Abstract: The study was conducted to assess the chilling effect on germination, hypocotyl and root length of two cotton cultivars. Hundred seeds of each Gossypium arboreum A2 genome (G1) and Gossypium hirsutum AD-1 (G2) were chilled at 0 °C for 0, 24, 48, 72 and 96 h and then placed in incubator at 32°C. Germination percentage significantly decreased with the increasing chilling period. However, the effect was pronounced on G1 than G2. The highest hypocotyl length after 96 h of chilling treatment was 64 mm for G1 and 91 mm for G2, under 48 h of chilling treatment. Root lengths of both varieties were maximum under the treatment of 24 h of chilling after 96 h. On the basis of germination percentage, hypocotyl and root length, it may be inferred that G2 was more cold tolerant variety than G1.
Introduction
Cotton is a major fiber crop and plays an important role in the economy of Pakistan. During 2001-2002, it was cultivated on 0.31 m ha with the total production of 10.9 million bales (Anonymous, 2000-2001). It also provides raw material to domestic industry comprising textile mills, ginning factories and oil expelling units.
Germination of cotton is sensitive to temperature (Baloch et al., 1999; Ahmad, 1999). The chilling effect is more pronounced in cotton than other crop species. Chilling injury occurs in cotton when the temperature drops below 15°C for a few hours during the first few days of germination (Lauterbach et al., 1999). Cotton seed is not uniformly sensitive to chilling at all stages of germination. In addition to immediate effect on emergence rate and stand development, chilling may also alter growth, fruiting pattern and yield (Edmisten, 2001). Cotton species is ranked as excellent, good, fair and poor cold tolerance showing 80, 70-80, 50 and below 50 imbibitional and metabolic emergence percents, respectively (Duesterhaus et al., 1999). Several cotton varieties with enhanced germination and seedling growth at low temperature have been previously identified (James et al., 2002). They compared different cotton varieties to low temperature sensitive varieties commonly grown on the Southern High Plains of Texas. They reported that antioxidants (for example glutathione) are responsible for cold tolerance. However, Barry et al. (2002) reported that enhanced chilling tolerance that has been demonstrated during short term laboratory treatments of unacclimated transgenic plants over expressing antioxidants may not be indicative of their performance in the field. Dmytro (2002) compared three transgenic lines with wild types against temperature less than 15°C. The results affirm the important role of increased photochemistry in PS II photo-protection observed in the transgenic plants. Cold tolerant cotton genotypes would allow earlier planting, thus producing more profit from reduction in sowing rates and obtaining greater yields of high quality fiber (Schulze et al.,1997). Keeping in view the facts that different cultivars respond differently towards chilling stress, it is imperative to quantify the effects of cold injury on growth parameters of cotton. Therefore, the specific objective of this study was to evaluate the chilling effect on germination and hypocotyl and root length of two cotton cultivars.
Materials and Methods
The laboratory study was conducted in the Cytogenetics Section, Central Cotton Research Institute, Multan during 2001-2002. Hundred acid delinted and sterilized seeds of two cotton cultivars, Gossypium arboreum A2 (G1) and Gossypium hirsutum AD-1 (G2) were chilled at 0°C for 0, 24, 48, 72 and 96 h. Twenty seeds of each cultivar were put in sterilized glass petridishes. Four sets of seeds were placed in the refrigerator. Treated and untreated seeds were put on moistened filter paper in rows, each containing ten seeds. After placing seeds the filter papers were rolled up and put them in sterilized glass beakers containing some amount of distilled water at the bottom for the germination and growth process. Then the seeds were incubated in dark at 32°C. The experiment was conducted in aseptic condition. The observations regarding germination, hypocotyl and root length were recorded after every 24 h till 96 h. Root length was measured following the techniques of Tennant (1975). Data were statistically analyzed by Steel and Torrie (1980). Germination percentage was calculated by the formula of Ahmad (1999).
Results and Discussion
Germination percentage: The increasing chilling period significantly decreased the germination of both the varieties (Table 1). However, the effect was more pronounced on G1 than G2 indicating that G2 is more cold tolerant than G1. Under all treatments the germination was higher in case of G2. Maximum germination (80%) of G2 can be attained under chilling period of 24 h. Edmisten (2001) reported that temperature below 40°C retarted the growth of seedlings. However, James et al. (2002) reported enhanced germination at low temperature. This may be due to presence the antioxidants pools (ascorbate and glutathione) enzymes which are responsible to enhance the germination in cold tolerant cotton varieties. Similar results of low germination under low temperatures were reported by others (Ahmad, 19990; Schulz et al., 1997; Baloch et al., 1999).
Hypocotyl length: The chilling period decreased the hypocotyl length for both cultivars (Table 2). The length of hypocotyl increased with the increasing interval of measurement from 24 and 96 h. Maximum hypocotyl length of G1 was 64 mm under the treatment of 24 h of chilling after 96 h while minimum length was 22 mm under the highest level of chilling (96 h) with 49% decrease in hypocotyl length as compared to control (Table 3). Maximum hypocotyl length of G2 was 91 mm under the treatment of 48 h of chilling after 96 h and minimum length was 34 mm under highest chilling effect of 96 h, with 37% decrease in hypocotyl length as compared to control.
| Table 1: | Effect of chilling for different periods on germination percentage of two cotton cultivars |
| Figures bearing the same letter(s) in the rows are not statistically different at P<0.05 | |
Edmisten (2001) reported that cotton is sensitive to temperatures below 10°C when it is absorbing water to begin germination. According to temperature below 10°C caused growth retardation for weeks into the season. Baloch et al. (1999) also reported that chilling retarted growth of cotton. However some varieties of cotton (G. hirsutum L.) may prove under chilling conditions. Similar results were reported by Schulz et al. (1997) and Duesterhaus et al. (1999). The mechanisms responsible to cold tolerance were proposed by various researchers. Dmytro et al. (2002) reported that antioxidant enzymes were responsible for chilling tolerance.
Root length: The chilling period also decreased the root length of both cultivars (Table 3). In contrast to hypocotyl, root length was increased with the increasing interval of measurement from 24 to 96 h. Under the treatment of 24 h of chilling after 96 h, the root length was maximum (108 mm) for G1 but under the highest level of chilling it was minimum (79 mm) with 14% less over control. While for G2, maximum (111 mm) root length was obtained under the treatment of 48 h of chilling and minimum (91 mm) under the highest level of chilling with 11% decrease as compared to control.
| Table 2: | Effect of chilling for different periods on hypocotyl length of two cotton cultivars |
| Table 3: | Effect of chilling for different periods on root length of two cotton cultivars |
| Figures bearing the same letter(s) in the rows are not statistically different at P<0.05 | |
Results obtained from the experiments conducted in North Carolina, USA showed that germination, growth and the root development were badly affected when that soil temperature dropped to less than 10°C (Edmisten, 2001).
Brown (2000) reported that low soil temperature adversely affected stand establishment, altered normal root development and caused cell damage that rendered seedling more susceptible to seedling diseases. Ahmad (1999) summarized the results of experiments conducted at CCRI, Multan and concluded that at low temperatures the root development of cotton is reduced. However, among the cotton varieties, G. hirsutum L. was found relatively more cold tolerant than others. Similar results were reported by others (Schulze, 1997; Baloch et al., 1999).
In conclusion, data regarding the germination percentage, hypocotyl and root length in G2 was ranked more cold tolerant than G1 because G2 is inter-specific hybridization of G1 and wild species of D. genome. Specific characters have been transferred from wild species to G2.