Flag leaf area and harvest index had positive and highly significant correlation with number of grains per spike while days to 50%heading, days to maturity, grain filling duration, thousand grain weight and tillers per meter had negative but non-significant correlation with it (Table 3).
Harvest index showed negative and non-significant correlation with plant height, grain filling duration, days to maturity and spike length (Table 3).
heritability for grain filling duration was higher than heritability due to polygene in BC1 BC2 and F2 for the crosses with highest environmental influence.
Grain filling duration is the time from anthesis to physiological maturity which determines the accumulation of dry mater and its partition into grain.
The main reasons of grain yield loss under heat stress conditions were reduction of grain filling duration (17%), grain filling rate (8%), grains weight (24%), SNPM (14%) and GNPS (14%).
DTF: Days to flowering, DTM: Days to maturity, GFR: Grain filling rate, GFP: Grain filling duration, SNPM: Spike number per [m.sup.2], GNPS: Grain number per spike, TGW: 1000 grain weight, HI: Harvest index, BY: Biological yield, GY: Grain yield.
Number of days from heading to physiological maturity was taken as grain filling duration. Grains from the each of the five spikes were threshed manually and counted.
Analysis of variance indicated that drought stress at different reproductive stages affected the plant height, spike length, chlorophyll content index, grain filling rate, grain filling duration, spike length, grains per spike, grain weight, grain yield, biological yield, harvest index, water use efficiency and transpiration efficiency of all tested wheat cultivars (Table 2).
Heat stress shortens the grain filling duration but accelerates the grain filling rate (Dias and Lidon, 2009).
For instance, an increase of 5C in temperature above 20C shortened the grain filling duration by twelve days in wheat with increase in grain filling rate (Yin et al., 2009).
Grain filling rate and duration are known to display genetic and environmental variation, although grain filling rate seems more genotype-dependent and grain filling duration is influenced mainly by post-anthesis temperature.
Conversely, grain filling duration seldom varied and, therefore, grain filling rate explained most of the variation in grain weight.