Ecotypic And Genotypic Variation In Rice Yield And Quality In Response To Different Elevated Temperatures

A recent increase in global warming has more frequently exposed crops to elevated temperatures which in turn have already offset a significant portion of yield increase arisen from various factors. This rising temperature is also posing great threats for productivity and quality related attributes of rice. The paucity of studies regarding agronomic performance of rice cultivars under field-scale elevated temperature is seriously lacking our current understanding of the potential consequences of high temperatures. Significant efforts are required to enable rice plant to cope with the menace of high temperature stress. As an immediate solution, use of plant growth regulators (PGRs) can be targeted to maximize plant tolerance against heat stress. Some PGRs such as glycine betaine (GB), salicylic acid (SA) and ascorbic acid (AsA) have been reported to improve tolerance against several abiotic stresses. It can be hypothesized that these PGRs might have the potential to alleviate the high temperature related negative effects in rice by improving its yield, quality and physiological performance. Moreover, grains located at different positions within the same panicle vary tremendously in several aspects. The response of the spikelets located at different positions within the same panicle to high temperatures and PGRs is not clear. Based on these scenarios, a set of different experiments ranging from growth chambers to field-level trials and from non-stressed to different elevated temperatures were conducted in this study whose details are given below.For quantifying the relative influence of H(D+N)T (high diel i.e. day plus night), HDT (high daytime), HNT (high nighttime) and AT (ambient) temperatures on agronomic performance, grain yield and various yield contributing factors, two field-scale experiments were conducted during2009and2010at the experimental field of Huazhong Agricultural University, Wuhan, China. In addition, the effect of these temperatures on quality related various traits such as milled grain dimensions, protein and amylose content and chalkiness were also evaluated. A newly developed system consisting of blowers plus heaters was used which could increase the temperature of the field by approximately2℃. Different rice cultivars (including various indica and japonica ecotypes) were used during both years. Starting from booting till harvest maturity, all genotypes were subjected to the above-mentioned four different temperature treatments during both years. Moreover, a micro-plot trial was conducted in2010within the main-plots (where temperature and PGR treatments were employed) of the field. Two PGRs i.e. GB and SA were foliarly applied to evaluate their effect on rice appearance quality such as grain dimensions and chalkiness and their potential role in ameliorating the negative effects of various elevated temperatures. In addition to these field trials, another experiment was carried out in controlled environment to study the interactive effect of HNT and foliar application of AsA on physiological traits of rice plant; by mainly focusing on its anti-oxidative and nutrient status. Two rice (Oryza sativa L.) model varieties Kasalath (indica) and Nipponbare (japonica) with different plant architecture were used in this trial. Moreover, from the micro-plot trial, panicles were randomly sampled to further determine the effect of HDT, HNT and H(D+N)T temperature treatments and two PGRs on the spikelets located at different positions on the panicle axis. Based on the length of the panicle the grains were divided into upper, middle and lower grains and their traits were studied. The following main results were obtained:(1) Among the tested high temperature treatments H(D+N)T proved to be comparatively more devastating as it severely affected almost all the investigated traits such as grain yield and its components, biomass and HI. Under our experimental conditions, an increase of about2℃in H(D+N)T resulted in16.3%and26.6%yield reduction in both indica and japonica ecotypes, respectively during2009. In2010, this yield reduction ranged from21.3%for indica to40.2%for japonica cultivars. The decrease in grain yield caused by HDT was4.1and4.0%for indica and japonica ecotypes respectively, in2009. The grain yield reduction caused by HDT in2010was3.2and9.1%. Similarly, HNT decreased grain yield by0and10.1%in2009, and by16.9and45.3%during2010for indica and japonica ecotypes, respectively. HDT reduced aboveground biomass more than HNT in2009, while in2010there was no much difference between the effects of these two treatments on aboveground biomass. HNT resulted in more reduction of spikelet fertility and HI than HDT during both years. Japonica ecotype appeared to be more sensitive to temperature increase than indica during both years in terms of grain yield and its components. In case of genotypic variation, the effect of temperature treatments on the studied traits was cultivar dependant.(2) The results about appearance quality revealed that elevated temperatures reduced kernel dimensions and increased chalkiness in most of the cultivars. There were significant variations in response of the two ecotypes to different temperature treatments in terms of appearance quality. Like yield related traits, H(D+N)T treatment caused more deleterious results than HDT and HNT in both ecotypes. H(D+N)T reduced grain length, width and area in comparison with AT by0.33,1.82and2.16%in indica, and by1.72,2.23and3.96%in japonica ecotypes, respectively, when averaged across the two years. Conversely, the mean value of grain length to width ratio (LWR) across both years was increased under H(D+N)T by1.56%for indica and by0.56%for japonica ecotypes, respectively. HNT increased grain LWR even more than H(D+N)T in japonica ecotype when compared with AT. Moreover, H(D+N)T increased the percentage of grains with chalkiness (PGWC) by16.65and17.85%, and increased percent area of chalky endosperm (PACE) by53.3and63.9%for indica and japonica ecotypes, respectively, when averaged across the two years. No consistent trends for high temperature treatments were observed for protein and amylose content, alkali spreading value, milling traits and head rice yield across the genotypes. The greater than one heat susceptibility index (HSI) value for japonica ecotype and relatively greater negative effect observed for different quality related parameters under heat treatments suggest that it was relatively more susceptible to elevated temperatures than indica.(3) The data obtained from PGR trials imply that for some genotypes SA was relatively more effective in ameliorating the negative effects of high temperature than GB. The grain treated with SA showed reduced chalkiness, and the grains dimensions were improved in some cultivars when compared with those ripened under elevated temperatures but without any PGR. The effect of GB was not so consistent and varied markedly with the tested cultivar and temperature treatment. The results of the growth chamber trial showed that both genotypes exhibited considerable similarity in response to both HNT and AsA. HNT increased the malondialdehyde (MDA) content of both varieties and exogenous AsA reduced it almost to the level of control night temperature (CNT). Similarly, the HNT associated increases in Na+and K+in both root and shoot were also lowered by exogenous AsA. The activities of anti-oxidative enzymes such as catalase (CAT) and peroxidase (POD) were also increased under HNT. While application of AsA led to reductions in their activities. POD was more sensitive to HNT than CAT and AsA also showed more pronounced effect on POD compared to CAT. Although preliminary results for the acclamatory role AsA under HNT are favorable, future detailed studies including diverse genotypes and different levels of AsA applied at various stages are imperative. (4) The findings of the field trial conducted to explore the variation in spikelets located at different positions on panicle axis, revealed that panicle length was increased by elevated temperatures; especially by HDT. The grain density was significantly greater on the middle part of the panicle than the upper and lower positions as more than40%grains were found on this position compared with less than30%for upper and lower positions, with no prominent response to temperature and PGR treatments. Similarly, spikelets positioned at upper location on panicle axis were considerably heavier than the middle, which in turn had greater weight than the lower grains, irrespective of temperature treatments. H(D+N)T and HNT tended to reduce the grain weight compared with HDT or AT. The fertility of spikelets of the tested genotypes at different positions showed tremendous variation. Two indica varieties (DTWX and ZX232) showed greater fertility for the upper spikelets across all temperature regimes, followed by middle and lower positions, respectively. While, the maximum ratio of fertile spikelets was observed in the lower grains in two japonica cultivars (JWR221and ZH8) followed by middle and upper grains, respectively, irrespective of temperature treatments. Some genotypes were responsive to the applied PGRs as the grain weight of the spikelets in the middle part of the panicle was increased and it reached to the level of superior spikelets in some genotypes. Moreover, PGRs partially reduced the variation caused by different temperatures. Based on HSI values, the upper grains proved to be comparatively more tolerant to elevated temperatures than the middle and lower grains.This study confirms the deleterious effect of elevated temperatures plus existence of genotypic and ecotypic variations in rice crop and provides new insights about the role of PGRs in alleviating the negative effects associated with high temperature. Future efforts to exploit the genetic diversity and identify the physiological pathways underlying the variation in response of rice cultivars under high temperature will maintain sustainable rice production even in warmer climates.