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 under360 and 800 µl l^-1CO₂. One of two nitrogen sources, nitrateor ammonium, was added to the nutrient medium at concentrationsof 3 mol m^-3. Root systems of the developing plants suppliedwith nitrate compared to those supplied with ammonium were characterizedby:(a)more biomass on the lower part of the root;(b)fewer lateralroots of first and second order;(c)longer roots;(d)higher specificroot length;(e)a smaller root diameter. The morphology of theroot systems of nitrate-fed plants changed in the presence ofelevated 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 totalleaf area in the presence of high carbon dioxide than in normalCO₂, due to an increase in epidermal cell size that led to developmentof larger leaflets with lower stomatal frequency. The observedchanges in the morphology of roots and shoots agreed with theresults observed for total biomass production. Nitrate-fed plantsincreased their biomass production by 100% in the presence ofelevated CO₂compared to 15% in ammonium-fed plants, indicatingthat the response of carob to high CO₂concentrations is verydependent on the nitrogen source. Under elevated CO₂, nitrate-grownplants had a larger content of sucrose in both roots and shoots,while no significant difference was observed in the contentof sucrose in ammonium-grown plants, whether in ambient or enrichedcarbon dioxide. Hence, the differences in soluble carbohydratecontents can, at least partly, account for differences in rootand shoot morphology.Copyright 1997 Annals of Botany Company Ceratonia siliquaL.; carob; ammonium; carbohydrate; carbon dioxide; nitrate; morphology; sucrose