Plant growth regulators on growth, sex expression and fruit yield Exogenous application of growth regulators may shift the sex expression towards femaleness by increasing the production of female flower and suppressing that of male flower in cucurbitaceous plants. Plant growth regulators have positive effect on the production of early flowering and yield. Growth regulators can decrease male and female flower ratio and increasing the number of fruits per plant and individual fruit weight as well as increase the total yield.
The role of plant growth regulators in various physiological and biochemical processes in plant is well known from its identification. Root and flower buds initiation, development of flowers and fruits are controlled by different physiological processes. In many crops like cucurbitaceous ones, these processes can often be altered to man’s benefit by proper application of plant growth regulators. The concept that plant growth and development are regulated by a substance produced in minute quantities is one organ that elicits a response in another was first suggested by Julius von Sachs, the father of plant physiology. The term plant growth regulators (PGRs) cover the broad category of organic substances (other than vitamins and nutrients) that in minute amounts, promote, inhibit, or otherwise modify physiological processes (Wareing and Phillips, 1978). The PGRs, where endogenous (phytohormones) or exogenous, elicit essentially the same plant responses. Presently, PGRs are used to control a host of physiological processes in crop production, including flowering and fruiting (fruit set and parthenocarpy), partitioning of assimilate, germination, growth suppression, and post harvest ripening (Weaver, 1975).
The principle in sex modification in cucurbits lies in altering the sequence of flowering and sex ratio. Besides the environmental factors, endogenous levels of auxins, gibberellins, ethylene and abscisic acid, at the time and the seat of ontogeny determine the sex ratio and sequence of flowering (Leopold and Kriedemann, 1975). Exogenous application of plant regulators can alter the sex ratio and sequence, if applied at 2 or 4 leaf stage, the critical stage at which the suppression or promotion of either sex is possible. Hence, modification of sex to desired direction has to be manipulated by exogenous application of plant regulators once, twice or even thrice, at different intervals (Devies, 1987).
Ravindran (1971) reported that the exogenous application of ethral (2-Chlorothyl phosphonic acid) at concentrations ranging from 200 to 600 ppm induced stunting growth, retardation and male sterility and the production of male flowers significantly reduced in bitter gourd. Similarly, in cucumber (Cucumis sativus L.), application of ethral up to 500 ppm (Bhandary et al., 1974) increased the female flowers. They stated that ethrel concentration up to 500 ppm delayed male flowering up to 14 days and advanced female flowering by up to 9 days, while number of male flower were also reduced and female flowers increased by the application of same treatments. Higher ethrel concentrations were detrimental, whereas sex expression of snake gourd could be altered by foliar application of ethephon (ethrel) at 250 ppm and fruit yield could also be increased (Cantliffe, 1976). Mishra et al. (1976) reported that in cucumber maximal suppression of staminate flowers was obtained by the application of 400 ppm of ethrel. In case of fruit yield and yield components like number of pistillate flowers, fruit numbers plants-1 , fruit size and fruit weight were increased in Trichosanthes anguina plants by the application of ethrel at 50 to 150 ppm. The best result was obtained with ethrel at 150 ppm (Ramaswamy et al., 1976). In Luffa acutangula seedlings treated with 500, 1000 or 2000 ppm of ethrel, Patnaik et al. (1974) reported that ethephon treated plants produced pistillate flowers only, but the number of fruits and total yield were inferior to those of untreated plants. Verma et al. (1980) reported that ethrel treatments (50, 100, 150 and 200 ppm) were the most effective in increasing the number of female flowers, producing the largest number of fruits and greatest fruit weight plant-1 in cucumber. They further reported that all the treatments reduced the number of male flowers.
Krishnamoorthy (1981) studied the effect of ethrel at 250 to 1000 ppm on growth, flowering and sex expression of Cucurbita pepo L. They stated that ethrel increased the number of female flowers and decreased of Mia et al. 363 male flowers. Li (1983) reported that ethrel at 200 or 300 ppm could lower the site of the first female flower, promote the appearance of female flowers, increase the number of fruits and leaf area, reduce the number of male flower, fruit setting and increase yields of three cucumbers. The ethylene-releasing chemical, ethrel enhanced the development of pistillate flowers and delays development of staminate flowers of monoecious cucurbits.
Sreeramulu (1987) treated the sponge gourd plants at 3-true leaf stage with ethrel solution (100 mg/L). He observed that ethrel not only increased the number of pistillate flowers, but also hastened the appearance of the first female flower. The effect of ethrel on the staminate flowers was the opposite, that is, it delayed the appearance of the first staminate flower and also decreased the total number of female flowers. The sex ratio (staminate: pistillate) is decreased from 12.1:1 to 6.8:1. Plants were sprayed with different growth regulators at the 2 and 4 true leaf stages. The total yield (2.39 kg plant-1 ) was the highest in plants treated with ethrel (ethephon) at 100 ppm. The average control yield was 0.69 kg plant (Arora et al., 1985). Singh and Choudhury (1988) stated that ethrel at 50 and 100 ppm induced the first pistillate flowers earlier and at lower nodes in cucumber and bottle gourd, but delayed the appearance of female flowers in water melon. Karim et al. (1990) treated hybrid seedlings of cucumber with water, ethephon (250 and 350 ppm) at 1, 2, 3 and 4 leaf stage. Seedlings treated with ethephon at any stage produced more female flowers than water treated plants. The maximum increase in the number of female flowers occurred with 250 ppm ethephon applied at the 2-leaf stage.