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Boron deficiency and phytohormone interactions have been studied in unfertilized cotton (Gossypium hirsutum) ovules grown in vitro. Such ovules required exogenous indoleacetic acid and/or gibberellic acid for fiber elongation. Boron also was required for maintenance of fiber elongation and normal morphogenesis throughout 14 days of culture. The amount of exogenous boron necessary for maximum fiber elongation varied among experiments, presumably in relation to endogenous boron levels at anthesis. Some ovular epidermal cells distant from the liquid medium could be induced to elongate into fiber even after 6 days in boron-deficient medium in response to the later addition of boron.Boron deficiency, in the presence of exogenous indoleacetic acid, was characterized by lack of fiber development on the inundated ovular surface and reduced fiber growth on the ovular surface exposed to air. In the presence of gibberellic acid, boron deficiency was characterized by complete absence of fiber and callusing of the entire ovular surface. When both indoleacetic acid and gibberellic acid were added, the lack of boron resulted in proliferation of callus laterally and upward from the inundated epidermis, accumulation of brown pigments (presumably phenolic compounds) in the callus, and restriction of fiber to a small area of the upper ovular surface.If fertilization is permitted (flowers left intact until the second day postanthesis) and ovules are aseptically transferred to culture, fibers continue to elongate in response to a completely defined, liquid, basal medium. This continued elongation of cotton fibers is markedly stimulated by exogenous gibberellic acid (GA3) but only slightly, if at all, by indoleacetic acid. It was concluded that IAA is the phytohormone of principal consequence synthesized in response to the processes of fertilization (2). If fertilization is prevented and ovules are transferred to culture, fibers fail to elongate in the presence of the same basal medium. However, fiber elongation does proceed when IAA and/or GA3 are included in the basal medium. IAA alone provides for moderate fiber elongation of unfertilized ovules, GA3 provides for less fiber elongation but more callus formation from the ovular surface, and when IAA and GA3 are furnished in combination, the fiber elongation response is approximately additive (2, 3). Thus, a situation could be created where cells are completely dependent upon exogenous phytohormones for elongation and boron deficiency could be imposed at any point in the phytohormone-dependent elongation phase. This report deals with various effects of boron deficiency and phytohormone combinations on the growth and development of unfertilized cotton ovules in vitro. MATERIAILS AND METHODSBoron is required for normal growth and development of all higher plants, although to date no single specific physiological role has been assigned to it. During the past 2.5 decades boron nutrition has been implicated in several general areas of plant metabolism: organic translocation, enzy...

Cotton (Gossypium hirsutum L.) ovules grown in a defined nutrient medium undergo normal morphogenesis, induding fiber production. In identical medium lacking boron, ovules callus and accumulate brown substances. Boron deficiency-like symptoms were induced by 6-azauracil and 6-azauridine in ovules growing in boron-sufficient media. Other nudeoside base analogs either reduced or had no effect on over-all growth, but did not cause typical boron-deficient callus growth of cotton ovules. Orotic acid and uracl countered the effects of 6-azauracil. Actinomycin D, fluorodeoxyundine, and ethidium bromide reduced not only fiber production on ovules growing in boron-sufficient media but also callusing of ovules in boron-deficient media.Similarties between symptoms of boron deficiency and 6-azauracil injury, and the ability of uracil to suppress both, suggest that boron defciency symptoms are related to reduced activity in the pyrimidine biosynthetic pathway. Changes in nucleic acid levels have been correlated with boron deficiency. Some workers have reported that RNA levels increase during boron deficiency (8-10) while others have observed a decrease in nucleic acid levels (5, 7). Chapman and Jackson (7) found that an early sign of boron deficiency in Phaseolus aureus root tip segments was increased incorporation of radioactive precursors into RNA. Only later, concomitant with increased ribonuclease activity and cessation of elongation, did a reduction in RNA levels occur. They point out that reports of boron deficiency-related reductions in RNA were in grossly deficient tissues (1,18) and that a much earlier and possibly more direct effect of boron deficiency was increased RNA synthesis (8, 9).

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The apparent involvement of ornithine docarboxylase (ODC) and putrescine in the early stages of fruit growth in tomato (Lycopersieon esculentum Mill.) has been previously described. Further evidence presented here supports the direct involvement of ODC and putrescine in the cell division process in tomato fruits. In tomato fruits grown in vitro, in which basic growth processes are inhibited, the activity of ODC and arginine decarboxylase (AI)C) and the level of free polyamines were reduced. While OI)(2 and ADC activity was correlated with the period of cell division in the tomato fruit, the free polyamine content was correlated with the DNA content, cell size, and fruit fresh weight. The addition of exogenous putrescine, however, did not restore the basic growth processes in the fruits grown in vitro.