Heat Stress at Different Grain Filling Stages Affects Fresh Waxy Maize Grain Yield and Quality

The effects of heat stress (mean day/night temperatures were 29/18 and 35/18 degrees C for control and heat stress treatments, respectively) at 1-10 or 11-20 days after pollination (DAP) on grain yield and quality of two waxy maize varieties, Suyunuo5 and Yunuo7, were studied. The decrease in grain number and fresh grain yield was more severe when heat stress was introduced early. The responses of grain weight and its components to heat stress were dependent on the variety and heat stress stage. Heat stress increased the starch granule size and volume percentage of diameter larger than 17 mu m in both varieties. Crystallinity, iodine binding capacity, and pasting and gelatinization temperatures were increased in both varieties under heat stress at 11-20 DAP; however, the response to early-stage heat stress was variety dependent. Peak viscosity and retrogradation percentage were increased by heat stress at either stage. These results suggest that heat stress decreases fresh grain yield and accelerates grain filling rate; it increases starch content, starch granule size, crystallinity, and iodine binding capacity as well as decreases the protein content in Yunuo7, thus making the grain less tender and prone to retrogradation. Warm conditions should shorten the time to harvest in fresh waxy maize production.