Degree Name

Master of Science (MS)

Semester of Degree Completion

1991

Thesis Director

Janice Coons

Abstract

Lettuce (Lactuca sativa L.) seed germination is inhibited when temperatures are higher than 25-30 C. The extent of this thermodormancy is affected by both genetic and environmental factors. Lettuce seed of four seedlots of "Empire" produced in the summers and winters of two years were sown in the field in late summer and early fall at Yuma, AZ to determine stand establishment with high soil temperatures. The seedlots produced in the summer had higher percent field emergence than those produced in the winter. Also, seed of these four seedlots were germinated in petri dishes in a growth chamber at 20, 25, 30 or 35 C to determine germination percentage and germination rate. Summer seedlots had significantly higher germination percentages and rates than winter seedlots within a temperature. As temperature was raised, significant decreases for the summer seedlots occurred at 35 C and significant decreases for the winter seedlots occurred at 25 or 30 C.

Lettuce seeds from the four seedlots of "Empire" and from six cultivars that varied in thermotolerance during previous studies also were germinated in seed envelopes to determine root length and rate of root elongation at 20, 25, 30 or 35 C. Summer seedlots had significantly higher root lengths and rates of root elongation than winter seedlots at 35 C. The tolerant cultivars, PI 251245 and Empire, had significantly higher root lengths and rates of root elongation than the sensitive cultivars, Severa and Dabora, at 30 and 35 C.

Electrical conductivity of seed leachates from seeds of the four seedlots and six cultivars was measured at 20, 25, 30 or 35 C with no significant interaction between seedlot and temperature. No significant difference in percent ion leakage between seedlots was found but the temperature effect was significant. No significant interaction between cultivar and temperature was found but the temperature effect was significant. At all four temperatures one of the intermediate cultivars (Salinas) had significantly higher percent ion leakage than one of the sensitive (Dabora) and one of the tolerant (PI 251245) cultivars.

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