Slow export of photoassimilate from stay-green leaves during late grain-filling stage in hybrid winter wheat (Triticum aestivum L.)

Hybrid winter wheat (cv. XN901) shows a strong hybrid vigor in terms of yield potential and its leaves and stems stay green when its kernels reach maturity. Much of its pre-stored carbon reserve is left unused in its stems and leaves. In this study, photosynthesis and translocation of carbon assimilates of the hybrid wheat were investigated at its late grain-filling stage. Feeding labelled (CO2)-C-14 to flag leaves of XN901 showed that its export to grains was much slower than that of an ordinary cultivar (Shaan 229) that is currently used in production. Within 24 h of labelling, Shaan 229 and XN901 exported 59 % and 40 %, respectively, of the C-14 from their flag leaves. At the sink side, 52.1 % of the fed C-14 in Shaan 229 was exported to its grains within the first 24 h while XN901 deposited only 15.8 % of the fed C-14 in the grains at the same period. Photosynthetic rates and chlorophyll contents in the flag leaves declined much more slowly in XN901 than in Shaan 229 during the late grain-filling stage. Hybrid wheat XN901 also had a longer grain-filling period (6 days longer), higher grain yield (15 % more), but lower harvest index (19 % less) than Shaan 229. Its grain-filling rate was lower but declined more slowly at the late grain-filling stage than that of Shaan 229. Results indicate that the strong hybrid vigour of XN901 leads to an unfavourably delayed senescence which results in the much unused carbon reserve in its straws. Raising harvest index should be the key aim to materialize its yield potential.