Many plants extend their flowering period through sequential flowering to maximize the number of flowers that survive herbivory and receive pollen. This sequential flowering in conjunction with producing flowers in excess of available fruiting resources allows plants to selectively retain flowers that have the potential of producing a high number of viable seeds. Further, hand-pollination studies of sequentially flowering plants show late-opening flowers develop fewer fruits than early-opening flowers. Early fruiting may minimize the risk of uncertainty in fruiting later in the flowering season and may allow plants to grow larger fruits with more viable seeds over the fruiting period. However, studies on naturally-pollinated plants show conflicting evidence for the effect of flowering sequence on fruiting patterns. These contradictions may be explained by plants’ decisions to allocate resources for fruiting early-opening flowers which may limit resources available for additional fruits. We conducted an experiment to detangle the effects of flowering sequence and resource commitments to existing fruits on fruiting patterns using sequentially flowering yucca plants. We hand-pollinated 84 inflorescences and compared fruiting among three flowering stages, early-opening flowers, late-opening flowers without already existing fruits, and late-opening flowers with already existing fruits. Results were compared with observations from naturally-pollinated inflorescences.
Results/Conclusions
We show that shifts in fruiting patterns in sequentially flowering plants can be explained and predicted by the presence of already existing fruits. Inflorescences with already existing fruits retained significantly fewer flowers compared to inflorescences without already existing fruits, regardless of the flowering sequence. Further, the proportion of fruits retained from early- and late-opening flowers on inflorescences without already existing fruits were similar. Therefore, the presence of fruits, and not flowering sequence, decreased the probability of fruiting of late-opening flowers. Naturally-pollinated inflorescences also showed that increase in the number of fruits already initiated from early-opening lower flowers, significantly decreased the probability of fruiting of late-opening middle and top flowers. Further, fruits from the early-opening bottom flowers of naturally-pollinated inflorescences showed the presence of partially fertilized fruits that were significantly smaller than other fruits. Fruiting early in the flowering season, even when flowers are partially fertilized, is likely a plant’s strategy to safeguard against the uncertain fates of late-opening flowers. Therefore, a plant’s resource allocation to already-initiated fruits that are likely more valuable assets to plants than flowers that have uncertain fates may be the cause for decrease in fruit retention later in the flowering season when fruits are already initiated.