Abstract

Media summary

Keywords

Leaf water potential, osmotic potential, photosyntheis, photodamage, photoinhibition, drought resistance

Introduction

The chickpea (Cicer arietinum L.) is generally grown under conserved moisture condition and the crop usually encounters into terminal drought during pod filling stage. The reduction in the grain yield in chickpea under drought has been reported to be associated with significant decrease in the above ground dry matter or vegetative biomass (Basu & Singh, 2003). Growth of the vegetative biomass in chickpea is highly sensitive to drought stress, which is clearly visible, even the crop experiences moderate stress during vegetative period. The reduction in the biomass could be due to either decrease in the photosynthesis or reduced leaf expansion rate or cumulative effect of both (Turner & Begg, 1981 ). On the other hand, the decline in photosynthesis in dehydrating leaves may be due to stomatal closure or else changing biochemical and biophysical pathways of photosynthesis (Boyer, 1970). The precise mechanism of reduction in the biomass even under moderate water stress is not known. Very little information is available regarding the effect of water stress on photochemical efficiency of PS II in chickpea, the species known to have for wide range of adaptability under drought condition. It is therefore imperative to analyse the factors at the level of non-stomatal components of photosynthesis influencing biomass in chickpea.

Methods

Six chickpea (Cicer arietinum L.) genotypes namely C 214, K 850, Katila, Phule G5, RSG 143-1 and Vijay were selected for physiological studies. The crop was sown under conserved soil moisture during October 2002 last week without any pre-sowing irrigation. Trial was conducted at experimental station of IIPR , Kanpur in Randomized block design (RBD) with three replications, where moisture treatment were kept as main plot and genotypes in the sub-plot. Two treatments involving different soil moisture regimes (i.e. irrigated and rainfed) were opted for studying the effect of drought. The irrigated treatment received two irrigations to ensure non-limiting conditions through out the crop growth. Rainfed crops were a allowed to grown on residual soil water. There was no rainfall during the season of experimentation. Soil moisture at different depths was monitored with a neutron moisture probe. Number of secondary branches originating from primary stems and leaf numbers were recorded manually. Biomass was estimated by weighing the oven-dried samples. Water relation characteristics such as relative water content (RWC), leaf water potential and osmotic potential were determined by standard methods or as described by Leport et al. (1998). Leaf expansion rate was calculated on the basis of measurement of length of tagged leaves at every 24-hour interval. Chlorophyll fluorescence of the leaves was measured by using Pulse Amplified Modulated Fluorometer (FMS 2 Hansatech, U.K.) according to Schreiber et al.. (1986). Quantum yield of leaves was measured at different level of PAR (photosynthetic active radiation) from low to high irradiances (about 10-500 µmol photons m^-2s^-1) using different levels of actinic light. The fluorescence data were analysed using FMS 2 soft wares.

Results

During terminal drought experienced by the rainfed crops in the month of March, the leaf relative water content (RWC) in rainfed situation decreased to about 15-25% lower than irrigated crop (Table 1). The water stress had no significant effect on effective quantum yield (Fv’/Fm’) or steady state minimal fluorescence, Fs at various irradiance levels (Table 1 and 2).

Table 1 : Effect of water stress and irradiance on effective quantum yield (Fv/Fm).

Genotype (G) LSD_0.05 = 0.017 significant; Treatment (T)= ns ; Irradiance (I) LSD_0.05 = 0.017 significant ; G x T (LSD_0.05)= 0.024 significant; G x I (LSD_0.05) = 0.042 significant ;T x I = ns ; G x T x I = ns

Table 2 : Effect of water stress on steady state minimal fluorescence, Fs.

Genotype (G) LSD_0.05 =25.96 significant; Treatment (T)= ns ; Irradiance (I) LSD_0.05 = 25.96 significant ; G x T (LSD_0.05) = 36.71 significant ; G x I (LSD 0.05) = ns ; T x I = ns ; G x T x I = ns

There was no indication of any damage in photosynthetic apparatus of water-stressed leaves, as Fs did not significantly change with respect to water regimes (Table 2). However, the decrease in quantum yield due to high irradiance irrespective of water stress was evident which may be attributed to the transient state of photo-inhibition.

The leaf elongation or expansion rate was not reduced significantly in rainfed crops in spite of the fact that leaf elongation is considered to be very sensitive to drought (Table 3). However, the investigation showed that leaf and secondary branch numbers were more adversely affected due to drought than leaf expansion rate and both these traits were highly correlated (P<0.01) with grain yield or pod numbers per plant (Table 4 & 5).

Table 3 : Leaf expansion rate in relation to water status

IR= Irrigated ; RF = Rainfed

Table 4 : Leaf and secondary branch numbers per plant at two levels of soil moisture

Table 5 : Associations (r) of leaf and secondary branch numbers with pods

** Significant at P<0.01

Conclusion

The results showed that neither quantum yield nor minimal fluorescence Fs were affected due to lowering of RWC in chickpea. This finding is not in agreement with other earlier reports that indicated decrease in the quantum yield (Fv’/Fm’) as a result of various abiotic stresses such as drought and high light in many plants (Wang et al, 2001; Basu et al, 1998), although crop growth conditions and intensity of drought might be different in different studies. Photochemical damage due to stress was usually reflected in either an increase in original fluorescence Fo/Fs, or decreases in maximum fluorescence (Fm), variable fluorescence (Fv) or quantum yield (Fv/Fm). This indicates the existence of some inherent mechanism that allows the photochemical process involving PS II to function normally in chickpea under water stress. This resistance could be due to inherent ability to maintain positive turgor with decrease in the RWC or leaf water potentials (Table 3). Earlier studies showed that the rates of photosynthesis in chickpea are markedly reduced by water deficits (Leport et al., 1999), which could be due to stomatal closure (Qifu Ma et al., 2001). The present results therefore suggested that non-stomatal components are relatively stable due to turgor maintenance in chickpea under mild drought. The maintenance of positive leaf turgor could be attributed to the osmotic adjustment (Turner and Jones, 1980).

References

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