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.