Petiole twisting in the crowns of Psychotria limonensis: implications for light interception and daily carbon gain

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Abstract
We used Y-plant, a computer-based model of crown architecture, to examine the implications of leaf reorientation resulting from petiole bending in Psychotria limonensis (Rubiaceae) seedlings. During this reorientation process, bending of the petioles of lower leaves that are potentially self-shaded by the upper leaves rotates the lamina around the stem's orthotropic axis so that self-shading is reduced. Simulations of daily light capture and assimilation revealed a 66% increase in daily C gain due to reorientation of the leaves as compared to simulations where the leaves remained in their characteristic opposite decussate pattern set by the phyllotaxy. This was due to enhanced carbon (C) gain of the lower leaves because of the reduction of shading by upper developing leaves in the same vertical plane. The light signal for this movement was experimentally examined by placing leaf-shaped filters above already fully expanded leaves and following the resulting shade-avoiding movements. The filters were either neutral density shade cloth that reduced the photon flux density (PFD) but did not alter the red to far red ratio (R:FR) or a film that reduced the PFD equivalently but also reduced the R:FR. Leaf reorientation was much more rapid and complete under the low R:FR as compared to the high R:FR indicating involvement of a phytochrome photosensory system that detected the presence of a shading leaf. Plants in gaps were found to lack a reorientation response indicating that the reorientation is specific to the shaded understory environment.