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The nature and the magnitude of genetic divergence were estimated in 52 rice genotypes using Mahalonobis’s D2 – statistics. The genotypes were grouped into 8 clusters showing fair degree of relationship between geographic distribution and genetic divergence. Cluster VIII showed maximum intra cluster divergence while inter cluster divergence was maximum between clusters IV and VIII. All the minimum and maximum cluster mean values were distributed in relatively distant clusters. Traits like plant height followed by days to maturity, panicle exertion, days to 50% flowering and panicle length were the major contributors to genetic divergence.
Genetic divergence, cold tolerance, rice, inter cluster distance
Rice occupies 44.80 million ha area with 99.180 million tones production which is 42.4% of total food grains and productivity of 2214 kg/ha, respectively in India. In Andhra Pradesh it is grown in an area of 43.75 lakh ha with a production of 14.21 lakh tones and productivity of 3248 kg/ha. Rice yields are low in Telangana region during Rabi season. In Telangana region the low temperature vary from 8oC to 16oC starting from December to first fortnight of February. The optimum day temperature before heading is 31o-32oC and optimum night temperature is 21o-22oC and in the period for 15 days after heading vary from 8o to 16oC . These critical temperatures vary with genotype, duration of critical temperature, diurnal changes and physiological status of the plant. Rice might suffer from low temperature at the seedling stage of the crop  Even low temperature at the crop establishment and tiller development stage (vegetative phase) of the crop might affect the growth and development of the crop. Conventional high yielding varieties have little tolerance to low temperature. The major constraint is lack of suitable varieties for this ecosystem. The development of stable rice varieties for this fragile ecosystem should be the major thrust in present situation. The estimate of genetic divergence in the available germ plasm is important for the selection of desirable donors for breeding programme. Hence the present study was conducted to achieve objectives to screen genotypes with good phenotypic acceptability and good emergence index with better tolerance to low or minimum temperature and to have a preliminary understanding about the nature of tolerance of genotypes with respect to low or minimum temperature. Several workers have emphasized the importance of genetic divergence for the selection of desirable parents [3, 4] Therefore, the magnitude of genetic divergence and characters contributing to the genetic divergence were studied for cold tolerance.
In the present investigation, it is suggested that hybridization programme within the divergent cluster V and VIII are expected to give promising and desirable recombinants in the segregating generations. Also, traits contributing to the maximum genetic divergence viz. plant height followed by days to maturity, panicle exsertion, days to 50% flowering and panicle length may be utilized in selecting genetically diverse parent.
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