Siberian wildrye seed yield limited by assimilate source

Siberian wildrye (Elymus sibiricus L.) is a cool-season, perennial bunchgrass widely used to increase forage production of semi-arid grasslands and to restore degraded lands. It is drought and cold tolerant, and has high yield potential and livestock acceptability. However, seed yield is low and variable and there is little information on the physiological processes limiting its seed yield. A split plot experiment was conducted to determine the effect of source-sink treatments under different nitrogen (N) applications on seed weight, seed setting rate, and dry matter and N remobilization during three successive growing seasons. The main plots were N level (0, 90, 180 kg N ha−1) and sub plots were source sink treatments (half trimmed and unaltered spikes). Leaf, stem, and spike dry weights and N concentration were measured at anthesis and seed physiological maturity, while seed weight and seed setting rate were measured at seed physiological maturity and milk stage respectively. Stem and spike dry weights were increased, whereas leaf dry weight was decreased between anthesis and maturity. N application significantly increased all plant parts of Siberian wildrye dry weight and seed weight compared with 0 kg N ha−1 application. The half-spike treatment increased seed weight and seed setting rate across all N levels, and stem dry weight accumulation, but there was no significant influence on N translocation. The results of this study indicate that Siberian wildrye seed yield is source limited since seed weight and seed setting rate were increased in the half-spike treatment with N application. Stem dry weight increased after anthesis suggesting that stems were sinks and competed for assimilates with spikes. Therefore, breeding for increased photosynthesis and partitioning of assimilates from stems to spikes may result in increased and more consistent seed yield.