Fruit Set, Pollen Fertility, and Combining Ability of Selected Tomato Genotypes under High- temperature Field Conditions

Selected tomato (Lycopersicon esculentum Mill) genotypes were evaluated for their fruit-setting ability under high-temperature field conditions. A temperature-controlled greenhouse study was conducted to determine the percent fruit set from the total number of flowers and fruit produced per plant. Ratings for set obtained under high- temperature field conditions were significantly (P = 0.001) correlated with percent fruit set determined under similar greenhouse conditions. Most of the Asian Vegetable Research and Development Center (AVRDC) selections, Bea- verlodge lines, 'Nagcarlan', and 'Red Cherry' could be considered heat-tolerant. Small-fruited, abundantly flowering genotypes were less affected by heat stress than larger-fruited cultivars. Prolonged periods of high temperature caused drastic reductions in pollen fertility in most genotypes, except 'Red Cherry' and L. esculentum var. cerasiforme (PI 190256). Stigma browning and stigma exsertion commonly occurred on all lines, except AVRDC CL-5915-553 and PI 190256. Diallel analyses indicated that pollen fertility and fruit set under high field temperatures were primarily under additive gene control. High day and night temperature are known to cause drastic reductions in tomato flowering and fruit set (Abdalla and Ver- kerk, 1968; Kuo et al., 1979; Rudich et al., 1977) and in fruit size and quality (El Ahmadi and Stevens, 1979a; Hanna and Hernandez, 1982; Iwahori and Takahashi, 1964; Levy et al., 1978). Flowering phases most sensitive to high temperature (40C for 3 to 4 h) were during meiosis (8 to 9 days before anthesis) and fertilization (1 to 3 days after anthesis) (Iwahori, 1965, 1966; Iwahori and Takahashi, 1964; Sugiyama et al., 1966). Failure of fertilization after exposure to heat mainly could be attributed to decreases in pollen germination and pollen tube elongation (Iwahori, 1966; Weaver and Timm, 1989). Wide genotypic variation among cultivars has been observed in the effect of high temperature on pollen and ovule production and viability, pollen dehiscence; pollination effectiveness (Shelby et al., 1978), style elongation (El Ahmadi and Stevens, 1979a; Rudich et al., 1977), splitting of the antheridial cone, and stigma exsertion (Levy et al., 1978). El Ahmadi and Stevens (1979b) reported the inheritance of fruit set under high temperature of heat-tolerant genotypes under greenhouse conditions to be additive with moderate heritability. Heritability was low for seed set, but high for stigma exsertion. Hanna et al. (1982) also found additive gene action to be more important than nonadditive gene action for percent fruit set, percent flower drop, and percent undeveloped ovaries under high field temperatures. Our objectives were to evaluate putative heat-tolerant tomato genotypes for their response to high-tem- perature field conditions in the southeastern United States, to select the best heat-set lines as genetic sources for incorporation in a breeding program, and to determine the inheritance of re- productive characteristics crucial to heat tolerance through dial- lel analyses.