In most plants bearing cleistogamous flowers, chasmogamous flowers are also borne by the plants. For example, Viola, Oxalis and Commelina contain both these kinds of flowers.( I am unaware of a species which bears only cleistogamous flowers, which would be very unfavourable for evolutionary success)
The Cleistogamous flowers help in reproduction with minimal energy and resource expenditure. It also maximises the chance of reproduction, which is an important factor where the agents of pollination are scarce. On the other hand Chasmogamous flowers provide variablity, hybrid vigour and generate better genotypes through recombination. Both of these strategies are useful in different environment. In adverse (less pollinators, energy stress) cleistogamy would be favourable, and in other cases where no energy stress prevails, chsmogamy is advantageous.
My question is, How can plants, if they can, regulate which strategy is supported? Is the ratio of cleistogamous to chasmogamous flowers constant irrespective of the environment, or does the prevailing environment have a say in deciding which of these two kinds of flowers will be preferred over the other? If it does, what is the regulatory mechanism underlying this control of flowering startegies?
One other related question is whether there are any species which bear only cleistogamous flowers? That would seem very unfavourable for evolution to act on.