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 A4+7 and A7 were effective in stimulating hypocotyl elongation at concentrations 10 to 15 times lower than that needed for a response to GA1 or GAs, Dark-grown dwarfs responded to about a 3-fold lower concentration of GA47 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 GA4+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.Gibberellin applied exogenously to genetically dwarfed plants can often restore or nearly restore normal stem elongation (1,3,5,6,9,17,27). Plants elongate less in light than in darkness, and GA also alleviates or nearly alleviates the light inhibition of growth in many species (19,20). In several, but not all cases, genetic dwarfs have been shown to contain lower levels of endogenous GA-like substances than their normal counterparts (1,10,28,29,32), indicating that the effects of dwarfing genes are often mediated via a block in GA biosynthesis. However, in the few comparisons of GA levels between dark-and light-grown plants, dark-grown plants have been found to contain similar (14, 15) or even lower (1) levels of GAs than plants grown in light, suggesting that lightgrown plants may be less sensitive to GAs. (7) found that a bush strain exhibited greater fresh weight of roots than an isogenic vine strain. No noticeable effect of the bush gene on root elongation was observed.We have described two monogenic dwarfs in watermelon, dw-l and dw-2 (18), the latter of which can be restored to normality with exogenously applied GA (17). In this paper we report on the response of dw-2 and normal seedlings to various concentrations of different gibberellins in terms of root and hypocotyl elongation, and demonstrate a unique inverse relationship of root to hypocotyl elongation in the dwarf seedlings. In addition, we have been able to distinguish anatomically, dark-enhanced versus GA-enhanced growth in dwarf and normal seedlings. MATERIALS AND METHODSPlant Material and Growing Conditions. Two strains of watermelon (Citrullus lanatus [Thunb.] Matsu.) were...

No abstract

Rates of cell proliferation were about 2.5 fold greater in gibberellic acid‐treated seedlings of dwarf watermelon than in untreated dwarf seedlings as determined by direct counts of pith cells in elongating hypocotyls of watermelon. The higher rates of cell proliferation observed in normal and GA‐treated dwarf seedlings were due both to shorter cell cycle times and increased growth fractions of cells than in dwarf seedlings. The shorter duration of the cell cycle in GA‐treated seedlings was primarily due to a reduced S period.

An inheritance study of 2 dwarf mutants in watermelon showed that dwarfness in each form is due to single, non-allelic, recessive genes designated dw-1 and dw-2. Dwarfness in dw-1 and dw-2 plants was primarily attributed to reduced length of internodes, but there was also some reduction in the number of internodes in the dwarfs compared to the normal plants. The shorter internodes in dw-1 dwarfs were due to both fewer and shorter cells than in the internodes of vine forms. In the dw-2 dwarf most of the reduction in internode length was due to fewer cells.

A study of vegetative growth and flowering was conducted on normal and 3 dwarf segregants, dw-1, dw-2 and dw-1:dw-2 (extreme dwarf) of watermelon [Citrullus lanatus (Thunb.) Matsumura and Nakai]. Con pared to normal segregants, stem elongation was reduced by about 50% in dw-1 segregants, 80% in dw-2 segre gants and 90% in extreme dwarfs. The primary effect of the dwarf genes was an additive reduction in internode length. The rate of leaf initiation was reduced in dw-2 and extreme dwarfs, but not in dw-1 dwarfs. Extreme and dw-2 dwarfs flowered at an earlier node number than either dw-1 or normal segregants. However, due to slower growth and abortion of early pistillate flowers, particularly in dw-2 and extreme dwarfs, the dwarf segregants displayed a temporal delay in flowering. Hence, the late-flowering characteristic of the dwarf segregants, especially in dw-2 and extreme dwarfs, imposes serious limitations on their usefulness for breeding early maturing dwarf cultivars.