THE INFLUENCE OF NITROGEN, LIGHT AND WATER-STRESS ON CO2 EXCHANGE AND ORGANIC-ACID ACCUMULATION IN THE TROPICAL C3-CAM TREE, CLUSIA-MINOR

The carbon balance and changes in leaf structure in Clusia minor L. were investigated in controlled conditions with regard to nitrogen supply and responses to low and high photosynthetically active radiation (PAR). Nitrogen deficiency and high PAR led to the production of smaller leaves with higher specific leaf dry weight (SLDW) and higher leaf water content, but with lower chlorophyll content. Nitrogen and PAR levels at growth also affected CO2 exchange and leaf area. In - N conditions, total daily net CO2 uptake and leaf area accumulation were slighly less for high-PAR-grown plants. In contrast, high-PAR-grown plants supplied with nitrogen showed about a 4-fold higher total daily CO2 uptake and about twice the total leaf area of low-PAR-grown plants. Although total daily net CO2 uptake of + N plants was only slightly higher than - N plants under the low PAR level, + N plants produced almost three times more leaf area but with lower SLDW. Under well-wanted conditions, low-PAR-grown plants showed only CO2 evolution during the night and malic acid levels decreased. However, there was considerable night-time accumulation of titratable protons due to day/night changes in citric acid levels. High-PAR-grown plants showed net CO2 uptake, malate and citrate accumulation during the dark period. However, most of the CO2 fixed at night probably came from respiratory CO2. Positive night-time CO2 exchange was readily observed for low-PAR-grown plants when they were transferred to high PAR conditions or when they were submitted to water stress. In plants grown in high and low PAR, CAM leads to a substantial increase in daily water use efficiency for water-stressed plants, although total net CO2 uptake decreased.