Comparison of Carbohydrate Compartmentation in Relation to Photosynthesis, Assimilate Export and Growth in a Range of Maize Genotypes

Abstract

A model derived from compartmental analysis of assimilate partitioning and export in maize source leaves was used for comparison of growth characters in a series of genotypes. The measurements were performed on fully expanded fourth leaves. The parameters measured were: net photosynthetic rate (PN), carbon flux to starch and size of two sucrose pools (Q1 and Q2), the transfer coefficients between the compartments, the leaf area and the shoot and root dry matter. Photosynthetic rate on a leaf area basis varied slightly (1:1.34). This variation is amplified (1:2.96) when carbon input is expressed on a leaf basis due to great genotypic variability in leaf area (1:2.33). Compartmental analysis showed that the sizes of sucrose compartments (Q1 and Q2) were also genotype dependent. Compartment Q1, which provides sucrose export, was always greater than Q2, which represents a transient storage. The transfer coefficients were rather constant among genotypes, the coefficient representing export (k₀₁) being much higher than the other two (k₁₂ and k₂₁). The proportion of the assimilates which was stored as starch during the light period represented 12-17.5% of the carbon input depending upon the genotype. This percentage of allocation to starch was negatively correlated with growth rate. The correlations between growth rate or total production and each physiological parameter (P_N, carbon output from Q1 and Q2, flux toward starch) were in general highly significant (0.75<r<0.94). The correlations were positive except for starch. A multilinear regression analysis showed that two parameters, net photosynthesis per leaf or net output from Q1 associated with carbon flux to starch, provided a nearly total explanation of intergenotypic variance. The results clearly demonstrate that in maize, the differences in net carbon fixation were not sufficient to totally predict growth rate and that knowledge of the additional carbon distribution and export parameters will give a better understanding of the system.