SEEDLING GROWTH OF CATAWBA RHODODENDRON .2. PHOTOSYNTHESIS AND CARBOHYDRATE ACCUMULATION AND EXPORT

Catawba rhododendron (Rhododendron catawbiense Michx.) seedlings of tw o provenances, Johnston County N.C. (35 degrees 45'N, 78 degrees 12'W, elevation = 67 m), and Yancey County, N.C. (35 degrees 45'N, 82 degre es 16'W, elevation = 1954 m), mere grown in controlled-environment cha mbers for 18 weeks with days at 18, 22; 26, or 30C in factorial combin ation with nights at 14, 18, 22, or 26C. Seedlings of the higher-eleva tion provenance generally exhibited higher net leaf Photosynthetic rat es,(P-N)s than those from the lower elevation at all temperature combi nations. Thus, it appears seedlings of the high-elevation provenance p ossess greater relative thermotolerance, expressed as net photosynthes is, than the low-elevation provenance. Eighty-seven days after initiat ion (DAI) of,the experiment, P-N showed a quadratic response to increa sing day temperature, with the maximum occurring at 22C, whereas P-N d ecreased linearly with increasing night temperature; At 122 DAI, P-N i ncreased linearly with increasing day temperature with Rights at 22 an d 26C. Highest P(N)s were at 30/22C and 26/22C. Carbohydrate export in creased with increasing day temperature, whereas the response to night temperature was minimal. High levels of nonstructural carbohydrates o ccurred at thermoperiods (22/22C and 26/22C) that optimize seedling gr owth. However, definitive trends relating seedling growth to P(N)s, le af carbohydrate levels, or to the amount of carbohydrate exported from the leaves were difficult to generalize due to numerous day x night i nteractions.