Carbohydrate storage and utilization during grain filling as regulated by nitrogen application in two wheat cultivars

Wheat (Triticum aestivum L.) cultivar LZ953 is one of a type of large-spike and large-grain cultivars with delayed leaf senescence and slow grain filling. Its grain filling is adversely affected by the dry hot winds that generally occur during the late stages of the crop in northern China. This study investigated the possible reasons for the slow grain filling and compared LZ953 with an early ripening cultivar, LM14, cultivated at high and low nitrogen (N) levels. The results showed greater photosynthetic rate, carbon (¹⁴C) accumulation, leaf area, and soluble sugar accumulation in stems and sheaths at grain filling in LZ953 than LM14. However, export of soluble sugar from the stem and sheaths, as well as starch accumulation in the grains, were smaller in LZ953. As a result, grain filling was slow during the first 14 days after anthesis (DAA). Nitrogen level had no significant effect on the grain-filling rate of either cultivar. Under high N, LZ953 absorbed more N after anthesis than LM14. In LZ953 soluble sugar storage in the stems and sheaths and starch accumulation were greatly reduced under high N. Nevertheless, the soluble sugar contents of the stem and leaf sheaths of LZ953 were greater than those of LM14. At 15 DAA, the ¹⁴CO₂ assimilation of LZ953 was significantly greater than that of LM14, while the percentage of assimilated ¹⁴C in the grains of LM14 was greater than that of LZ953. The proportion of ¹⁴C partitioned to the grains of LZ953 was reduced by increased N application. Our results suggest that carbohydrate utilization, starch synthesis, or both limit the rate of early grain filling in LZ953. This would lead to a negative feedback on the export of soluble sugar from vegetative organs.