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

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Abstract
The effects of light and temperature on flowering and pollen tube growth were studied in watermelon [Citrullus lanatus(Thunb.) Matsum. and Nakai, cv. Early Yates] plants grown in controlled environment cabinets. All female flowers were pollinated in one group of plants; none was pollinated in the other group. Temperature increase from 25 °C to 35 °C with daylength of 14 h and light intensity of 32 klx caused increase in flower number per plant, proportion of male flowers, ovary length and diameter, ovule number per ovary, rate of pollen tube growth and percentage of penetrated ovules at 24 hand 48 h after pollination. Very few flowers were produced at 40 °C, but there was a high proportion of male flowers. Increase in daylength from 14 h to 24 h at 25 °C with light intensity of 32 klx also increased number of flowers per plant, ovary length and diameter and number of ovules per ovary but sex expression and rate of pollen tube growth were unaffected. Reduction in daylength from 14 h to 8 hat 25 °C and light intensity of 32 klx and reduction in light intensity from 32 klx to 8 klx at 25 °C and 14 h daylength both produced an increase in the percentage of immature ovules. The presence of fruit on the vine resulted in fewer flowers per plant and in reduced ovary legnth and diameter under all conditions tested. The results are discussed in relation to the fruiting response of the plant.