Leaf Orientation of Soybean Seedlings. I. Effect of Water Potential and Photosynthetic Photon Flux Density on Paraheliotropism

Paraheliotropic (light avoiding) leaf movements can reduce transpirational water loss by lowering light interception by the leaf through action of the pulvinus. We investigated effects of plant water potential (ψ w ) and photosynthetic photon flux density (PPFD) on paraheliotropism in young seedlings of soybean [ Glycine max (L.) Merr. cv. Northrup King S1346], The effect of paraheliotropism on leaf temperature, ψ w , and stomatal conductance (g e ) was determined. The angles of unifoliolate and lower trifoliolate leaves were measured for plants at a range of ψ w values under different sources of illumination (sun, metal halide lamp, and fluorescent light bank) at PPFD ranging from 500 to 1600 μmol m ‐2 s ‐1 . Leaf temperatures and g c were measured for selected plants, including those with'leaves restrained in a horizontal position. The angle between unifoliolate leaves, measured at the base of the leaves, was highly correlated with ψ w . Both unifoliolate leaves and the terminal leaflets of trifoliolate leaves exhibited less paraheliotropism at a given ψ w when PPFD was reduced. Paraheliotropism was associated with improved plant water status and lower leaf temperatures, but g c did not differ between paraheliotropic leaves and those restrained in a horizontal position. Paraheliotropic leaf movement and stomatal closure acted in parallel, commencing at approximately −0.4 MPa. Paraheliotropic leaf movements in soybean seedlings provide a mechanism that, with stomatal closure, can reduce transpiration under conditions of low ψ w and high PPFD.