We are now witnessing changing environmental conditions and these changes will likely continue into the coming decades due to projected increases in temperatures on the earth surface. Recent model projections suggest that the global mean surface air temperatures will increase by 1.4 to 5.8 °C by 2100. The consequences of these high temperatures include reductions in crop yields. Soybean is one of the major crops grown in the US, where high temperatures (>35 °C) during reproductive growth decreased yield. Two experiments were conducted to determine the effects of temperature on soybean pollen germination properties and identify responses to temperature among genotypes. Pollen collected from 44 genotypes grown outdoors in large pots was subjected to in vitro temperature treatments that varied from 15 to 50 °C at 5 °C increments in Experiment I. The mean cardinal temperatures (Tbase, Topt, and Tmax), averaged over 44 genotypes, were 13.2, 30.2 and 47.2 °C for pollen germination and 12.1, 36.1 and 47.0 °C for pollen tube growth. The relationship between Tbase and Topt for pollen germination was high indicating that genotypes with high Tbase also had higher Topt. The relationship was weak between Tbase and Tmax. The Topt for pollen tube growth was 5.8 ¢XC higher than the Topt for pollen germination. The study also showed significant genotypic variability for physiological parameters studied, but no significant correlations between the observed physiological parameters with any of the pollen germination or pollen tube growth parameters were studied. This indicated that physiological parameters, measured under ambient conditions may not be useful to identify reproductive tolerance to high temperatures in soybean. Total response index (TSRI), the sum of individual responses of all pollen parameters differentiated genotypes and their tolerance to high temperature. A heat-tolerant genotype (DG 5630RR) identified using TSRI technique, when grown at optimum and high temperature conditions in experiment II, were actually less sensitive to high temperatures compared to heat-sensitive and heat-intermediate sensitive genotypes indicating that pollen can be used as screening tool for heat tolerance. The identified high-temperature tolerant genotypes, based on pollen germination parameters, might be useful in current and future soybean breeding programs.

Pollination success of animal-pollinated flowers depends on rate of pollinator visits and on pollen deposition per visit, both of which should vary with the pollen and nectar "neighborhoods" of a plant, i.e., with pollen and nectar availability in nearby plants. One determinant of these neighborhoods is per-flower production of pollen and nectar, which is likely to respond to environmental influences. In this study, we explored environmental effects on pollen and nectar production and on pollination success in order to follow up a surprising result from a previous study: flowers of Ipomopsis aggregata received less pollen in years of high visitation by their hummingbird pollinators. A new analysis of the earlier data indicated that high bird visitation corresponded to drought years. We hypothesized that drought might contribute to the enigmatic prior result if it decreases both nectar and pollen production: in dry years, low nectar availability could cause hummingbirds to visit flowers at a higher rate, and low pollen availability could cause them to deposit less pollen per visit. A greenhouse experiment demonstrated that drought does reduce both pollen and nectar production by I. aggregata flowers. This result was corroborated across 6 yr of variable precipitation and soil moisture in four unmanipulated field populations. In addition, experimental removal of pollen from flowers reduced the pollen received by nearby flowers. We conclude that there is much to learn about how abiotic and biotic environmental drivers jointly affect pollen and nectar production and availability, and how this contributes to pollen and nectar neighborhoods and thus influences pollination success.

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In contrast to the wealth of knowledge concerning sucrose-rewarded learning mechanisms, the question of what bees learn when they collect pollen from flowers has been little addressed. Pollen-rewarded learning is of interest not only in furthering our understanding of associative conditioning pathways in the insect brain, it may also shed light on the role that cognitive processes may have played in shaping the early evolutionary relationship between plants and their pollinators, given that pollen is thought to have been the ancestral reward for flower visitors. Thus the central aim of this thesis was to demonstrate the conditions under which pollen may reinforce learning of floral features in two model species, the honeybee (Apis mellifera) and bumblebee (Bombus terrestris). Having developed a number of paradigms for the study of pollen-rewarded learning, here I ask what bees might learn during pollen collection, both in terms of the sensory characteristics of pollen itself and additional cues paired with this reward. Freely flying bees were shown to be sensitive to differences in the type of pollen offered for collection and were able to associate the presence of a coloured stimulus with both the availability and quality of the pollen reward. The sensory pathways involved in the evaluation of pollen were also investigated. When bees were restrained, in order to more tightly control exposure to the reward, pollen was not found to support learning in an olfactory conditioning task. Furthermore, when delivered in solution with sucrose, pollen was found to inhibit learning relative to bees rewarded with sucrose alone. It seems that pollen contains compounds which are perceived as distasteful by bees and that through the contamination of nectar, pollen may influence bees foraging decisions via differential learning and recognition of floral cues.

Bibliography: leaves 97-114. "Self-incompatibility (SI) is an important genetic mechnism to prevent the inbreeding of flowering plants and also an excellent system for studying cell-cell recognition and signal transduction."

Bibliography: leaves 97-114. Electronic publication: Fulltext available in PDF format; abstract in HTML format. "Self-incompatibility (SI) is an important genetic mechnism to prevent the inbreeding of flowering plants and also an excellent system for studying cell-cell recognition and signal transduction." Electronic reproduction.[Australia] :Australian Digital Theses Program,2001.