Effects of Drought Stress on Flower Number in ‘Summer Queen’ Passion Fruit

To determine the effects of drought stress, especially light drought stress, on flower number in passion fruit, one-year-old passion fruit plants grown in 7.5 L plastic pots were subjected to different soil water content treatments, namely wetness, light drought, and heavy drought for two months. Average, maximum, and minimum soil water contents (v/v) were 44, 47, and 41% in the wetness treatment, 23, 40, and 11% in the light drought treatment and 11, 33, and 6% in the heavy drought treatment. Flower number decreased as the strength of drought stress increased, although the number of nodes and flower buds did not. Flowering periods were from June 27 to July 19 in the wetness treatment and June 26 to July 16 in the light drought treatment with three peaks around July 1, 6, and 13. In the heavy drought treatment, the flowering period was from July 11 to 18 with one peak. The flower bud number was not affected by drought stress. Light drought stress did not suppress vegetative growth, such as vine length, leaf number, leaf length, or photosynthetic rate, although heavy drought stress did. Stomatal conductance was suppressed by light drought only at 12:00PM and by heavy drought throughout the day. Leaf water potential was decreased by heavy drought at 3:00PM, but not by light drought. In the wetness and light drought treatments, visible wilting was not observed, and in the heavy drought treatment the plants wilted before irrigation, although they recovered about 15 min after irrigation. In conclusion, even light drought stress, which did not suppress vegetative growth, reduced the flower number in passion fruit. Drought stress suppressed flower bud development but not differentiation.