PHYSIOLOGICAL-BASIS OF THE RESPONSE OF HARVEST INDEX TO THE FRACTION OF WATER TRANSPIRED AFTER ANTHESIS - A SIMPLE-MODEL TO ESTIMATE HARVEST INDEX FOR DETERMINATE SPECIES

The fraction of biomass partitioned to grain is critical to crop yield. Methods to predict this fraction are required for crop modelling. The relationship between harvest index (HI) and the fraction of water transpired after anthesis (theta-E) provides a simple basis for the estimation of HI for determinate species. In this paper, empirical and theoretical aspects of this relationship are examined and a method is developed to estimate HI. The model proposed is: HI(pv) = theta-Ee/[1 - (a - b-theta-Ee)] Subscripts pv and E indicate that HI and theta-E are normalized by production value and vapour pressure deficit, respectively. The model also differentiates between source and sink restrictions to yield. The parameter a quantifies the 'potential' contribution of pre-anthesis assimilates to grain, and 1/(1 - a + b) the 'potential' harvest index; both are genotype-dependent. The model accounted for 75% (P < 0.001) of the variance of HI of wheat plants with HI ranging from 0 to 0.47 and theta-E from 0.05 to 0.47, and 81% (P < 0.001) of the variance of HI of sunflower plants with HI from 0.13 to 0.46 and theta-E from 0.04 to 0.41.