C4 photosynthetic carbon metabolism in the leaves of aromatic tropical grasses - I. Leaf anatomy, CO2 compensation point and CO 2 assimilation.

A few species of Cymbopogon and Vetiveria are potentially important tropical grasses producing essential oils. In the present study, we report on the leaf anatomy and photosynthetic carbon assimilation in five species of Cymbopogon and Vetiveria zizanioides. Kranz-type leaf anatomy with a centrifugal distribution of chloroplasts and exclusive localization of starch in the bundle sheath cells were common among the test plants. Besides the Kranz leaf anatomy, these grasses displayed other typical C4 characteristics including a low (0-5 µl/l) CO2 compensation point, lack of light saturation of CO2 uptake at high photon flux densities, high temperature (35°C) optimum of net photosynthesis, high rates of net photosynthesis (55-67 mg CO2 dm(-2) leaf area h(-1)), little or no response of net photosynthesis to atmospheric levels of O2 and high leaf (13)C/(12)C ratios. The biochemical studies with (14)CO2 indicated that the leaves of the above plant species synthesize predominantly malate during short term (5 s) photosynthesis. In pulse-chase experiments it was shown that the synthesis of 3-phosphoglycerate proceeds at the expense of malate, the major first formed product of photosynthesis in these plant species.