Differential response of two wheat genotypes to nitrogen supply. II. Mesophyll cell characteristics and photosynthesis of laminae at full expansion

Abstract

Two wheat genotypes, viz. Uniculm 'Gigas' (V1) and Kalyansona (V2) were raised to maturity under two levels of nitrogen supply @ 30 (N1) and 120 (N2) kg ha^-1, under pot culture conditions. Laminae numbering 1 to 8 on the shoot were examined for area, net photosynthetic rate (P_n) and mesophyll cell characteristics, at full expansion. N1 resulted in greater reduction in the area of upper laminae in V1 than in V2. The flag and penultimate laminae maintained similar P_n rates at both N levels in V1 despite a large reduction in area at N1. However, V2 exhibited about 25% reduction in P_n rates of the flag laminae at N1 as compared with N2. Mesophyll cell number per unit area gradually increased with laminae order in the genotypes. The greater number of mesophyll cells in the upper laminae was essentially due to a reduction in cell size and surface area as a result of low N availability. The large differences in laminae size due to N treatments, particularly in V1, are attributed to mesophyll cell numbers. However, the maintenance of P_n rates in V1, even at low N supply, is a possible outcome of higher activity of the mesophyll cell constituents in this genotype.