| Above optimal temperature and drought commonly appear during grain filling in wheat growing areas of China, and are the most adverse ecological factors limiting grain yield and quality. Thus, fully understanding effects of post-anthesis high temperature (HT) and drought on grain quality and yield and their physiological mechanisms are of significance for high yield and quality in wheat cultivation. The present studies, combined pot culture, rain-sheltered culture and field experiments with strong- and weak-gluten wheat cultivars, were carried out to characterize effects of HT and drought and their interactions on activity of key enzymes for synthesis, concentration, and components of starch in grains, to clarify starch pasting properties and flour processing quality and yield of different wheat cultivars exposed to different HT and drought regimes, and to. understand regulating effects of irrigation on grain yield and quality.Details are as follows:1. Two winter wheat (Triticum aestivum) cultivars differing in gluten strength, a strong-gluten cultivar Yumai 34 and a weak-gluten cultivar Yumai 50, were used in pot culture to investigate the effects of post-anthesis HT and drought and their interactions on activity of key enzymes involved in grain starch synthesis. The results indicated that both post-anthesis HT and drought decreased activity of UDPG-PPase, ADPG-PPase, SSS and SBE in grains. The effects became stronger and stronger with the development of grain filling. Obvious difference in behavior of these enzymes was observed between the two cultivar types under HT or drought conditions. For strong-gluten cultivar Yumai34, significant difference (P<0.01) in activity of UDPG-PPase, ADPG-PPase, SSS, and SBE was found among water treatments, but was only found in activity of UDPG-PPase and ADPG-PPase among HT treatments, indicating greater influence of drought than HT. For weak-gluten cultivar Yumai 50, effects of HT were greater on activity of the key enzymes than.those of drought, as activity of UDPG-Ppase, ADPG-Ppase, and SSS decreasedsignificantly under high temperature conditions, but under drought stress only activity of SSS decreased (PO.05).2. In the same pot experiment, studies were carried out to evaluate effects of HT and drought and their interactions on photosynthetic capacity and sucrose synthesis in flag leaves. The results indicated that both HT and drought deceased Fv (variable fluorescence), Fm (maximal fluorescence), Fv/Fm, and Fv/Fo (minimal fluorescence) in flag leaves, which indicated an inhibition in primary light energy conversion, potential photosynthetic activity, and electron transport of PS II. Thus significant decrease of net photosynthetic rate (Pn) was found both under HT and drought conditions. But difference in effects was found in different grain-filling stages. A significant decrease both in Fv/Fo and Fv/Fm in flag leaves was found just during the treating process, and after the treatment an adapted recovery of Fv/Fo and Fv/Fm appeared in the leaves. In late development stages, significant decrease was observed in fluorescence parameters, indicating a serious damage to PS II in flag leaves and early senescence of the plant. The results also showed that decrease in Fv/Fo and' Fv/Fm was additive under both stress conditions, and interactions of HT and drought on Fv/Fm and qN were found. Studies on key enzymes involved sucrose synthesis in flag leaves indicated that both HT and drought decreased activities of SPS and SS. For weak-gluten cultivar yumai50, significant difference (PO.01) in activity of SS and SPS was found among HT treatments, but was only found in activity (P<0.05) of SS among water treatments, indicating that greater influence of HT than drought. For strong-gluten cultivar Yumai 34, significant difference (PO.05) was found in activity of both SS and SPS among HT treatments, and was found (P<0.01) in activity of SPS among water treatments. It seemed that activity of SS was less influenced under drought stress for Yumai 34. The result indicated that damage of PS II and decrease in Pn and activities of key enzymes involved in sucrose synthesis in flag leaves were the main factors restricting starch synthesis and grain yield and quality.3. In the same experiment stated above, effects of post-anthesis HT and drought and their interactions were investigated on concentration, components, and pasting properties of starch in wheat grains. The results indicated that HT and drought reduced content of amylose, amylopectin, and total starch in grains. Under adverse conditions, as decrease wasmuch greater in amylose content than in amylopectin content, the ratio of amylose content to amylopectin content decreased. HT and drought resulted in an increase in starch peak viscosity (PV), hold-through (HT), final viscosity (FV), setback and pasting temperature, and elongated pasting time, but resulted in a decrease in breakdown of flour. Responses of two-type cultivars with different starch contents and pasting properties to HT and drought were also located. Yumai 50 was proved more sensitive to HT and drought than Yumai 34. The results also showed that, for most starch parameters (starch content, components and pasting properties), effects of post anthesis HT and drought were additive, and interactions of these two factors appeared on some parameters. HT stress in the early stage of grain-filling (5 days after anthesis, DAA) had greater effects on starch concentration and components than that in the middle stage (17 DAA). But for pasting properties, HT in the middle stage had greater effects than that in the early stage.Correlation analysis showed that activity of key enzymes was positively correlated to starch concentration, amylose content, and grain weight and yield after 15 DAA (PO.05). The correlation of activity of key enzymes to starch pasting properties differed between the two cultivars. Activity of most key enzymes for starch synthesis was positively correlated to starch viscosity parameters (PV, HT and FV)(P<0.05), and negatively correlated to pasting temperature and pasting time in Yumai 34. But in Yumai 50, the correlation was quite the reverse.4. In field experiment, three wheat cultivars, Yumai 34, Yumai 50, and Annong 9805 (also a weak-gluten cultivar), were used to evaluate effects of HT stress on flour and food quality. HT stress was obtained by covering plastic greenhouses (boxes) in the field at different grain filling stages. The results indicated that HT stress in the early stage of grain filling increased flour water absorption, forming time, stable period, and farinogram index both in Yumai 34 and Annong 9805, but HT in the late stage significantly decreased most farinograms and extensograms while increasing flour softness. For Yumai 50, both early HT and late HT stress decreased farinograms and extensograms significantly. It was suggested that HT stress in the late grain filling stage had greater influence on flour quality. Evaluation on food quality showed that HT stress reduced bread volume and bread score, especially in the strong-gluten cultivar Yumai 34. Relatively, HT stress had smallerinfluence on noodle score, but obviously reduced noodle tenacity and elasticity. Little influence was observed on biscuit score in the experiment.5. Effects of different irrigation regimes on quality of flour and food were studied under rain-sheltered conditions. The results indicated that, in the strong-gluten cultivar Yumai 34, no significant difference in farinograms and extensograms of flour was observed among treatments with irrigation times from 0 to 3. Maximum farinograms were obtained in treatments just irrigated once at the jointing stage. But quality traits of flour significantly decreased with increasing irrigation times up to 4. But in Yumai 50, a weak-gluten cultivar, farinograms and extensograms of flour increased with increasing irrigation times up to 2, but decreased when irrigation times increased from 3 to 4. Particularly, stable period of flour decreased significantly in treatments with irrigation times of 3 (P<0.05) or 4 (PO.01). Drought (non-irrigation treatments) reduced bread scores by 17.1-19.7% and 7.3% in Yumai 34 and Yumai 50, respectively. But drought had little influence on biscuit score. Post-anthesis irrigation experiments showed that once irrigation after anthesis did not reduce flour quality if the irrigating quota prior to anthesis was less than 135 mm. But flour quality significant decreased with the increase in irrigation times from 2 to 4. Under once irrigation, put-off of irrigation was adverse to both yield and quality of grain.6. Both post-anthesis HT and drought significantly decreased grain weight and yield (PO.05 or PO.01). And significant interactions of HT and drought were also fond in grain weight and yield. Different-type cultivars had their own ways to respond to HT and drought. In strong-gluten cultivar Yumai 34, effects of drought were greater than those of HT. But in Yumai 50, HT had much greater influence on grain weight and yield than drought. Stress in the early grain filling stage had greater effects on grain yield than that in the middle or late stages. Under rain-sheltered conditions, treatments with 3 irrigations (irrigated at jointing, heading and grain filling stage) obtained the highest yield. Significant difference in grain yield was observed between treatments with 3 irrigations and no irrigations. However, no significant difference was found among treatments with 1-4 irrigations. Under limited irrigation conditions prior to anthesis (<135 mm), 1 or 2 post-anthesis irrigations significantly increased grain yield. Taking flour quality into account, one post-anthesis irrigation resulted in the highest yield and the best quality. Correlation analysis indicated...
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