| Abscisic acid (ABA) and ethylene are important plant hormones involved in linking plant responses to stress. This study was to clarify the roles of ABA and ethylene in regulating grain filling under different soil moisture conditions. Two high-yielding rice cultivars currently used in the local production, Wuyunjing 8 (japonica) and Yangdao 6 (indica), were either pot- or tank-grown. Three treatments, well watered (WW), moderate soil-drying (soil water potential at -10- -30 kPa, MD), and severe water-deficit (soil water potential at —40—60 kPa, SD), were imposed from 9 d post-anthesis until maturity. The changes of ABA and ethylene concentrations in rice grains subjected to various soil moisture treatments during grain filling were investigated, and the effects of chemical regulators on the concentrations of ABA and ethylene in the grains were observed. The main results are as follows:1. Both MD and SD significantly reduced mid-day (11:30) leaf water potential. There were no significant differences in predawn (06:00) leaf water potential between MD and WW treatments, but the differences were obviously between SD and WW, indicating that MD plants could rehydrate overnight whereas SD plants could not. Changes in photosynthetic rate of the flag leaves were similar to those of leaf water potential. Both MD and SD reduced photosynthetic rate, but the differences between MD and WW were not significant, suggesting that photosynthesis is not seriously inhibited when pant subjected to MD. The two cultivars behaved the same.2. Both MD and SD enhanced the remobilization of non-structural carbohydrate from stems (including sheaths) and increased its contribution to grains. MD significantly increased grain filling rate whereas shortened grain filling period, and the increase in grain filing rate outweighed the loss of shortened grain filling period, leading to an increase in grain weight. SD significantly reduced grain-filling rate and shortenedgrain filling period, resulted in the reduction of grain weight.3. MD had no significant effect on the milling quality, appearance quality, cooking quality and nutrient quality of rice, whereas SD exhibited adverse influence on the milling quality and appearance quality. MD increased the peak viscosity and breakdown value, and reduced hot viscosity, final viscosity, and setback value, and SD had the opposite effect. Such results suggest that MD may improve the eating quality of rice.4. ABA concentration in the grains was very low at the early grain filling stage, and reached a maximum when the grain-filling rate was the highest. In contrast to ABA, concentrations of ethylene and 1-aminocylopropane -1-carboxylic acid (ACC) in the grains were very high at early grain filling stage and sharply decreased during the linear period of grain growth. Both MD and SD increased ABA concentrations in the grains. MD reduced, whereas SD remarkably increased, ACC concentrations and ethylene evolution rate. MD increased, whereas SD reduced, the ratio of ABA to ACC, implying that there is a greater enhancement of ABA concentration than ethylene production in the MD treatment, whereas the increase of ethylene outweighs the increase of ABA in the SD treatment. Both the concentration of ABA and the ratio of ABA to ACC were significantly and positively correlated, whereas the ethylene evolution rate and ACC concentration were significantly and inversely, correlated with grain-filling rate.5. Application of cobalt ion (Co(NO3)2), an inhibitor of ethylene synthesis, at 9-13 d post-anthesis significantly reduced ACC concentrations and ethylene evolution rate of the grains, and the results were reversed when ethephon (ethylene-releasing agent) was applied. Spraying with fiuridone (an inhibitor of ABA synthesis) reduced ABA concentration, but increased ACC and ethylene levels in the grains, and application of ABA had the opposite effect.6. Spraying with Co(NC?3)2 or ABA significantly enhanced the activities of sucrose synthase, ADP glucose pyrophosphorylase and soluble starch synthase, and increase...
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