Pollen germination of cherimoya ( Annona cherimola Mill.). : In vivo characterization and optimization of in vitro germination

Abstract

Cherimoya ( Annona cherimola Mill.) is a subtropical fruit tree highly dependent on pollination and in most parts of the world hand pollination is a common agricultural practice. However, erratic results following hand pollination have often been associated with low pollen viability. In spite of this dependence on good pollen viability, a good method to evaluate pollen viability is not yet available. This deficiency may be related to the fact that little is known about pollen germination occurring in vivo. In this work pollen germination is first characterised in vivo and the temperature effect on pollen germination evaluated. Subsequently, a medium for pollen germination has been optimized and the conditions for in vitro pollen germination determined. Mature pollen is trinucleated and is shed from the anther in tetrads. Upon pollen hydration, either on the stigma or in the germination medium, pollen is liberated from the tetrad. While the kinetics of the process are quicker in vitro, both processes occur rapidly and are completed within 2 h in vitro and within 5 h in vivo. Pollen germination is optimum at 20–25°C, that is the same temperature required for good fruit set in the orchard. Pollen requires prehydration prior to in vitro germination and it only germinates when the stamens are also added to the medium, that requires the provision of calcium and boron as well as sucrose at 5–10%. Provided these factors are taken into account, pollen germination can reach levels similar to the ones attained in vivo.