Some Effects of Light Intensity, Daylength and Temperature on Flowering and Pollen Tube Growth in the Watermelon (Citrullus lanatus)

The effects of light and temperature on flowering and pollentube growth were studied in watermelon [Citrullus lanatus(Thunb.)Matsum. and Nakai, cv. Early Yates] plants grown in controlledenvironment cabinets. All female flowers were pollinated inone group of plants; none was pollinated in the other group. Temperature increase from 25 ^°C to 35 ^°C with daylengthof 14 h and light intensity of 32 klx caused increase in flowernumber per plant, proportion of male flowers, ovary length anddiameter, ovule number per ovary, rate of pollen tube growthand percentage of penetrated ovules at 24 hand 48 h after pollination.Very few flowers were produced at 40 ^°C, but there was ahigh proportion of male flowers. Increase in daylength from14 h to 24 h at 25 ^°C with light intensity of 32 klx alsoincreased number of flowers per plant, ovary length and diameterand number of ovules per ovary but sex expression and rate ofpollen tube growth were unaffected. Reduction in daylength from14 h to 8 hat 25 °C and light intensity of 32 klx and reductionin light intensity from 32 klx to 8 klx at 25 ^°C and 14h daylength both produced an increase in the percentage of immatureovules. The presence of fruit on the vine resulted in fewerflowers per plant and in reduced ovary legnth and diameter underall conditions tested. The results are discussed in relation to the fruiting responseof the plant.