Variation in Carbon Isotope Discrimination and Its Relationship to Specific Leaf Area and Ribulose-1,5-Bisphosphate Carboxylase Content in Groundnut Genotypes

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 (&sigmaf;L) 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 &sigmaf;L 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 &sigmaf;L (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 &sigmaf;L or Δ.