An empirical test of bet-hedging in the heterocarpic desert annual Pectocarya heterocarpa (Boraginaceae)

Background/Question/Methods Persisting in a variable and harsh environment is a major challenge for annual plants, which must complete their entire life cycle in just a few short months. Bet-hedging reduces variation in reproductive success, which allows plants to persist in unpredictable environments. Pectocarya heterocarpa (Boraginaceae) is a winter annual native to southwest North America that has the unique feature of producing three seeds types, one subterranean and two aerial. The basal seeds are inserted directly into the ground and remain attached to the maternal skeleton. Seed heteromorphism is frequently assumed to be a form of bet-hedging, but there is limited empirical evidence to support this. In 2010, we identified 150 plants each from basal and aerial seeds following germination on Tumamoc Hill in Tucson, AZ, and removed all other plants within 10cm. We expect that the basal seed types are produced early in development as a conservative bet-hedging strategy, and that aerial seeds are produced later and exhibit a riskier strategy with higher dispersal and germination fractions. Using type-specific vital rates and historical demographic data, we will use stochastic modeling to test the hypothesis that seed heteromorphism is a bet-hedging mechanism in P. heterocarpa.

Results/Conclusions We found evidence that basal cleistogamous flowers are made early in the life cycle, with plants producing buds at 5-weeks after germination, prior to production of chasmogamous flowers that produce aerial seeds. All maternal skeletons collected prior to germination in 2010 had basal seeds still attached (mean = 10.38 +/- 1.55 S.E.) regardless of maternal size. Basal seeds had high germination fractions in 2010 (mean = 0.35 +/- 0.03) and 99% survival prior to floral initiation. One month following germination, plants from aerial seeds are larger than those from basal seeds (one-way t-test, p = 0.029). Plants will be monitored for survival and reproduction throughout the 2010 growing season and collected following fruit maturation to analyze biomass allocation patterns and seed types made. Our preliminary data suggests that basal seeds provide reproductive assurance in a risky environment. By making these seeds early, while survival is still high, plants ensure that they have reproduced, even if they die soon after. These basal seeds also exhibit a conservative, slow growing strategy, compared with the more risky, aerially dispersing seeds. These results suggest that seed heteromorphism is a bet-hedging mechanism in P. heterocarpa, which reduces risk of reproductive failure and ensures representation in the next generation.