THE EFFECT OF NITROGEN-SOURCE ON PHOTOSYNTHESIS OF CAROB AT HIGH CO2 CONCENTRATIONS

Carob seedlings (Ceratonia siliqua L. cv. Mulata), fed with nitrate or ammonium, were grown in growth chambers containing two levels of CO2 (360 or 800 mu 1 1(-1)), three root temperatures (15, 20 or 25 degrees C), and the same shoot temperature (20/24 degrees C, night/day temper ature). The response of the plants to CO2 enrichment was affected by e nvironmental factors such as the type of inorganic nitrogen in the med ium and root temperature. Increasing root temperature enhanced photosy nthesis rate more in the presence of nitrate than in the presence of a mmonium. Differences in photosynthetic products were also observed bet ween nitrate- and ammonium-fed carob seedlings. Nitrate-grown plants s howed an enhanced content of sucrose, while ammonium led to enhanced s torage of starch. Increase in root temperature caused an increase in d ry mass of the plants of similar proportions in both nitrogen sources. The enhancement of the rates of photosynthesis by CO2 enrichment was proportionally much larger than the resulting increases in dry mass pr oduction when nitrate was the nitrogen source. Ammonium was the prefer red nitrogen source for carob at both ambient and high CO, concentrati ons. The level of photosynthesis of a plant is limited not only by atm ospheric CO2 concentration but also by the nutritional and environment al conditions of the root.