Wide Hybridization in Plants pp. 359-364 (1960, English trans. 1962)

INTERSPECIFIC HYBRIDIZATION OF WATERMELONS
M. K. Gol'dgauzen
Candidate of Biological Sciences
Moldavian Research Institute of Irrigated Agriculture *
*Moldavskii institut oroshaemogo zemledeliya.

The variety composition in the U.S.S.R. of table watermelons is far from ideal. Many varieties have excellent qualities together with a number of substantial shortcomings: a poor keeping quality, often unsatisfactory transport qualities, susceptibility to a number of diseases, an insufficient drought resistance, etc.

Interspecific hybridization, with the aid of which shortcomings of the existing watermelon varieties can be eliminated and entirely new economically important types can be synthesized, is imperative for a radical improvement of the assortment of table watermelons now in use.

We worked on hybridization of watermelon species for several years at the Central Asian Experimental Station of the All-Union Institute of Plant Cultivation (Tashkent). Our work was interrupted and then resumed at the Moldavian Research Institute or Irrigated Agriculture and Vegetable Cultivation (Tiraspol').

Three main watermelon species were investigated: Citrullus edulis Pang., C. colocynthoides Pang., and C. colocynthis (L.) Schr.

In the following we give a brief description of the mentioned species. Citrullus edulis is a table watermelon with a juicy, sweet fruit pulp; it is well known, widely cultivated, and was introduced into cultivation long ago. C. colocynthoides is a fodder watermelon with a compact unsweet pulp, which is only in the early stages of being cultivated. If the history of the introduction of table watermelon to the territory of Old Russia has been lost through the many centuries which have passed, the history of fodder watermelon belongs to our time and counts no more than three decades. The fodder watermelon is interesting for hybridization because of the following: 1) a good keeping quality of the fruit; its fruit can be kept for one, two, and even three years, due to a high content of pectins (from 10 to 19% of the dry weight); 2) large fruit and high yields; the fruit reach a weight of 15 to 20 kg, and the yield amounts to 500-800 centners/ha, which is twice or three times the normal yield from table watermelons; 3) immunity to a number of diseases and lesser demands on cultivation conditions. But, together with these mentioned qualities, there are also certain shortcomings: fairly late ripening and a fruit pulp which is not sweet and is too compact.

Citrullus colocynthis is a wild plant with a white, bitter, poisonous pulp. It is characterized by a wide area of distribution, thanks to its exceptional adaptability to the severe conditions of waterless deserts. The colocynth has a larger number of traits which are significant for breeding purposes than fodder watermelon, namely: 1) an outstanding, unique drought resistance; 2) many-fruitedness; 20 or even 30 fruit are produced on one plant, whereas in other watermelon species no more than two to five fruits are produced; 3) the fruit may be stored for long periods; 4) short, trailing stems; the vines of the colocynth are approximately half the length of those of table watermelon; 5) immunity to many diseases.

The following negative characters are inherent in colocynth: bitterness and toxicity of the fruit pulp, the small size of the fruit (100 to 400 g) and fairly late ripening, later even than in fodder watermelon C. colocynthoides.

It is seen from the above that the species C. colocynthoides and C. colocynthis possess a number of valuable characters which are absent in C. edulis. This warrants their promising use in hybridization with varieties of table watermelon.

Direct and backcrosses between the three described species were carried out by us, with the use of a few forms and varieties of each species.

The percentage of success in intercrosses between watermelons varies between 3 and 31. There is a markedly lowered number of seeds in the hybrid fruit, i.e., there are sometimes 4 to 40 seeds per fruit instead of the normal 400 to 600. Hybrid seed is often of a poor shape, underdeveloped, or has an entirely normal external appearance but germinates badly. This is illustrated by Table 1.

Table 1
RESULTS OF CROSSES BETWEEN WATERMELON SPECIES

Crossed species Crossability
(%)
Seed in hybrids
(% of normal seed number)
Germination of
hybrid seed (%)
C. edulis x C. colocynthoides 31 23-44 57
C. colocynthoides x C. edulis 13 44-52 54-95
C. edulis x C. colocynthis 3 22-31 62
C. colocynthis x C. edulis 22 5-4 35
C. colocynthis X C. colocynthoides 18 1,5-11,5 -
C. colocynthoides x C. colocynthis 7 - -

The table shows that in reciprocal crosses of the same species combination, crossability, seed set, and viability of the seed are apt to differ. The first-hybrid generation C. edulis x C. colocynthoides is marked by a dominance of C. colocynthoides characters and also by a distinct heterosis of the vines, leaves, and size and weight of the fruit (Table 2).

Table 2
HETEROSIS IN LENGTH OF THE TRAILING PLANT AND WEIGHT OF THE FRUIT
OF F1 HYBRIDS BETWEEN TABLE WATERMELON AND FODDER WATERMELON

Indices Table watermelon variety
Belyi dlinnyi [White Long]
Fodder
watermelon
First
Generation
Weight of the fruit (kg) 3-4 7-80 15-25
Length of stem (m) 2-4 3-5 7-10

The second generation does not exhibit much variability; most of the forms essentially approach C. colocynthoides in all characters, i.e. they are late-ripening and have a coarse compact unsweet fruit pulp and a thick peel. Forms, in which the specific characters of C. edulis dominate, constitute only 1 to 3% of the entire composition of the second generation. In order to increase the number of forms of the edulis type, backcrosses to table watermelon varieties have to be made already in the first generation. The heterosis phenomenon, which is observed in the first generation, disappears in the second generation.

In evaluating the group of C. edulis x C. colocynthoides hybrids from a breeding point of view, one is bound to note the great practical significance of such crosses. In the first place, forms with a less compact, less coarse pulp and with a greater sugar content can be obtained, i.e., forms of an improved fodder type. At present, some breeders work in this direction to improve fodder watermelon. Secondly, small numbers of plants are obtained which approach table watermelon in the sugar content of their fruit, but whose fruit pulp is more compact than that of C. edulis; this is a most valuable feature in transport and storage qualities. As has already been said, the compactness of the pulp derives from the high pectin content of the fruit inherent in C. colocynthoides.

In the following we give data on the segregation of sugar and pectin content characters in table X fodder watermelon hybrids (Table 3).

Table 3
SEGREGATION OF SUGAR AND PECTIN CONTENT CHARACTERS IN HYBRIDS BETWEEN TABLE AND FODDER WATERMELON
(ACCORDING TO DATA OF V. V. ARASIMOVICH)

Parents Total sugars
(%)
Pectins (%
of dry weight)
(variety Tsukatnyi [Candied]) 2.24 10.87
(variety Kleklei) 9.35 0.91
  5.69 3.56
  5.71 1.44
  1.52 6.50
  6.11 4.23
  3.50 6.30
  4.55 1.63
  5.22 1.92
F2 plants 5.21 3.26
  5.31 1.86
  6.03 3.30
  5.49 -
  5.43 2.10
  5.19 1.45
  6.01 6.93
  6.13 0.63

Owing to the considerable immunity of fodder watermelons, forms of the table type which are immune to diseases can be obtained through hybridization of C. edulis with C. colocynthoides. Almost all the modern breeding of watermelons for immunity in the U.S.A. is based on hybridization of water watermelons with fodder ones.

Quite recently a new variety of table watermelon, which is resistant to fusarium wilt, has been produced in the Soviet Union at the Bykov Melon Production Experimental Station. It must be noted, however, that the development of new varieties of table watermelon through hybridization with fodder types involves prolonged selection and measures to induce homozygosity, with the obligatory use, as has already been said, of a backcross to C. edulis. The production of improved varieties of fodder watermelon is less laborious and can be accomplished within a shorter time.

Hybrids of C. edulis x C. colocynthis. The first generation of these hybrids exhibits an almost complete dominance of C. colocynthis characters. First-generation plants are late-ripening colocynth forms with a bitter pulp, though with somewhat enlarged fruit and leaves. It is noteworthy that, in crosses between C. edulis and C. colocynthis as in crosses with C. colocynthoides, the length of the vegetative period depends to a considerable degree on the respective reciprocal crosses (Table 4).

The particular characteristic of F1 hybrids between table watermelon and colocynth is the pronounced heterosis in the length of the vines; the main stem of the colocynth is on an average 1 to 2 m in length, that of the table watermelon — 2 to 4 m, and that of F1 hybrids — 10 to 12 m.

The second generation differs only slightly from the first; colocynth characters dominate in the segregation, but some forms manifest an influence of the table watermelon: the fruit are enlarged, the fruit pulp acquires a pink color, and a sweet taste appears. There is an independent segregation of bitterness and sweetness characters respectively, and forms desired by the breeders can be obtained in the second generation, i.e., forms which are sufficiently sweet but, at the same time, slightly bitter. In the second generation, there is no fruit which is entirely free of bitterness. It must be said, that breeding work is made much more difficult by this bitterness of the fruit. Upon adequate selection, colocynth characters decreased quantitatively with each subsequent generation: the fruit became larger, the pink color of the pulp was intensified, and bitterness also disappeared, but the pulp still remained coarse and weakly sweet. We are of the opinion that backcrosses are absolutely indispensable in work with colocynth, and they are best carried out already in the first generation, as in work with fodder watermelon.

Crosses between table watermelon and colocynth for breeding purposes are more promising than crosses with fodder watermelon, but the work takes longer. Hybridization with colocynth is particularly promising in the creation of watermelons for storage which can be preserved for a long time without losing sugar; in fact, such forms, accumulate sugar during storage, which is a completely novel feature hitherto unknown in the assortment of table watermelons. This is illustrated by Table 5.

Table 4
INFLUENCE OF PARENT USE IN CROSS (SEED OR POLLEN PARENT)
ON THE LENGTH OF THE RIPENING SEASON OF FIRST GENERATION FRUIT

Crossed species Number of days from
fruit set to maturation
C. edulis x C. colocynthoides 52
C. colocynthoides x C. edulis 68
C. edulis x C. colocynthis 60
C. colocynthis x C. edulis 94

A distinct and considerable accumulation of sugars is observed during the first two months of storage and a very insignificant loss of sugars — 0.5 to 1% — in the subsequent two to three months.

In addition to producing storage forms of the table type by hybridizing C. edulis with C. colocynthis, one also obtains many-fruited forms, some of which have sufficiently large fruit; we obtained individuals, which surpassed their parents considerably in yield (Table 6).

Table 5
CHANGES IN SUGAR CONTENT OF HYBRID FRUIT OF WATERMELON DURING STORAGE
(ACCORDING TO DATA OF V. S. BONDAREVA)

Dates of analysis First generation of backcrosses of colocynth to table watermelon varieties
Kubinets Krymskii pobeditel'
[Crimean Victor]
Belyi dlinnyi Melitopol'skii
142
sugar content of fruit (%)
At picking time 6.52 6.73 6.64 6.15
2 weeks after picking 7.29 8.50 8.68 8.17
2 months after picking 7.27 - 7.48 7.94
3 months after picking 6.60 7.61 7.34 6.66
4 months after picking 5.52 6.49 5.94 6.11
5 months after picking 5.19 6.26 - 5.17

Table 6
NUMBER OF FRUIT AND THEIR WEIGHT IN INTERSPECIFIC HYBRIDS OF WATERMELON

Parents and F2 Number of fruit
per plant
Weight of fruit
(kg)
C. edulis (Variety Vengerskii [Hungarian]) 2-5 3-6
C. colocynthis up to 30 0.1-0.4
F1 hybrid Vengerskii x C. colocynthis 13 2-4
C. edulis (Variety Tsel'nolistnyi)* 2-3 2-4
F2 hybrid Tsel'noslistnyi x C. colocynthis    

* [This is the Russian transliteration for the entire-leafed variety or hybrid.)

As was to be expected, forms of the table type, which are highly drought resistant and suitable for cultivation under desert conditions, are synthesized by hybridization of table watermelons with colocynth.

By crossing table watermelon variety Tsel'nolistnyi [Entire-leaved]. with colocynth we have created a new variety, Pioner pustyn' [Pioneer of the Desert]. This variety showed an exceptional drought resistance when tested under severe desert growing conditions in the Aral desert (Chelkar), in the salty desert of Kazakhstan (Balkhazh), and under the dry farming conditions of Kirgiziya (Vasil'ev State Farm, near Frunze).

Thus, though it may be a lengthy and complicated process interspecific hybridization holds high promises for the creation of entirely new types of watermelon varieties.