American Journal of Botany, 30(9): 707-711 (Nov 1943)
W. E. Lammerts

IN ORDER to understand the reason for the experiments reported below, a brief account of previous experiences in growing embryo-cultured peach seedlings is necessary. As reported (Lammerts, 1942) embryo-cultured peach hybrids were brought into flower two years after pollination, thus establishing a two-year breeding cycle. Most of these hybrids were obtained by crossing fairly early ripening commercial peaches with varieties having a relatively short chilling requirement. Thus the Babcock usually ripens in July and, when the embryos were cultured within a few weeks after ripening of the fruit, no difficulty was experienced in growing them to a height of 11/2-2 feet before they became dormant. They were then hardened off and given cold storage treatment. When removed to the greenhouse, they grew rapidly to a height of three to four feet by May. They were then transplanted to the field where growth continued until early in November when many flower buds were formed.

In the spring of 1937 an extensive series of crosses was made, many of which involved the use of varieties which do not ripen their fruits until late in August and early in September. Seedlings from embryos of these fruits cultured during the middle of September and early October became dormant when only two to three inches high, often forming the peculiar rosette-like growth reported by Flemion (1934), Davidson (1934), and Tukey (1934). When these populations were transplanted to the field in the spring, after cold storage treatment and subsequent growth in the greenhouse, many seedlings became dormant and remained so until about the middle of August or even September, making but little growth. Very few flower buds were formed and many of the trees did not flower at all the following spring. The tendency to remain dormant after transplanting to the field was most marked in the populations resulting from crosses of varieties having a long chilling requirement. Obviously some other system of handling seedlings is necessary in order to bring all kinds of peach seedlings so abundantly into flower two years after pollination as to make their use as female parents entirely practicable. The experiments reported below were devised to determine what this system should be.

MATERIAL AND METHOD.—The peach seedlings used in the experiments were from (1) St. Helena, which is practically evergreen and has a very short chilling requirement; (2) a double flowering pink variety having a somewhat longer chilling requirement; (3) an unnamed variety much like Muir grown by Mr. Etter of Ettersburg, and having a fairly long chilling requirement; and (4) Muir known to have a relatively long chilling requirement.

The experiments performed were designed to test the reaction of seedlings of the above varieties with varying chilling requirements to various conditions of photoperiod and temperature. The embryos were cultured in the greenhouse according to the methods already reported (Tukey, 1934; Lammerts, 1942). After removal to three-inch pots the seedlings were placed under differential light treatments, and watered weekly with balanced nutrient solution. In the sixteen-hour photoperiod experiments 200-Watt C Mazda lamps placed about three feet above the plants were used to supplement the normal day length. The temperatures were held at 60 to 65°F. minimum at night and reached maximums of 80 to 100°F. during the day, except when outdoor temperatures were used (table 1, Exper. 10). The trees grown under these conditions were transplanted to the field May 28, 1942, after hardening off and chilling at 40° F. for six weeks. They were grown under rather poor field conditions in order to see if an abundance of flower buds would be formed in spite of the fact that the embryos from which they were grown were cultured so late in the fall.

In the experiments with the Muir seedlings one group was transferred to the California Institute of Technology controlled light and temperature chambers—Here the normal day length was supplemented by light furnished by three kinds of lamps in each battery arranged in the following ratio:

1 Cooper Hewitt Fluorescent "daylight" lamp.
4 General Electric "daylight" lamps.
1 General Electric 3500 K "white" lamp.

TABLE 1. Summary of experiments and results obtained by exposing indicated varieties
of embryo-cultured peach seedlings to various conditions of photoperiod and temperature.

Experiment number Variety and Number of plants Chilling requirement Kind and time of treatment Length of time before dormancy began Range in height
when dormancy began
Tree or seedling behavior
1 St. Helena
50 plants
Very short 16-hour photoperiod; 60-65°F. minimum, 80-100°F. maximum. Begun Nov. 1, 1941 135 days 33-62 in. Many flower buds. Trees remained evergreen winter of 1942-43
2 Double pink flowering
27 plants
Short Same as Experiment 1 120 days 21-46 in. Many flower buds. Trees dropped leaves winter of 1942-43, but leafed out early spring of 1943
3 Etter's seedling
23 plants
Medium long Same as Experiment 1 but begun Dec. 1, 1941 87 days 2-14 in. Many flower buds. Trees dropped leaves winter of 1942-43, and leafed out late in spring of 1943
4 Muir
7 plants
Long Same as Experiment 1 but began Feb. 19, 1942 31 days 2 1/2-9 in. No flower buds formed. Trees made little growth because of very poor soil
5 Muir
7 plants
Long 8-hour day, outdoors; 35-40°F. minimum, 70-85°F. maximum. Begun Feb. 19, 1942 14 days 2-3 1/2 in. No flower buds formed. Some thickening at the apex
6 Muir
7 plants
Long 8-hour day in greenhouse; 55-60° minimum, 80-85°F. maximum. Begun Feb. 19, 1942 14 days 2-3 1/2 in. No flower buds formed. Much thickening at the apex
7 a Muir
5 plants
Long 8-hour day, 68° and night. Begun Feb. 19, 1942 90 days 2-4 in. No flower buds formed. Thickened at the apex
8 a Muir
4 plants
Long 8-hour day, 80°F. day and 68°F. night. Begun Feb. 19, 1942 231 days 10-12 3/4 inches No flower buds formed. No rosettes formed. Very slow but continued growth
9 Muir
7 plants
Long Normal day in green house; 55-60°F. minimum, 80-85°F. maximum. Begun Feb. 19, 1942 14 days 2 1/2-4 in No flower buds formed. Rosette formation and much thickening at the apex
10 Muir
7 seedlings
Long 16-hour photoperiod, outdoors; 35-40°F. minimum, 70-85°F. maximum. Begun Feb. 19, 1942 14 days 2 1/2-4 in. No flower buds formed. Rosette formation and slightly thickened at the apex
11 a Muir
5 seedlings
Long 24-hour photoperiod; 68° F. day and night. Begun Feb. 19, 1942 231 days 34-38 in. Some flower budsformed which developed after cold storage treatment
12a Muir
5 seedlings
Long 24-hour photoperiod; 80° F. day and 68°F. night. Begun Feb. 19, 1942 231 days 27-38 1/2 inches No flower buds formed. Growth not quite as good as in Experiment II

a Experiments performed at California Institute of Technology in greenhouse where temperature was controlled as indicated to within 2 or 3 degrees.

2 Grateful acknowledgment is made to James Bonner of the California Institute of Technology for so kindly furnishing the facilities for the experiments performed there and for taking care of the peach seedlings during the experiment. The author is also indebted to Guy L. Philp of the College of Agriculture, Davis, Calif., for seed of the Muir peach, and to Jess Watt of the Armstrong Nurseries, Ontario, for seed of the other varieties used.

The intensity furnished by this array of lamps was about 400 foot-candles per square foot when three feet above the plants. Toward the end of the experiment the growing tips received about 1000 footcandles. The temperatures were held at about 68°F. day and night in one chamber and 80'F. day, 68°F. night in the other. After June 8, 1942, the temperatures in both chambers were raised to 80°F. day and night.2

Experiments.—The experiments are summarized in table 1. The most striking feature was the marked differential response of the four varieties of seedlings to the sixteen-hour photoperiod at 60° to 65° minimum temperature (Experiments 1 to 4, table 1). This response was inversely proportional to the chilling requirement of the varieties used. As these varieties are heterozygous, the variation in height within each population was no doubt the result of variation in seedling genotype. Though the Muir seedlings did not form rosettes under the sixteen-hour photoperiod, they grew only 2 1/2 to 9 inches high and did not form flower buds in the field following cold-storage treatment. An interesting relation between temperature and photoperiod is shown in the reaction of the Muir seedlings. A sixteen-hour photoperiod and low outdoor temperatures of 35° to 40'F. minimum resulted in little growth, the seedlings becoming entirely dormant in about two weeks (Experiment 10). Very little thickening of the terminal portion of the shoots occurred. Under greenhouse temperatures of 55° to 60°F. minimum, 80° to 85°F. maximum, and normal February and March day length (Exper. 9), the seedlings also became dormant but much thickening occurred as shown in figure la and 2a. Seedlings grown under eight-hour day and held at 80°F. during the day and 68°F. at night continued growing very slowly (Exper. 8). The normal upward growth of the seedlings was affected, and as shown in figure lb, the little growth which did occur was as much lateral as upward.

In this connection an experiment performed this spring is of interest. Seedlings of the Muir embryo cultured October 1, 1942, were allowed to go dormant under normal greenhouse temperatures of about 52°F. minimum and decreasing day length. The seedlings were quite dormant by December 1, 1942. On January 25, 1943, four of each of these seedlings were placed in chambers controlled as follows: (1) eight-hour day, (2) normal day, (3) sixteen-hour photoperiod, and (4) twenty-four-hour photoperiod. The temperatures were held at 70° to 75°F. at night and reached maximums of 80° to 95°F. during the day. All resumed growth in eight to fifteen days, most resuming growth in eight days. Evidently the change to higher temperature alone in spite of short day is sufficient to break the dormancy of embryo-cultured seedlings from non-after-ripened seedlings. The growth, however, in the short day chambers as in Experiment 8 was very slow. Under twenty-four-hour photoperiod and 68° or 80°F. minimum (Exper. 11 and 12, table 1), growth and elongation were quite rapid as shown in figure le. In the experiment the seedlings grew better at 68°F. than at 80°F., though this may have been due merely to the greater difficulty experienced in controlling red spider at the higher temperatures. An experiment now in progress with ten different varieties including Rio Oso Gem, which has an even longer chilling requirement than Muir, indicates that all respond and grow rapidly in height when exposed to a twenty-four-hour photoperiod at 70° to 75°F. minimum and 80° to 90°F. maximum, using 100-Watt C Mazda lamps to supplement the normal day length. Some seedlings of varieties with a relatively long chilling requirement, such as Humboldt, showed a tendency to rosette formation, but rapidly resumed growth, only a few leaves being curled and twisted. In one of the control group of seedlings grown at greenhouse temperature of about 52°F., and normal winter day of eight to nine hours, the seedlings of St. Helena and Shalil, both having a short chilling requirement, also grew very well, while the varieties with a long chilling requirement made little growth and some formed rosettes.

Fig. 1. Muir peach seedlings embryo cultured Dec. 15, 1941, and grown as follows: (a) normal February and March day length and greenhouse temperatures of 55-60°F. minimum and 80-85° maximum (Experiment 9); (b) 8-hour day and 80°F. day, 68°F. at night (Experiment 8) and (c) 24-hour photo period and 68° F. day and night (Experiment II).
Fig. 2. Closeup photograph of seedling in figure la showing pronounced thickening at the apex as compared with seedling in figure lb which continued slow growth, at higher minimum temperature.

DISCUSSION.—The marked increase in rate of growth of woody plants when grown under long photoperiod and high minimum temperature has been reported by several investigators, particularly Kramer (1936). However, this response to long photoperiod does not seem to have been taken advantage of by fruit breeders as a means of shortening the breeding cycle of trees. The peach is remarkably responsive to increased photoperiod and higher minimum temperatures, though cherries and apricots, as well as cherimoyas and avocados, have also shown marked response. Since recessive factors, such as those determining the nectarine type of fruit or double flowers in the peach, can usually not be combined with factors for good fruit quality, shape, or large size until at least the F2 or back cross generation has been grown, the need of developing techniques to bring F1 hybrids into flower and fruit production in the shortest possible time is obvious. In the case of the peach, the experiments reported above indicate that the best procedure for successfully growing embryo-cultured seedlings, particularly those from varieties with a long chilling requirement is to grow them at temperatures of 70° to 75°F. minimum in continuous light. A 100-Watt C Mazda lamp is adequate for the extra light treatment. Varieties with a shorter chilling requirement respond very well to a sixteen-hour photoperiod. When the seedlings have reached a height of about two to four feet, depending on the varietal response, they may be hardened off by exposing them to short day treatment at 45°F. after which they are given six weeks cold storage treatment in a dark room at 40°F. The cold storage treatment should be started by about the middle of February, so that the dormant seedlings may be transplanted directly to the field in early April. This allows enough time for the seedlings to grow to a height of six to eight feet by fall and form many flower buds. Accordingly, they will flower abundantly enough the following spring to give adequate progenies when used as female parents. By using continuous light treatment even seedlings of late varieties with a long chilling requirement, cultured as late as October 15, may still be grown large enough to be used as female parents two years after pollination.

Of the eleven varieties of peaches so far tested, seedlings from those with a short chilling requirement were less influenced by short day treatments and lower temperatures than were those from varieties with a long chilling requirement. Their response to increased photoperiod is also much greater. Should this correlation prove general it would seem that hybrids could be accurately classified in the seedling stage with respect to their chilling requirements when mature trees. This important practical possibility is being investigated.

It would, furthermore, appear that embryo-cultured seedlings from non-after-ripened seeds might be very useful for a study of the reason why deciduous trees become dormant in the fall, as their responses to short days seems to parallel that of mature trees. The differential onset of dormancy of peach varieties has long been observed. In Southern California, at least, varieties with a long chilling requirement become dormant long before those with a short chilling requirement, such as the double flowering peaches and St. Helena (Lammerts, 1941). In the case of embryo-cultured seedlings dormancy is obviously a response of the plant to short days and low minimum temperatures. This suggests that the differential onset of dormancy of mature trees in the late summer and fall may likewise be due in part to a differential response to shorter days and cooler night temperatures, varieties with a short chilling requirement being less influenced by short days and lower temperatures. Should further investigation show that this correlation between seedling and mature tree response is general, the usefulness of embryo-cultured seedlings in studying the problems of the cause of dormancy in deciduous trees is apparent. Instead of working with relatively few large cumbersome trees, hundreds of small seedlings could be analyzed as to possibly significant chemical changes following exposure to various light and temperature treatments.


Embryo-cultured peach seedlings from non-after-ripened seed often form rosette growths and go dormant very early, especially when cultured in the fall. This behavior is most pronounced in seedlings from varieties with a long chilling requirement, and, accordingly, it has hitherto not been possible to establish a two-year breeding cycle for hybrids of this type.

Experiments are reported which show that seedlings from varieties with a long chilling requirement make rapid growth instead of forming rosettes when placed under continuous light at temperatures of 70 to 75°F. minimum. When transplanted to the field in April following hardening off and exposure to six weeks cold storage in a dark room at 40°F., these seedlings make rapid growth and flower abundantly the second spring after pollinations are made.

In the experiments reported the response of the embryo-cultured peach seedlings to longer photoperiod and higher minimum temperatures was inversely proportional to the chilling requirement of the varieties from which the seed was obtained.

It is suggested. that if this correlation is general peach hybrids might be accurately classified in the seedlings stage with respect to their chilling requirement when mature trees, thus saving much field space and expense in the growing of seedlings in the field when varieties with a short chilling requirement are the main objective of the breeding program.

The possible usefulness of embryo-cultured peach seedlings in the study of dormancy problems is discussed.