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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.

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    $\begingroup$ +1 interesting question. I am sure there are some articles presenting more or less complex (depending on the number of parameters that we authorize to vary) mathematical models to explain to frequency of cleistogamous and chasmogamous flowers. Eric Charnov's book The Theory of Sex Allocation is a classical book in this kind of discussion and it maybe addresses your question. $\endgroup$
    – Remi.b
    Dec 31, 2013 at 9:23
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    $\begingroup$ This book from Stuart West might also address the question $\endgroup$
    – Remi.b
    Dec 31, 2013 at 9:23
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    $\begingroup$ did you check this article $\endgroup$
    – WYSIWYG
    Jan 1, 2014 at 8:09

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This is a very good question, but I think the reason that it's not being answered is because it is in a sense too broad: different plant groups maintain balances of cleistogamous and chasmogamous flowers, and they modulate that balance through different mechanisms. Many of these mechanisms (genes involved, environmental cues, developmental pathways) may not be fully understood.

Culley and Klooster (2007) categorize cleistogamy depending on the degree to which "the prevailing environment has a say":

  • In dimorphic cleistogamy CL and CH flower differ in the time or place of production, with CL flowers produced in conditions (underground, low light levels, early in the season) that are potentially unfavorable for outcrossing.
  • In induced cleistogamy potentially CH flowers that experience conditions such as drought or low temperatures fail to open and self-pollinate, becoming, in effect, CL flowers.

You should check out the Culley and Klooster (available online if you make a jstor login) – they discuss complete cleistogamy which addresses your last question. They report several completely CL species in their Table 1, and give references.

More generally, many different plant groups maintain balances of self-pollination and outcrossing (i.e. "real sex"), through an even more diverse set of mechanisms.

Even more generally, many plants and some animals maintain balances of sexual reproduction and clonal reproduction, through an even more diverse set of mechanisms. For instance, vegetative reproduction (e.g., strawberry runners) is very common in many plant groups; facultative and obligate parthenogenesis in animals also occurs.

Culley, Theresa M. and Matthew R. Klooster (2007). The Cleistogamous Breeding System: A Review of Its Frequency, Evolution, and Ecology in Angiosperms. Botanical Review. Vol. 73, No. 1, pp. 1-30

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