The photoprotective role of thermonastic leaf movements in Rhododendron maximum: potential implications to early spring carbon gain
Russell, Raymond Benjamin
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Rhododendron maximum L. is a dominant subcanopy species in the southern Appalachian Mountains. R. maximum undergo distinct thermonastic leaf movements (TLM). The purpose of these movements has not yet been determined. Previous studies have suggested TLM are a photoprotective mechanism for the dynamic light environment of the subcanopy in a deciduous forest during winter. The present study aimed to determine the effects of restricting TLM on photoinhibition, net photosynthesis, and other gas exchange parameters, particularly during the early spring. After restricting TLM on certain leaves, we observed the above parameters from autumn 2005 to late spring 2006. Our results indicated that photoinhibition increased (lower Fv/Fm) in treatment leaves over reference leaves throughout the winter. The difference became greater during the early spring, when reference leaves began to return to normal levels of photochemical efficiency and treatment leaves sustained low Fv/Fm. Net photosynthesis was lower for treatment leaves than reference leaves. This became most significant during the early spring, when maximum carbon gain is possible. Finally, gas exchange parameters as measured by light and CO2 response curves did not indicate any significant difference between treatment and reference leaves post canopy closure. Out results suggest that TLM are an important mechanism for photoprotection, allowing leaves of R. maximum to recover quickly during the early spring and maximize their early spring carbon gain.
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