The Use of Seed Germination Rate as an Indicator of Heat Tolerance in Spinach (Spinacia oleracea L.)

Spinach is a cool season crop with very specific optimum temperatures requirements. Heat tolerant cultivars would allow production to be improved so that spinach may be grown in early summer or as a year-round vegetable crop in greenhouses or high tunnels. Breeding for heat tolerant varieties requires that heat tolerance can be assessed efficiently, and seed germination under heat stress could potentially serve as a screening tool. The objective of this study is to determine whether germination rate could be used to indicate and screen for heat tolerance. Two experiments are conducted in this research in growth chamber conditions. In experiment 1 of seed germination test, four cultivars are evaluated for seed germination at 20 °C (the optimum temperature for spinach germination) and at 35 °C (the temperature for heat stress) using a randomized complete-block design (RCBD) with three replicates. Seeds are pretreated for surface sterilization and then placed in petri dishes with moistened blotter paper. To avoid water loss, dishes are placed in plastic zip closure bags. Germinated seeds are counted and removed on 7-day intervals starting the seventh day after germination and continuing through 21 days. Seed germination is recorded and percentage is calculated. Based on experiment 1, experiment 2 incorporates the seed germination test into the whole spinach plant growth. Seeds are sown in trays filled with pre-moistened Sun Gro Horticulture Sunshine Mix #1 and allowed to germinate for 10 days at 20 °C. Then, the seedling group for heat stress is transferred to 35 °C while the control group is maintained at 20 °C. Plants are grown for an additional 20 days. Shoot height, leaf number, and the dry weight of the biomass are used for evaluating growth. Germination rate is considered a predictor of heat tolerance if the performance of the cultivars in experiment 1 of the seed germination test is analogous to those in the experiment 2 of the whole plant test.