Seedling Growth of Catawba Rhododendron. II. Photosynthesis and Carbohydrate Accumulation and Export

Abstract

Catawba rhododendron (Rhododendron catawbiense Michx.) seedlings of two provenances, Johnston County, N.C. (35°45′N, 78°12′W, elevation = 67 m), and Yancey County, N.C. (35°45′N, 82°16′W, elevation = 1954 m), were grown in controlled-environment chambers for 18 weeks with days at 18, 22, 26, or 30C in factorial combination with nights at 14, 18, 22, or 26C. Seedlings of the higher-elevation provenance generally exhibited higher net leaf photosynthetic rates (PN)s than those from the lower elevation at all temperature combinations. Thus, it appears seedlings of the high-elevation provenance possess greater relative thermotolerance, expressed as net photosynthesis, than the low-elevation provenance. Eighty-seven days after initiation (DAI) of the experiment, PN showed a quadratic response to increasing day temperature, with the maximum occurring at 22C, whereas PN decreased linearly with increasing night temperature. At 122 DAI, PN increased linearly with increasing day temperature with nights at 22 and 26C. Highest PNs were at 30/22C and 26/22C. Carbohydrate export increased with increasing day temperature, whereas the response to night temperature was minimal. High levels of nonstructural carbohydrates occurred at thermoperiods (22/22C and 26/22C) that optimize seedling growth. However, definitive trends relating seedling growth to PNs, leaf carbohydrate levels, or to the amount of carbohydrate exported from the leaves were difficult to generalize due to numerous day × night interactions.