PARENT SESSION
Posters P8C C4 and CAM. Abstracts (685-698)

Effects of growth light conditions on the organization of the photosynthetic apparatus in the C₄ plant leaves. Yushi Tazoe^*,1, Ko Noguchi¹, Ichiro Terashima¹, ¹ Department of Biology, Graduate School of Science, Toyonaka, Osaka, Japan

ABSTRACT- Increasing CO₂ concentration around Rubisco in bundle sheath chloroplasts (BSC) enables C₄ plants to perform an efficient CO₂ assimilation under high light conditions. However, leaves of C₄ plants growing in dense stands do not always receive high light. Under low light conditions, C₄ photosynthesis would be partly limited due to an inactivation of some C₄ enzymes. In this study, to investigate how C₄ photosynthesis responds to low light conditions, and whether C₄ plant has shade acclimation mechanisms, we grew Amaranthus cruentus L. (NAD-ME subtype) in high- (HL) and low-light (LL) conditions. We measured amounts of various photosynthetic proteins, CO₂ assimilation rate and carbon isotope ratio of the leaves. We also examined leaf structure. In LL leaves, chlorophyll content per leaf area was greater, chlorophyll a/b ratio was lower, and BSC grana were more developed than in HL leaves, indicating acclimation of LL leaves to the shade. On the other hand, amounts of Rubisco and C₄ enzymes (PEPC, PPDK) and a light-saturated rate of photosynthesis at 360 ppm CO₂ per leaf area were similar irrespective of the growth light conditions. These results showed that LL leaves also performed C₄ photosynthesis at the light-saturation condition. However, carbon isotope discrimination () in LL leaves was greater than in HL leaves (LL 11‰, HL 8‰). Therefore CO₂ leakage from the BS compartment would occur in LL leaves under LL condition. Cell size, intercellular air spaces and interveinal distance were greater in LL leaves, which would facilitate CO₂ leakage. Since CO₂ leakage from the BS compartment depends on the balance between C₄ enzyme and Rubisco activity, we are currently measuring these enzyme activities and CO₂ response curve of leaf photosynthesis. We will also measure the CO₂ leakiness by the concurrent measurements of gas exchange and carbon isotope discrimination.

KEY WORDS: PEPC, shade acclimation, C₄ photosynthesis, Rubisco