Cotton fibers (Gossypium hirsutum L.) developing in vitro responded to cyclic temperature change similarly to those of fieldgrown plants under diumal temperature fluctuations. Absolute temperatures and rates of temperature change were similar under both conditions. In vitro fibers exhibited a "growth ring" for each time the temperature cycled to 22 or 150C. Rings were rarely detected when the low point was 280C. The rings seemed to correspond to alternating regions of high and low cellulose accumulation. Fibers developed in vitro under 34°C/280C cycling developed similarly to constant 340C controls, but 34°C/220C and 340C/15°C cycling caused delayed onset and prolonged periods of elongation and secondary wall thickening. Control fiber length and weight were finally achieved under 34°C/220C cycling, but both parameters were reduced at the end of the experiment under 34°C/150C cycling. Fibers developed under all condifions had equal bundle tensile strength. These results demonstrate that: (a) cool temperature effects on fiber development are at least partly fiber/ovule-specific events; they do not depend on whole-plant physiology; and (b) cultured ovules are valid models for research on the regulation of the field cool temperature response.The cotton fiber (Gossypium hirsutum L.) is an elongated epidermal cell of the cotton ovule with a thickened secondary wall composed of almost pure cellulose. Its development is characterized by two overlapping phases, primary wall synthesis to accomplish fiber elongation and secondary wall synthesis to accomplish fiber thickening (25). It has been known for decades that field-grown fibers exposed to cool temperatures (generally at night) have prolonged periods and reduced overall rates of elongation and thickening (9,10,14,28) and "growth rings" in their secondary walls (2,18 vidual fibers (1 1). However, little research has been directed toward determining the mechanism of this response despite its adverse economic consequences (8) and fundamental importance to understanding the regulation of cell wall deposition.The temperatures that affect wall deposition (e.g. 22°C) are well above those typically associated with chilling injury in plants, including cotton (12, 24), suggesting that a particular temperature-sensitive step might be identifiable. Differences between existing cultivars (9-11, 14, 28) shows that the cool temperature response has a manipulable genetic component. Ectothermic organisms such as plants are precisely adapted to regulate metabolism optimally at the temperatures normally encountered (16). There is evidence that plants are biochemically adapted to have relatively narrow temperature optima for certain enzymes, so that metabolism may not necessarily have a simple linear relationship with temperature (5). Since cotton evolved under hot subtropical temperatures, lowering the optimum temperature for certain processes might be possible if key regulatory genes could be identified, isolated from another organism with a lower temperature optimum, and tran...

The effects of temperature on rates of cellulose synthesis, respiration, and long-term glucose uptake were investigated using cultured cotton ovules (Gossypium hirsutum L. cv Acala Si1). Ovules were cultured either at constant 340C or under cycling temperatures (12 h at 34'C/12 h at 15-400C). Rates of respiration and cellulose synthesis at various temperatures were determined on day 21 during the stage of secondary wall synthesis by feeding cultured ovules with [14C]glucose. Respiration increased between 18 and approximately 34C, then remained constant up to 40"C. In contrast, the rate of cellulose synthesis increased above 18 C, reached a plateau between about 28 and 37 C, and then decreased at 40'C. Therefore, the optimum temperature for rapid and metabolically efficient cellulose synthesis in Acala S11 is near 28'C. In ovules cycled to 150C, respiration recovered to the control rate immediately upon rewarming to 340C, but the rate of cellulose synthesis did not fully recover for several hours. These data indicate that cellulose synthesis and respiration respond differently to cool temperatures. The long-term uptake of glucose, which is the carbon source in the culture medium, increased as the low temperature in the cycle increased between 15 and 28 C. However, glucose uptake did not increase in cultures grown constantly at 340C compared to those cycled at 34/280C. These observations are consistent with previous observations on the responses of fiber elongation and weight gain to cycling temperatures in vitro and in the field.The cotton fiber is a single elongated epidermal cell of ovules of Gossypium species. Fiber development is typically divided into two stages, primary wall formation (to accomplish elongation) and secondary wall synthesis ( Current address: University of Iowa, Ames, IA. lose in the secondary wall that is required for fiber maturation (10, 17), and temperatures less than 250C are low enough to induce resistance to the deleterious effects of subsequent exposure to 50C in cotton seedlings (20). The research reported here is focused on the period of secondary wall deposition because of the adverse effect of cool night temperatures in northern cotton growing regions on fiber cell wall thickening, which is a major determinant of crop quality and value (10). Although general mechanisms explaining the inhibitory effects of low, nonfreezing temperatures on plant growth have been suggested (14), specific mechanisms by which cellulose synthesis in cotton is decreased are not known. Nor is it known if cellulose synthesis is affected similarly or differently than overall metabolism. One or more factors required for cellulose synthesis could be disrupted by cool temperatures, including (a) production, transport, or uptake of substrate (7, 9); (b) provision of sufficient energy through respiration; (c) function of enzymes due to membrane phase changes (18, 27) or direct kinetic effects (13); and (d) function of the endomembrane system and cytoskeletal elements.One goal of our research is to un...

Growth kinetics and levels of auxin substances were studied in three cotton cultivars, designated as long, medium and short staple cultivars. Fibre length and dry weight plotted against boll age showed sigmoidal patterns and were fitted to a logistic curve by computer curvilinear regression analysis. The final length of the fibre in different cultivars was the product of the rate of elongation per day and the total period of elongation. Further, considerable overlap between the elongation and the secondary thickening phases was recorded. No relationship between auxin substances and rate of fibre elongation was discernible. The peak levels of auxin substances in all the cultivars were recorded before or about the time when elongation had just started, and it is concluded that the auxin synthesized during the elongation phase is consumed in elongation growth. Thus there is necessarily no relationship between remaining auxin and growth.

Summary Potassium requirements for growth --dry matter (DM) and leaf area (LA) and related processes --relative leaf growth rate (RLGR), relative growth rate (RGR), net assimilation rate (NAR) and crop growth rate (CGR) were determined by plant analysis during the entogeny of wheat. Wheat (Triticum aestivum cv. HD 2329) plants were supplied with different amounts of K from deficient to adequate through nutrient solution. Samples were taken at specific stages for K determinations. The DM and LA were recorded at 45d, 75d and 105d. The growth related processes RGR, NAR and CGR were estimated between 30-45d, 45-75d and 75-105d. In case of RLGR the observations were carried out between 15-30d, 30--45d and 45-75d. These physiological processes and grain yield were correlated with K concentration in whole plant at 30 and 45d and top two leaves at 75 and 105d.The results indicated that K status in plants influences growth mostly through leaf area formation which inturn influences successively RLGR, RGR and CGR and finally grain yield. For vegetative growth the optimum concentration required in plants was always lower than the optimum for grain production.

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