Petiole mechanics, light interception by Lamina, and “Economy in Design”

Abstract

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.

A functional analysis of the crown architecture of tropical forest Psychotria species: do species vary in light capture efficiency and consequently in carbon gain and growth?

“…6) but here there may also be tradeoffs. Displacement of the leaf area away from the point of attachment of the petiole increases the moment of force, requiring additional investment in support (Givnish 1986;Niklas 1992;Niinemets and Fleck 2002). Leaves of P. marginata have the greatest displacement of lamina area away from the petiole attachment point, which causes the increased Ea-This species also has a low SLA as compared to the other species (Table 2), which is consistent with a greater investment in support.…”

Isolation and handedness of helical coiled cellulosic thickenings from plant petiole tracheary elements

“…The vascular bundles contain fibrillar material, but also the coiled tracheary elements (c) which should be strong in compression. Of course, the cross-sectional shape (Pasini 2008) and distribution of the components in the petiole will all contribute to the effective stiffness and strength of the petiole in supporting the leaf in its role of light interceptor (Niklas 1992). While the contribution of the coiled tracheary elements to the mechanical performance of the petiole is speculative, there seems little doubt that the coiled structure contributes to the ability of the petiole vascular system to maintain a negative pressure as the leaf is buffeted by the wind.…”

Biotic and abiotic consequences of differences in leaf structure

“…Vallardes & Pearcy (1998) propose that shade plants would be more damaged by photoinhibition, from high PAR interception on leaves of limited electron-transport capacity. Niklas (1992) argues that in one species of plant, petiole investment is excessive for light interception, summed over the day. I suggest that the architecture might be closer to optimal if one were to account for sunlight interception being more valuable early and late in the day, when vapourpressure differences are smaller and WUE is larger.…”