Resource and pollen limitation: plant size-dependent reproductive patterns in Physalis longifolia.
The effect of pollen addition and reduction on the fruit and seed production of ground cherry (Physalis longifolia) plants was examined experimentally in the field over an entire flowering season. Pollen levels were manipulated for entire plants by augmenting natural pollination with hand-pollinations and reducing natural pollination by taping half of the flowers closed. These manipulations make it possible to determine if plants are simultaneously resource and pollen limited, a condition that suggests the plant is maximizing seed production given the resources available for female reproduction. Effects of pollen manipulation were dependent on the size of the plant. Large plants responded to reduced pollination by increasing flower production and the number of seeds per fruit; as a result, total seed production did not differ from that of large control plants. In contrast, small plants showed no change in response to reduced pollination, yet seed production did not differ significantly from that of controls, as seed production in both groups was low. Large hand-pollinated plants did not differ significantly from large control plants in any components of yield. In contrast, small hand-pollinated plants had significantly higher percentage fruit set and produced significantly more fruit and seeds than did small control plants. Differences in seed production reflected different proportions of plants that set some rather than no fruit. This may result from low pollinator visitation rates to small plants with small floral displays. Experimental results for large plants are only consistent with the hypothesis that large plants are simultaneously resource and pollen limited. To the extent that there is a trade-off between flower production and seed provisioning, large P. longifolia plants are allocating resources so as to maximize maternal seed production. In contrast, small plants are only pollen limited.
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Lawrence