| High temperature stress during panicle initiation period significantly often reduces the number of spikelets per panicle,seed setting rate and grain weight in rice,resulting in the decrease of rice yield.Nitrogen fertilizer can regulate the grain yield formation,panicle growth and development in rice.At present,the effect of nitrogen fertilizer on panicle differentiation under high temperature is still unclear.Therefore,the purpose of this experiment was to investigate the effect and physiological mechanism of increased panicle nitrogen fertilizer application rate on rice yield under high temperature treatment during panicle initiation stage.This experiment was carried out under potted condition.The high temperature resistant cultivars Shanyou 63(SY63)and Nagina22(N22),and high temperature sensitive cultivars Huanghuazhan(HHZ)and Liangyoupeijiu(LYPJ)during flowering were selected for the experiment.Nitrogen panicle fertilizer was applied at the second stage of panicle initiation,and then plants were treated with high daytime temperatures for 15 d.The average temperature was29.6/26.8°C(day/night)in the control treatment(CK),and 35.6/27.5°C(day/night)under the high temperature treatment(HDT).The panicle nitrogen fertilizer treatments were low panicle fertilizer application rate(LN,0.012 g N/kg soil),medium panicle fertilizer application rate(MN,0.072 g N/kg soil)and high panicle fertilizer application rate(HN,0.144 g N/kg soil).The main results were as follows:(1)Increasing panicle nitrogen fertilizer application rate increased rice yield under high temperature during panicle initiation stage.Under LN treatment,compared with the control temperature treatment,high temperature decreased the yield by 46.6% in LYPJ,17.0% in HHZ,and 19.2% in N22,respectively,while the yield of SY63 was not significantly affected by high temperature treatment.Under both MN and HN treatments,compared with the control temperature treatment,the yield of LYPJ was reduced by 34.0% and 24.8% by high temperature treatment,respectively,but the yield of HHZ,SY63 and N22 was not significantly affected by high temperature treatment.Rice yield was tightly associated with seed setting rate and spikelet fertility,as supported that significantly positive correlations between rice yield with seed setting rate and spikelet fertility.Compared with the control temperature treatment,high temperature significantly reduced the seed setting rate and spikelet fertility of rice under LN treatment,but small reductions were observed under MN and HN treatment.Rice spikelet fertility was positively correlated with pollen fertility.Under high temperature treatment,the pollen fertility under LN treatment slightly decreased compared with that under MN and HN treatment.Therefore,our results show that the increasing panicle nitrogen fertilizer application rate under high temperature treatment alleviates the adverse effects of high temperature on rice yield by improving the pollen fertility and spikelet fertility.(2)Increasing panicle nitrogen fertilizer application rate reduced plant temperature under high temperature during panicle initiation stage.The stem temperature of LYPJ plants treated with MN and HN under high temperature treatment was 1.4℃ and 2.5℃ lower than that of plants treated with LN.The stem temperature of HHZ plants treated with MN and HN under high temperature treatment was 0.1℃and 0.6℃ lower than that of plants treated with LN.The stem temperature of SY63 plants treated with MN and HN under high temperature treatment was 2.6℃ and 3.2℃lower than that of plants treated with LN.The stem temperature of N22 plants treated with MN and HN under high temperature treatment was 0.7℃ and 0.9℃ lower than that of plants treated with LN.The results show that increasing panicle nitrogen fertilizer application rate could reduce the temperature of plant stem under high temperature treatment during panicle initiation stage.Rice spikelet fertility was negatively correlated with stem temperature.For the possible explanation,compared with LN treatment,MN and HN treatments significantly increased the leaf area and effective tillers number,and significantly increased the leaf transpiration rate under high temperature treatment,thus reducing stem temperature and improving the spikelet fertility.These results suggest that increasing panicle nitrogen fertilizer application rate alleviate the adverse effects of high temperature on rice yield by reducing stem temperature.(3)Increasing panicle nitrogen fertilizer application rate under high temperature during panicle initiation stage enhanced accumulation of non-structural carbohydrates(NSC)in panicles.Compared with LN treatment under high temperature treatment,MN and HN treatments significantly increased the leaf net photosynthetic rates of LYPJ,HHZ,SY63 and N22.Compared with the control temperature treatment,the high temperature treatment had no significant effect on the panicle dry weight of SY63 and N22,but significantly reduced the panicle dry weight of LYPJ and HHZ under the three nitrogen fertilizer application treatments.Under high temperature treatment,the panicle dry weight under LN treatment slightly decreased compared with that under MN and HN treatment.The variation trend of NSC in panicles was consistent with that of dry matter.Compared with the control temperature treatment,high temperature significantly reduced the panicle NSC concentration of LYPJ and HHZ under LN treatment,but small reductions were observed under MN and HN treatment.Reduction of spikelet fertility and pollen fertility caused by high temperature treatment significantly and positively correlated with the reduction of NSC concentration in panicles,respectively.The results suggested that increasing panicle nitrogen fertilizer application under high temperature treatment could alleviate the decrease of dry matter weight and NSC in panicles,resulting in improved pollen fertility and spikelet fertility of rice.(4)Increasing panicle nitrogen fertilizer application rate under high temperature during panicle initiation stage promoted accumulation of sucrose in panicles.Compared with the control temperature treatment,the high temperature treatment had no significant effect on the panicle sucrose concentration of SY63 and N22 under the three nitrogen fertilizer treatmes,but significantly reduced the panicle sucrose concentration of LYPJ and HHZ.Reductions of spikelet fertility and pollen fertility caused by high temperature treatment were significantly and positively correlated with the reduction of sucrose concentration in panicles,respectively.Compared with the control temperature,the high temperature treatment had no significant effect on the activities of sucrose synthesis related enzymes(SPS and SSs),and sucrose hydrolysis related enzymes(AI,NI and SSc)in panicle of SY63 and N22,but significantly reduced those of LYPJ and HHZ under the three nitrogen fertilizer treatments,resulting in the decrease of sucrose concentration in panicles of LYPJ and HHZ.In addition,reductions of spikelet fertility and pollen fertility caused by high temperature treatment were significantly and positively correlated with the reduction of SSs activity in panicles,respectively.Under high temperature treatment,the panicle SPS and SSs activities of LYPJ and HHZ under LN treatment slightly decreased compared with that under MN and HN treatment.Therefore,ours results suggest that the increasing panicle nitrogen fertilizer application rate under high temperature treatment alleviated the decrease in the activities of SPS and SSs in panicles,resulting in increased the concentration of sucrose,accompanied the alleviated heat impact on rice yield by improving the pollen fertility and spikelet fertility of rice. |
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