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Cotton is generally regarded as a crop of the hot, semiâ€arid regions of the world, but is also an important crop in aridâ€irrigated regions and extends to semiâ€humid tropics. Cultivars are needed that can endure and recover from heat stress so as to minimize yield loss in hot, semiâ€arid areas and to reduce the water needed in irrigated production. An understanding of the response of cultivars to heat stress is also important in modeling cotton growth. Several studies have shown that high temperature affects seedling, vegetative, and reproductive stages, including yield and fiber quality of cotton.
Optimum temperatures for seed germination and seedling development of cotton range from 28 to 30_C. The base temperature for seed germination is near 12_C, while that for growth is about 15.5_C. Cool temperatures during germination and initial growth are a problem in several locations in the United States particularly across the Mississippi Delta region. Genotypic diVerences for germination and root development under cool soil temperatures have been observed (Mills et al., 2005). The optimum range of temperature (day/night) for cotton root growth is 30/22-35/27_C, and high temperatures (40/32_C) altered the distribution of roots, causing shallower roots, even under optimum water and nutrient conditions (Reddy et al. , 1997 b,c)â€¦. Burke (2001) reported that seedling heat tolerance is essential in most dryland cotton production areas because producers plant cotton when moisture becomes available. Similarly in North India, the soil temperature and wind velocity at sowing time are very high, resulting in rapid loss of soil moisture (Lather et al., 2001). Under these conditions, emerging cotton seedlings have poorly developed root system, with a primary tap root and the beginnings of lateral root development. Burke (2001) observed that when seedling temperature increases above optimal levels, acquired thermotolerance system is induced. Maximum protection levels are induced when plant temperature reaches 37.7-40_C, but at higher temperatures protection levels decline rapidly. McMichael and Burke (1994) found that the diVerences in the temperature optima appear to be associated with dynamic changes in seedling development, which may be related to changes in stored seed reserves. Numerous functions of roots, including uptake of nutrients and water, assimilation and synthesis of metabolites, and translocation, are very sensitive to temperature. Root temperature may be more critical than shoot temperatures for plant growth because roots have a lower temperature optimum and are less adaptable to extreme fluctuations (Nielsen, 1974). Synthesis of cytokinins which originate predominantly in roots is
among the most sensitive processes (Paulsen, 1994).
Leaf area development is highly sensitive to temperature. Optimum temperature e for leaf area developm ent is 26 _ C (Reddy et al. , 1992a ,b). Leaf expansion in cotton occurs at a greater rate in the dark than in the light (Krieg, 1981). At 20 days after emergence (DAE) the leaf area of plants grown at 28_C was found to be six times more than that of plants grown at 21 _ C (Reddy et al. , 1997b ,c). Reddy et al. (1992a ) report ed that in uplan d cotton main stem elongation, leaf area expansion, and biomass accumulation rates were very sensitive to temperature at about 21 DAE. The temperature optimum for stem elongation, leaf area expansion, and biomass accumulation was 30/22_C. Development rates, as depicted by number of main stem nodes produced, number of fruiting branches and fruiting branch nodes were not as sensitive to temperatures above 30/22_C as were growth rates. The length of fruiting branches increased as temperature increased to 30/22_C and then decreased about 25% among plants grown at the two higher temperatures (35/27 and 40/32_C), due to shortening of branch internodes. Growth of fruiting branch length responded to temperature in a similar fashion to main stem elongation and to fruiting branches produced when temperature treatments were imposed at first flower (Reddy et al., 1990). Heitholt (1994) speculated that extremely high air temperatures from 31 to 44 days after planting, which reached 34_C or greater each day, reduced canopy growth. In other studies, Reddy et al. (1991b, 1992a,b) observed that both upland and Pima cotton main stem elongation rates and node development rates responded significantly to temperature. The temperature optimum for fruiting branch growth, square and boll production, and retention was 30/22_C. Above 30/22_C, average fruiting branch length was less and square initiation was completely inhibited at 40/32_C, while vegetative branch length kep t increa sing up to 40/22 _ C (Reddy et al. , 1992a ,c). In Ind ia, Sikka an d Dastur (1960) gave the optimum range for vegetative growth of Asiatic cotton as 21-27_C and cool nights are needed for the best results, but given good moisture conditions the plant can stand temperatures even as high as 43-46_C.
Flowering intervals on vertical and horizontal branches are influenced by temperature (Munro and Farbrother, 1969; Reddy et al., 1997c). Mauney
(1966) found similar relationships between temperature and flowering interval. Farbrother (1961) found that the horizontal flowering interval was approximately 11 days in Uganda, where temperatures in the field are fairly uniform throughout the year. Ehlig and LeMert (1973) observed that the number of flowers per meter of row declined approximately 3 weeks after periods when the maximum temperature exceeded 42_C. Heat stress during flowering resulted in square and flower drop when day temperatures exceeded 30_C (Reddy et al., 1992c). At day temperatures above 40_C, all the squares and flowers were aborted and dropped in several upland cotton cultivars (Reddy et al., 1991a). Pima cotton was more sensitive to high temperature than upland cotton and some of the Pima cotton varieties failed to produce fruiting branches and reproductive sites when the average daily temperature was 36_C (Reddy et al., 1995a, 1997c, 2004, 2005). Although upland type cotton did produce fruiting branches and formed squares at high temperature, it did not successfully produce bolls (Reddy et al., 1991b, 1992a).
Highâ€temperature stress prior to and during flowering significantly influences several reproductive processes leading to decreased fruit set in cotton. Oosterhuis (1999) observed that high temperature could lead to
decreased pollen viability and fertilization and this eVect usually occurred approximately 17 days before flowering. Similar observations were previously reported (McDonald and Stith, 1972; Meyer, 1969; Powell, 1969; Sarvella,
1966). High temperature of 32_C at 15-17 days before anthesis caused pollen sterility in temperatureâ€sensitive male sterile lines. Even fertile lines begin to show sterile anthers when temperatures were above 38_C (Meyer, 1969). The exact stage of development at which the sensitivity occurs is not known; howeve r, based on the tim escale of Sarvel la (1964) or Quintan ilha et al. (1962) it occurs after, rather than during, meiosis.
Rawson (1992) and Ziska et al. (1997) demonstrated that higher temperatures could accelerate crop development and reduce the time during which carbon (dry matter) is gained. Hodges et al. (1993) observed that most of the shortening of development time occurs during the boll growth period, resulting in smaller bolls, lower yields, and poor quality lint. At high temperature, crop developmental rate will proceed at much faster rate. Accordingly, the time required to produce squares, flowers, and mature fruits was reduced by an average of 1.6, 3.1, and 6.9 day _C_1 of increased temperature, respectively (Reddy et al., 1997c). Furthermore, assuming that temperature increase will be equally distributed throughout the growing season, a 5_C increase in average global temperature should speed development from emergence to maturity by 35 days (Reddy et al., 1997a,b). Therefore, high temperatures can have a detrimental eVect on boll development. Stockton and Walhood (1960) found that boll size and fiber length decreased with increasing temperatures.
Brown et al. (2003) proposed that environmental stresses, particularly water deficit, and temperature stress were mainly responsible for yearâ€toâ€year variability in cotton yield (Lewis, 2000). Oosterhuis (2002) observed that high temperature during day, followed by high night temperatures,might exacerbate this detrimental eVect and provide an important cause of yield variability. Johnson and Wadleigh (1939) reported increases in yields with increases in July average maximum temperature up to 35_C and decreases in yields as the July average maximum temperature exceeded 35_C. There was a strong negative correlation between high temperature and cotton yield in Arkansas (Oosterhuis, 2002). Haig ler et al. (2005) report ed that in cen tral and south Tex as, high tempe ratur e cou pled with water stre ss during boll filling resulted in relatively short fibe rs wi th high micr onaire (increase rough ness). Lint index, lint percentage, and lint per boll were decreased by either high (37_C) or low (13_C) night temperatures (Gipson and Ray, 1976).
It is evident from literature that growth of cotton is highly influenced by temperature. Studies by Jackson (1967) in Sudan (Northeast Africa) revealed that relative growth rate (RGR) and net assimilation rate (NAR) increased with increasing temperature during August through midâ€October, but subsequent decrease in temperature decreased NAR. Rajan et al. (1973) studied the impact of temperature on growth components at the seedling stage within the range of 10-35_C and showed that NAR, leaf area index (LAI), and leaf area ratio (LAR) increased with increasing temperature. Studies by Singh et al. (1987) showed that increasing temperature decreased crop growth rate (CGR) and mean LAI, but improved NAR, specific leaf weight (SLW), and leaf weight ratio (LWR). It appears that if temperature regimes(s) experienced by the crop were supraoptimal it decreased leaf area and biomass production.
At high temperature, severe cellular injury and even cell death may occur, which could be attributed to a catastrophic collapse of cellular organization (Schafft et al., 1999). At moderately high temperatures, injuries or death may occur only after long-term exposure. Direct injuries due to high temperatures include protein denaturation and aggregation and increased fluidity of membrane lipids.
Relative cell injury level from leaf disks at high temperature has been suggested as a screening technique for heat tolerance in plants (Sullivan, 1972). Several studies suggested the effectiveness of cell membrane thermo stability in terms of relative cell injury level in detecting genetic variability in heat tolerance in warm season crops (Ismail & Hall, 1999). This technique is simpler, quicker and less expensive than the whole plant screen. Potentially it can be used with early vegetative stage leaf tissue from plants grown in field nursery environments (Ismail & Hall, 1999).