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.