Consequences of chilling stress on watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] germplasm lines at seedling stage

A total of sixty one germplasm lines of watermelon were selected based on differences in fruit size. First year (2012), the young seedlings were exposed to low temperature stress 20/10 ± 2°C (day/night) for 15/9 hours (day/night, 21:00 to 06:00 HR) inside the growth chamber for 1-week, then second stress treatment 10/5 ± 2°C was given, and seedlings were maintained at this for continuous 10 days. The control seedlings were throughout maintained at 28 ± 2°C till the end of experiment. Thirteen best performing germ plasmlines were selected based on different growth and physiological traits. Second year (2013), these young seedling were subjected to chilling stress (5/5 ± 2°C, day/night) for 15/9 hours (day/night, 21:00 to 06:00 HR). The seedlings were raised in plug trays filled with commercial soil mixture and kept moist with Hoagland nutrient solution on daily basis. Based on the present experimental findings, 10102, 10124, 10398, and 10491 were selected as best performing germplasm lines for more most of traits (shoot dry weight per plant, stem length per plant, leaf number per plant, root length per plant) studied in chilling stress, and also maintained high cellular membrane integrity, chlorophyll fluorescence, protein content, and peroxidase activity. Hence, chilling stress 5°C was considered to be as better critical range for facile means of selecting tolerant and sensitive germplasm lines at young seedling stage in watermelon based on different growth parameters studied.