AN INVESTIGATION OF FLOWER FORMATION IN ADULT AND JUVENILE CITRUS TREES

IN 'THE DEVELOPMENT of many woody plants the seedlings pass through a juvenile stage marked by an absence of flowers and by other distinctive morphological features which involve leaf form, branch habit, growth habit, or degree of thorniness. In citrus the non-flowering, or juvenile stage of seedlings, as compared with young trees produced by buds from adult trees, is characterized by great vigor of growth, erect habit, and numerous large thorns (fig. 1 and 2). The species of citrus, closely related genera, and a few other trees sometimes show the anomalous behavior of flowering within the first year of the juvenile phase. Jost (1907) mentions the case of an oak which normally flowers for the first time at 60 to 80 years of age, but which occasionally produces flowers in its first year and then dies. The anomalous flowering of very young juvenile citrus seedlings, especially prevalent in the grapefruit (Citrus paradisi Macf.), occurs when the seedlings are a few months to one year old and usually in the winter and early spring months (Dec.-Mar.). A single terminal flower is borne on the main axis of the plant (fig. 3). These flowers are frequently abnormal in structure and rarely produce fruit, though occasionally fruit containing viable seed are matured (fig. 4 and 5). Vegetative growth after flowering is continued from one or more shoots from axillary buds. These seedlings do not normally flower again until they have passed through the juvenile phase of growth which may last many years. The flowering of very young juvenile seedlings suggests the possibility of arbitrarily reproducing at a still later stage of seedling development the conditions required for flower formation. Normally, citrus seedlinngs flower for the first time at 5 to 10 years of age. Frost (1943), in describing citrus breeding work in California, states that at least 5 or 6 years are required from seed to earliest fruiting. E. M. Savage,3 from long experience in growing citrus seedlings in central Florida, estimates that the time usually required for seedling trees on their own roots to flower is about as follows: tangerines (C. reticulata Blanco) 5 to 7 years, sweet orange (C. sinensis [L.] Osbeck) 6 to 7 years, grapefruit 7 to 8 years, tangelo (C. paradisi X C. reticulata) 5 to 8 years. Occasional seed1 Received for publication June 20, 1946. 2 Physiologist, associate physiologist, and assistant botanist, respectively, Division of Fruit and Vegetable Crops and Diseases, Bureau of Plant Industry, Soils and Agricultural Engineering, Agricultural Research Administration, U. S. Department of Agriculture. 3Personal communication. lings, especially those of grapefruit and tangelo, may require considerably longer periods. Savage grew a tangelo seedling that fruited for the first time at 22 years of age. All parts of the citrus tree do not pass through the juvenile stage at the same time. Frost (1943) has observed that the trunk of a thorny seedling and the proximal portion of its main branches retain for a long time the ability to produce thorny shoots. On the other hand, the shoots from the uppermost branches show a tendency toward progressive reduction in thorniness and increase in flowering. The long delay of citrus seedlings in coming into bearing is a serious handicap to rapid progress in citrus breeding. While this problem is common to the breeding of most of the tree fruits, it is particularly vexatious in the case of citrus. Usually the number of hybrid (zygotic) seedlings resulting from a cross is greatly exceeded by the number of associated nucellar seedlings, and often it is not possible to distinguis-h between them until thev have fruited. Thus, many seedlings of no value from a breeding standpoint must be grown until thev bear fruit. Most of the reported attempts to hasten flowering of seedling trees or of varieties that are slow in coming into bearing have been based upon empirical methods, though some of them, nevertheless, appear logical in the light of recent work which indicates that flowering, at least in some plants, is controlled by flower-forming substances produced in the leaves. Treatments to promote early flowering that have been reported include ringing, heavy top or root pruning, confining root systems in small pots, stunting the plant by withholding water and nutrients, frequent transplanting, artificially controlling photoperiod, subjecting to low temperature, grafting on dwarfing stock, inarching seedlings to adult plants, and various combinations of these treatments. The effects of these treatments on earliness of bearing are, in general, somewhat conflicting. In most instances only negative results were obtained. The investigations reported in this paper are primarily concerned with attempts to induce flowering of juvenile citrus seedlings. Coincidently with these experiments a study was also made of the relation of day-length and of season of the year to flower formation in adult citrus. It was thought that knowledge gained from a study of adult trees would be of aid in estimating the best time to apply certain treatments to juvenile seedlings. It should be noted that, with few exceptions, the observations reported in this paper are on the effects