Petiole mechanics, light interception by Lamina, and "Economy in Design"

Computer simulations were used to assess the influence of palmate leaf morphology, decussate phyllotaxy, and the elastic moduli of petioles on the capacity of turgid and wilted twigs ofAesculus hippocastanum to intercept direct solar radiation. Leaf size, morphology, orientation, and the Young's and shear moduli (E and G) of petioles were measured and related to leaf position on 8 twigs whose cut ends were placed in water ("turgid" twigs) and 8 twigs dried for 8 h at room temperature ("wilted" twigs). Petioles mechanically behaved as elastic cantilevered beams; the loads required to shear petioles at their base from twigs were correlated with the cross-sectional areas of phyllopodia but not with petiole length or tissue volume. Empirically determined morphometric and biomechanical data were used to construct "average" turgid and wilted twigs. The diurnal capacity to intercept direct sunlight for each was simulated for vertically oriented twigs for 15 h of daylight, 40° N latitude. The daily integrated irradiance (DII) of the wilted twig was roughly 3% less than that of the otherwise comparable twig bearing turgid leaves. Simulations indicated that the orientation of turgid leaves did not maximize DII. More decumbent (wilted) petioles increased DII by as much as 4%. Reduction in the girth, E, or G of petioles, or an increase in petiole length or the surface area of laminae (with attending increase in laminae weight), increased petiolar deflections and DII. Thus, the mechanical design of petioles ofA. hippocastanum was found not to be "economical" in terms of investing biomass for maximum light interception.