Staminate Bloom Date and Temperature Responses of Pollen Germination and Tube Growth in Two Walnut (Juglans) Species

Pollen was collected from ten cultivars of Juglans regia and three cultivars of the later-blooming species, J nigra Extensive phenological data were available for these cultivars. Cultivars were chosen on the basis of staminate bloom date to include the earliest and latest blooming individuals available and a representative range throughout the bloom season. Mean staminate bloom dates for the cultivars examined covered a period of 46 days over which time mean daily temperatures rose 6 C. In order to determine if adaptations to temperature were expressed by the gametophyte generation, pollen was subjected to controlled temperatures from 5 to 40 C in 2 to 4 C increments and analyzed for germination percentages and pollen tube elongation. A positive relationship was found for pollen germination percentage and mean staminate bloom date such that earlier blooming individuals showed lower minimum temperature thresholds for germination, and optimum temperature for pollen germination was positively correlated with mean staminate bloom date. Differences in pollen tube growth, determined separately from hydration and germination responses, were less clear. Most J. regia clones had lower minium temperature thresholds for growth than the J. nigra clones, but there was no clear relationship to earliness of bloom within the species. No differences were discerned in optimum temperatures for pollen tube growth either between the two species or within species.