Experimental evidence is presented to show that the 18O enrichment in the leaf biomass and the mean (time-averaged) transpiration rate are positively correlated in groundnut and rice genotypes. The relationship between oxygen isotope enrichment and stomatal conductance (g(s)) was determined by altering g(s) through ABA and subsequently using contrasting genotypes of cowpea and groundnut. The Peclet model for the 18O enrichment of leaf water relative to the source water is able to predict the mean observed values well, while it cannot reproduce the full range of measured isotopic values. Further, it fails to explain the observed positive correlation between transpiration rate and 18O enrichment in leaf biomass. Transpiration rate is influenced by the prevailing environmental conditions besides the intrinsic genetic variability. As all the genotypes of both species experienced similar environmental conditions, the differences in transpiration rate could mostly be dependent on intrinsic g(s). Therefore, it appears that the delta18O of leaf biomass can be used as an effective surrogate for mean transpiration rate. Further, at a given vapour pressure difference, delta18O can serve as a measure of stomatal conductance as well.
Two pot experiments were conducted in two different seasons at the University of Agricultural Science, Bangalore, India, to study (a) the relationship between chlorophyll concentration (by measuring the leaf light-transmittance characteristics using a SPAD metre) and transpiration efficiency (TE) and (b) the effect of leaf N on chlorophyll and TE relationship in peanut. In Experiment (Expt) I, six peanut genotypes with wide genetic variation for the specific leaf area (SLA) were used. In Expt II, three non-nodulating isogenic lines were used to study the effect of N levels on leaf chlorophyll concentration-TE relationship without potential confounding effects in biological nitrogen fixation. Leaf N was manipulated by applying N fertiliser in Expt II. Chlorophyll concentration, TE (g dry matter kg 21 of H 2 O transpired, measured using gravimetric method), specific leaf nitrogen (g N m 22 , SLN), SLA (cm 2 g 21 ), carbon isotope composition (Á 13 C) were determined in the leaves sampled during the treatment period (35-55 days after sowing) in the two experiments. Results showed that the leaf chlorophyll concentration expressed as soil plant analytical development (SPAD) chlorophyll metre reading (SCMR) varied significantly among genotypes in Expt I and as a result of N application in Expt II. Changes in leaf N levels were strongly associated with changes in SCMR, TE and Á 13 C. In both the experiments, a significant positive relationship between SCMR and TE with similar slopes but differing intercepts was noticed. However, correction of TE for seasonal differences in vapour pressure deficit (VPD) between the two experiments resulted in a single and stronger relationship between SCMR and TE. There was a significant inverse relationship between SCMR and Á 13 C, suggesting a close linkage between chlorophyll concentration and Á 13 C in peanut. This study provides the first evidence for a significant positive relationship between TE and leaf chlorophyll concentration in peanut. The study also describes the effect of growing environment on the relationships among SLA, SLN and SCMR.
Variation in cahn isotope discrimination (Δ) and ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) content per unit leaf area was examined in leaves from upper and lower positions in the canopy of six groundnut (Arachis hypogaea L.) genotypes, grown under irrigated and mild water-deficit conditions in the field. The leaf mass per unit leaf area (ρL) and soluble proteins in leaves were determined at 80, 96, 111 and 127 days after sowing (DAS), while Δ and Rubisco were determined at 80 DAS only. The mean Δ ranged from 18.2 to 20.20 among genotypes, representing a significant (P < 0.01) variation. Rubisco content per unit leaf area also varied significantly (P < 0.01) with genotype and leaf position. There was a trend to an increase in Rubisco content under water deficit, but the effects were not significant. Leaves at the top of the canopy had a higher Rubisco content and lower Δ, than leaves at the bottom of the canopy. Genotype × leaf position interaction was significant for Δ and Rubisco, indicating the importance of leaf position in selecting for water-use efficiency (W), using leaf traits in groundnut. Rubisco content and Δ were negatively related (r2 = 0.65, P < 0.01). There was a significantly positive correlation between Rubisco content and ρL in the upper leaves (r2 = 0.60, P < 0.01 ), but not in the lower leaves in the canopy. However, the overall relationship between Rubisco and ρL (r2 = 0.40) was not as strong as it was between Rubisco and Δ. The results suggest that, in groundnut, the basis of genotypic variation in was mostly (> 60%) attributable to Rubisco content per unit leaf area. In view of the leaf positional effects on Δ and Rubisco, the upper leaves in the canopy should be used for selecting genotypes for W based on leaf traits like ρL or Δ.
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