Effects of Post-Anthesis High-Temperature Stress on Carbon Partitioning and Starch Biosynthesis in a Spring Wheat (Triticum aestivum L.) Adapted to Moderate Growth Temperatures

This study investigates carbon partitioning in the developing endosperm of a European variety of spring wheat subjected to moderately elevated daytime temperatures (27 degrees C/16 degrees C d/night) from anthesis to grain maturity. Elevated daytime temperatures caused significant reductions in both fresh and dry weights and reduced the starch content of harvested grains compared to plants grown under a 20 degrees C/16 degrees C d/night regimen. Accelerated grain development caused by elevated temperatures was accounted for by representing plant development as thermal time (degrees C DPA). We examined the effects of high-temperature stress (HTS) on the uptake and partitioning of [U-C-14]-sucrose supplied to isolated endosperms. HTS caused reduced sucrose uptake into developing endosperms from the second major grain-filling stage (approximately 260 degrees C DPA) up to maturity. Enzymes involved in sucrose metabolism were unaffected by HTS, whereas key enzyme activities involved in endosperm starch deposition such as ADP-glucose pyrophosphorylase and soluble isoforms of starch synthase were sensitive to HTS throughout grain development. HTS caused a decrease in other major carbon sinks such as evolved CO2, ethanol-soluble material, cell walls and protein. Despite reductions in the labeling of carbon pools caused by HTS, the relative proportions of sucrose taken up by endosperm cells allocated to each cellular pool remain unchanged, except for evolved CO2, which increased under HTS and may reflect enhanced respiratory activity. The results of this study show that moderate temperature increases can cause significant yield reductions in some temperate wheat cultivars chiefly through three effects: reduced sucrose uptake by the endosperm, reduced starch synthesis and increased partitioning of carbon into evolved CO2.