Relationship Between the CO_2 Compensation Point and Photorespiration in Soybean Leaves

Effects of several factors on the CO 2 compensation point (Γ) in soybean leaves were observed by using a CI 301 photosynthetic gas analysis system to elucidate the relationship between the CO 2 compensation point and photorespiration (Rp). Both the CO 2 compensation point and photorespiratory rate increased with increasing light intensities above growth light intensity (Fig.2). The change in the CO 2 compensation point was mainly due to phosphate limitation occurring under strong light as demonstrated by the phosphate feeding experiment (Fig.3). Mannose 10 mmol/L (Table 1), 0.2% PEG 6000 (Fig.5), and DL glyceraldehyde 10 mmol/L (Table 3) all raised the CO 2 compensation point in soybean leaves, but photorespiratory rate was decreased by mannose (Table 1) and PEG (Fig.5) treatment, and increased slightly by DL glyceraldehyde treatment. KH 2PO 4 5 mmol/L lowered both the CO 2 compensation point and photorespiratory rate in soybean leaves (Fig.4). Moreover, the CO 2 compensation point was higher in aging leaves than that in those just fully expanded, but photorespiratory rate was lower in the former than that in the latter (Table 2). These results indicate that photorespiration is an important factor affecting the CO 2 compensation point, but is not the sole one in C 3 plant leaves. The CO 2 compensation point always changes with the ratio of photorespiratory rate to carboxylation efficiency (CE) in the same direction.