Biomass accumulation and partitioning of newly developed Green Super Rice (GSR) cultivars under drought stress during the reproductive stage

Drought is a major abiotic threat in rice production; thus, there is a need to develop adaptable rice varieties that can withstand drought stress and still produce high yield in non-stressed environments. Green Super Rice (GSR) cultivars address this issue. These cultivars are bred through an innovative introgression breeding strategy that requires less irrigation water and chemical inputs without compromising grain quality and yield. This study verified the physiological efficiency and performance of newly developed GSR cultivars that previously showed favorable response to drought during advanced yield trials. Five drought tolerant GSR cultivars and two checks were subjected to continuously flooded (CF) and drought-stressed environments during the dry seasons of 2011 and 2012 at the International Rice Research Institute (IRRI) experimental farms in Los Ba˜nos, Philippines. The cultivars’ ability to allocate assimilates and accumulate biomass under drought stress during the reproductive stage was verified. Leaf area index (LAI), biomass dry weight, and panicle yield were measured at the panicle initiation (PI), flowering (FL), and physiological maturity (PM) of the sample cultivars. All the cultivars performed satisfactorily in the CF environment with grain yield ranging from 5 to 11.5 tons ha−1. Water stress during the reproductive stage significantly reduced grain yield by 75–88% in the moderate drought (soil water tension between 100 and 300 kPa in upper 15 cm soil layer) and 77–96% in the severe drought (soil water tension >300 kPa in upper 15 cm soil layer) experiments. The shortened reproductive duration mainly contributed to the significant reduction in yield under drought stress. Two GSR cultivars, GSR IR1-5-S10-D1-D1 and IR83142-B19-B, responded well in severe drought environments, with grain yield almost similar to the drought check (1.79 tons ha−1). Under moderate drought stress, there was a relative yield advantage of 25% and 40% for the two GSR cultivars over the drought check, respectively. Yield advantage across environments, varying from fully irrigated to drought-stressed, was 31–36%. These two GSR cultivars were effective in mobilizing stored carbohydrates from the vegetative organs to the panicles and not shortening the duration from flowering to maturity, to allow all reserved carbohydrates be allocated to storage organs as a mechanism to cope with drought stress. Lower leaf area index (LAI), which allowed balanced biomass accumulation and lower transpiration, without a significant decrease in grain filling duration, was another drought-coping strategy. These physiological responses and characteristics apparently enabled the GSR cultivars to withstand drought stress; these are key indicators for varietal selection in drought-prone environments, particularly in severe drought stress in the reproductive stage. Despite the poor ability of some cultivars to cope with severe drought, three out of five selected GSR cultivars produced grain yield (2.0–2.9 tons ha−1) that was the same or higher than the drought check in moderate drought stress. The introgression breeding technique applied in the newly developed drought-tolerant cultivars through the GSR breeding strategy was found to be effective. It could produce high yields in both CF and water-limited environments, and thus, it could serve as a model for other breeding programs to adopt.

干旱是水稻生产中的主要非生物胁迫威胁，因此亟需培育既能抵御干旱胁迫，又能在非胁迫环境下保持高产的适应性水稻品种。绿色超级稻（Green Super Rice, GSR）品种正是为解决这一问题而研发的。此类品种通过创新的导入育种策略培育而成，可在不降低籽粒品质与产量的前提下减少灌溉用水与化学投入。本研究验证了此前在高级产量试验中对干旱表现出良好响应的新型GSR品种的生理效率与田间表现。2011与2012年旱季，在菲律宾洛斯巴诺斯的国际水稻研究所（International Rice Research Institute, IRRI）试验农场，对5个耐旱GSR品种与2个对照品种设置持续淹水（Continuously Flooded, CF）与干旱胁迫两种环境处理。试验验证了供试品种在生殖阶段干旱胁迫下的同化物分配与生物量积累能力。在供试品种的穗分化期（Panicle Initiation, PI）、开花期（Flowering, FL）与生理成熟期（Physiological Maturity, PM）分别测定叶面积指数（Leaf Area Index, LAI）、干生物量与穗部产量。所有供试品种在持续淹水环境中均表现良好，籽粒产量介于5至11.5吨每公顷。生殖阶段的水分胁迫会显著降低籽粒产量：中度干旱（上层15cm土壤水张力介于100至300kPa）处理下产量降幅为75%至88%，重度干旱（上层15cm土壤水张力>300kPa）处理下降幅为77%至96%。生殖周期缩短是干旱胁迫下产量显著下降的主要原因。其中两个GSR品种GSR IR1-5-S10-D1-D1与IR83142-B19-B在重度干旱环境中表现优异，籽粒产量与耐旱对照（1.79吨每公顷）基本持平。在中度干旱胁迫下，这两个GSR品种相较耐旱对照分别具有25%与40%的相对产量优势。各环境下（从完全淹水到干旱胁迫）的产量优势为31%至36%。这两个GSR品种可有效将营养器官储存的碳水化合物转运至穗部，且不会缩短开花至成熟的时长，以此将所有储备碳水化合物分配至贮藏器官，作为应对干旱胁迫的策略。较低的叶面积指数可实现生物量的均衡积累与蒸腾作用的降低，同时不会显著缩短籽粒灌浆时长，这是另一项耐旱策略。上述生理响应与特征显然使GSR品种能够抵御干旱胁迫，这些指标是易旱环境（尤其是生殖阶段重度干旱环境）下品种选育的关键参考依据。尽管部分品种应对重度干旱的能力较差，但5个入选GSR品种中有3个在中度干旱胁迫下的籽粒产量（2.0至2.9吨每公顷）达到或高于耐旱对照。本研究证实，通过GSR育种策略应用导入育种技术培育的新型耐旱品种切实有效。此类品种可在持续淹水与水分受限环境中均实现高产，因此可作为其他育种项目借鉴的范本。