Small Scale Variation of Leaf Growth

Carbohydrate production of the growing leaf itself might affect the homogeneity of growth distribution within the lamina (Walter et al. 2005): When comparing leaves of Populus deltoides that were exposed to the sun with leaves that were completely shaded, but growing on sunlit trees, it was observed that relative growth rate distribution across the lamina was more patchy and temporal fluctuation of average relative leaf growth rate was much stronger in leaves that grew in the shade. At night, leaves of both populations showed comparable temporal fluctuations of relative growth rate. This leads to the speculation that in shaded leaves the fluctuating but strong import of carbohydrates from source leaves led to spatial and temporal heterogeneities of growth that were dampened in the case of sunlit growing leaves by the stabilizing effect of carbohydrates produced "on-site".

Transient variations of leaf growth are almost always seen in laboratory experiments immediately after switching lights on or off (Walter and Schurr 2005). These variations have to be considered in the context of immediate alterations of the hydraulic properties of the plant (closing or opening of stomates, turgor changes) and of a change in apoplastic pH: Switching lights off leads to a transient acidification of the apoplast (Muhling et al. 1995) that is connected with an amplification of cell-wall extensibility (Cosgrove 1999).

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