*Correspondence: Bing Liu, E-mail: bingliu@njau.edu.cn

Spatial gradients of protein and starch in wheat grains and determine grain milling characteristics, and flour utilization. The increasing compound heat stress and drought stress (HDS) due to global climate change threaten wheat grain yield and quality parameters, but the impacts of extreme climate events on the gradients of protein and starch in wheat grains remain unclear.

In this study, two-year environment-controlled experiments with four heat stress levels (17/27, 21/31, 25/35, and 29/39 °C) and three drought stress levels (25%, 55%, and 75% of field capacity), were conducted to investigate effects of HDS on the gradients of protein and starch concentration within the five grain layers.

The results show that HDS resulted in significantly higher protein concentration, whilst resulting in lower starch concentration in wheat grain layers. Among the five layers, the endosperm layers had the highest increase in protein concentration under HDS, but the decrease of starch concentration under HDS ranked as husk > aleurone > endosperm layer. HDS altered the protein and starch concentration of the five grain layers unevenly.

There were significant linear relationship between relative protein and starch concentration with accumulated heat degree days (AHDD). With a 1°C·d increase in AHDD, the protein concentration in the five grain layers increased by 0.42-3.76%, while the starch concentration decreased by 0.02-1.65%, depending on the drought stress levels.

Our results indicate that HDS will significantly impact the grain quality parameters for flour milling process and human diet, and will provide important insights on adapting wheat quality to climate change.