Euphytica 25 (1976) 321-328
JUVENILITY IN HYBRID
D. P. DE VRIES
Institute for horticultural Plant Breeding (IVT). Wageningen, the Netherlands
Received 3 November 1975
1Also presented at the Juvenility Symposium held at West-Berlin. November 1976.
In Hybrid Tea-roses relations between growth and juvenile period (J.P. number of days from seed germination to flower bud appearance) were studied for three years.
Plants showing a flower bud for the first time are significantly longer than those without a bud. In comparison with plants with long J.P.'s, plants with short J.P.'s have shorter shoots both at bud appearance and at first flowering, flower sooner, are significantly longer when measured on one date, and yield about three times more cut flowers in 6 months. It was shown that the J.P. depends on plant development and does not govern this process.
Mainly initiated by breeders' difficulties, many studies were made of juvenility in woody plants. Recently a comprehensive review was published by ZIMMERMANN (1972) followed by an annotated bibliography by ULYATT (1974).
In most species the juvenile period is relatively long. Attempts to influence the duration of the period have been more or less successful. Particularly in perennial fruits, pre-selection for short juvenile periods has been shown to be a possibility of shortening the breeding cycle. Relationships of the juvenile period with tree size, precocity and possibly fruit yield were established for apple and pear (VISSER, 1967; VISSER & DE VRIES, 1970).
In Hybrid Tea-rose most seedlings flower a few weeks after seed germination and, as the main selection is done for flowering characteristics, the juvenile period has never been subjected to research. Made within the scope of a breeding research in roses, our present study revealed a number of relations with the juvenile period, many of which have parallels in other crops.
MATERIAL AND METHODS
The plant material consisted of a number of Hybrid Tea (H.T.)-rose seedlings raised in 1972, 1973 and 1974 in a breeding programme for glasshouse roses. Just after germination, in the first half of March, plants in the cotyledon stage were transplanted into pots and left on benches until first flowering occurred. By the end of May when all seedlings had flowered they were transplanted into beds in the same greenhouse and treated as cut roses. The trials ended in the last week of December. On the plants on the bench the following data were recorded:
a) date of germination.
b) date of flower bud appearance. c) date of first flowering,
d) at the dates of b) and c): plant length in cm and number of leaves,
e) in 1972 only: at two-day intervals, the number of fully grown leaves, length of the leaved shoot, length of the flower stalk.
In the greenhouse. the number of cut flowers ( flower yield) of each plant was recorded after bottom break formation.
These recordings were made in 1972 on 9 populations with a total of 350 individuals, in 1973 on 6 populations with 177 individuals and in 1974 on 8 populations with 536 individuals.
It should be noted that when in the next pages reference is made to e.g. '1971 populations', seedling populations are indicated of which the progenitors were mated in 1971 but the progeny of which was observed in 1972, owing to half a year fruit growth and subsequent stratification.
Throughout this paper the number of days from germination to flower bud appearance will be referred to as the juvenile period (J.P.).
Development of the seedling. Fig. 1 illustrates the development of the leaved shoot, the flower stalk and the number of leaves of H.T.-rose seedlings. Data were derived from the growth measurements in 1972.
During the first 18 days after germination a rosette of 3-4 true leaflets is being formed just above the cotyledons; from then shoot growth with elongation of the internodes starts. After 27 days the apical flower bud appears; the juvenile period has now ended. Although only 9 days later (36 days from germination) the ultimate number of six leaves is clearly perceptible, this number was already present at bud appearance. After bud appearance, the flower stalk also shows a rapid growth, its rate being about the same as that of the leaved shoot. The growth curves of both shoot and flower stalk are distinctly sigmoid. When flowering starts 54 days after germination, the flower stalk is slightly longer than the shoot (11 versus 10 cm), together giving the plant a length of 21 cm.
|Fig. 1. Development from germination to first flowering of the number of leaves, the leaved shoot and the flower stalk of a H.T.-rose seedling.|
Parameters of the juvenile period. Rose seedlings may differ largely as to their J.P., both within and between years, but for all populations observed their distribution over the classes of J.P. appears to be a normal one. Average values for the J.P.'s were in 1972: 32.9 (a = ±6.7). in 1973: 31.1 (a = ±5.6) and in 1974: 24.8 (a = ±3.3) days.
|Fig. 2. Relation between the number of days after germination and the length of H.T.-rose seedlings at bud appearance (•). and the length of seedlings without a bud (A) at the same dates.|
Our data showed striking differences between the length of the plants at flower bud appearance. In Fig. 2 the average shoot length at bud appearance in 1972 of groups of seedlings with J.P.'s of 21 to 54 days, is compared with the average length of the seedlings that did not yet show a flower bud at that moment. It can be seen that in every instance plants that have a bud for the first time, are about 25 mm longer than those that do not have a bud.
Fig. 3 illustrates the average growth in length of the leaved shoot of groups of seedlings with 5 different J.P.'s (21, 24, 30. 36,42 days) in 1972, involving 245 individuals. With the longer J.P.'s, the growth curve initially runs horizontally longer than with the shorter ones, but once shoot elongation has set in, the growth rate of each J.P. group of plants is similar (about 5.5 mm/day). This rate is maintained longer in plants with a long J.P. than those with a short J.P. It can also be seen that at flower bud appearance, plants with a long J.P. are longer than those with a short J.P. The latter phenomenon was observed throughout our trials, as is shown in Fig. 4. This figure illustrates for the 1971. 1972 and 1973 populations, the relation between the time of bud appearance (in the classes: 15-<l8 to 45-<48 days) and plant height at that moment (in percentages of the annual population average), in the three years plant height at bud appearance increases with the J.P. It can also be seen that there is a tendency towards shorter J.P.'s with the years.
|Fig. 3. Growth curves of the leaved shoots, of H.T.-rose seedlings with J.P.'s of 21, 24, 30, 36 and 42 days, in 1972. (O= bud appearance, X= flowering).|
In Fig. 3 it can also be seen that with increasing J.P.'s, rose seedlings have increasingly longer shoots at first flowering; highly significant correlations between the two parameters were found annually and are presented in Table I. Also in Table 1. the positive correlations between shoot length of the plants at bud appearance and the number of leaves at that time are presented, demonstrating that through the years plants that are longer at bud appearance have more leaves.
|Fig. 4. Relation between the time of, and plant length at bud appearance of H.T.-rose seedlings. expressed as a percentage of the yearly population average (1971, 100% - 5.6 cm, 1972, 100% = 5.5 cm, 1973, 100% - 5.1 cm).|
Table 1. Correlations between shoot length at bud appearance and at 1st flowering, between shoot length at bud appearance and the number of leaves at 1st flowering, between length of the J.P. and time of 1st flowering, between time of 1St flowering and flower yield, in Hybrid Tea-roses.
|Parameters||Coefficients of correlation1|
|1||shoot length at bud appearance - shoot length at 1st flowering||0.599
|2||shoot length at bud appearance - number of leaves at 1st flowering||0.721
|3||length of J.P. - time of 1st flowering||0.621
|4||time of 1st flowering - flower yield||-0.675
1All correlations significant at P<0.001.
The length of the juvenile period also determines the time of first bloom, as can be seen in Fig. 3: again very significant positive correlations between the two parameters were found annually (see Table 1).
Fig. 5 illustrates for seedlings of the 1971 populations the relation between the juvenile period of groups of plants and their length on the 42nd day after germination. This date was chosen because the earliest plants just did not flower, while several late ones did not have a bud, thus creating a large variation in length. A highly significant inverse correlation demonstrates that the longer the J.P.'s, the shorter the plants will be.
Another character of the juvenile period in roses is its relation to flower production. Fig. 6 illustrates that in all three years of observation there was a very significant negative correlation between the average J.P. of groups of plants and their flower yield from June till the end of December. Thus rose seedlings with shorter J.P.'s produce far more flowers than those with longer ones; it can be seen that yield may vary annually from 5070 below to 50% above average. Also very significant negative correlations were found annually between the number of days from germination to first flowering (time of 1st flowering) and flower production (Table 1).
|Fig. 5. Relation between the J.P. and average plant length on the 42nd day after germination, of groups of H.T.-rose seedlings in 1972.||Fig. 6. Relation between the J.P. and flower yield of H.T. -rose seedlings, expressed as a percentage of the yearly population average (1971, 100% = 12.3 1972, l00 % = 11.2, 1973, 100% = 11.3 flowers per plant).|
Hybrid Tea-roses show few, if any, juvenile characters. In accordance with GOEBEL'S (1896) definition the crop has a homoblastic development. Therefore the seedlings are mainly interesting for their strikingly short juvenile period, which offers possibilities for study seldom found in other woody crops. An added advantage is that the plants were grown under controlled environmental conditions.
Our definition of the juvenile period as the number of days from seed germination to flower bud appearance, may seem inexact because from seed germination to its visible result some time elapses, and also because flower bud appearance occurs some time after its initiation. However, both stages being very conspicuous, their time interval provides a workable standard.
No measures were taken to shorten the J.P. of our seedlings, but the average period decreased with the years. In each year different seedling populations were grown, but as the same germ plasm was used through the years, the decrease in average duration will be mainly environmental and due to an increase in the know how of plant care. VISSER (1964), investigating juvenility in apple and pear, also found better cultural practices to promote growth and to shorten the J.P.
When measured on one date, H.T.-rose seedlings of the same age show a relation between length and J.P. similar to the one found in apple and pear (VISSER, 1964; SAURE, 1970), viz. the longest (= thickest in fruit trees) plants have the shortest J.P.'s. However, when the length of the same seedlings was measured at bud appearance (comparable with thickness in the year of flowering in apples), in roses as in apples (SAURE, 1970), the shortest plants have the shortest J.P.'s.
In the foregoing the juvenile period was described in relation to a number of phenomena in the development of the seedling. Hereafter it will be discussed whether these phenomena are dependent on the J.P. or determine it.
The earliest discernable phenomenon after germination is the successive appearance of leaves, which are initially arranged in a rosette. Just after the last leaf is formed, both in seedlings and in varieties (HORRIDGE & COCKSHULL, 1974) the flower bud will be initiated. Seedlings differ considerably as to the period they stay in the rosette stage (Fig. 3), but have in common that just before all leaves are developed internode elongation begins. The period of leaf formation takes less time in seedlings with few leaves than in those with many leaves, Consequently the former plants will sooner start shoot elongation, will sooner show a flower bud and will be shorter at that moment than the latter. The former seedlings are said to have a shorter juvenile period than the latter. The after-effects of these processes are that growing at about the same rate from flower bud appearance to first flowering, seedlings with few leaves will flower sooner and at a smaller size than seedlings with many leaves. The difference in time will be increased by plants with many leaves and internodes, needing more time for the full development of these organs.
In roses. and probably in many other crops that are heterozygous for most characters. the number of leaves, internode length and the number of petals etc. show a continuous distribution, suggesting a multigenic base. It will be due to this genetical make up, that various juvenile periods can be observed.
As to roses, it is concluded that the juvenile period is the result of the genetically controlled development of the seedling rather than an independent entity governing these processes.
In view of the foregoing, the concept of a minimal leaf number for flowering (LANG, 1965) appears to be a dubious one in roses. However, it was observed that even when grown under the stress of extremely low light intensities, no seedlings were found with less than 4 leaves (DE VRIES, 1974).
Some authors (WAREING, 1959; VISSER, 1970) mention a minimum size for flowering in each crop, i.e. in roses the plant length at bud appearance. It was seen that in a heterozygous crop this character is a variable, mainly dependent on the genetically determined number of leaves, and as such modifiable by environment.
One of the main aims in rose breeding being higher flower production, selection for plants with short juvenile periods is essential, as they may produce three times more flowers in the same time span than plants with long J.P.'s. But, because highly productive plants tend to produce shorter (= less leaves and internodes) shoots, particularly the breeder of greenhouse varieties should, among the plants with short J.P.'s, select for exceptions to the rule: plants with long shoots.
As the observation of flower bud appearance is a rather laborious one, it should be remembered that, applying the time from seed germination to first flowering as a selection criterion, about the same plants are selected.
I wish to express my thanks to Miss Truus Lucassen and Dr Ramiro Gil Ortega (now at Zaragossa -Spain) for their unfailing assistance in measuring plants in 1972.