Outcrossing Distance Effects in Delphinium Nelsonii: Pollen Loads, Pollen Tubes, and Seed Set

Abstract

Seed production of a flower depends both on the quantity of pollen received and on its quality as manifested in rates of germination, ovule fertilization, and seed maturation. Disentangling effects of these two factors is complicated by the fact that seed set is a decelerating function of pollen quantity. One potential element of quality is the genetic similarity of pollen donor and recipient. In ten experiments with Delphinium nelsonii, seed set varied with outcrossing distance, a correlate of genetic similarity. Over all experiments, a 10—m distance significantly (P < .02) outperformed shorter and longer distances by 23—33%, suggesting quality differences. To explore this possibility, we fit decelerating negative—exponential regressions to relationships among stigma pollen load, pollen tube number reaching the ovary, and seed set in a subset of experiments having 1—, 10—, and 100—m treatments. Regression parameters suggested that 10—m pollen is most efficient at producing tubes and seeds, and analysis of residuals from pooled regressions (a nonlinear ANCOVA) showed that 10—m residuals were most positive. The latter effect was significant for pollen load—seed set relationships (P = .009, five experimental replicates) and pollen tube—seed set relationships (P = .021, two replicates), but not pollen load—pollen tube relationships (P = .128, three replicates). These results illustrate the utility of nonlinear regression in distinguishing pollen quantity and quality. They reinforce the conclusion that outcrossing distance affects pollen quality in D. nelsonii, and that an optimal outcrossing distance between 1 and 100 m maximizes seed set on average in pollinations using one donor per carpel. Insofar as quality differences persist in natural pollen mixtures that arrive on stigmas, outcrossing distance should affect reproductive success through both paternal and maternal sexual functions.