Is flower/corolla closure linked to decrease in viability of desiccation-sensitive pollen? Facts and hypotheses: a review of current literature with the support of some new experimental data

Abstract

Pollen hydration status at dispersal depends on many parameters and on this basis two main groups of grains may be distinguished: those with a low water content are usually named orthodox or desiccation-resistant, while those with a higher water content at dispersal are known as recalcitrant or desiccation-sensitive due to their reduced mechanisms to keep water constant. On exposure, the latter lose water and die quickly. Although the end of flower receptivity may occur in different ways, in species possessing recalcitrant pollen it often consists in corolla closure. In addition to data available from current literature, also a new set of experiments was performed: the viability of pollen of eight entomophilous species having nectar as pollinator reward, desiccation-sensitive pollen and corolla closure at the end of receptivity, irrespective of pollination, was tested from the onset of anthesis until flower closure using a fluorochromatic reaction. Pollen viability fell sharply in all species, albeit at different rates, depending on initial water content, and may be also on types of carbohydrate reserves and inherent enzymes interconverting them. On the basis of experimental data and the literature, the authors speculate on the effects of corolla closure: it avoids dispersal of pollen with reduced viability, it avoids collection of unconsumed nectar while facilitating its reabsorption by the nectary parenchyma and its utilization for other purposes, and it avoids contamination by moulds and bacterial spores that could penetrate the plant via the nectar and nectary parenchyma.