LEAF CAVITY CO2 CONCENTRATIONS AND CO2 EXCHANGE IN ONION, ALLIUM-CEPAL

Onion (Allium cepa L.) plants were examined to determine the photosynt hetic role of CO2 that accumulates within their leaf cavities. Leaf ca vity CO2 concentrations ranged from 2250 mu L L(-1) near the leaf base to below atmospheric (< 350 mu L L(-1)) near the leaf tip at midday. There was a daily fluctuation in the leaf cavity CO2 concentrations wi th minimum values near midday and maximum values at night. Conductance to CO2 from the leaf cavity ranged from 24 to 202 mu mol m(-2) s(-1) and was even lower for membranes of bulb scales. The capacity for onio n leaves to recycle leaf cavity CO2 was poor, only 0.2 to 2.2% of leaf photosynthesis based either on measured CO2 concentrations and conduc tance values or as measured directly by (CO2)-C-14 labeling experiment s. The photosynthetic responses to CO2 and O-2 were measured to determ ine whether onion leaves exhibited a typical C-3-type response. A line ar increase in CO2 uptake was observed in intact leaves up to 315 mu L L(-1) of external CO2 and, at this external CO2 concentration, uptake was inhibited 35.4 +/- 0.9% by 210 mL L(-1) O-2 compared to 20 mL L(- 1) O-2. Scanning electron micrographs of the leaf cavity wall revealed degenerated tissue covered by a membrane. Onion leaf cavity membranes apparently are highly impermeable to CO2 and greatly restrict the ref ixation of leaf cavity CO2 by photosynthetic tissue.