Changes in the morphology of roots and leaves of carob seedlings induced by nitrogen source and atmospheric carbon dioxide

Carob seedlings were grown hydroponically for 9 weeks under 360 and 800 μl l^-1 CO₂. One of two nitrogen sources, nitrate or ammonium, was added to the nutrient medium at concentrations of 3 mol m^-3. Root systems of the developing plants supplied with nitrate compared to those supplied with ammonium were characterized by: (a) more biomass on the lower part of the root; (b) fewer lateral roots of first and second order; (c) longer roots; (d) higher specific root length; (e) a smaller root diameter. The morphology of the root systems of nitrate-fed plants changed in the presence of elevated carbon dioxide concentrations, resembling, more closely, that of ammonium-fed plants. Total leaf area was higher in ammonium- than in nitrate-fed plants. Nitrate-fed plants had greater total leaf area in the presence of high carbon dioxide than in normal CO₂, due to an increase in epidermal cell size that led to development of larger leaflets with lower stomatal frequency. The observed changes in the morphology of roots and shoots agreed with the results observed for total biomass production. Nitrate-fed plants increased their biomass production by 100% in the presence of elevated CO₂ compared to 15% in ammonium-fed plants, indicating that the response of carob to high CO₂ concentrations is very dependent on the nitrogen source. Under elevated CO₂, nitrate-grown plants had a larger content of sucrose in both roots and shoots, while no significant difference was observed in the content of sucrose in ammonium-grown plants, whether in ambient or enriched carbon dioxide. Hence, the differences in soluble carbohydrate contents can, at least partly, account for differences in root and shoot morphology.