Initial growth of seedlings of flame azalea in response to day/night temperature

Seedlings of flame azalea [Rhododendron calendulaceum (Michx.) Torr] were grown for 12 weeks under long-day conditions with days at 18, 22, 26, or 30C for 9 hours in factorial combination with nights at 14, 18, 22, or 26C for 15 hours. Total plant dry weight, top dry weight, leaf area, and dry weights of leaves, stems, and roots were influenced by day and night temperatures and their interactions. Dry matter production was lowest with nights at 14C. Root, leaf, top, and total dry weights were maximized with days at 26C in combination with nights at 18 to 26C. Stem dry weight was maximized with days at 26 to 30C and nights at 22C. Leaf area was largest with days at 18 and 26C in combination with nights at 18 or 26C. Within the optimal day/night temperature range of 26C/18-26C for total plant dry weight, them was no evidence that alternating temperatures enhanced growth. Shoot : root ratios (top dry weight : root dry weight) were highest with days at 18 and 30C. Leaf area ratio (total leaf area : total plant dry weight) was highest and specific leaf area (total leaf area : leaf dry weight) was largest when days and nights were at 18C and were lower at higher temperatures. Regardless of day/night temperature, leaf weight ratio (leaf dry weight : total plant dry weight) was higher than either the stem weight ratio (stem dry weight : total plant dry weight) or root weight ratio (root dry weight : total plant dry weight). Net leaf photosynthetic rate increased with day temperatures up to 30C.