COMMERCIALIZATION OF HYBRID-VIGOUR IN THE TOMATO.

New Zealand Journal of Agriculture, Volume 53, Issue 3, 21 September 1936, Page 139

COMMERCIALIZATION OF HYBRID-VIGOUR IN THE TOMATO.

J. W. Hadfield and R. A. Calder,

Agronomy Section, Plant Research Station,

Palmerston North

Hybrid-vigour. Fertilization in plants is effected by the union of germinal material present in the pollen (male) and the ovules (female) resulting in a new individual which derives its germinal constitution from its parents. The germinal material carries genes which are transmitted to the progeny, and it is the expression of these genes in the progeny that determine its appearance and behaviour. Certain of the genes are responsible for the expression of such characteristics of the plant as maturity, vigour, fruit-production, and disease resistance. The more desirable reactions of these characteristics are generally partly or wholly dominant over their alternatives, and therefore if brought in by means of crossing they become manifest in the first generation. It is unlikely that any single variety will carry a full complement of the large number of genes responsible for the expression of these desirable characteristics, but any one variety will carry’certain of them, and the genes that are not carried by it may be carried by some other varieties. This opens up the possibility of combining in one individual the desirable characteristics of the two varieties, thereby increasing the productivity of the hybrid beyond that of either parent. Success depends, of course, on the possibility of finding two varieties which are of mutual help in this respect, each bringing in its quota to be combined in the hybrid. This expression of vigour is termed hybrid-vigour, or heterosis. Varieties which are alike in appearance may have much the same origin, and therefore carry much the same range of vigour factors. In such cases the hybrid may show little or no increased vigour. On the other hand, varieties dissimilar in appearance are likely to be of dissimilar origin, and when crossed give a greater measure of hybrid-vigour. This generally proves to be the case.

The possibility of capitalizing hybrid- depends upon its extent .and the cost of producing hybrid seed. It is likely to be most attractive commercially when a little labour results in the production of a large amount of seed. The tomato is an example in which one cross will furnish a large amount of seed, and it is found that the measure of improvement resulting from certain crosses more than compensates for the -cost involved in producing the hydrid seed. Generally speaking, the hybrid is earlier, produces a heavier yield, and the fruit is intermediate in appearance between the two parents. Plants like wheat and peas that are self-fertilized become “ fixed,” .and the progenies of a single plant resemble the parent. The tomato is dominantly self-fertilized and therefore, generally speaking, its ■characteristics are “ fixed.” In this event the first-generation hybrids .are all alike, although differing in some respects from either parent. This uniformity is not permanent, for in the second and subsequent .generations segregation occurs and many variations arise. It is from this segregating material that the plant breeder makes his selections .and later attempts to “ fix ” them. Naturally the same position arises in the case of the hybrid-vigour. Its expression is at its maximum in the first generation following the cross, and is not permanent because in subsequent generations segregation occurs and hybrid-vigour becomes less progressively with each generation. The problem then resolves itself into finding varieties which when •crossed result in some definite improvement above either parent along the lines of increased yield, earlier maturity, or improved quality. Several workers have investigated the possibilities of commercializing, hybrid-vigour in tomato-production. In general, hybrid-vigour has been demonstrated, and in certain cases this has been reflected in an increased yield of fruit and in precocity. Alabouvette and Titard (France) found the greatest increase in two varieties with the least morphological resemblance. New Zealand Trials. The trials to be discussed were conducted at the Plant Research Station, Palmerston North. Four varieties, selected at random, were usednamely, Large Red, Early Cluster, Kondine, and Sunrise. Kondine and Sunrise proved to be very similar in nearly all respects, and Early Cluster was not markedly different from these. Large Red, however, proved quite distinct in maturity, growth, yield, and shape of fruit. It was unfortunate for the purposes of this trial that three of the varieties were so similar to one another. The varieties were crossed one with another, both ways, in 1933-34, and the resulting hybrid seed was sown in 1934-35. The seedlings were planted out, staked, and pruned to a single stem, in a manner similar to that adopted generally by commercial growers. Each plot consisted •of five plants, the hybrid plot being grown alongside the parent plots for purposes of comparison, and the whole trial replicated three times. The fruit was picked and weighed when it showed colour, and usually two pickings each week were found necessary. This allowed determination of not only the total yield but also the earliness of maturity . general OBSERVATIONS on RESULTS. - It was observed throughout that the hybrid plants grew more vigorously than the parents, but no measure of this was attempted. The

hybrid fruit was, in appearance, intermediate between the parents,, although this observation was possible only in the crosses with Large Red, which was sufficiently distinct from the other varieties (Fig. 1). In certain cases the hybrid plants gave a greater yield than either parent, and in nearly all instances were noticeably earlier. As yield and maturity are the two most important factors, thesewill be considered in respect to each cross. There were fourteen weighings taken altogether, and the presentation of such a mass of detail would be confusing. The yields are therefore presented at three periods and as a total. The first period is between 9th January, 1935, and 28th January, 1935. During this month tomatoes were scarce, and retailed as high as Bd. per pound ; therefore any increase in yield at this time was worth nearly double any in the succeeding period. The second period extends from 29th January, 1935, to 4th March, 1935, when tomatoes averaged a retail value of about 4d. per pound. During the final period, sth March, 1935, to 22nd March, 1935, tomatoes could be purchased at as low as 2d. per pound, and at the final picking on 22nd March, 1935, all fruit, whether ripe or not, was picked owing to the danger of . frost. It is necessary therefore to bear in mind the importance of early maturity in assessing the value attending the use of hybrid seed. The yields given are those from three plots each of five plants, and in all cases the mother plant that was used in the cross is mentioned first and the pollen-bearing plant second.

Kondine is the heavier-yielding parent and Large Red the earlier. Both crosses yielded more heavily than the heavy-yielding parent, and in maturity proved to be about equal to the early-maturing parent. There has therefore been a distinct improvement, because it has been possible to increase yield and at the same time to maintain early maturity.

Of the parents in this cross Early Cluster outyields Large Red, but the latter is decidedly earlier in maturity. In Early Cluster x Large Red the yield is about the same as the heavy-yielding mother plant (Early Cluster), but is markedly earlier and approaching the maturity of Large Red. In the reciprocal cross Large Red X Early Cluster the yield is definitely above the maternal parent, but precocity not quite so marked. It may be said of this hybrid that it approximates the yield of the heavier-yielding parent and approaches the maturity of the earlymaturing parent. It does then to a large extent combine the good qualities of both parents.

The parental varieties of this cross are seen to be somewhat alike in yield and maturity. Both crosses have given a small and probably non-significant increase over both parents, but the most striking difference is to be seen in maturity. The hybrid is definitely earlier than either parent. The general similarity of the parents would not lead to the expectation of any marked increase in yield.

It has already been pointed out that these varieties appeared almost identical, and the hybrids have given a very small, non-significant increase over the parents. In earliness of maturity the hybrids are, however, definitely superior to both parents. Of other crosses made, Large Red X Sunrise was intermediate in yield between the parents, and in maturity was definitely earlier than Sunrise. The reciprocal cross was unfortunately lost, and comparisons cannot be made. In the trial of Early Cluster X Sunrise, the latter variety proved to be both earlier and heavier-yielding than Early Cluster, and the

hybrid almost attained the heavy yield of Sunrise, and surpassed it in earliness of maturity. The reciprocal cross was in this instance also lost.

Summary of Results.

(1) The use of first-generation hybrid seed of tomato does in the case of certain crosses result in a yield higher than either parent; failing this the trend is towards the higher-yielding parent. (2) The use of such seed does in the case of certain crosses hasten maturity beyond either parent or, failing this, the maturity has a decided trend towards the earlier-maturing parent. (3) Taking into consideration the combination of increased yield and earlier maturity, it would appear that sufficient improvement may be expected to warrant the cost of producing hybrid seed for commercial purposes. (4) There is evidence that crosses between dissimilar varieties are likely to result in a greater measure of benefit than are those made between varieties of great similarity. (5) These trials were designed to explore the possibilities of the utilization of hybrid-vigour. This being sufficiently proved, the next step must be to discover those varieties which, when combined, result in the greatest measure of benefit. Technique of Tomato Hybridization. The tomato is normally a self-fertilized plant. The pistil, or female organ, is a thin green projection arising from the ovary; it terminates in the stigma, which, when receptive, exudes a sticky secretion. This structure is enclosed within five anthers, or male organs, set on very short, stout stems or filaments. These anthers are joined laterally and are arranged in a cone-shaped- formation with an opening at the apex. They are two-lobed and, when ripe, split along the line of dehiscence occurring on the inner side of each lobe. Thus the pollen is exerted inwardly, whence it comes in contact with its own stigma (Fig. 2). It is essential for crossing purposes to know the stage in the development of the flower at which this pollen distribution occurs, for emasculation or removal of the anthers of the female parent must take place prior to this period. By a careful examination of a few flowers, the approximate stage at which pollen is freed can be recognized. It is generally when the flowers are about half-bloom ; subsequent to this self-pollination will occur (Fig. 3). When a flower suitable for crossing has been selected, most of the others on the same cluster should be removed ; this allows for more favourable development. Emasculation may be effected quite easily with a pair of tweezers, though care must be taken not to burst the anther or to damage the pistil. If carried out at the half-bloom stage, the stigma is generally in a condition for immediate pollination, but, when effected earlier, which is preferable, a day or two should elapse before foreign pollen is applied ; in this' case it is advisable to cover the exposed stigma with a paper cover until ready for pollination. The application of pollen is quite a simple operation. As it is advisable that the pollen for crossing should be as fresh as possible, flowers between the half-bloom and the full-bloom stage should be utilized. If the anthers are divided, an examination of their inner

surfaces will reveal whether or not the line of dehiscence has begun to split and thus whether or not the pollen has been exerted. The most satisfactory supply of pollen is available at the commencement of the splitting process. It may be applied by brushing the stigma

of the prepared female individual with the inner surface of the anthers ; if the stigma is receptiveand this can generally be determined by its appearance— pollen adheres and fertilization takes place. For genetical investigation it is advisable to cover the effected cross with a waterproof paper cover, but for commercial purposes this is not necessary. If a number of different crosses are being made consecutively, it is essential to wash the hands and any instruments with alcohol after each separate cross ; this renders ineffective any adhering pollen. Saving Seed. The fruit should be picked when ripe and placed on one side till it is slightly over-ripe. It should then be pulped in a jar and allowed to ferment for about five days to liberate the seed from its surrounding tissues. When fermented a large quantity of water is added, and, after being well stirred, the pulp and flesh is skimmed off the top. If fermentation has been thorough the seed sinks to the bottom, and, by repeatedly adding water and decanting it, clean seed is obtained. The seed should be spread out to dry on some absorbent material. The seed should be dried thoroughly before it is stored away. The seed remains viable for several years, and hybrid seed need not be produced more frequently than every other year. In determining the number of crosses to make it may be assumed that an average tomato gives two hundred seeds. To check this seventeen medium to small fruits were pulped. These yielded 3,300, or 194 seeds per fruit. Germination of good tomato-seed is generally about 99 per cent.

Yield of Fruit, in Pound's. • — ' 9 th January, Yield of Fruit, in Pound's. 9th January, 1935, to 28th January, 93529th January, 1935, to . 4th March, 19355th March, 1935, to 22nd March, 1935Total. Kondine . . .. II -6 80 -8 31-4 123-8 Large Red 30-7 53’9 16-0 100 • 6 Kondine X Large Red 26-0 87-4 ■ 32-3 145 Large Red X Kondine 32’8 74-8 23'7 I3I-3

Table Large Red crossed with Kondine.

— Yield of Fruit, in Pounds. 9th January, 1935, to 28th January, 193529th January, 1935, to 4th March, 19355th March, ' 1935, to 22nd March, 1935Total. Early Cluster . . . . 16-4 89-0 41-1 146-5 Large Red 30’7 53'9 16 -o ioo-6 Early Cluster X Large Red 26-2 92 -2 31-7 150-1 Large Red X Early Cluster 25-4 77-0 29-4 131-8

Table 2.—Large Red crossed with Early Cluster.

— . Yield of Fruit, in Pounds. 9th January, 1935, to 28th January, 1935. 29th January, 1935, to 4th March, 19355th March, 1935, to 22nd March, 1935Total. Early Cluster 13’5 98-5 43’6 155-6 Kondine ■14-2 98-6 34’0 146-8 Early Cluster x Kondine . . . . IQ -2 106-4 36-9 162-5 Kondine X Early Cluster . . 32’4 97'8 37'5 167-7

Table 3. -Early Cluster Crossed with Kondine.

— Yield of Fruit, in Pounds. 9th January, I935> to ' 28th January, 193529th January, 1935, to 4th March, 19355 th March, 1935, to 22nd March, 1935Total. Kondine .. . . 14-2 98-6. 34-o . 146 • Sunrise . . 12 -g 98-7 29’4 141 -o Kondine X Sunrise X Sunrise 26-3 26’3 84-0 84-0 37'8 37’8 148 • 1 148 • I Sunrise X Kondine Kondine 25’4 25’4 86 • 7 86 • 7 37’7 37’7 149-8 149 • 8

Table 4. —Kondine crossed with Sunrise.