Physiological Factors Controlling Growth of Rhododendron Species
Carole Saravitz, Ph.D.
North Carolina State University
Raleigh, North Carolina
Summary of Research
Growth of rhododendrons in the wild usually occurs in understory settings in soils high in organic matter and low in availability of mineral nutrients. To gain a better understanding of the growth of rhododendrons in such conditions, plants were grown in hydroponic culture where pH can be controlled precisely and mineral nutrient uptake can be measured. Plant growth was monitored by measuring the uptake of essential mineral nutrients and then replenishing the nutrients in the solution as well as recording final total plant weight. In the first experiment, Rhododendron calendulaceum plants were placed in continuous flow hydroponic culture containing a complete nutrient solution and solution pH was adjusted to 4.0, 4.7, 5.4 or 6.1. Both nitrogen (N) uptake and final total plant dry weight were related to the pH of the nutrient solution. Greatest dry mass and greatest N uptake occurred at the lowest pH (4.0) and both total plant dry weight and N uptake decreased with increasing pH.
In a second experiment, Rhododendron catawbiense plants were placed in continuous flow hydroponic culture containing either nitrate (NO3-) or ammonium (NH4+) as the N source and solution pH was adjusted to 4.0 or 6.0. The source of N in the solution affected the response of the plants to pH. As in the first experiment, when NO3- was used as the only N source, plants gained greater dry weight when grown in nutrient solution with the pH adjusted to 4.0 compared to plants growing in solution with the pH adjusted to 6.0. Interestingly, rhododendron plants grown in nutrient solution containing NH4+ as the sole source of N responded differently to pH. Dry matter accumulation was not significantly different between plants growing at pH 4.0 or 6.0 when plants were grown in nutrient solution containing NH4+ only. Thus rhododendron plants were less sensitive to pH when they are grown with ammonium N than when grown with nitrate N. Our findings suggest that including NH4+ in fertilizer applications may prevent difficulties in N uptake if soil pH rises.