THE EFFECT OF DELPHINIUM NELSONII POLLEN ON SEED SET IN IPOMOPSIS AGGREGATA, A COMPETITOR FOR HUMMINGBIRD POLLINATION

Sympatric plant species can compete for pollination services in several ways. For example, pollinators may move between species and deposit heterospecific pollen on stigmas, which in turn may reduce the efficacy of conspecific pollen. We explored this possibility by determining the effect of Delphinium nelsonii poIlen on seed set in Ipomopsis aggregata. These montane herbs are pollinated by hummingbirds, experience heterospecific pollen deposition in nature, and suffer reduced seed set in each other's presence. We hand-pollinated flowers of I. aggregata with either pure conspecific pollen or a mixture of pollen of the two species. Resulting pollen loads on stigmas ranged from 0-865 D. nelsonfi grains and from 10-336 I. aggregata grains; mean seed set per flower was 11.3. There was no detectable effect of D. nelsonii pollen load on I. aggregata seed set. It is possible that seed set reductions seen in previous studies of competition for pollination between these species were caused by pollen wastage, pollen layering on the pollinatorl or the temporal sequence of pollen arrival at the stigma. This content downloaded from 207.46.13.85 on Sat, 28 May 2016 06:02:42 UTC All use subject to http://about.jstor. org/terms 1145 July,1985] KOHN AND WASER-EPPECT OF DELPHINIUM POLLEN ON IPOMOPSIS METHODS In June 1983, we dug up 29 I. aggregata plants in bud stage from a pasture near Almont, Gunnison Co., Colorado (elevation 2,450 m). Plants were potted in native soil and moved 35 km to a greenhouse at the Rocky Mountain Biological Laboratory, Gothic, Colorado (RMBL, 2,900 m elevation). Ten plants were subsequently used as conspecific pollen donors and the rest as pollen recipients. We also potted 20 D. nelsonii plants from a meadow near the RMBL to serve as heterospecific pollen donors. Potting at different elevations was necessary to obtain plants of the two species that would flower simultaneously. We removed the first 10 flowers that opened on each of the 19 recipient I. aggregata plants and counted ovules by examining squashed ovaries at 25 x . Subsequent flowers were handpollinated as described below, except that ovules were again counted for up to 10 flowers per plant that developed after the pollination experiment was terminated. We also counted ovules of the first and last 10 flowers produced by 10 plants in the field in 1984. Recipient flowers were emasculated before anthers dehisced to avoid contaminating stigmas with self pollen, which is incompatible (Waser and Price, 1983). We pollinated almost daily, between 0800 and 1230 hours, from 27 June to 16 July. Pollen was collected from most male-phase flowers on donor plants and usually came from flowers that had opened on the day of collection. We mixed pollen from all donors and applied it with fine watercolor brushes, with each flower receiving either pure conspecific pollen or a mixture of conspecific and heterospecific pollen. We alternated pollination treatments (i.e., pure-mixed) to flowers on each recipient; if an odd number of flowers were receptive, first and last flowers received mixed loads. We used reflexed stigma lobes as an indicator of receptivity. Tests for the presence of esterase (1-ANS, Mattson et al., 1974; FDA, Heslop-Harrison and HeslopHarrison, 1970) indicate that this is as accurate as possible, but there may have been some variation in receptivity that remained uncontrolled. Three days after flowers were pollinated, we removed and stained stigmas, squashed them gently, and counted pollen at 100 x . Grains of the two species are different shapes and sizes (45-55 ,um diam for I. aggregata vs. 20-25 ,um for D. nelsonfi). Subsequently, we removed undehisced fruits and counted seeds. We excluded from analyses fruits attacked by fly larvae (Zimmerman, 1979), but included undamaged flowers that failed to set fruit or seed. Finally, we assessed natural conspecific and heterospecific pollen loads by examining 240 stigmas from five diffierent I. aggregata patches near the RMBL. We also counted pollen loads on 46 I. aggregata stigmas from Waser's (1978a) previous hand-pollination experiment. These values were then compared to those obtained in the present experiment. RESULTS-Mean ovule number was higher for the first 10 flowers on pollen recipients than for flowers measured after the end of pollinations(means = 28.1 vs.21.8; paired t = 11.8, df= 18, P < 0.001, two-tailed). Thus, the maximum number of seeds a flower could set declined on average during the experiment. Mean ovule counts in the field in 1984 were 23.3 for early flowers and 19.7 for late flowers on the same plants (paired t = 3.50, d.f. = 9, P < 0.01, two-tailed). Of 454 hand-pollinated stigmas from the 19 pollen recipients, 172 contained I. aggregata pollen alone and 282 contained pollen of both species. Pollen loads ranged from 0-865 D. nelsonii grains (x = 108.1, SD = 141) and from 10-336 I. aggregata grains (x = 87.3, SD= 51.1). Amounts of pollen of the two species on individual stigmas were positively correlated (r = 0.26, df = 281, P < 0.001). The mean seed set of hand-pollinated fruits was 11.3 (SD = 5.6, N= 454). Stigma loads of D. nelsonii and I. aggregata pollen produced by our experiment fell within the ranges of 1-977 and 3-357 obtained for the two species, respectively, in Waser's (1978a) hand-pollinations. Field-collected stigmas from 1983 carried 0-309 heterospecific grains, a somewhat smaller range than in the experiment, and 0-478 conspecific grains (x = 119, SD = 80.5, N = 240), some of which were certainly incompatible self grains. Similarly, R. Mitchell (pers. comm.) found 0-156 D. nelsonii grains and 36-372 I. aggregata grains on I. aggregata stigmas collected during natural flowering overlap of the species in 1984. The dose-response relationship for seed set resulting from pure I. aggregata pollen (Fig. l) should be fit reasonably well by a negative exponential function of the form y = a [1expf-bx)], which is asymptotic to some maximum value of y (seed set) as x (pollen load) increases. The values in Fig. 1 are poorly fit by a linear model (Table 1). Log-transfolnzation of pollen loads and an exponential function do explain slightly more of the variance (Table 1). Even the best-fitting model explains very little of the variance in seed set, however. Multiple linear regressions with log-transformed pollen loads (Table 2) indicate that there is no detectable effect of D. nelsonfi pollen on This content downloaded from 207.46.13.85 on Sat, 28 May 2016 06:02:42 UTC All use subject to http://about.jstor. org/terms TABLE 2. Multiple regression of log-transformed pollen load on seed set in the 454 flowers receiving mixed conspecific and heterospecific pollen. The regression model is y = alogx + blogz + c, where x = number of I. agegats pollen grains, y = seed set, z = number of D. nelsonii grains, and a and b = partial regression coefficients. The table includes tests of the null hypothesis that regression parameters equal zero I. REGRESSION PARAMETERS Parameter Variable df estimate SE t P a 1 6.7 1.1 6.1 0.0001 b 1 -0.01 0.06 0.1 0.9 c 1 1.2 2.0 0.6 0.5