. The standard curve of enzyme activity was

prepared using L- phenylalanine as substrate.

2.9. Determination of total phenols

The estimation of total phenols was assayed according to Bray and Thorpe

(1954). The leaves were homogenized in ethanol and the supernatant was mixed with


International Research Journal of Management Science & Technology http://www.irjmst.com 5

Folin-Ciocalteu’s phenol reagent (FCR). The intensity of the blue color was read at 650

nm in a UV-visible spectrophotometer. Standard curve was prepared using pyro-catechol.

2.10. Statistical Analysis

The experiment was carried out twice 2009-10 and 2010-11. In the short term,

host resistance is one of the main driving forces influencing disease development

(Chakarborty, 2005). Therefore in order to correlate effect of enhanced [ CO2] on leaf

chemistry and incidence of disease, one year results have been discussed in this study.

The data were analyzed using the Statistical Package for Social Sciences version 17

(SPSS Inc., Wacker Drive, Chicago, IL) for windows. All results are given as mean, ±

standard deviation of six replicates. The student’s t – test was used to calculate the

significance between results. An asterisk (*) indicates that means are statistically

different at P<0.05 between the ambient and elevated [CO2] concentrations.

3. Results

3.1. Disease incidence

The incidence of naturally occurring disease was determined for field grown

plants in both ambient and elevated [CO2] treatments beginning one week after seedling

emergence and continuing through the onset of leaf senescence. Field surveys revealed

the development of two foliar diseases in decreasing order of severity: Leaf rust caused

by Puccinia triticina and stripe rust caused by Puccinia striiformis. Elevated [CO2] had a

strong effect on the susceptibility of wheat plants to foliar pathogen Puccinia triticina .

Incidence of white rust caused by Puccinia triticina was 75.8% higher under elevated

[CO2] as compared to plants in ambient CO2 conditions (Fig. 1). In same pattern, the

incidence of Puccinia striiformis increased 33.4% under elevated [CO2].

3.2. Leaf epidermal characteristics

Significant changes in structural characteristics of wheat leaves grown under

elevated [CO2] was observed. Plants grown under elevated [CO2] exhibited 47.5%

increase in wax on leaves when compared to control plants. Scanning electron

micrographs of the adaxial surface of leaf revealed alteration in stomatal structure.

Significant differences were observed in stomatal density, stomatal aperture length and



width. Stomatal density decreased by 69% in elevated [CO2] over ambient CO2. Stomatal

aperture also showed significant reduction in length and width under elevated [CO2]

compared to ambient CO2.

3.3. Concentration of carbon, nitrogen, sulphur and C/N ratios

As expected, the carbon concentration of plants grown under elevated [CO2] was

significantly higher than that of plants grown under ambient CO2. However, nitrogen

concentration reduced, this led to concomitant increase in C/N ratio under elevated CO2.

Plants grown at elevated [CO2] had no significant effect on uptake of sulphur – resulting

in decreased N/S ratio.

3.4. Carbohydrate and protein concentrations

The concentration of carbohydrates was about two times higher under elevated

[CO2] in comparison to control (Fig. 2). On the other hand, there were negligible

differences in the protein concentration between both the atmospheric treatments.

3.5. Phenylalanine ammonia lyase (PAL) activity and total phenols concentration

Cultivation of mustard plants at enhanced CO2 led to significant increase in PAL

activity. Correspondingly, there was significant increase in total phenols concentration

(22.6%) under elevated [CO2] (Fig. 3).



Fig. 1.

Effect of elevated [CO2] on per cent disease incidence of two naturally occurring foliar

disease in wheat. The mean values were plotted with (±) S.D. of six replicates.

Significant differences are indicated by: *P<005.



Fig. 2

Effect of elevated CO2 on (A) carbohydrates; and (B) protein concentration (mg g-1 F W)

in leaves of wheat. The mean values were plotted with (±) .D. of six replicates.

Significant differences are indicated by: *P<005.

0

5

10

15

20

25

30

35

40

45

50

Con

ce

ntr

ation

of

tota

l p

he

no

ls(

mg

g-1

)

A

*

Ambient CO2 Elevated CO2



Fig. 3.

Effect of elevated CO2 on (A) total phenols concentration; (B) phenylalanine ammonia

lyase activity. The mean values were plotted with (±) S.D. of six replicates. Significant

differences are indicated by: *P<005.

4. Discussion

Changes in atmospheric composition of CO2 altered disease expression in wheat,

but responses of the two pathosystems varied considerably. Incidence of both rust

infection increased under elevated [CO2] but with different aspect. Though effect of

enhanced CO2 on growth physiology can be largely generalized but plant pathogen

interaction cannot be generalized because of their complex nature. There are examples of

both necrotrophic and biotrophic pathogens showing lower as well as increased disease

levels at enhanced concentrations of CO2 (Eastburn et al., 2010).

Puccinia triticina and Puccinia striiformis which causes rust diseases initiates

infection via stomata into the leaves of wheat. The slight increase extent of incidence in

wheat is possibly due to reduced stomatal density and stomatal length under elevated

[CO2]. The two main fluxes across the leaf epidermis are water vapor and CO2. As CO2

rises for given water budget, plant affords to reduce its stomatal conductance without

suffering a reduction in carbon assimilation rates. Two main pathways driving this

response are smaller stomatal pores (Bettarini et al., 1998) and a reduction in stomatal

numbers (Royer, 2001) – also observed in this study. Chakraborty et al., (2000) have

suggested that changes in stomatal structure and function induced by elevated [CO2] may

alter foliar disease in future because many pathogens enter through stomata – as the case

may be in wheat- Puccinia interaction in this study. Not only does stomatal closure

reduce the pore size for stomatal-infecting pathogens to enter the plants, but it could also

alter the microclimatic conditions on the leaf surface via localized humidity reductions

around stomatal pores (McElrone et al., 2005).



Cultivation of mustard plants at elevated [CO2] resulted in significant increase in

epicuticular wax. Plant epicuticular waxes are complex mixtures of primarily straight

chain aliphatic hydrocarbons with a variety of substituted groups (Walton, 1990). The

concentration of carbon based compounds (epicuticular wax and phenols) increased

under elevated [CO2] in mustard. This is consistent with the carbon-nutrient balance

hypothesis that postulates the accumulation of carbon-based defense metabolites at

increased C/N ratio (Karowe et al., 1997; Gleadow et al., 1998; Hamilton et al., 2001).

The amount of epicuticular wax on leaf surface is one of the factors in Triticum spp.

responsible for differences in susceptibility to two different pathogen (Conn and Tewari,

1984).

Plants grown under elevated [CO2] result in decreased nitrogen concentration

(Matros et al., 2006, Plessl et al., 2007). Our results on wheat comply with the earlier

reports. Plants grown under elevated [CO2] showed increase in C/N ratio with

corresponding increase in concentration of carbohydrates and dilution of nitrogen

concentration. Growth at elevated [CO2] leads to significant increase in the foliar

carbohydrate content which indicates a source/sink imbalance and increased C/N ratio

(Long et al., 2004). Higher carbohydrate concentration in leaves may be responsible for

increased incidence of white rust infection caused by Puccinia spp as enhanced sugar

content of leaves under elevated CO2 improves the status of sugar dependent pathogens

(Eastburn et al., 2011).

Elevated [CO2] increased the concentration of total phenols and induced higher

PAL activity. Measure of PAL activity and phenol concentration have been used as

biochemical markers of defense in plants and may be useful for gaining a better

understanding of the effect of enhanced CO2 on defense chemistry of plants. PAL is a key

enzyme of phenylpropanoid metabolism in plants. The enzyme catalyses de-amination of

L-phenylalanine into trans-cinnamic acid. The trans-cinnamic acid serves as a precursor

of various secondary metabolites including phenols, phenylpropanoids and monomers of

lignin and salicylic acid (Creasy and Zucker, 1974). Incidence of foliar diseases induced

PAL activity in Brassica juncea; however, it was significantly higher in plants grown

under elevated [CO2]. Increased PAL activity induces accumulation of phenolic

compounds via phenylpropanoid pathway. Quantitative analysis of total phenols in



mustard leaves showed increased concentration in FACE compared to ambient CO2

which may be due to increased PAL activity and is also consistent with carbon-nutrient

balance hypothesis discussed. Our observations of PAL activity are in accordance with

Matros et al., (2006) where strong and significant increase in level of PAL activity in

tobacco plants was observed under elevated [CO2].

The leaf chemistry is known to be more sensitive to high CO2 concentrations than

to low CO2 concentrations (Williams et al., 2000). Both nutrients and secondary

metabolites in plant leaves are important in determining the leaf quality as food for

pathogens (Scriber and Slansky, 1981). Elevated [CO2] led to increase in carbon based

compounds (carbohydrates, phenols, wax). However, to clearly define the portion of

investment in growth or defense, or between different classes of defenses, a complete

understanding of biosynthetic pathways is needed. In conclusion, our findings suggest

that exposure to enriched CO2 atmosphere can alter defensive responses in wheat against

pathogens and these changes may be due to real impact of CO2 level on the defense

chemistry of plants.

Acknowledgement

The research work is supported by Ministry of Environment & Forests, Government of

India. We are thankful to Director, Indian Agriculture Research Institute, New Delhi for

providing FACE facility. Thanks are also extended to Prof. K. S. Rao, Department of

Botany, University of Delhi for CHNS facility, respectively. Yogesh Yadav gratefully

acknowledge Department of Science and Technology in the form of Junior Research

Fellow.

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