Since Darwin (19th century), scientists have asked why some plants close their flowers. A lot of work has been made in order to identify cues that might induce flower closure. These functional (proximate) factors, such as temperature, light, circadian rhythms, etc., however, do not uncover an evolutionary (ultimate) origin of flower closure – why the flower responds to these stimuli. Why do plants invest time and energy to open and close their flowers?
Some studies showed that plant species with petal movements typically grow in habitats with large diurnal temperature changes or high dew and rainfall, and/or flower early in spring. These preliminary findings suggest that flower closure could have something to do with protecting reproductive organs inside the flower. To date, nobody has investigated whether flower closure protects pollen grains, so we did an experiment with Crocus discolor. Moreover, we asked whether crocuses close their flowers in order to avoid wasting pollen from anthers by the wind. This species has a center of distribution in the Western Carpathian Mountains of Slovakia and Poland. It grows on meadows and pastures from submontane to subalpine zones and bloom since March till May. Because flowering takes place in the spring in open, unprotected areas, it is reasonable to suggest that flower closure has a protective function.
The pollen viability experiment was made by treating flowers with a wire to prevent flower closure. Control flowers were left intact. After 5 days, we removed pollen from the anthers and checked its viability under a binocular microscope. Pollen grains were transferred from anthers to glass slides and mixed with a solution which makes viable grains black and dead grains yellow. We counted grains from both treated and control flowers and found that there were more viable grains in the control flowers than in treated flowers.
Secondly, we investigated whether open flowers are more vulnerable to pollen loss caused by wind. Although crocuses are not wind-pollinated, mountain areas are often typically windy. Experimental flowers were treated with a wire identically as in the previous experiment. Untreated flowers were left intact. After 48 hours, we removed anthers from all flowers and stored them in ethanol. Later, the total number of pollen grains were counted in the laboratory, and we compared the number of pollen grains between treated and untreated flowers. No difference was found between these groups, so it seems that pollen grains are heavy and not negatively influenced by wind.
Our results suggest that flower closure protects grains from unfavorable environmental conditions, but not against the wind. Actually, we do not exactly know why pollen grains are most viable when the flowers are closed. We can only hypothesize that certain microclimatic conditions inside corolla could keep pollen grains alive. Perhaps humidity inside the corolla could be different from an environment outside the flower, but this topic needs further, in-depth investigation.
The further question now is whether these findings could be generalized to other plant species, particularly those flowering in the spring. Pollen grains, in fact, differ by water content. For example, the crocus has a lot of water in pollen grains, which suggest that these grains could be vulnerable to dissipation, but a lot of other species with flower closure have grains with low water content, and dissipation will most probably not be an ultimate reason why flowers are closed. Further research involving more species with flower closure is required to resolve this question before any generalizations can be made.
These findings are described in the article entitled Flower closure enhances pollen viability in Crocus discolor G. Reuss, recently published in the journal Flora.