| Rice (Oryza sativa L.) is staple food for most of Asian population especially poors, therefore, food security and rice production are synonyms in many Asian countries. Yangtze River Valley (YRV) of China is the oldest niche for rice production and backbone of the food security for the sheer magnitude of Chinese population. Temperature influences all aspects of crop growth and development, shaping yield throughout the growing season. Northern parts of China are benefited from increase in temperatures where temperatures are still within optimum range for rice production, while southern parts of China, where temperatures already exceed the optimum. Elevated temperatures increase the likelihood of reduced yield through shorten the grain filling period, increased spikelet sterility and reduced grain weight. For all these reasons to increase crop resilience towards climate stresses and maintain productivity and yield under conditions of altered seasonal fluctuations and temperatures will require region-specific strategies. Anthropogenic activities in the second half of the last century lead global mean temperature to increase by 0.74℃ and further 1.1-5.4℃ rise by the end of this century is predicted under Intergovernmental Panel on Climate Change (IPCC) B1 and A2 scenarios (IPCC,2007). Due to less radiant heat loss as a result of increased cloudiness nighttime temperatures are expected to increase at a faster rate than daytime temperature.At regional levels, IPCC scenarios are not helpful and uncertainties exist in the degree of warming and may have positive or negative impacts on different crops. Therefore assessment of long-term regional trend in temperature is mandatory to understand potential impacts of future climates. Here, we have put a deliberate effort to analyze the historical weather data of Jiangsu Province located at lower reaches of YRV to ascertain changing trend in temperature extremes and its derived stress days and nights during RGP as proxy of rice sterility in this region. Asymmetric seasonal changes in daily maximum (TMX) and minimum (TMN) temperatures resulted in reduced diurnal temperature range (DTR) during studied period. Mean relative humidity also decreased at different temporal scales, though regional variations exist. Frequency of hot nights (HN) and heatwave-B (HW-B) substantially increased in the last 20 years resulted in increased occurrence of severe hot days (SHD) and severe heatwave (SHW) when both TMX and TMN are higher than described critical limit. Hot nights (HN) were observed only in the second half (SH) of the dataset, while their intensity has been increasing with time. Episodic sterility occurs in the absence of extreme heat events, and we have hereby confirmed the increasing trend of HNs and days with both higher TMX and TMN (SHD, severe hot days) associated with this crucial issue. Stagnant observation of TMX, hot days (HDs), and heatwave-A (HW-A) while, increasing trend of TMN, HN, (HW-B), SHD and (SHW) are main features of changing climate in the region.Experiments aimed to study responses of crops to rising temperature are mainly conducted under controlled cover conditions like greenhouses, open-top chamber etc. These indoor facilities cannot mimic predicted climatic conditions and can significantly alter crop microclimate. These potential artifacts can have undesirable impacts of plant growth and developmental processes. Use of infrared heaters (IRH) is one of the attractive alternatives which can provide reliable and reproducible canopy warming for different crops in similar way they are heated by solar radiation. As compared to warming in greenhouses, open-top champers and other similar warming facilities, these IRHs have minimum impacts on crop microclimate. Kimball et al. (2008) arranged six IRHs in an array of 3-meter-dimater to uniformly warm wheat canopy. Due to lower turbulence and closed stomata, IRH can cause greater increase in canopy temperature during nighttime than during daytime. Rice production is suffering from asymmetric rise in day and night temperature which is evident from warming trend observed for Jiangsu province in this study.A free-air temperature enhancement (FATE) facility to simulate temperature changes by the year 2100 was envisioned, constructed and modified for rice paddies in the Lower Reaches of Yangtze River Valley, China. Modifications in the original design of heater array through the use of concrete-anchored posts, improved software, overhead wires, extensive grounding, and monitoring with a thermal camera, the technology was safely and reliably extended to paddy rice fields. FATE system provides canopy warming closely corresponding with the scenarios for year 2100, and analogue to diurnal temperature gradient in paddy fields. The system maintained canopy temperatures increases within 0.5℃ of day-and nighttime set-point differences of 1.3 and 2.7℃ 65%of the time for B1 scenario and 68 and 55% of 2.7 and 5.7℃ set-point difference target for A2 scenario.To investigate effect of warming predicted under IPCC B1 and A2 warming scenarios, on rice yield and quality parameters, two year experiment was conducted at Danyang FATE site. Two indica rice hybrids differing in response to high temperature stress, Teyou-559 (TY) and Shanyou-63 (SY) were used. During rice grain filling stage, four temperature regimes, ambient reference (CK), high daytime temperature (HDT, +1.3℃), high nighttime temperature (HNT,+2.7℃), high diel temperature (HDNT, +1.3/2.7℃) resembling IPCC low emission B1 scenario, were imposed. Diurnal deviation from target canopy temperature was inversely related to wind speed. Two hybrids responded differentially to daytime, nighttime and diel post-anthesis warming and shortened grain filling duration, reduced grain yield and thousand-grain weight. These results indicate that daytime (-4%), nighttime (-7%) and diel (-6%) warming under low emission scenario will have differential decrease in rice production. This reduction is mainly due to differential decrease in grain weight. SY proved to be more susceptible to daytime warming, while TY was affected more by nighttime and diel warming. Effect of warming on vegetative dry mass was insignificant, indicating observed reduction in yield is attributed to impeded translocation of photosynthates.Three temperature regimes ambient reference (CK), high temperature during reproductive stage (HTR) and high temperature during grain filling (HTGF) (CK +2.7/5.7℃), resembling IPCC high emission A2 scenario, were imposed using upgraded FATE arrays. Both hybrids responded differentially to warming and reduced grain yield, spikelet fertility and thousand-grain weight. Heat stress during reproductive stage profoundly induced sterile spikelets. High temperature induced spikelet sterility was greater in TY and spikelet located at upper panicle branches were affected more. A compensatory increase in the weight of remaining grains was not observed in the warming which reduced spikelet fertility. Warming treatment HTR caused 49% and 19% reduction in yield for TY and SY respectively. However, yield losses due to HTGF warming was lower (-18% and-12%) than under HTR for both hybrids. Effect of warming on vegetative dry mass was comparatively lower which indicates that reduction in yield was attributed to spikelet sterility and impeded translocation of photosynthates. These results indicate that warming under high emission scenario will have more detrimental impacts on rice production, especially during reproductive stage and TY is more susceptible to this warming.In addition to changes in yield components, Post-anthesis B1 warming induced changes in the grain appearance quality, protein fractions (albumins-globulins, prolamin and glutelins) and amino acid contents of both hybrids. Warming during grain filling enhanced formation of chalky grain and reduced the perfect grains. Grain internal structure was also damages by the warming. Genotypic differences in their protein fractions and amino acid contents existed among the tested cultivars. Warming treatments induced increased in protein contents and amino acids significantly. Amino acids like cysteine, isoleucine, lysine and phenylalanine were not affected by warming treatments for both cultivars, while other amino acids were significantly affected by temperature treatments. Different asymmetric temperature conditions exist between the quantities of each protein fraction. From these results it was concluded that grain appearance quality and internal structures were significantly deteriorated by post-anthesis asymmetric warming and genotypic variations in protein fractions and amino acid contents were also influenced by temperature during grain filling.High temperature resistant cultivar, Shanyou-63 had greater amino acid contents as compared to sensitive cultivar Teyou-559. Sulfur containing essential amino acids (Cysteine and methionine) decreased under nighttime warming, however increased or remained unchanged under diel warming, which indicates that nighttime warming had negative impact on sulfur metabolism and accumulation during grain filling. Different response of asymmetric warming conditions exists between the quantities of each protein fraction. From these results it was concluded that grain appearance quality and internal structures were significantly deteriorated by post-anthesis asymmetric warming. Protein and amino acid accumulation under high temperature during grain filling might be due to heat-stable translocation of amino acids into developing grain.This effort presents an overview of long-term changing pattern of temperature extremes and vulnerability of rice production at regional and to determine avenues to best address the pressure facing rice cultivation today and rescue food supply in the face of climate change. New crop varieties, with high temperature resistant, and adjustment in agronomic practices may play a role in sustainable rice production in the predicted growing conditions with higher temperatures. |
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