Chapter 1: INTRODUCTION Cucurbits form an important group of vegetable crops grown extensively across the world. More than 20 types of cucurbits are known to be grown in different parts of the country owing to variable consumer preferences. Cucurbit group consists of a wide range of vegetables and either used as salad (cucumber), for cooking (all the gourds), for pickling (cucumber), as dessert fruits (muskmelon and watermelon) and as preserved items (ash gourd). Although, the cucurbits are of immense economic importance as food plants and it has been difficult to estimate their importance mainly due to the absence of reliable statistics regarding their area and production in India. They are extensively grown in varied mixed cropping pattern in long and meandering river-beds. The area under cultivation of cucurbits is 0.42 million ha with the production of 4.5 million tones. The perusal of figures given in table-1 indicates that the average productivity (kg/ha) of cucurbits in India is lower than the world average for most of the crops and far below the productivity achieved by the developed countries. The use of cucurbits as food is not primarily for calories, mineral or vitamin values, since they are either the poor and modest sources of these
Table 1: Area, production and productivity of cucurbits during 2000 Crop Area Production Productivity Highest (1000 ha) (1000 Mt) (Kg/ha) India World Pumpkin, Squash 355 3400 9577 12570 73377 and Gourds (Netherlands) Cucumber and 18 209 11750 11021 664286 Gherkins (Netherlands) Cantaloupes and 31 641 20447 16847 23629 (USA) Melons Watermelon 19 250 13158 21830 33633 (Japan) nutrients. However, there are few exceptions like bitter gourd richer in vitamin C, pumpkin containing high carotenoid pigments, kakrol high in protein and chow-chow fairly high in calcium. Some cultivars of squashes and pumpkins are relatively high in energy and carbohydrates. Biochemically the cucurbits are characterized by bitter principles called cucurbitacins. Chemically cucurbitacins are tetracyclic triterpenes having mixtures carrying taxonomic significance. Highest concentrations of such principles are found in fruits and roots, leaves being normally or slightly bitter. Bitter principles found in roots differ from those of fruits. The nutritive value of seeds is due to their high oil and protein content. The seed oil of cucurbitaceae falls into 2 groups; one with the palmitic-oleic-
linoleic acid composition and the other with conjugated triene acids, pumicic acid and alpha-oleostearic acid. At least 150 fatty acids are known to occur as components of seed oils. They are of two categories, one with simple saturated and unsaturated fatty acids, and the other with highly unsaturated fatty acids with conjugated double bonds. Luffagulin, a novel ribosome inactivating peptide is isolated from the seeds of ridge gourd which inhibits cell-free translation with an IC50 of 3.5 nm. (Yeung and Li, 1991). Being warm season crops, cucurbits are of tropical origin, mostly in Africa, tropical America and Asia, chiefly the South-East. Phylogeny of this large group of crops has not been studied adequately, possibly with an exception of Cucurbita. Luffa is an essentially old world genus, consisting of two cultivated and two wild species and there is only one wild new world species. It is rather difficult to assign with accuracy the indigenous area of Luffa species. It has a long history of cultivation in tropical countries of Asia and Africa. The name Luffa or Loofah is of Arabic origin because the sponge characteristic has been described in Egyptian writings and Chinese name Szkua - dish cloth gourd or towel gourd signifies its mention in early Chinese literature. India has been recognized as one of the primary centers of origin and rich centers of diversity of Luffa spp. Sanskrit name Koshataki indicates its early cultivation in India. It is also known as jhingery in Bengali,
urai in Hindi, heeray in Kannada, wetakolu in Sinhala and kabatity in Ilocano. Cytogenetic investigations have been conducted by Dutt and Roy (1969, 1971) and Roy et al. (1975) on two cultivated species Luffa acutangula (ridge or ribbed gourd) and Luffa cylindrica (sponge gourd) and two wild species, Luffa graveolens and Luffa echinata. Chromosome counts in all the species were found to be the same (2n=26) and comparative morphology of the wild and cultivated species and chromosome pairing in interspecific hybrids, made Roy and his associates to suggest Luffa graveolens as the prime species which has given rise to the two cultivated monoecious species Luffa acutangula and Luffa cylindrica. The other related species comprised Luffa operculate grown in tropical America, Luffa umbellate, Luffa pentandra, Luffa gigante, Luffa scabra and Luffa narylandica. The sex expression, i.e., monoecism and dioecism, have, however, evolved independently because the advanced sex form of dioecism could be found only in wild primitive species Luffa echinata. Some researchers raised a hermaphrodite variety of Luffa acutangula to that of a specific status Luffa hermaphrodita, which has not been accepted because of its easy crossing ability and fully fertile hybrids with Luffa acutangula. In general, both way crosses between Luffa acutangula and Luffa cylindrica successfully give rise to F 1 hybrid plants. High pollen sterility
exists in the interspecific hybrids and seed setting is poor. Fertility is restored when F 1 hybrids are crossed with either of the parents. Without artificial pollination, however, the two species do not hybridize, in view of the sterility barriers, which are effective in preventing exchange of genes between the two species. The family Cucurbitaceae is a moderately large one, comprising about 117 genera and 285 species distributed in warmer regions of both the hemispheres (Jeffrey, 1983). In India, Chakravarty (1982) reported the occurrence of 36 genera and 100 species have been described. The taxonomic classification of Cucurbitaceae has been dealt with differently by various botanists. Jeffrey (1980) of Kew Gardens revised his earlier classification, collecting information from palynology, seed-coat anatomy and phytochemistry. He divided the family into two sub-families (I) Zanonioideae and (II) Cucurbitoideae which was subdivided into eight tribes : (i) Melothrieae Cucumis ; (ii) Schizopeponeae ; (iii) Joliffieae Momordica ; (iv) Trichosantheae Trichosanthes ; (v) Benincaseae Coccinia, Benincasa, Lagenaria, Citrullus, Luffa ; (vi) Cucurbiteae Cucurbita ; (vii) Cyclanthereae Cyclanthera and (viii) Sicyoeae Sechium. Chakravarty (1982), however, classified the family into three tribes, (i) Cucumerineae comprising the genera Trichosanthes, Lagenaria, Luffa,
Benincasa, Momordica, Cucumis, Citrullus and Coccinia, (ii) Orthospermeae consisting of Cyclanthera and Dicaelospermum and (iii) Zanonieae includes Sechium and others (Table 2). The gourd family has a distinct set of morphological features which can be easily distinguish it from other plant families. Generally, the plants have a fairly long tap root with lateral roots confined to top layer of 60 cm. Hence, these are adapted to grow in river-beds to utilize sub-terranean moisture and also some of them (mostly semi-wild) have xerophytic habit. The stems are branched (3 to 8), prostrate or climbing and the branches cover large areas. Leaves are simple, mostly 3 to 5 lobed, variously shaped, palmate, cordate or reniform. Tendrils are borne on the axils of leaves, simple or bifid in Citrullus, Cucurbita, Luffa, Sechium and Lagenaria and they are absent in bush or erect types. The inflorescence in the cucurbits is axillary, solitary or clustered or recemose. Flowers are mostly unisexual, large and showy, mostly monoecious (staminate and pistillate flowers develop separately in the same plant). In addition, dioecious F 1 hybrids, andromonoecious and hermaphrodite forms are also available. The cultivated forms of cucumber are mostly monoecious, while gynoecious (pistillate flowers only) stocks have been developed for seed production but in muskmelon, cultivated forms are
Table 2: Common Name, Botanical name, Family, Plant habit and parts used as vegetable. Crop Botanical Name and Family Plant habit Plant part Somatic chromosome used Number Ash gourd Benincasa hispida (Thumb.) Cucurbitaceae Annual Fruits Cogn., 2n = 24 Bitter gourd Momordica charantia L. Cucurbitaceae Annual Fruits 2n = 24 Bottle gourd Lagenaria siceraria (Mol.) Cucurbitaceae Annual Fruits Standl., 2n = 22 (44) Cucumber Cucumis sativus L., Cucurbitaceae Annual Fruits 2n = 14 (24) Kunduru or Coccnia grandis (L.) Cucurbitaceae Perennial Fruits Ivy gourd Voigt., 2n = 24 Long melon Cucumis melo L., 2n = 24 Cucurbitaceae Annual Fruits Musk melon Cucumis melo L., 2n = 24 Cucurbitaceae Annual Ripe fruits Pointed gourd Trichosanthes dioica Roxb., Cucurbitaceae Perennial Fruits 2n = 22 Pumpkin Cucurbita moschata (Duch. Cucurbitaceae Annual Fruits Poir), 2n = 24 (40 & 48) Ridge gourd Luffa acutangula (L.) Cucurbitaceae Annual Fruits Roxb., 2n = 26 Round melon or Citrullus latanus var. Cucurbitaceae Annual Fruits gourd fistulosus L., 2n = 22 (24) Smooth gourd Luffa aegyptica L. Cucurbitaceae Annual Fruits 2n = 26 Snake gourd Trichosanthes anguina L. Cucurbitaceae Annual Fruits 2n = 22 Summer squash Cucurbita pepo L.. 2n = 40 Cucurbitaceae Annual Fruits Squash C. mixta Pang., 2n = 40 Water melon Citrullus lanatus (Thumb) Cucurbitaceae Annual Ripe fruits Matsum, 2n = 22 Winter squash Cucurbita maxima Duch., Cucurbitaceae Annual Fruits 2n = 24 (40)
mostly andromonoecious (bisexual and staminate flowers in different nodes). In Lagenaria, Cucurbita, Citrullus, Luffa, Momordica and Trichosanthes, mostly monoecious forms are observed. However, the dioecious forms (staminate and pistillate flowers in separate plants) which are in cultivation comprising cropss like pointed gourd, ivy gourd or kakrol also exists. In Luffa, male inflorescence is in recemes of several flowers, peduncle is 15-35 cm long, female flowers are solitary, borne axillarily as male flowers, flowers are 4-5 cm in diameter, fragrant, calyx is 5 partite, glandular, petals are yellow, obovate, 2-2.5 cm long, stamens are 3, ovary is inferior, filiform, with 10 longitudinal ribs possessing swollen glands, style is short, stigma is trifid, flowers opening in evening, fruits are club shaped, crowned by enlarged sepals, style is persistent, angled fruits in ridge gourd are 10-ribbed, many seeded, seeds are black and pitted (Peter, 1998). The anthesis and dehiscence is between 17-20 hours. Pollen fertility is at its maximum on the day of anthesis and lasts till 2-3 days after anthesis in winter and one and half days in the rainy season. Stigma is receptive 6 hours before and 84 hours after anthesis (Singh, 1957). Hermaphrodite form which bears only bisexual flowers, like Satputiya cultivar of Luffa acutangula, is rare. The fruits of Luffa are used only in the immature stages as on maturity, the fruits become bitter and inedible.
The ridge gourd is closely related to smooth gourd. The morphological features of F 1 plants involving these two parents were of intermediate nature. The hybrid had 2n=26 chromosomes and showed various irregular metaphase configurations including a ring of 4 chromosomes. These two species are closely related and the structural changes of the chromosome have played an important part in their evolutionary differentiation. Genetics of a few qualitative traits have been worked out by Singh et al. (1948) in ridge gourd (Table 3). Table 3: Genetics of qualitative traits in ridge gourd. Characteristics No. of genes Type of gene action Colour of node Monogenic (NN) Blue colour dominant to green Seed surface Monogenic (PP) Pitted surface dominant to smooth Sex expression Digenic Monoecious dominant to hermaphrodite Fruit bearing Monogenic Non clustering dominant over clustering habit Corolla colour Monogenic (YY) Orange yellow dominant to Lemon yellow Fruit surface Monogenic (RR) Ridged dominant over smooth Fruit taste Monogenic (BB) Bitterness dominant over non-bitterness Type of stamen Monogenic (FF) Partially fused stamen dominant over free Flowering in cucurbits normally starts in about 40-45 days after sowing depending upon the weather conditions. There are three types of flowers, staminate (androecious), pistillate (gynoecious) and hermaphrodite.
The node number of the first female or perfect flower is important. It is an index of earliness of the cultivar and there are variations in a given species in this regard. The probable sequence of flowers on the plant is undeveloped male, normal male, normal female, inhibited male, giant female and parthenocarpic female flowers. There may be few deviations in the sequence depending upon the genotypes and environmental factors. The sequence of flowering follows a set pattern. Kalloo (1988) has described following the sex forms (Figure 1): 1. Trimonoecious (.. ): Rare in breeding lines of cucumber, muskmelon and ridge gourd 2. Monoecious (. ): Predominant one found in ash gourd, bottle gourd, bitter gourd, cucumber, muskmelon, pumpkin, ridge gourd, round melon, squash and watermelon 3. Androecious ( ): Rare 4. Andromonoecious (. ): Muskmelon mainly 5. Gynoecious ( ): Cucumber and muskmelon breeding lines 6. Gynomonoecious (. ): Rare in the breeding lines of cucumber, muskmelon and ridge gourd and 7. Hermaphrodite ( ); Ridge gourd In a typical monoecious sex form, the number of male flowers produced is in far greater proportion than the pistillate flowers. This is called as sex ratio, which may range from 25 to 30:1 to 15:1; the later condition is advantageous and economical, because it results in greater number of pistillate flowers per
Figure 1: Diagrammatic representation of flowering habits in cucurbits plant and consequently higher fruit set and yield. Sex ratio is highly sensitive to environment and high nitrogen. Long days and high temperature greatly promote more number of male flowers. The hermaphrodite sex is considered as a primitive form, where only bisexual or perfect flowers are produced (Seshadri, 1990). Generally, in monoecious plants, staminate flower appears first and pistillate flowers at the later stages. In monoecious cucumber, staminate, mixed and pistillate phases are normally noticed. The sequence of flower types on main shoot in squash has been described by Nitsch et al. (1952), (Figure 2). Andromonoecy is predominant in muskmelon and very rare in watermelon, ridge gourd, smooth gourd and cucumber. Androecy and
gynomonoecy are rarely seen in the segregating population of ridge gourd, cucumber and muskmelon. Flowering habit may be explained in different forms such as: (1) gynomonodioecius- a group consisting gynoecious and monoecious individuals; (2) gynodioecious- a group consisting gynoecious and hermaphrodite plants; (3) androdioecious- a group consisting androecious and hermaphrodite plants; (3) dioecious- a group consisting androecious and gynoecious plants. All of these forms are very rare except dioecious. Generally, these are artificially synthesized in buffalo gourd whereas gynodioecy and androdioecy have been artificially synthesized in cucumber. Figure 2: Schematic presentation of the sequence of flower type on main shoot in a squash (cv. Acorn)
Cucurbitaceous vegetables form a distinct group among the crosspollinated vegetable crops, in respect of method of improvement. Besides the wide range of sex forms and sex expression, which favours out breeding, they are not completely cross-pollinated and as stated earlier, natural self pollination does take place even though in a less degree. Another characteristic of this group of crops is that they do not suffer much from inbreeding depression or do so only negligibly, in contrast to other cross-pollinated crops. This phenomenon allows breeding cultivars through pure line or single plant selection. The other breeding methods are mass selection, pedigree, bulk population, interspecific hybridization, polyploidy and heterosis breeding. Hybrids between varieties of ridge gourd show heterosis for yield; hybrids show marked increase in fruits per plant when the hermaphrodite variety satputiya is used as one of the parents. Important characteristics used in the ridge gourd improvement programmes are sex ratio, yield and seed production potential. Due to complexity in flower forms, it is essential to study the mechanism of sex expression in ridge gourd for the development, stabilization and utilization of different sex forms in crop improvement programme.
In view of the information available on the genetics of flowering and its economic exploitation in ridge gourd, the present study was undertaken with the following objectives, 1. To obtain basic information on the type of gene action operating in the inheritance of sex expression in ridge gourd. 2. To assess the possibility of sex modification using plant regulators in gynoecious sex form of ridge gourd for its maintenance. 3. To estimate the magnitude of heterotic responses in respect of green fruit yield per plant.