Response of seedlings of a dwarf and a normal strain of watermelon to gibberellins.

Hypocotyl and root elongation in a dwarf and a normal strain of watermelon (Citrullus lanatus [Thunb.] Matsu.) in the absence or presence of different gibberellins was investigated in seedlings grown under gold fluorescent light or in darkness. The normal strain, "Sugar Baby," responded only slightly to the gibberellic acids employed. At appropriate concentrations all of the gibberellic acids were capable of normalizing growth in the monorecessive dwarf strain, WB-2, in darkness or in light. Gibberellins A(4+7) and A(7) were effective in stimulating hypocotyl elongation at concentrations 10 to 15 times lower than that needed for a response to GA(1) or GA(3). Dark-grown dwarfs responded to about a 3-fold lower concentration of GA(4+7) than those grown in light.In contrast to hypocotyl elongation, root elongation was greater in the dwarf than in the normal strain. Concentrations of gibberellic acids which enhanced hypocotyl elongation of WB-2 plants, inhibited root growth proportionately.Anatomically, the response of the dwarf strain to GA(4+7) was primarily in terms of increased cell division. Dark-enhanced elongation of both SB and WB-2 was due almost solely to increased cell elongation. The results suggest that, at least in some species, a major component of dark-enhanced growth (or light-inhibited growth) is physiologically distinct from gibberellic acid-stimulated elongation.