Responses Of Hormones To High Day And Night Temperatures And Their Relationships With Yield

Both the daytime and nighttime temperatures are increasing as a result of global warming. The possibility of occurrence of high temperature at the heat-sensitive stages (panicle initiation, heading and grain filling stage) for rice is increasing. Most of the studies on the effects of high temperature on rice are concentrated in the high day temperature. Little is known about how high night temperature affects rice production. High temperature during heading and maturity stages of rice growth decrease grain yield by reducing the number of spikelets per panicle and grain filling percentage. However, its mechanism is still unclear. In this study, three heat-tolerant cultivars (Shanyou63, T226, N22) and three heat-sensitive cultivars (Liangyoupeijiu, T219, IR64) were cultivated and subjected to high temperature during panicle initiation, heading and grain filling periods, respectively. The natural temperature and the optimum day and night temperature combination were taken as controls (CK and NT), and three high temperature treatments included high night temperature (TN), high day temperature (TD) and the whole day of high temperature (TD+N).The aims of this study were to observe the effects of high night and day temperature during different stages on yield and its genotypic variation; to study the responses of roots morphology and hormones under different forms of heat treatments; to analyze the relationship between hormones changes and the yield formation by high temperature, with the purpose to reveal the mechanism for changes of hormones in roots affecting the number of spikelets per panicle and grain filling percentage, and provide theoretical guidance for rice production and breed of heat-tolerant cultivar. The main results are as follows:1. High temperature during panicle initiation affected grain yield and other physiological indices significantly.1) Analyzing of the yield and its component factors, day and night high temperature during panicle initiation both remarkably reduced grain yield. Compared to control, the influence of TD+N on yield was greater than other two high temperature treatments, and the reduction for heat-tolerant cultivars under TD+N was less than that for heat-sensitive cultivars. To be specific, both day and night high temperature promoted the branchs and spikelets degeneration and reduced their survived number. On the other hand, the high temperature decreased grain length, grain width and finally reduced the grain weight, especially in TD+N.At this stage, SY63achieved a higher yield under all high temperature treatments; The lower degeneration of T226under TN lead to its resistant to high night temperature; and N22was more resistant to high day temperature. Interestingly, IR64showed some thermostability at this stage.2) In addition, TD+N reduced the photosynthesis of SY63and IR64, while TN decreased both dry matter accumulation and photosynthesis of SY63and N22, but increased for IR64. High temperature also impeded the synthesis of non-structural carbohydrate (NSC) in stem. And there is a higher content NSC residual in stem at mature for LYP9, SY63under TN and TD+N, while a lower content for IR64. The xylem sap was significantly increased in all varieties by TN and TD+N.3) Moreover, the high temperature obviously decreased the concentration of zeatin+zeatin riboside (Z+ZR), indole-3-acetic acid (IAA) and gibberellins (GA), and increased the concentration of abscisic acid (ABA) in xylem sap and roots. The relationships of hormones in xylem sap to both yield and spikelets per panicle are significant and positive, respectively. Similar correlations were observed between hormones in roots and both yield and spikelets per panicle.The possible reasons that SY63could keep high yield under high temperature treatments were attributed to higher leaf photosynthetic rate, higher content of NSC in stem, well developed roots system, higher vigorous roots activity and higher contents of Z+ZR and IAA in roots. N22was sensitive to high night temperature, possibly because it had an extremely low content of Z+ZR and IAA in roots by TN.2. High temperature during heading also affected grain yield and other physiologicalindices significantly.1) High temperature at heading significantly reduced grain filling percentage and yield of all cultivars. The relative yield was less for the heat-sensitive cultivars than for the heat-tolerant cultivars, with a more reduction under TD+N treatment and a less reduction under TN. The changes of pollen fertility and grain filling under high temperature were similar. The pollen fertility was decreased by high temperature both day and night, while more pronounced decline by TD.The heat damage increased with time.2) High temperature also decreased the dry matter accumulation of LYP9and IR64, and increased NSC content in stem at mature, the increase amplitude was higher for heat-sensitive cultivars. Except for IR64, the roots/shoot ratio of LYP9, SY63and N22got smaller by high temperature. In addition, the increase of xylem sap was observed in all cultivars, especially in TD+N. 3) The variation trends of hormones under high temperature were basically similar between heading stage and panicle initiation stage:the mean concentrations of Z+ZR, GA, IAA were lower in high temperature treatments than in CK, while ABA was higher. The results of the correlation analysis between yield and grain filling percentage and hormones showed that:the yield and grain filling percentage had no significant correlation with the hormones concentration in xylem sap, but was significantly positive correlated with Z+ZR concentration in roots and significantly negative correlated with ABA concentration in roots. The relationship of their relative values was same. Under TD+N, the Z+ZR content in roots of tolerant cultivars was significantly higher than control, and the ABA content was significantly lower than control, which contributed to remission root senescence and yield loss.3. The effects of high temperature during grain filling on yield were less than the othertwo stages mentioned above.1) Compared to control, the TN treatment did not reduce yield significantly, but the TD and TD+N treatments significantly reduced grain filling percentage, grain weight and yield. The reduction was less for the heat-tolerant cultivars than for the heat-sensitive cultivars. High temperature had significant effects on grain length, depended in genotypes, whereas no significant effects on grain width.2) Compared to other two stages, the roots morphological indices had no remarkable changes among treatments during grain filling, and the bleeding content was lower by high temperatures. The high temperature treatments also obviously decreased Z+ZR and IAA contents in roots and spikelets during grain filling, and heat-tolerant cultivar had a higher relative IAA concentration in spikelets. The yield, grain filling and grain weight were significantly positive correlated with Z+ZR and ABA concentration in roots and spikelets, and significantly negative correlated with IAA concentration in roots. The results suggested that high contents of Z+ZR and ABA in roots and grains may be an important factor for stable grain yield for heat-tolerant cultivars during grain filling.