Source–Sink Adjustment: A Mechanistic Understanding of the Timing and Severity of Drought Stress on Photosynthesis and Grain Yields of Two Contrasting Oat (Avena sativa L.) Genotypes

Abstract

There is limited knowledge about the effects of the severity and timing of drought stress on oat (Avena sativa) yield and the critical stages at which water management could be effectively implemented. A controlled study was conducted to identify a variety-specific critical stage, and to examine the physiological mechanisms of drought stress on photosynthesis (Pn) and yield formation in two contrasting genotypes. We found that compared with sufficient water supply (AW), grain yield was reduced by 36%, 69% and 44% in ‘Shadow’, and by 31%, 33% and 41% in ‘Bia’ under the severe stress imposed either at jointing, heading or post-anthesis stage. The grain/leaf area (LA) ratio increased by 18–32% and biomass distribution to stems + leaves decreased by 5.2–6.2% of Bia under moderate stress (MS) as compared to AW. This, along with the improved harvest index, led to a comparable yield. Under AW, Shadow displayed 13–16% larger LA duration (LAD) and had significantly higher Pn at the heading and post-anthesis stages, leading to 13% to 20% more spikelets panicle−1 and 13–21% greater groat yields than Bia. It can be concluded that (1) water stress at heading for Shadow and at post-anthesis for Bia was detrimental to grain yield through reduced LA and LAD with a reduced sink size, (2) under the MS, greater grain yield of the hulled Bia was attributable to a stronger sink activity, and (3) higher groat yield of the naked Shadow under AW was associated mainly with a higher source activity and more spikelets panicle−1.