Nitrogen form preference of six dipterocarp species

We investigated the nitrogen form preference of six dipterocarp species: Anisoptera costata Korth., Dipterocarpus obtusifolius Teijsm. ex Miq., Hopea odorata Roxb., Neobalanocarpus heimii (King) P. Ashton, Shorea faguetiana Heim, and Shorea roxburghii G. Don. Seedlings were supplied with nitrogen as nitrate, ammonium, or both in sand culture in a controlled environment. Except for N. heimii, all species showed greater shoot growth when supplied with ammonium than with nitrate. Higher root mass ratios were observed in all species with nitrate, which would be an adaptive response to limited nitrogen uptake. The five species, which preferred ammonium, showed a higher light-saturated photosynthetic rate with ammonium supply. The lower light-saturated photosynthetic rate with nitrate supply was a result of lower photosynthetic capacity, as indicated by a lower CO2-saturated photosynthetic rate. The lower leaf nitrogen content in seedlings supplied with nitrate would be the cause of the lower photosynthetic performance. Nitrate reductase activity in leaf and root of D. obtusifolius, N. heimii, and S. roxburghii showed generally low inducibility with nitrate.