Influencing Mechanism Of Heat Stress After Pollination On Grain Starch Quality Formation Of Waxy Maize

The experiments were conducted at the intelligent greenhouse of Agricultural College, Yangzhou University in 2014 and 2015. The effects of heat stress after pollination on grain starch quality formation were studied using Suyunuo5 and Yunuo7 as materials. The main results are as follows:Heat stress during grain filling decreased the fresh grain weight and volume, grain dry weight and grain moisture content. The decrease of fresh grain volume was more pronounced for Yunuo7 and later grain filling stages. The grain dry weight of all treatments was gradually increased with grain filling before 22 d, the value under normal temperature after 22 d after pollination (DAP) gradually increased, while the value under heat stress conditions stopped.With grain filling, the grain protein content sharply decreased before 22 DAP, and similar among three stages after 22 DAP. Compared to control, the protein content under heat stress was lower at 7 DAP and higher at 30-40 DAP. With grain filling, the grain albumin content gradually decrease, glutelin content gradually increase, globulin content decrease first and increase later, while prolamin content rose first and fall later, respectively. The starch content sharply increased before 15 DAP, and slowly increased thereafter, and the value was decreased by heat stress after 22 DAP.At 7 DAP, the pericarp accumulated some amyloplast and it gradually disappeared with caryopsis development. The pericarp thickness reduced and the amyloplast in the pericarp was degraded. Compared with control, the pericarp thickness and the number of amyloplast in pericarpic cells were decreased under heat stress conditions at the same stage. The starch granule present circular at early stages, the amyloplast in endosperm cells gradually enlarged with the caryopsis development, the amyloplast almost filled in the endosperm at 22 DAP, and the shape changed to irregular due to squeeze each other. The accumulation of starch under heat stress conditions was earlier than under normal temperatures at early stage, and the starch granule size was larger than its counterpart under normal temperatures.Heat stress had no influence on starch crystalline types, and all samples at different stages and different temperature presents typical "A" type pattern. Compared with control, the starch crystallinity under heat stress was higher at early grain filling stage and the value was lower at later stages. With grain filling, the starch iodine binding capacity gradually decreased, and the maximum absorption wavelength was sharply decreased from 7 to 15 DAP and kept consistent thereafter. Heat stress increased the both maximum absorption wavelength and starch iodine binding capacity at different stages for both varieties.The activities of ADP-glucose pyrophosphorylase, soluble starch synthase and starch branching enzymes rose first and fall later and peaked at 15-22 DAP with grain filling. The activities of those three enzymes were decreased by heat stress for both varieties. Compared with control, heat stress increased and decreased the RNA expression quantity of AGP, SSIIa, SSIIb, and SBEIIb at 7 and 22 DAP for Suyunuo5, respectively. The expression quantity of AGP and SSIIb for Yunuo7 under two stages were similar to Suyunuo5, while the expression quantity of SSIIa and SBEIIb were decreased by heat stress under both stages.The results of grain protein expression spectra indicated there are 365 and 490 points for Suyunuo5 and 473 and 544 points for Yunuo7 at 7 and 22 DAP, respectively. There are 9 and 17 different points at 7 and 22 DAP for Suyunuo5 and which account for 2.47% and 3.47% for all detected points, respectively. The different points for Yunuo7 at 7 and 22 DAP were 18 and 24, which account for 3.81% and 4.41% for all detected points, respectively. The 68 different expression protein points by mass spectrum identification representative 48 proteins, and could summary to 35 types.The starch peak viscosities were similar among different stages, the breakdown viscosity rose first and fall late with grain filling, while pasting temperature was gradually decreased. The pasting temperatures at different stages were increased by heat stress. The thermal characteristics rose first and fall late with grain filling. The thermal characteristics were changed by heat stress. Gelatinization enthalpy, onset and peak temperature of starch at 22 DAP were decreased by heat stress. However, the gelatinization enthalpy, onset and peak temperature, retrogradation enthalpy and percentage at maturity (40 DAP) were increased by heat stress.