LITTLE IS KNOWN about the factors which determine tuber formation in the potato and in other tuberous plants. Most of the work with potatoes has dealt with the finding of conditions under which the plant produces its maximum yield, breeding new varieties for yield and disease resistance, and with studies dealing with the tuber itself. Several authors (Dostal, 1945; Deusse, 1947) have postulated the existence of a specific, hormone-like, transmissible factor for tuberization. Doposzheg-Uhlar (1911) attempted the isolation of a substance of the above nature from Gloxinia, but without conclusive results. On the other hand, others have suggested that tuber formation is a matter of surplus carbohydrates formed by the potato plant. (Driver and Hawkes, 1943;Arthur et al., 1930.) Bernard (1902 attempted to show that tuberization is brought about by the symbiotic relationships of the plant and a fungus. At present this fungal theory has not been entirely discarded.-Previous work on the effect of temperature and photoperiod on the tuberization of the potato plant has shown that these two environmental factors affect tuberization in some instances increasing, in others depressing the yield of tubers. A comprehensive review of the subject has been published by Driver and Hawkes (1943). Summarizing the results obtained by the writer, on the effect of these two factors on the tuberization of the Kennebec potato plant, it was found that short days (8 hr. of light) with low night temperatures, are promotive of good tuber yields, while in long days (16 hr. or longer of light) with high night temperatures no tubers are formed. Consequently, it is possible to control tuber formation in the potato plant at will by the appropriate combination of light and temperature conditions. The experiments to be reported in this paper indicate that tuber formation in the cultivated potato (Solanum tuberosum 1.) results from a stimulus formed or activated by specific conditions of temperature and photoperiod. The stimulus remains active in the plant for a certain time even when the plant is moved to conditions unfavorable for tuber formation. Furthermore, the stimulus appears to be transmitted through a graft union. Sugars as such are not the sole determining factor for tuberization.

No abstract

No abstract

The stem and root of certain plant species often become modified into structures whose form and function differ considerably from that of the typical organ. Moreover, though the functions of these two organs, when in their typical form, differ considerably among themselves, they assume upon modification a new, identical and most important one, that of food storage. Many of the plants with such modified stems or roots, e.g., the potato and the sugar beet, have become important food and feed crops and are good examples of plants with a highly efficient mechanism for the storage of energy in form of food.In addition to storing food, some of the modified stems and roots function as organs of vegetative propagation. The provision of two reproductive mechanisms enables these plants to survive in difficult environments. Thus, it is not surprising to find their natural habitats to be swampy grounds, desert areas, :freezing regions and other dif:ficult environments.Since they function frequently as organs of vegetative propagation, designed to survive adverse environmental conditions, tubers and tuberaus roots quite commonly pass through a period of dormancy. They can therefore be considered as vegetative resting stages, and the physiology of tuber formation as part of the physiology of dormancy. However, since tuber formation has received more attention than the formation of other vegetative resting organs a separate treatment seemed indicated. Our physiological knowledge of the formation of another, but related type of resting organs, the bulbs, is more limited; in particular, nothing is known about internal factors involved in bulb formation. Because of this paucity of data, the discussion of bulb formation has been included in the chapter on dormancy in plant organs 1 .The tuber and the corm are examples of modified stems, and the tuberous and the fleshy root are examples of modified roots. Advancement in the understanding of the mechanism of formation of the modified root has been slow. Most of the work on the study of tuberization has dealt with stem tubers, and here the research has centered around the potato plant (Solanum tuberosum L.). Thus this review of the physiology of tuberization will be mainly concerned with this species.

1982. The activity and interaction of brassinolide and gibberellic acid in mung bean epieotyls. -Physiol. Plant. 54: 239-243.The growth rate of mung bean epicotyls was used for evaluating the effect of brassinolide on cell elongation. Growth above that of control plants was observed at 10"^*' M and above. Gibberellic acid showed an additivity relationship with low concentrations (10"' -10~* M) of brassinolide in this test system and the two growth promoters may therefore act independently at the cellular level Because of relative ease of operation, great sensitivity and the short time required for assessing biological activity, this assay could be used in conjunction with the bean second intemode bioassay for evaluating the activity of brassinolide and its analogs, as weE as of other growth promoters.