The effect of plant size on the expression of cleistogamy in Mimulus nasutus

Abstract

1. Most dimorphic angiosperm species produce chasmogamous flowers adapted for allogamy and cleistogamous flowers adapted for autogamy. Plant size is an important internal environmental factor and several studies have suggested an adaptive strategy whereby cleistogamous flowering ensures a basic seed output while larger plants can spare resources to make additional investment in the more expensive but presumably more fit outcrossed seed produced by chasmogamous flowering. This study tests the effect of plant size on the percentage of chasmogamy in a species, Mimulus nasutus, where both the cleistogamous and chasmogamous flowers are autogamous and so seed from chasmogamous flowers has no genetic advantage. 2. We measured change in expression of cleistogamy/chasmogamy in glasshouse‐grown plants of different sizes and architecture. Size was manipulated by varying the length of time for which the plants were grown under short days. Architecture was manipulated by removing stems. 3. We found that increased size led to an increase in the percentage of chasmogamy in M. nasutus. This is unexpected given the lack of genetic advantage of outcrossing and suggests that this effect can occur as a direct result of increased resource availability without the necessity of selection for progeny fitness. 4. An increase in plant size was also found to lead to an increase in resource sinks (developing flowers and seed capsules on other parts of the plant). As increased plant size can therefore produce both increased total resources and increased number of resource sinks, the net resources available to a developing flower will be a balance of these two effects. Resource re‐allocation is shown to occur between flowers on a stem, between stems and between flowers and seed set. In each case, this results in a net decrease in resources to new flowers and so results in there being only sufficient resources for cleistogamous flowering. 5. We discuss the difficulties of using species with varying growth forms to test the hypothesis that expression of allogamous chasmogamy increases with plant size because of the genetic advantage of outcrossing.