Effect of drought stress on flowering characteristics in rice (Oryza sativa L.): a study using genotypes contrasting in drought tolerance and flower opening time

Rice is most susceptible to heat and drought stress at flowering stage, but flowering characteristics under drought stress are not well characterized. This study investigated flowering characteristics of rice genotypes contrasting in their flower opening time (FOT) and level of drought tolerance. Near-isogenic lines for the early-morning flowering trait (IR64+ qEMF3) and for drought tolerance (IR87707-445-B-B-B), and their recurrent parent cultivar (IR64) were used. IR64+ qEMF3 had stable earlier FOT than IR64 and IR87707-445-B-B-B under drought stress conditions. Drought stress occasionally affects FOT depending on genotype. The number of open spikelets was higher in IR87707-445-B-B-B than in IR64 and IR64+ qEMF3, and the difference among genotypes increased as the rice plants were subjected to more severe stress levels. Panicle temperature increased under drought stress conditions and was similar among genotypes when it was measured at the same time of day, demonstrating that earlier FOT in IR64+ qEMF3 must be beneficial to avoid heat stress at flowering under drought stress conditions. However, IR64+ qEMF3 did not exhibit drought avoidance, as evidenced by the root mass at depth. To assess the potential for the EMF trait to complement ongoing drought breeding efforts, heat tolerance among 13 advanced drought breeding lines and released cultivars was tested. Wide variation in heat tolerance at flowering was observed and, notably, none of the 13 lines possessed the EMF trait. This study therefore proposes that a breeding strategy that transfers the EMF trait into drought tolerant lines could enhance the resilience of rice spikelets to the combined stresses of heat and drought at flowering. Abbreviations: Analysis of variance (ANOVA), dry season (DS), early-morning flowering (EMF), flower opening time (FOT), near-isogenic line (NIL), panicle water potential (PWP), quantitative trait locus (QTL), wet season (WS)水稻在开花期对高温与干旱胁迫最为敏感，但干旱胁迫下的水稻开花特性尚未得到充分表征。本研究针对开花时间（Flower Opening Time, FOT）与耐旱水平存在显著差异的水稻基因型，探究其开花相关特性。本试验所用材料包括携带清晨开花（early-morning flowering, EMF）性状的近等基因系（near-isogenic line, NIL）IR64+ qEMF3、耐旱近等基因系IR87707-445-B-B-B，及其轮回亲本品种IR64。干旱胁迫条件下，IR64+ qEMF3的开花时间相较于IR64与IR87707-445-B-B-B始终更早且表现稳定。干旱胁迫对开花时间的影响因基因型而异。IR87707-445-B-B-B的开放小穗数高于IR64和IR64+ qEMF3，且随着胁迫程度加剧，不同基因型间的差异逐渐扩大。干旱胁迫下植株穗温度升高，且在每日同一时刻测定时，各基因型间的穗温度无显著差异，这表明IR64+ qEMF3较早的开花时间可有效帮助其规避干旱胁迫下开花期遭遇的高温胁迫。不过从深层根系生物量来看，IR64+ qEMF3并未表现出干旱规避特性。为评估EMF性状对现有干旱育种工作的补充潜力，本研究测试了13个先进干旱育种品系与已审定品种的开花期耐热性。结果显示，供试材料的开花期耐热性存在广泛变异，且值得注意的是，这13个品系均未携带EMF性状。因此，本研究提出一种育种策略：将EMF性状导入耐旱品系，可提升水稻小穗对开花期高温与干旱复合胁迫的抗逆能力。缩写说明：方差分析（Analysis of variance, ANOVA）、旱季（dry season, DS）、清晨开花（early-morning flowering, EMF）、开花时间（Flower Opening Time, FOT）、近等基因系（near-isogenic line, NIL）、穗水势（panicle water potential, PWP）、数量性状位点（quantitative trait locus, QTL）、湿季（wet season, WS）