TUBERIZATION IN HEAT TOLERANT HYBRID HT/92-621 UNDER CONTROLLED TEMPERATURE CONDITIONS

Hybrid HT/92-621, suitable for cultivation in peninsular and coastal parts of India as well as for early planting in north-western plains, has recently been recommended for release as the first heat tolerant variety of CPRI. Although the hybrid is able to tuberize under high night temperature conditions, yet this character has not been quantified in detail. Therefore, tuberization in leaf bud cuttings of this hybrid was studied along with Kufri Chandramukhi (control) at 24 and 27°C day temperature in combination with 18, 20, 22 and 24°C night temperatures. HT/92-621 was able to tuberize at all the night temperatures but good tuberization was observed up to 22°C night temperature, whereas, cv. Kufri Chandramukhi gave good tuberization at 18°C. The tuber fresh weight was higher at 24°C than 27°C day temperature in both the varieties. Traditionally potato has been classified as a cool climate crop with an optimal mean temperature of 17°C for good yield. High temperature induces development of plants with thin stems, longer inter-nodes, small leaves, and long stolons. High temperature also increases the rate of dark respiration in plants which doubles with every 10°C increase in temperature. As a result, more of the carbohydrates are used up as respiratory substrate. Higher temperatures reduce tuber yield by overall reduction of plant development due to heat stress or by reduced partitioning of assimilates to tubers (Nagarajan and Bansal, 1990). Minimum night temperature plays a crucial role during tuberization in potato and largely determines whether plants will tuberize or not. Tuberization is reduced at night temperatures above 20°C and there may not be any tuberization at 25°C and above. Exposure of potato plants to heat stress alters the hormonal balance in the plants. Gibberellins content increases in the shoots and stolons which promotes shoot growth and inhibits tuberization and bulking. Consequently, most of assimilated carbon is partitioned to above ground vegetative parts, at the cost of the tubers (Basu and Minhas, 1999). Potato plants show variation in their ability to form tubers at different night temperatures. Some of the genotypes are better able to tuberize at higher night temperatures compared to others. This feature was exploited to develop a screening technique for heat tolerance (Nagarajan and Minhas, 1995). Crosses were made among exotic heat tolerant and local high yielding cultivars and the progenies were screened for heat tolerance. The promising genotypes were exposed to heat stress during early (September) planting conditions at Modipuram. Based on the results of three years of yield trials, hybrid HT/92-621 was found to be most promising. It was included in All India Co-ordinated Potato Improvement Project during 2000 and was tested at 5 locations in peninsular India for three years. During 2003 this hybrid was recommended for release as a heat tolerant variety (Minhas et al., 2003). Although hybrid HT/92-621 gives significantly higher yield than other cultivars under higher temperature conditions, yet the heat tolerance, in terms of its ability to tuberize at higher night temperatures, has not been quantified. Therefore, two experiments were conducted to evaluate the tuberization behaviour of HT/92-621 along with control cultivar Kufri Chandramukhi at (i) 27°C day temperature and 18, 20, 22, 23, 24 and 25°C night temperatures and, (ii) 24°C day temperature and 18, 20, 22 and 24°C night temperatures. Both these experiments were conducted in growth chambers (Conviron, Model E-15, Canada). The day length was 14 h for both the experiments. Potato plants were grown in growth chambers for 35-40 days at 24°C day/night temperature and 24 h photoperiod (non-tuber inducing conditions), so that no tuberization stimulus was formed. Single node cuttings were taken from 4 th to 7 th node. Twenty cuttings (4 replications of 5 cuttings) of each genotype were used for each temperature treatment. Each cutting was planted in sand filled plastic cups. Cuttings were exposed to temperature treatments for 21 days, after which cuttings were checked for axillary tuber formation. Observations were taken on the number of cuttings bearing tubers, number of tubers formed, whether tubers were sessile or on stolon, tuber diameter, tuber fresh weight and length of stolons.

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