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=Leaf Placement in a Canopy=

Problem Solving
Leaf placement is an emergent self-organizing behavior. Plants integrate many different environmental signals and initiate specialized responses. Some examples of these signals include: CO2 concentrations, R/FR light ratio, and irradiance. As well as plant hormones, abscisic acid, auxin, cytokinins, and gibberellins. The integration of these signals creates optimal leaf placement, in order to achieve the greatest level of light acquisition. This resource acquisition is important in increasing fitness. Specified leaf placement is a demonstration of a plant’s ability to integrate signals and problem solve. This integration of environmental signals minimizes self-shading in plants. Without specified leaf placement, branches and leaves from a single individual can prevent each other from acquiring light, and reduce the plant’s ability to compete with neighbors.

Bud Outgrowth
Bud outgrowth is the initiation of new lateral branches. It is the primary control in leaf placement. Axillary bud growth is controlled by both light intensity and hormones. Plants grown in shade (low red light to far-red light ratio) will experience reduced axillary bud growth and increased apical stem growth. Apical dominance refers to growth dominated by the apical stem. Reduced apical dominance will result in increased radial growth of the plant. Buds require higher intensity light in order to initiate growth, and there is a specific R:FR threshold that the bud must reach in order to develop. One experiment has shown clear communication between buds on an individual plant. When buds on an upper stem were subjected to dark conditions, buds on a lower stem on the plant shoot sprouted. Communication between buds alters growth and is important in the integration of signals that drives leaf placement. It is the plant’s perception of light quality that induces or restricts growth from a particular bud. The presence of neighbors is sensed by lower light quality. Absorption of red and blue light by neighbors and the reflection of far-red light radiation by their green tissues decreases the R:FR, signaling to the plant the presence of a competing neighbor. Abscisic acid and Gibberellins are the hormonal controls of bud growth. Gibberellins promote bud growth and stem elongation, while abscisic acid inhibits bud growth. Another hormone, Cytokinins, has also been found to stimulate bud outgrowth, and reduce apical dominance.

Branching Angle
Branching in plants is initiated by bud outgrowth, but is strongly regulated by apical dominance (Wilson). Apical control inhibits lateral branching and cambial activity, which controls branch diameter growth of existing branches. Branches grown in high intensity light can escape apical control, and grow larger. In plants, branching angle increases as the organism grows, and branches grow slower at increased angles. So, distal branches will have slower rates of growth than younger branches growing much more vertically.

Leaf Growth Communication and Senescence
Highly specific leaf placement requires holistic integration of thousands of leaves, in one individual tree. Examples of communication between mature and developing leaves are evident through responses to CO2 concentrations. Mature leaves exposed to higher levels of concentration of CO2 reduces stomatal density, developing leaves mimic the stomatal density of mature leaves. Forest leaves can alter their leaf angle within minutes in response to varying levels of irradiance. Low red to far red light ratios can indicate self-shading and induce leaf senescence. Leaves producing less auxin become more sensitive to a hormone called ethylene. Ethylene induces the breakdown of cell walls of leaf stem. Another hormone, Cytokinins, however have the opposite effect of ethylene and, early on in leaf development this senescence can be reversed if conditions change (observed in Tobacco).

