Relation between Midday Depression of Photosynthesis and Leaf Water Status in Soybean Cultivars

Physiological attributes relating to cultivar difference in midday depression of appparent photosynthetic rates (AP) of soybean leaves were examined in terms of leaf water status. In a 3-year experiment with 9 (1990) or 10 (1991, 1992) cultivars grown on the naturally rain-fed field and rain-intercepted field, the following parameters were measured twice a day (0900–1100 h, 1300–1500 h): AP, stomatal conductance (Gs), water potential (Ψw), osmotic potential (Ψs), pressure potential (Ψp), relative water content (RWC) and exudation rate (Ex) during flowering to early podfilling. The degree of midday decline of AP (APdec), which was expressed as a ratio of AP measured at 1300–1500 h to AP at 0900–1100 h, was significantly correlated with the decline of Gs on 5 out of 9 occasions. On an occasion where the vapor pressure deficit was extremely large, the cultivar with the highest APdec (Peking) maintained higher Ψw compared with cultivars with lower APdec (Sogazairai etc.). When soil water availability was restricted by intercepting rainfall, Peking tended to have lower RWC and to maintain higher Ψp by lowering Ψs than Sogazairai, suggesting that leaf water status was responsible for the cultivar difference in APdec. On occasions where there was adequate rainfall, however, midday decline of AP often occurred with little decline of Ψw. A positive correlation of Ex with APdec and Gsdec among cultivars was observed. Thus, cultivar difference in the degree of midday depression of photosynthesis appeared to be associated with leaf water status, although other physiological attributes could often predominate, probably depending on the environmental conditions.