Plant Growth Regulators (PGR): Cornerstone in modern cucurbit hybrid seed production
Authors: Rajesh Kumar Sharma¹ and Bharat Taindu Jain^2
1Ph.D. Scholar, Division of Seed Science and Technology, ICAR-IARI, New Delhi-110012
²Ph.D. Scholar, Dept. of Genetics and Plant Breeding, CCSHAU, Hisar-125004

Introduction:

Cucurbit crops are mostly produced for their immature or mature fruits. However, a relatively high proportion of cucurbit fields are destined for seed production, either for seed consumption or for propagation material. Production of cucurbits for seed consumption (as a snack food) is very popular in some parts of the globe and almost unknown in others. Open-pollinated cultivars (inexpensive seeds) are used in almost all crops for seed consumption. In the last decades the proportion of hybrids has dramatically increased because of the advantages of heterosis, improved technology and utilization of controlled environments. Cucurbit crops are insect pollinated, and the introduction of honey bee colonies into cucurbit seed production fields is an essential step for success. However, isolation is required to prevent out crossing, both for open pollinated cultivars and hybrid cucurbit seeds. Understanding the role of phytohormones and growth regulators in sex expression of cucurbits has greatly facilitated hybrid seed production.

Sex expression in the Cucurbitaceous family is variable across genus and species as well as within species. The knowledge of sex expression for any cultivar among cucurbit crops is essential in planning the best methods for seed production. Sex expression is regulated by phytohormones and an effective way to manipulate this trait is by external supply of these natural hormones or their artificial derivates (plant growth regulators - PGR) to the plant. Application of the correct PGR at the appropriate time and concentration is a cornerstone in modern cucurbit seed production. Cucumbers are an excellent example of significant advances in manipulating sex expression over the last decades. Most old cucumber cultivars were monoecious; having many staminate and few pistillate flowers on the same plant. Modern cucumber cultivars have a strong tendency to produce female flowers exclusively (up to complete gynoecy) combined with different degrees of parthenocarpy.

Sex modification through hormones and chemicals:

Sex expression in dioecious and monoecious plants is genetically determined and can be modified to a considerable extent by environmental and introduced factors such as mineral nutrition, photoperiod, temperature, phytohormones. Amongst these, phytohormones have been found to be most effective agents for sex modification and their role in regulation of sex expression in flowering plants has been documented. The morphological differences in various sex types and their specific metabolic characteristics result from the possession of specific patterns of proteins, enzymes and other molecules. Modification of sex expression in cucurbits has been induced both by changing the environmental conditions and by applying treatments with growth regulators.

1) Use of growth regulators to promote maleness:

Self-pollination is required to maintain inbred gynoecious lines, but these genotypes do not produce pollen. The solution to this problem came with the discovery of growth regulators able to induce formation of male flowers in these lines. The first compound used to induce maleness in gynoecious lines was GA. Some recommendations were three applications of GA3 at 1000 ppm or GA4/7 at 50 ppm beginning at the two leaf plant stage and spraying biweekly. A problem observed with GA use is that different gynoecious lines vary in response to GA application and, in some cases, the number of induced male flowers was not sufficient for hybrid seed production. Additionally, GA applications typically cause excessive stem elongation or malformed male flowers. Because silver ions inhibit ethylene action, an alternative to GA is silver nitrate. This compound induces male flowering in gynoecious lines for extended periods and is often more effective than GA. A single application of silver nitrate solution at 600 mg per liter before the first flower opens has been recommended (George, 1999). Silver thiosulfate is another alternative and appears to be less toxic than silver nitrate.

2) Use of growth regulators to promote femaleness:

There are different compounds that induce of femaleness in cucurbits. However, ethephon, a compound that releases ethylene gas, has been used the most in hybrid seed production. The main practical advantage of ethephon is its persistent effect on some species. The response to ethephon varies among species and cultivars. For example, in monoecious summer squash and cucumber, it can induce the formation of only female flowers for enough time to permit hybrid seed production by open pollination. In Cucurbita maxima, using ethephon dramatically reduces the labor required for removing male flowers in hybrid seed production. Ethephon is less effective in promoting femaleness in melon and watermelon. The most important use of ethephon in commercial hybrid seed production is in summer squash (Cucurbita pepo). for ethephon, is applied in concentrations of 250 to 500 ppm when the plants have 1, 3 and 5 true leaves. These applications suppress formation of male flowers during the pollination period. Because of the importance of avoiding self-fertilization in hybrid seed production, the plants should be checked periodically and any male flowers that appear should be removed before anthesis. Once development of male flowers in the female line has been suppressed, bees can be used for pollination. Five to six hives per hectare are recommended to ensure successful pollination. In the field, the proportion of female and male plants is usually 3 to 4 female plants per each male plant. An important practical implication of using ethephon in hybrid seed production of cucumber or summer squash is that the plants must be very uniform when the product is applied. This requires good agronomic management of the crop, which includes adequate soil preparation, sowing, irrigation, fertilization, and weed and pest control. If the plants are uneven in growth at the time of ethephon application, some plants will receive the product at differing stages of development causing those plants to produce male flowers that results in undesired pollen contamination.

Environmental sex modification:

Environment has greater influence on the sex modification. But due to introduction of photosensitive varieties or hybrids in modern era of agriculture it has gained less importance. However in seed production it has its influence on sex expression. In cucumber, high temperature and long day length (> 14 hours) favours male flowers. High temperature will extend the flowering of female flowers. As the temperature increases form 19 ⁰C to 23 ⁰ C the node for first female flowers has also increased from 9.6 to 16.5 numbers. This clearly indicates that high temperature favours male and delays female flowering.

Male sex expression of several plant species is favoured by high temperatures and female sex expression by low temperatures. Male sterile mutant of tomato developing male sterile flowers at a minimum temperature of 30°C and normal flowers at lower temperatures. In Brussels sprouts of low temperature effect on the development of the androecium. In onions a slight production of viable pollen by normally male sterile plants above 20 °C.




About Author / Additional Info:
I am currently pursuing PhD in Seed Science and Technology from Indian Agricultural Research Institute, New Delhi.