Heat stress during grain filling in maize: Effects on kernel growth and metabolism

The average temperature in the U.S. Corn Belt during the grain-filling period of maize (Zea mays L.) is above optimum for maximum grain yield. The objectives of this study were to determine the effects of an extended period of high temperature during grain filling on kernel growth, composition, and starch metabolism of seven maize inbreds, Plants were exposed to heat stress (33.5/25 degrees C) or control (25/20 degrees C) day/night temperature treatments in a greenhouse from 15 d after pollination (DAP) until maturity, and the experiment was conducted in triplicate over time. Root zone temperature was maintained at 25/20 degrees C in both treatments. No significant interaction occurred between genotype and temperature treatments for nine grain traits, Heat stress lengthened the duration of grain filling on a heat unit (HU) basis, but an overcompensatory reduction in kernel growth rate per HU resulted in an average mature kernel dry weight loss of 7% (P = 0.06), Proportionally similar reductions occurred for starch, protein, and oh contents of the kernel. Heat stress also reduced kernel density. A survey of 11 enzymes of sugar and starch metabolism extracted from developing endosperm revealed that ADPglucose pyrophosphorylase, glucokinase, sucrose synthase, and soluble starch synthase were most sensitive to the high temperature treatment. However, upon adjusting enzyme activities with measured temperature coefficients (i.e., Q(10)) only ADPglucose pyrophosphorylase exhibited reduced activity. Results indicate that chronic heat stress during grain filling moderately restrains seed storage processes and select enzymes of starch metabolism to similar degrees across multiple maize inbreds.