Adaptive Germination Responses by Walnut (Juglans) Pollen to Temperature during Pollen Development
In winter-deciduous tree species, temporal variation in budbreak varies widely within species and is presumably related to requirements for winter chilling, level of chilling experienced and the post-dormant heat requirement (Couvillon and Erez, 1985; Spiegel-Roy and Alston, 1979). Temperature ranges and optima for pollen germination and pollen tube growth vary widely among species. Intraspecific variation in pollen responses to temperature is less well documented, and we have been interested in determining the extent to which variation in sporophyte responses to temperature (as in time of budbreak) is expressed in the gametophyte. In an investigation of two closely related Prunus species, P. dulcis (almond) and P. persica (peach), with different bloom dates, we (Weinbaum, et al., 1984) determined that there were no differences in pollen tube growth responses to temperature, but that temperatures at which pollen germination occurred maximally differed between the two species (16 C in the earlier blooming almond vs. 23 C in peach). No differences were evident within either species. Budbreak and bloom occur considerably later in Juglans spp. (walnut) than in Prunus, and the temporal range for these phenomena are much greater. Historical data from extensive collections of almond and walnut clones available at the University of California show that the range in mean bloom date is 22 days for almond clones (Weinbaum et al, 1984) and 50 days among J. regia clones (Hendricks, et al, 1985). Therefore, we have been investigating the response to temperature by pollen among clones and cultivars of walnut. This paper summarizes some of our results (Luza, et al., 1987; Polito, et al., 1991).
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