The Effects Of Daytime And Nighttime Warming On Nitrogen Accumulation And Use Efficiency In Rice

Rice is one of the most important crops in China and many other countries. Global warming has increased the average temperature and the frequency of extreme hot days during rice growing season, which poses a great impact on rice production. In order to reduce the effects of climate change through developing appropriate N fertilizer management practice, it is critical to study the effect of increasing (day/night) temperature on grain yield and N use efficiency in rice under different N treatments, and to examine if N application could alleviate the detrimental effects of increasing temperature. In this study, four different rice cultivars were grown in pots at three temperature treatments, including daytime warming treatment (HDT), nighttime warming treatment (HNT), and natural temperature treatment (CK). The warming treatments were imposed from tillering to maturity. Grain yield, the total biomass, dry weight of different parts, N accumulation and N use efficiency were measured. Main objectives of this study were:(1) to investigate the variations in rice yield, aboveground biomass, nitrogen accumulation and use efficiency under HDT and HNT treatments,(2) to characterise the effects of different N application levels on grain yield, N accumulation and use efficiency under warming conditions,(3) To compare the difference of heat-tolerant and-sensitive cultivars in yield, nitrogen accumulation and use efficiency under warming conditions. The main results obtained in this study are as following:1. In this study, the average temperature of daytime and nighttime under warming treatments were higher than the CK treatment by2-3℃. The percentage and the number of extreme hot days in HDT and HNT were significantly increased during the duration of warming treatment. Compared with CK treatment, the number of extreme hot days, in which daily maximum temperature is greater than38℃and daily minimum temperature greater than30℃, was significantly increased in HDT and HNT treatments, respectively.2. Rice yield decreased significantly under both HDT and HNT due to the reduction of total spikelets per plant, seed setting rate and1000-grain weight, and the lower seed setting rate was the main reason for the lower yield under the warming treatments. There were significant genotypic differences with regard to the tolerance to warming stress between different cultivars. Under the warming treatments, the seed setting rate was reduced by14-70%in N22,25-89%in Shanyou63(SY63), and51-98%in Liangyoupei9 (LYP9), which indicated that N22has the strongest tolerance to warming stress following by SY63. LYP9was most sensitive to the warming stress.N fertilizer treatments affected the tolerance of rice to warming stress. Under the warming treatments, seed setting rate was reduced by27-86%under no N fertilizer application, while it was20-56%under the medium N fertilizer application, and24-65%under the high N fertilizer application. The results indicate that lack and excess N fertilizer application may both reduce the tolerance of rice to warming stress.3. At heading stage, compared with CK, the warming treatments reduced the aboveground biomass by4-22%in different cultivars under different N treatments in2012. The decrease in aboveground biomass under the two warming treatments was not significant in2013. Under CK treatment, the percentage of leaves biomass in total biomass was14.7-21.5%and22.2-24.9%for different cultivars under different N treatments in2012and2013, respectively, while it was15.8-26.4%and22.4-29.1%under the warming treatments. The results indicate that the proportion of dry matter in the leaves at heading stage may increase under the warming treatments.At maturity stage, the warming treatments had little effect on rice aboveground biomass. The percentage of grain dry weight in total biomass under the CK was36.1-47.7%and7.7-36.8%for different cultivars under different N treatments in2012and2013, respectively, while it was0.7-32.9%and1.6-23.6%under the warming treatments. This indicates that the biomass allocated to grains decreased at maturity stage under the warming treatments. As the seed setting rate was significantly decreased under the warming treatments, the dry matter accumulated before and after flowering could not be transported from stems and leaves to panicles during the filling stage, so the dry weight of stems and leaves was significantly higher at maturity stage under the warming treatments in comparison with that under the CK.4. The warming treatments had little effects on the nitrogen accumulation of rice at both heading and maturity stages. At heading stage, the percentage of nitrogen accumulation in leaves under the CK is38.9-48.3%and45.4-49.7%for different cultivars under different N treatments in2012and2013, respectively, while it was40.6-52.4%and46.5-54.2%under the warming treatments. This indicates that the allocation of N to the leaves at heading stage was increased under the warming treatments.At maturity stage, the percentage of N accumulation in grains under the CK is59.2-73.7%and19.1-58.3%for different cultivars under different N treatments in2012and2013, respectively, while it was2.3-59.9%and4.4-44.0%under the warming treatments. This indicates that the allocation of N to the grains decreased at maturity stage under the warming treatments. When the seed setting rate was significantly decreased under the warming treatments, N could be not transported from stems and leaves to panicles during the filling stage, so N accumulation in panicles was significantly decreased and N accumulations in stems and leaves were significantly increased at maturity stage under the warming treatments.5. Due to the fact that the biomass and N accumulation at maturity stage did not change under the warming treatments, N use efficiency for biomass (NUEb) at maturity stage did not change. The warming treatments reduced grain yield significantly, as a result, N use efficiency for grain yield (NUEg), N harvest index (NHI), partial factor productivity of applied nitrogen (PFP) were significantly decreased under the warming treatments. This study indicates that the warming treatments had little effects on the N accumulation of rice, and the seed setting rate was the key factor that influenced N use efficiency under the warming treatments. Improving the tolerance to warming stress of rice cultivars and appropriate N fertilizer application may reduce the negative effects of warming on rice production.