Satisfactory crops are produced if the temperature averages a little higher, but the trees show some symptoms of a prolonged dormancy trouble.

Varieties like Elberta that need 850 hours for flowerbuds blossom normally after 49.6 in December and January, but with a 950-hour leafbud requirement, Elberta trees leaf out normally only after a 48.3 in December- January. Dixired trees, which need 1,050 hours for leafbuds, foliate normally only if December January temperatures average 47 or lower. These temperature relations apply only in the Southeast.

High temperatures during the rest-breaking period counteract the effects of chilling instead of advancing development. Buds on trees in the shade of some object require less chilling than exposed trees.

Warm, sunny days increase the chilling requirements of buds. A few days of extremely high temperature during the first part of the rest may produce abnormalities in flowerbuds of peaches that subsequent chilling fails to overcome. Typical abnormal blooming occurs, even though the winter has been cold.

Leafbuds differ from flowerbuds in that subsequent chilling may be effective in overcoming the influences of high temperatures. The resting condition is a reversible process in leafbuds and to a limited extent in flowerbuds.

BUD DROP, a serious problem for some species of fruit, particularly apricot, is associated with warm winters. The temperature relation is not the direct cause, however.

Anthers in buds of stone fruits start to deteriorate and turn brown in late winter and early spring about the time visible swelling of buds occurs. The condition spreads to other flower parts, and as many as 95 percent of apricot buds may fall off. A 50-percent bud drop in peaches is not uncommon.

Among pome fruits having mixed buds, like the apple and the pear, all or only some of the flowers may fall off. A spur is left that has only a cluster of leaves or leaves and a reduced number of flowers.

The condition has been correlated with high temperatures in early fall, although bud drop is also greater in winters having fewer chilling hours. Generally it is more serious in peach varieties that have a long chilling requirement, but exceptions occur, particularly in varieties having a moderate chilling requirement.

Peach varieties of Peento origin that require little chilling may be affected severely by bud drop. High maximum temperatures during the winter are not responsible for the condition.

THE REST influence seems to be centered largely in the buds. It was once thought the cambium had no rest and only the bud was affected. The idea was disproved when active apple scions grafted on unchilled trees did not grow so well as those grafted on trees where the rest period had been broken by chilling. Both series of stocks had been disbudded to eliminate any bud influence on growth.

Thus far no adequate substitute has been found to replace chilling temperatures in breaking the rest of buds on orchard trees.

Heat, freezing, ether, alcohol, hydrochloric acid, oil, sodium nitrate, chloroform, ethylene dichloride, ethylene chlorhydrin, and other organic compounds have some influence if the rest has been partly broken. Mechanical injury or wounding is effective.

Spraying orchards with an emulsified oil, either lubricating, linseed or seal oil, has hastened b1ossoming and foliation of apple and pear trees.

Fortifying the oil with dinitrocresol increases effectiveness. Too often in this country injury results from this treatment without adequate bud response. The basis of the stimulation seems to be largely an influence of injury. The closer a treatment comes to killing a bud, the greater appears to be its stimulating properties.

As yet there is no safe, cheap, and dependable means of preventing prolonged dormancy on orchard trees where the winters are warm.

THE CAUSE of the rest has been a subject for investigation for many years. Diffusible auxins, sometimes thought responsible, generally are higher in rest-broken buds and deficient in buds during the rest. It is unlikely therefore that they are agents. C. H. Hendershott and D. R. Walker, of the North Carolina State College, identified a growth inhibitor in dormant peach flowerbuds as naringenin (5,7,4' trihydroxyflavanone). It was eight times more plentiful in buds collected December 1, 1958, than in those collected February 2, 1959.

THE SOUTHERN limits of successful production of many fruits and nuts in the United States is determined by the chilling requirements of their buds. When prolonged dormancy and subsequent crop losses occur too often, production becomes unprofitable, and susceptible kinds or varieties of fruit disappear from an area.

Acceptable varieties of fruits having relatively short chilling requirements are being developed, and the frontiers of the commercial production of these fruits are being extended southward. The problem in this country is of consequence only in the southern border States, including North Carolina, South Carolina, and Georgia.

Varieties of peaches vary widely in their chilling requirements. The southernmost area of production of the leading varieties is in south-central Georgia, but less acceptable varieties are adapted to the winter climate of Miami, Fla.

The Babcock variety, which has a short chilling requirement, is planted extensively in southern California. Maygold is grown commercially as far south as northwestern Florida. Jewel and the new Tejon, Red Ceylon, and Rochon varieties require less chilling than Babcock or Maygold. Many good varieties are in the class that need 750 hours of chilling or are adapted to December January winters that average 51 . Among them (in order of ripening) are Springtime, Earligold, Hiland, Robin, Redcap, Keystone, Sunhigh, Southland, July Elberta, Merrill 49'er, and Redskin.

Breeders in time will develop varieties suitable for commercial production in all southern regions.

Nectarines sports of peaches are like peaches in their adaptability to a warm climate, but no extremely short-chilling varieties are known. Silver Lode and Panamint have a moderately short chilling requirement.

Apples generally require more chilling than peaches, but the range among varieties is wide. Some varieties, as Hume and Beverly Hills, are adapted to the warm winters of southern California. Important apple districts in the United States generally have cold winters, and winter chilling is a minor aspect.

Pears are similar to apples in chilling requirements of varieties. Leading producing areas are in cooler climates. Baldwin, Garber, Kieffer, Leconte, Orient, and Pineapple are varieties having short chilling requirements.

THE ORIENTAL plums (Prunus salicina) require relatively little chilling, compared with other fruits. Varieties like Mariposa and Burmosa are adapted to winters of southern California. The European plum varieties (Prunus domestics) blossom much later as a group than the Oriental plums. Tragedy and Sugar require less chilling than others.

American plums, Prunus americans, have long chilling requirements. Prunus angustifolia are native throughout the Southeast. Bruce and Six Weeks, commercial varieties that are hybrids of Prunus salicina and Prunus angustifolia, are adapted to warm areas of the Southeast.

Apricot buds require relatively little chilling and begin development even before almonds. Apricot trees shed their flowerbuds following warm winters, however, and the crop suffers. Thus, although the species is adapted by chilling requirements to warm winters, bud drop prevents successful commercial production in the warm sections.

Because buds of the sweet cherry and sour cherry (Prunus avium and P. cerasus) have moderately high chilling requirements, they generally are unsuited to warm sections.

Grape buds require little chilling to break their rest. The eastern grape, Vitis labrusca, is native to areas having long, cold winters, but it has an extremely short chilling requirement.

The European grape, Vitis vinifera, requires no chilling after the vine becomes dormant to make normal growth. A period of 10 to 12 weeks, however, elapses before growth starts. Time appears to be the controlling factor in breaking the dormancy of vinifera buds.

Almonds have short chilling requirements. Their rest is broken early in the winter. This trait, plus the fact the buds will grow at rather low temperatures, makes the frost hazard a serious problem for growers of almonds.

Trees of Persian walnut, Juglons regia, and filberts are sometimes affected by lack of winter chilling in warmer climates. Placentia variety of walnut is the best known among those having shorter chilling requirements. The Barcelona variety of filbert generally suffers least from lack of chilling. Pecans, which are native to southeastern United States, require little winter chilling. Some forms of pecan growing wild in Indiana need more chilling than those that grow in Mississippi.

FLOWERING and the production of Fruits and seeds in many deciduous trees therefore depend on a certain rhythmic response to temperature during the dormant season. Doubtless many other plants not so well studied are similarly affected by temperature in the formation of seed.

JOHN H. WEINBERGER is a horticulturist in the Fruit and Nut Crops Research Branch, Crops Research Division, Department of Agriculture, Fresno, Calif.