Sun and Shade Effects on the Leaf of Guarea (Meliaceae): Plasticity of a Branch Analogue

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Abstract
The compound leaf of Guarea has been described as a branch analogue because the leaf-tip bud, unique among angiosperms, rhythmically produces new pinnae for several years. To test phenotypic plasticity in these leaves, young trees were grown under full sun (SUN), partial shade of ca. 50% sun (PSH), and deep shade of 1.7% sun (DSH). Pinnae produced in PSH or DSH have greater area, thinner blades, fewer palisade layers, and smaller specific weight (dry weight/area) than those produced in SUN. SUN plants produced more and shorter new leaves than shaded plants. Plants were also transferred to different light conditions. Leaves under the first condition produced new pinnae under the second condition. Both the original and new pinnae on the same leaf had characteristics similar to those produced on entire leaves that were grown only under a single condition. One leaf grown under SUN .fwdarw. PSH first had SUN pinnae and later flushed new PSH pinnae; the reverse was true for pinnae on a PSH .fwdarw. SUN leaf. Thus, successive pinnae on a single leaf develop structural features characteristic for the prevailing light conditions. Such leaf plasticity is similar to that in branches.