Temperatures and Fruits and Seeds

JOHN H. WEINBERGER.

DURING the warm months nearly all deciduous trees and many other plants in the United States form leaves, manufacture their food, and grow actively. During the colder months leaves are lost, visible growth ceases, and the plants rest. Temperature, like light, thus may influence the dormancy of buds and consequently flowering and the production of fruits and seeds.

This pattern of behavior is necessary for survival, because all actively growing tissues in plants are harmed or killed by temperatures only a little below the freezing point. Plants could not survive in most parts of this country if somewhere in their system they did not have a mechanism to keep them safely dormant through periods of freezing temperatures.

The mechanism involved is called rest, and the stage at which buds are unable to grow (even though external conditions are favorable for growth) is the rest period.

The rest is of particular advantage in the fall and early winter, when temperatures may be warm enough for growth but freezing temperatures are apt to occur. The buds need the rest to hold them dormant while they are becoming hardened by exposure to increasingly colder temperatures and are acquiring resistance to cold. The hardening process will not take place while the tree is growing.

The rest begins fairly early, soon after terminal growth ceases. As leaves are formed on the actively growing twig, buds develop in the axil of the leaf where the petiole is attached to the shoot. The bud is well formed by the time the leaf is fully developed.

The rest seems to develop in buds from the base upward, and individual buds at the base of the shoot may be in the rest while the shoot is still growing terminally. The fact that lateral buds on a shoot do not grow during the summer does not indicate they are in the rest. Hormones from the growing tip may check development of the buds. Cutting off the tip of the shoot would remove the inhibitory influence, and the bud may be forced into growth.

The rest influence in the bud develops gradually as the shoot becomes more mature and the bud older. The inhibitory influence from the shoot apex, which keeps them dormant, becomes less essential. The rest is easily broken early in bud development by leaf injury, wet weather, pruning, or other stimuli.

The practice of June budding in commercial nurseries is based on the principle that newly formed buds may readily be forced into growth. The resting influence becomes stronger as the summer progresses, and buds are less readily forced into growth.

Nurseries successfully practice dormant budding in late summer without stimulating growth of the transferred bud during that season. Although buds are essentially dormant during the summer and winter, they are still undergoing changes and development without making externally visible growth, if temperatures are above the minimum for growth.

Certain of the buds, because of internal influences, are transformed into flowerbuds, usually during late summer. During fall and winter, while their rest is being broken, these flower-buds continue to develop flower initials very slowly.

All buds are well into their rest at time of leaf-fall, although there is no causal relationship between leaf-fall and rest. The shortened length of daylight of the fall season causes the leaves of many species of trees to drop.

Additional illumination postpones leaf-fall, but even under continuous illumination on trees under street lights foliage is not retained indefinitely. In other species, length of day, or photoperiod, has no effect on the trees entering dormancy. The cooler weather of fall, or simply the physiological aging of the leaf, is responsible for leaf-fall in some species.

Evergreen trees undergo periods of active growth followed by periods of dormancy, or quiescence. This is more of a cyclic growth habit, and the dormant buds of evergreens are not necessarily in a resting condition.

MUCH OF THE WORK on the mode of breaking of the rest period of buds has been done with deciduous fruit trees, particularly the peach, Prunus persica. Other species of deciduous trees would differ somewhat in the intensity of the rest or duration of rest, but their pattern of behavior would be similar.

The peach is grown commercially in areas where the rest is sometimes not completely broken, and serious economic losses occur. The rest period of peaches is broken in the natural sequence of events during winter. The buds then are able to resume normal growth with the arrival of warm temperatures of spring.

Periods of low temperatures are responsible for breaking the rest. Chilling weather may occur even before the leaves are shed, and emergence from the rest then begins.

As the periods of chilling accumulate, the emergence is gradual, and it becomes increasingly easier to force the buds into growth by external stimuli. Extreme cold is not necessary. Temperatures of 45 F. are adequate, and even a temperature of 50 is effective but less efficient.

Probably the optimal temperature for breaking the rest of peach buds is around 40 fully as effective as freezing temperatures. In climates with long winters like those of the Northern States, the rest is broken long before the winter is over often by early it December. The time at which it is broken is determined easily by bringing shoots from the orchard into a warm room, placing the cut ends in water, and observing the development of the buds.

When peach trees have had their chilling requirements partly satisfied, many buds are in position to develop very slowly at temperatures favorable for growth.

If no further chilling occurs, straggly development and a condition called prolonged dormancy results. Blossoming may extend over 2 months. The flowers are generally weak and underdeveloped, and the style and stigma may fail to grow, although the ovary develops. Pollen development is weak. Many anthers fail to dehisce. Fruit set is poor. Some early-opening blossoms may have developed fruits the size of walnuts while the foliage is still sparse. Variable percentages of flowerbuds will drop off without opening.

Leafbuds generally require more chilling than flowerbuds do. If their chilling requirements are not satisfied, leafbuds may remain dormant or may make a small flush of growth and then become dormant.

In certain districts of southern California where winters are warm and summers are cool, growth sometimes starts before the rest is completely broken, fails to elongate normally, and then reverts to a resting condition. The tree is never completely out of the rest, and the rest inhibition dwarfs the tree.

The older buds in the center of the trees are the first to break dormancy when chilling is inadequate. They have undergone two or more chilling periods and need less chilling the second time than i-year-old buds. The next buds to develop are terminal shoot buds. Buds on shoot spurs and lateral buds are last.

Flowerbuds on certain varieties may drop off completely after extremely warm winters in the Southeast. No leafbuds develop. The tree is left completely dormant until very late spring. Then high temperatures of late April and May exert their influence in breaking the rest of the leafbuds and forcing them into sudden growth. Normal development follows during the summer.

The problem of warm winters in the commercial production of peaches was described by S. H. Rumph, the originator of the Elberta peach, as early as 1890. Thirty years later research workers proved that lack of chilling is responsible.

In the Proceedings of the Georgia Horticultural Society for 1890, Mr. Rumph. reported:

"The fruit growers in this section will long remember 1890 as the most peculiar and disastrous season ever witnessed. The extremely dry winter or some other unknown cause prevented peaches from blooming in February and March at their usual time. Our first cold wave during the winter came January 16, and then the thermometer registered only 30 degrees above zero. . . . Belle, Elberta . . . commenced blooming about April 5 . . . but the blooms were almost void of petals and the trees . .. did not leaf until late in April and May. It was a strange sight to see orchards of Alexanders without a leaf, as late as May 20th, the time that this variety usually ripens. . . ."

The mean temperature for November to February of that winter was 7.2 above average.

THE TIME element, or the length of time during which chilling occurs, is important.

An accumulation by early February of 750 hours during which temperatures were 45 or lower at Fort Valley, Ga., is sufficient to break the rest period of buds of Southland peach. That is the minimum requirement when the chilling is spread over 3 months. The rest could be broken earlier, by January 22 on occasion, but in this shorter period an accumulation of 1 thousand hours of chilling is necessary. Continuous chilling breaks the rest in the shortest length of time, but actually requires a greater number of hours of chilling than if the chilling alternated with warmer temperatures and was spread over a period of 3 months.

Most other important varieties of peaches have chilling requirements, equal to that of Southland or greater. Mayflower has one of the longest requirements, about 1,200 hours. On the other extreme are varieties like St. Helena and Red Ceylon, which are nearly evergreen and require very little chilling.

The chilling requirements of peach varieties expressed in hours of chilling apply only to the southeastern United States where the observations were made.

Weather patterns are different in the Southwest and California, and other measures or adjustments must be applied.

In the Central Valley of California, the principal peach-growing area of the West, winter weather is characterized by fogs or low clouds, which keep temperatures constant at around 50 or slightly lower for days at a time. Each hour of chilling under such conditions appears to be nearly 50 percent more effective than under normal conditions in the Southeast. After winters with little fog, the responses of peach buds in the two areas seem to be alike.

Temperatures in December and January are much more important in breaking the rest of peach buds in the Southeast than temperatures in November or February. The mean temperatures for December and January are just as reliable an indicator of the progress of rest breaking as hours of chilling, and the two measures may be used interchangeably.