3.1 Effects of drought stress on flower development and fruit set
To assess the effects of drought stress at reproductive stage on flower development and fruit set, tomato plants were drought stressed by withholding watering and allowed to grow at soil moisture 6 ± 3% for 4 d, then normal watering resumed (Figure 1A). On suspending watering, the first emerged buds were at about binucleate stage of pollen development (Figure S1A). At the end of the drought stress (DS), drought-stressed plants were severely wilted and stunted (Figure S1B). The first significant impact of drought stress on reproductive development was on flower production. Under DS, significant reduction in the number of opened flowers/day occurred between 1 d before suspending DS and 7 days after rewatering (DARW) with nearly zero flower production between 1-7 DARW. The second period of flower production occurred between 8 and 19 DARW, peaking on 11 DARW, suggesting that drought stress can postpone flowering (Figure 1B). Further, we found that 25% of flower buds on DS plants became yellowish and distorted in appearance and finally aborted. Strikingly, bud abortion occurred mainly at three specific stages of pollen development: tetrad (TED), early uninucleate microspore (EUM) and vacuolated uninucleate microspore (VUM) (Figure 1C), indicating that these stages are most sensitive to drought stress. In addition, 38.6% of opened flowers aborted resulting altogether in a total of 63.4 % flower (flower buds and opened flowers) abortion in DS plants, which was significantly higher than that in WW plants (Figure 1D). Consistently, fruit set in the DS plants was 36.6% which was markedly lower than 77.5% in WW plants, a reduction of 52.8% (Figure 1D). Subsequently, fruit yield per plant in DS plants was decreased by 57.4%. Together, our data suggest that bud and flower abortions are the main contributors to yield loss under drought condition.
All anthers subjected to preanthesis drought stress for 4 d had significantly shorter stamen at anthesis compared to WW stamen (Figure 1E). They also displayed three types of phenotypes: type-1 appeared very similar to the normal flower; type-2 had anther lobes often reflexed and freed from the style in the neck region; type-3 had extremely shortened stamen and mostly with stigma extended above the cone. Most of the flowers were type-1, followed by type-3 and then type-2 represented by 65.3%, 27% and 7.7% respectively. Interestingly, type-1, 2 and 3 flowers often but not always occurred on the same truss, with type-3 flower first appearing, followed by type-2 and then type-1 flowers (Figure 1E). Although the lengths of all preanthesis DS anthers were significantly shorter, the decrease was more dramatic for the type-3 anther which stamen was 1.6 mm shorter than the pistil compared to 0.2 mm higher than the pistil in the WW flower (Figure S1D), suggesting that the stigma exsertion is attributed to extreme shortening of the stamen and not elongation of the style. In short, drought stress affected the reproductive development of tomato in diverse ways.