Changes in apparent photosynthesis, CO2 compensation point and dark respiration of leaves of some Poaceae and Cyperaceae species with senescence

CO₂ exchange characteristics of detached mature and senescentflag leaves and of bracts in some Poaceae and Cyperaceae species,respectively, were studied using a closed IR system. Senescentleaves, 30 to 45 days after flowering, showed lower rates ofapparent photosynthesis and dark respiration, and higher CO₂compensation points (CCP) than those measured at the floweringstage. In senescent C₄-Poaceae, the increase of CCP was small(from 4.8 to 10.1 ppm on the average) with little influenceof temperature, and the photorespiration level, 0.4 mg CO₂/dm²/hr,was as low as that in mature leaf, indicating the presence ofnormal C₄-characteristics. On the other hand, a C₄-Cyperaceae,Cyperus microria Steud., showed extensive increases of CCP (from9 to 41 ppm) and photorespiration (from 0.8 to 2.1 mg CO₂/dm²/hr)with senescence. (Received August 25, 1979; )