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Water that may contain salt often is used to cool soil to help overcome high-temperature inhibition of lettuce germination. This study was done to determine how lettuce cultivars differ in their germination response to high temperature and NaCl. Ten lettuce (Lactuca sativa L.) cultivars (Grand Rapids, Climax, Coolguard, Empire, Great Lakes 659-700, Mesa 659, Salinas, Vanguard 75, Red Coach 74, and Wintersupreme) were germinated at 20, 25, 30, or 35C with 0.0, - 0.3, - 0.6, - 0.9, - 1.2, or - 1.5 MPa NaCl. With no NaCl, germination percentages and rates decreased significantly at 35C for all cultivars except `Salinas', which decreased at 30C. With higher concentrations of NaCl, decreases in germination percentages and rates were observed at lower temperatures. Cultivar differences in response to temperature were present with no NaCl but became larger in the presence of NaCl. `Great Lakes 659-700' and `Mesa 659' were most sensitive to high temperature and salt. `Coolguard' and `Empire' were most tolerant to high temperature and salt, with some tolerance also present in `Grand Rapids' and `Vanguard 75'. Relative tolerance of cultivars to NaCl as shown by germination percentages and rates was consistent with growth of roots.
Abstract
Seed of 11 phenotypes of tomato (Lycopersicon esculentum Mill.) were germinated in petri dishes at a constant 25, 30, 35, or 40C or at 12-hr alternating temperatures of 40/25, 40/30, and 40/35C. At constant high temperatures, the greatest differences between phenotypes occurred at 35C, based on percentage germination, but at 30C, based on germination rates. Phenotypes that germinated best at a constant 40C were ‘Nema 1200’, P28693, and ‘UC-82-L’. Phenotypes that germinated best at 40/25C were P28693, P28793, and ‘UC-82-L’. Germination increased greatly when temperatures were lowered by 10 to 15C for 12 hr per 24 hr, compared to a constant 40C. Tetrazolium tests showed that a high percentage of seeds that had not germinated after 14 days of imbibition at 35 or 40C was still viable, whereas, at the lower temperatures, fewer nongerminated seeds were viable.
Lettuce (Lactuca sativa) is one of the most important U.S. fresh-market vegetables with year-round consumption. For winter markets, lettuce is produced in the southwestern states with plantings in early fall when soil temperatures are high. Seed germination of lettuce, however, is inhibited by soil temperatures over 25 °C. The objective of this study was to test the emergence and growth of five lettuce cultivars using two seedlots produced in winter or summer to provide information for improving stand establishment at high temperatures. Seeds of five cultivars (`Empire', `Parris Island Cos', `Waldmann's Green', `Prizehead', and `Dark Green Boston') produced in Yuma, Ariz., during summer or winter months were used. Seeds were planted in a greenhouse mix in plastic trays and grown in a growth chamber at 23, 25, 30 and 35 °C. After 4 weeks, number of emerged plants, number of leaves, height, fresh weight, dry weight, and leaf area were measured. At lower temperatures (23 and 25 °C), more plants emerged and plants developed more rapidly than at higher temperatures (30 and 35 °C). More plants emerged of `Empire' and `Parris Island Cos' than of `Waldmann's Green' or `Dark Green Boston'. Growth varied greatly for the different cultivars. At 35 °C, only `Empire' winter seed emerged. At other temperatures, summer seed lots generally were better than or equal to winter seedlots. This information suggests that seeds developed during the summer are more vigorous at emergence than those developed during the winter.