Rate of change in harvest index during grain-filling of wheat

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Abstract
SUMMARY: A constant rate of change in harvest index (dHI/dt = k) has recently been incorporated into several crop simulation models, so that final grain yield can be calculated from final biomass and the duration of grain growth. Implicit is the assumption that dHI/dt is conservative across treatments and environments. This assumption was tested using data from five experiments grown in the United Kingdom (1973, 1978, 1994) and New Zealand (1992, 1993). The experiments included commercial spring and winter wheat cultivars introduced during the last 100 years and nitrogen, irrigation, sowing date, temperature and CO2 treatments. In all cases, the time course of harvest index (HI) had an initial lag phase, a linear phase and a maturation phase. The linear phase was stable in field-grown crops, except for a reduction in slope after lodging in some crops. Values for dHI/dt, taken as the slope of the linear phase, varied with variety and available nitrogen, were stable for a given variety among years, and were unaffected by water stress. Variation in dHI/dt among varieties was independent of their year of introduction, although those with the Rht2 semi-dwarfing gene generally achieved a higher final HI due to a reduced lag phase. Differences in the duration of the linear phase also caused differences in the final HI after drought. The upper and lower limits of dHI/dt for fieldgrown crops were 1·37 and 0·64% d-1 but, under normal fertility conditions, the variation was between 0·90 and 1·19 % d-1. Results indicated that dHI/dt could provide an effective semi-empirical relationship for predicting grain yield in simulation models. The consistent, linear nature of this relationship suggests a physiological maximum for dHI/dt, for a given species and variety. It may be possible to exploit varietal differences in dHI/dt, and in the lag phase, for yield improvement.