Tomato pollen development: stages sensitive to chilling and a natural environment for the selection of resistant genotypes
Abstract The time during which pollen development is most sensitive to chilling was investigated. Five cultivars of tomato (Lycopersicon esculentum Mill.) bearing flower buds at different stages of development were kept at 7°C for 1 week under 12-h light periods, during which time growth stopped. After returning the plants to minimum temperatures of 18°C, the presence of chromatin in the pollen was assessed daily as the flowers reached anthesis. The results suggested that there are two stages of acute sensitivity to cold during pollen development, each of which results in cold-stressed plants having pollen empty of chromatin. The first and most sensitive stage is about 11.2 d (SE = 0.3 d) before anthesis, and this is followed by a second stage of sensitivity about 5.6±0.2 d before anthesis. Flowers that had wholly developed under simulated natural temperatures that decreased diurnally from a maximum of 18°C to a minimum of 7°C also had defective pollen, but pollen of normal appearance was regained within 14°d on return to higher temperatures. Plants of L. esculentum, and a form (LA 1363) of the wild species L. hirsutum from high altitudes in the Andes, as well as F1 and F3 generations of their hybrid, were grown to the flowering stage at an altitude of 600 m in Hawaii and then grown for a further 30°d at 2000 m, where night temperature was below 10°C. The high altitude environment severely affected the quality of pollen produced and its release from the stamen in L. esculentum, but not in L. hirsutum LA 1363. The results with the hybrids suggested that such tropical mountain environments can be used as a natural phytotron in the selection of chilling resistance that is only expressed in the mature plant.