3.2 Drought stress reduces the fertility of the male organ
To ascertain whether drought affected the male gametophyte, pollen fertility was determined. On average, the pollen viability of the DS plants was reduced by 31.9% (Figure 2A). Surprisingly, the pollen viability of DS flowers was similar to that of WW flowers after 4 d of DS suggesting starch accumulation might be reduced but not prevented in pollen of maturing anthers. In contrast, type-2 and 3 flowers recorded zero pollen at anthesis after rewatering (Figure 2A).
Because low fruit set coincided with overall low pollen viability of DS plants, it was speculated that male sterility was the main contributor of bud and flower abortions and subsequently the poor fruit yield performance of DS plants. To confirm our speculation, reciprocal cross-pollinations were performed. When the stigmas of flowers on WW plants were pollinated with pollen of flowers from DS plants (WW× DS), fruit set was 55.5% lower than in WW plants, which was consistent with higher flower abortion rate (Figure 2B), indicating that DS severely affected the male fertility. In contrast, when the stigmas of flowers on DS plants were pollinated with sound pollen of flowers from WW plants (DS × WW), flower abortion and fruit set were 14.3% and 85.7% which were moderately lower and higher respectively than those in WW plants (Figure 2B), indicating that the female fertility was not significantly affected. These results suggest that drought stress-induced irregularities in male organ are the main cause of the low fruit set/yield of drought-stressed tomato plants.