Variable Photosynthetic Metabolism in Leaves and Stems of Cissus quadrangularis L . ' Received

By measuring titratable acidity, gas exchange parameters, mesophyli succulence, and lSC/UC ratios, we have shown that Cissus quadrgudaris L. has C3-like leaves and stems with Crassulacean acid metabolsm (CAM). In addition, the nonsucculent leaves show the diurnal fluctuations in organic acids termed recycling despite the fact that all CO2 uptake and stomatal opening occurs during the day. Young succulent stems have more Cs photosynthesis than older stems, but both have characteristics ofCAM. The genus Cissus will be a fnitful group to study the physiology, ecology, and evolution of C3 and CAM since species occur that exhbt characteristics of both photosynthetic pathways. CAM is the photosynthetic mechanism in which CO2 uptake and stomatal opening occur at night. The CO2 is fixed initially into malic acid which is decarboxylated during the following day. The CO2 is then fixed into carbohydrate via the C3 cycle (4). It has been shown that CAM is an adaptation to arid environments, since the ratio ofwater transpired relative to CO2 absorbed is very small (4). Several combinations of C3 and CAM photosynthesis have been reported. Old leaves of Kalanchoe show more CAM and less C3 photosynthesis than do young leaves (4). CAM in Kalanchoe blossfeldiana increases relative to C3 metabolism when grown on SD (6). Membryanthemum crystallinum (14) and Portulacaria afra (12) are examples of plants which can shift from C3 to CAM when water stressed. M. crystallinum has a high level of acidity and shows acid flux only when water stressed (14). P. afra, however, sustains a high level of acidity even when well watered, although acid flux occurs only when it is water stressed (12). Frerea indica is the only reported case in which different photosynthetic pathways are segregated in different anatomical structures of the plant (5). Deciduous leaves of F. indica show C3 metabolism, while the succulent stems have CAM (5). We report here yet another variation between C3 and CAM in Cissus quadrangularis (Vitaceae). The succulent stem shows increased CAM photosynthesis with age while the leaves have the C3 mode with some recycling of respired CO2. MATERIALS AND METHODS Plant cuttings of Cissus quadrangularis L. were grown in a sandy loam soil in a greenhouse. Plants were watered as needed to avoid stress. 'Supported in part by National Science Foundation grants PCM 8200366 and ATM 79-24591. Mesophyll succulence was calculated from the ratio of tissue water to total Chl (2) in fresh leaf and stem samples. Total water content was determined by comparing the weights of fresh samples and samples dried in a microwave oven for 10 min. Titratable acidity was determined by grinding tissue with a hand tissue grinder and titrating to pH 7.0 with 0.01 N KOH solution using an automatic titrator. Diurnal CO2 uptake and stomatal conductance were measured under greenhouse conditions with a dual isotope porometer (3). Whole tissue samples were prepared for carbon isotope ratio determination by grinding in liquid N2 and freeze drying. Starch was extracted from the ground samples by the method of Pucher et al. (9). Powdered tissue and starch samples were combusted by a modified version (8) of the Stump and Frazer method (10). CO2 was purified from the combustion products by cryogenic distillation and its '3C/12C ratio determined by mass spectrometry. Carbon isotope ratios are expressed as 813C values, where 813C [(C/12 )samP I ] X 100 13c1312C taXnda0% The standard is the Peedee belemnite carbonate. Precision of the 813C measurements was ±0.2%o. RESULTS Organic acid fluctuations (Fig. la) of about 100 ,eq titratable acidity per g fresh weight in the stem of C. quadrangularis were quite marked and typical of CAM plants (4). The leaves also showed some acid fluctuation, but the magnitude was only about 30 ,iq/g fresh weight. This amount of organic acid fluctuation has been observed in succulent plants not showing CAM and can be explained by recycling of respired CO2 during the night period (13). Leaves of C. quadrangularis did not fix CO2 (Fig. lb) and had negligible stomatal conductance (Fig. lc) during the night. Stems, however, had considerable CO2 uptake and stomatal conductance during the day and night period. Previous work has shown that the leaves of C3 species have mesophyll succulence (g H20/mg Chl) less than 1.0 whereas the green photosynthetic tissues of CAM plants have mesophyll succulence between 1.0 and 10.0 (4). Based on analysis of mesophyll succulence, the leaves of C. quadrangularis are typical of C3 plants and the stems are typical ofCAM species (Table I). Carbon isotopic ratios of different tissues of C. quadrangularis are shown in Table II. Isotope ratios of whole leaves are typical of C3 plants, while those of stems are typical ofCAM plants (1, 7). Younger stems had lower mesophyll succulence (Table I) and 813C values for total organic carbon than older stems (Table II). This suggests that younger stems have a higher proportion of C3 metabolism to CAM than older stems. Isotopic ratios of total 677 www.plantphysiol.org on July 22, 2017 Published by Downloaded from Copyright © 1983 American Society of Plant Biologists. All rights reserved. Plant Physiol. VoL 71, 1983