Paraheliotropism in two Phaseolus species: combined effects of photon flux density and pulvinus temperature, and consequences for leaf gas exchange

Phaseolus species exhibit paraheliotropic (light-avoiding) leaf movements in response to environmental stimuli. We investigated the response of two ecologically contrasting species, Phaseolus vulgaris (mesic) and Phaseolus acutifolius (xeric), to various combinations of photosynthetic photon flux density (PFD) and pulvinus temperature to compare the control and consequences, in terms of photosynthetic gas exchange, of paraheliotropism under identical conditions. Leaflet angles were measured in a cuvette that exposed the leaf to controlled PFD and pulvinus temperature. A gas exchange system was used to determine carbon assimilation, stomatal conductance and water use efficiency (WUE) in response to environmental factors and leaf movements. The effect of tilting leaves was compared with an equivalent reduction of PFD for horizontal leaves. Both species showed significant (but different) responses to PFD and temperature, with increased light avoidance at high levels of both factors. P. acutifolius exhibited greater light avoidance under equivalent conditions. The gas exchange consequences of changing leaf angles were similar but not identical to those of an equivalent modification of radiation level. The photosynthetic cost of the leaf movements was negligible, but there was a substantial increase in WUE, especially for P. acutifolius. Thus, the benefits of paraheliotropism are achieved at virtually no cost to the plant.