THE EFFECT OF PLANT SIZE ON THE EXPRESSION OF CLEISTOGAMY IN MIMULUS-NASUTUS

1. Most dimorphic angiosperm species produce chasmogamous flowers adap ted for allogamy and cleistogamous flowers adapted for autogamy. Plant size is an important internal environmental factor and several studie s 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 tes ts the effect of plant size on the percentage of chasmogamy in a speci es, Mimulus nasutus, where both the cleistogamous and chasmogamous flo wers are autogamous and so seed from chasmogamous flowers has no genet ic advantage, 2. Mie measured change in expression of cleistogamy/chas mogamy in glasshouse-grown plants of different sizes and architecture, Size was manipulated by varying the length of time for which the plan ts were grown under short days. Architecture was manipulated by removi ng stems. 3. We found that increased size led to an increase in the pe rcentage of chasmogamy in M. nasutus. This is unexpected given the lac k of genetic advantage of outcrossing and suggests that this effect ca n occur as a direct result of increased resource availability without the necessity of selection for progeny fitness. 4. An increase in plan t size was also found to lead to an increase in resource sinks (develo ping flowers and seed capsules on other parts of the plant). As increa sed plant size can therefore produce bath increased total resources an d 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 on ly sufficient resources for cleistogamous flowering. 5. We discuss the difficulties of using species with varying growth forms to test the h ypothesis that expression of allogamous chasmogamy increases with plan t size because of the genetic advantage of outcrossing.