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Heat-tolerant and -sensitive Lycopersicon esculentum Mill. and L. pimpinellifolium (Jusl.) Mill. genotypes were grown in the greenhouse under optimum- (27/23C, day/night) and high-temperature (35/23C) stress regimes. Heat tolerance levels in the genotypes were established by determining percent fruit set at high and optimum temperatures. Under optimum temperature, fruit set ranged from 41% to 84% and from 45% to 91% in the heat-sensitive and heat-tolerant genotypes, respectively. Under high temperature, no fruit set in the most heat-sensitive genotypes. Fruit set in the heat-tolerant genotypes ranged from 45% to 65%. In vitro germination and tube growth of pollen taken from genotypes grown under optimum temperature conditions were determined before and after subject- ing the pollen to 45C for 1, 2, and 4 hours. The response of pollen to heat treatments was genotype dependent and not a general predictor of fruit set under high-temperature stress. 0-0-43-8-1 and AVRDC-CLN-475BC1F2-265- 4-19 obtained from the Asian Vegetable Re- search and Development Center, Taipei, Tai- wan. The L. pimpinellifolium accessions tested were plant introductions (PI) 270450, 270441, 365914, 205009, 365916, and 365917 ob- tained from the U.S. Dept. of Agriculture, Plant Introduction Station, Geneva, N.Y. Seeds were planted in 72-cell flats (cell size, 4 × 4 × 6 cm) filled with a 1 peat : 1 horticultural grade vermiculite (w/w) mixture (Jiffy Mix Plus; Jiffy Products, Batavia, Ill.) and were grown in the greenhouse under a 27/23 ± 2C day/night cycle. At the two true-leaf stage, five plants of each genotype were transferred to a green- house and maintained under a heat-stress re- gime of 35/23 ± 2C (day/night). A second set of five plants each was maintained in another greenhouse under optimum temperatures of 27/23 ± 2C (day/night). These two sets of plants received identical management prac- tices (fertilizing, watering, staking, and daily mechanical flower pollination) for growing tomatoes commercially in the greenhouse. Percent fruit set was determined by scoring flower counts on all inflorescences present and fruit produced until at least six inflores- cences had set fruit. A fruit was considered set when its diameter reached ≥0.5 cm (Abdul- Baki, 1991; Shelby et al., 1978; Weaver and Timm, 1989). Plants grown under our optimum tempera- ture conditions served as a pollen source in the in vitro tests for determining the effect of heat exposure on pollen germination and pollen tube growth. In vitro germination and pollen tube growth from each genotype were deter- mined as described by Abdul-Baki (1992), using pollen samples collected two to three times a week from the first five to six inflores- cences. The pollen was removed from the flowers by mechanical vibration and mixed; eight 0.3-mg subsamples were placed in mois- ture-tight, 35-mm petri dishes and were sub- Heat stress adversely affects the vegetative and reproductive growth processes of tomato plants and ultimately reduces yield and fruit quality (Abdul-Baki, 1991; Dane et al., 1991; Wessel-Beaver and Scott, 1992; Yakir et al., 1984). Development of tomato cultivars with improved fruit set under high temperatures would be valuable for tomato crop production in regions where the temperature during part of the growing season reaches ≥35C or higher (Johnson and Hall, 1953; Stevens and Rudich, 1987). Under field conditions in hot, tropical, or arid regions, high temperatures ( ≥35C) can prevail for days and may extend into a portion of the dark period of the 24-h light-dark cycle (Abdalla and Verkerk, 1968). Efforts to intro- duce heat tolerance to the heat-sensitive com- mercial cultivars have prompted interest in developing criteria for evaluating germplasm from domesticated and wild tomato species for heat tolerance (Opena et al., 1979; Villareal et al., 1978; Weaver and Timm, 1989). The most commonly used criterion for tomato heat tolerance is the ability of plants to set fruit following exposure to high temperature. The effort and cost incurred in field evaluation of germplasm for heat tolerance is high. Identify- ing techniques that could be used to screen many plants quickly and economically for high-temperature tolerance would be valuable in identifying heat-tolerant germplasm.

Pollen Viability and Fruit Set of Tomato Genotypes under Optimumand High-temperature Regimes