Dormancy appears to mark a boundary between the development of the sporogenous tissue and the occurrence of meiosis for further microspore development. Breaking of dormancy occurs following a clear sequence of events, providing a developmental context in which to study winter dormancy and to evaluate differences in chilling requirements among genotypes.

Situations of high flower bud drop and low fruit set without apparent causes are common in fruit trees. The term flower quality has been coined to explain differences among flowers in their capacity to set fruit, but the causes underpinning these differences are largely unknown. This lack of knowledge is based on the fact that these differences are established a posteriori and there are no criteria to determine a priori what will make a flower to set a fruit or to drop. In this work, we profit of the empirical knowledge that there are fruiting and non-fruiting shoots to explore to which extent flower bud differentiation and bud development will affect the subsequent fruit set. For this purpose, the processes from flower bud differentiation to fruit set were sequentially analyzed in both types of shoots, over two years. More than half of buds from long shoots aborted development and dropped before flowering. At anthesis, most of the remaining flowers showed underdeveloped pistils that failed to sustain pollen germination or pollen tube growth along the pistil. This unsuccessful development resulted in clear differences in fruit set between both types of branches. These results highlight that flower bud differentiation and development play an important role for fruit set and that developmental timing appears critical to reach anthesis with a fully developed pistil.

In temperate woody plants, flowering time is adjusted to the environmental temperature, and survival to cold winter temperatures is achieved through dormancy. But also chilling temperatures during dormancy are required for proper flower bud development and flowering. The time the flower bud remains dormant is both genetically and environmentally controlled, and is a major factor determining the adaptation of species and cultivars to particular ecological conditions because chilling requirements vary greatly among genotypes. Recently this adaptation is jeopardised with climate changing conditions. However, little is known on the biological milestones underpinning these events, and when differences in flowering time are established. In this work we evaluate the hypothesis that anther meiosis time is related to winter cold temperatures, and that differences in flowering time are set up by differences in the time when pollen meiosis occurs. For this purpose, anther development has been characterized in five apricot cultivars with different chilling requirements. The work was done over two years with different weather conditions, a cold and a mild winter. The sporogenous tissues differentiated prior to dormancy and remained in this apparently quiescent stage during the winter. Once chilling requirements were fulfilled, meiosis closely followed and was highly correlated to breaking of endodormancy. Meiosis was completed within one week and was followed by a change in the colour of the anthers from green to yellowish, which could be a useful visual indicator to know that breaking of endodormancy had already occurred. The fact that this sequence of events was consistent in all the cultivars, and different climatic years analyzed, supports the hypothesis that winter cold temperatures are related to the time of pollen meiosis, which in turn reflects in different flowering times.

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Apricot (Prunus armeniaca L.) flowers at the end of the winter but flower bud differentiation occurs the previous summer. During the winter, the flower bud enters dormancy and resumes growth prior to flowering. The time the flower bud remains dormant is genetically and also environmentally controlled and is a major factor determining the adaptation of particular cultivars to particular ecological conditions. In apricot, as in other temperate fruit species, chilling is required for buds to emerge from rest and chilling requirements vary greatly among cultivars. However, very little is known on the physiological changes underlying these events. To explore the relationship between pollen development and chilling requirements, anther and pollen development have been characterised in several apricot cultivars with different chilling requirements. For this purpose, sequentially fixed flower buds from dormancy to flowering were histochemically examined in two consecutive years. While the pattern of pollen development is highly conserved, differences in timing were observed among the different cultivars and years. These differences are discussed in relation to the chilling requirements for each cultivar.

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