Variation in susceptibility to wind along the trunk of an isolated Larix kaempferi (Pinaceae) tree.

• Premise of the study: The force of the wind is a significant hazard to the survival of trees and can affect tree morphology. However, the actual distribution of the stress that wind causes to a tree trunk is not well understood in spite of its expected importance to tree morphology. The uniform stress hypothesis (i.e., tree trunks take a form that equalizes the distribution of stress along the outer surface of the stem) has been tested indirectly as a model, placing high importance on the mechanical safety of a trunk, and rejected theoretically. But stress on a tree's trunk has not yet been measured directly.• Methods: Actual strains at the surface of the trunk of an isolated Larix kaempferi tree due to wind loads were measured at seven heights on the trunk for 1 yr.• Key results: During the measurement period, wind-induced stress was higher in the upper portions of the trunk than in the lower portions, regardless of wind speed, and the difference increased as wind speed increased. The deflection of the trunk recorded at each position was also larger in the upper portions than in the lower portions.• Conclusions: The results indicate that the upper portions of the trunk of an isolated Larix kaempferi tree are more susceptible to wind than are the lower portions. These results do not support the uniform stress hypothesis and suggest that another limitation (either mechanical or nonmechanical) acts on the morphology of the tree.

Tree branching: Leonardo da Vinci's rule versus biomechanical models.

This study examined Leonardo da Vinci's rule (i.e., the sum of the cross-sectional area of all tree branches above a branching point at any height is equal to the cross-sectional area of the trunk or the branch immediately below the branching point) using simulations based on two biomechanical models: the uniform stress and elastic similarity models. Model calculations of the daughter/mother ratio (i.e., the ratio of the total cross-sectional area of the daughter branches to the cross-sectional area of the mother branch at the branching point) showed that both biomechanical models agreed with da Vinci's rule when the branching angles of daughter branches and the weights of lateral daughter branches were small; however, the models deviated from da Vinci's rule as the weights and/or the branching angles of lateral daughter branches increased. The calculated values of the two models were largely similar but differed in some ways. Field measurements of Fagus crenata and Abies homolepis also fit this trend, wherein models deviated from da Vinci's rule with increasing relative weights of lateral daughter branches. However, this deviation was small for a branching pattern in nature, where empirical measurements were taken under realistic measurement conditions; thus, da Vinci's rule did not critically contradict the biomechanical models in the case of real branching patterns, though the model calculations described the contradiction between da Vinci's rule and the biomechanical models. The field data for Fagus crenata fit the uniform stress model best, indicating that stress uniformity is the key constraint of branch morphology in Fagus crenata rather than elastic similarity or da Vinci's rule. On the other hand, mechanical constraints are not necessarily significant in the morphology of Abies homolepis branches, depending on the number of daughter branches. Rather, these branches were often in agreement with da Vinci's rule.

Actin reorganization underlies phototropin-dependent positioning of nuclei in Arabidopsis leaf cells.

In epidermal and mesophyll cells of Arabidopsis (Arabidopsis thaliana) leaves, nuclei become relocated in response to strong blue light. We previously reported that nuclear positions both in darkness and in strong blue light are regulated by the blue light receptor phototropin2 in mesophyll cells. Here, we investigate the involvement of phototropin and the actin cytoskeleton in nuclear positioning in epidermal cells. Analysis of geometrical parameters revealed that, in darkness, nuclei were distributed near the center of the cell, adjacent to the inner periclinal wall, independent of cell shape. Dividing the anticlinal wall into concave, convex, and intermediate regions indicated that, in strong blue light, nuclei became relocated preferably to a concave region of the anticlinal wall, nearest the center of the cell. Mutant analyses verified that light-dependent nuclear positioning was regulated by phototropin2, while dark positioning of nuclei was independent of phototropin. Nuclear movement was inhibited by an actin-depolymerizing reagent, latrunculin B, but not by a microtubule-disrupting reagent, propyzamide. Imaging actin organization by immunofluorescence microscopy revealed that thick actin bundles, periclinally arranged parallel to the longest axis of the epidermal cell, were associated with the nucleus in darkness, whereas under strong blue light, the actin bundles, especially in the vicinity of the nucleus, became arranged close to the anticlinal walls. Light-dependent changes in the actin organization were clear in phot1 mutant but not in phot2 and phot1phot2 mutants. We propose that, in Arabidopsis, blue-light-dependent nuclear positioning is regulated by phototropin2-dependent reorganization of the actin cytoskeleton.