**5. Conclusion**

456 Soybean Physiology and Biochemistry

flowers) or petiole girdling suggest that a decrease of stomatal conductance or Rubisco activity or Rubisco content in leaf, or both decreases of Rubisco activity and Rubisco content in leaf can be responsible for the decrease in leaf photosynthetic rate under excessive photosynthetic source capacity (Mondal et al., 1978; Setter & Brun, 1980; Setter et al., 1980; Wittenbach, 1982, 1983; Xu et al., 1994; Crafts-Brandner & Egli, 1987; Cheng et al., 2008). As described in the Introduction, excising sink organs or high CO2 treatment can have side effect(s) other than inducing excessive photosynthetic source capacity. In the present study using intact soybean plants in which excessive photosynthetic source capacity was constructed by treatment of continuous exposure to light, visible damages such as leaf decolorization and wilt were not observed. Treatment of continuous exposure to light did not affect significantly leaf chlorophyll, total protein and water contents analyzed. However, as mentioned above, totally, the effects of indirectly constructed excessive photosynthetic source capacity on leaf carbohydrate status, photosynthetic rate, stomatal conductance, Rubisco activity and photosynthetic gene expressions including Rubisco gene expression are similar to those of excessive photosynthetic source capacity that is constructed by treatment

Regarding the detailed mechanism(s) of why leaf photosynthetic rate declines under excessive photosynthetic source capacity, recent studies using transgenic plants show that hexokinase could be involved in carbohydrate-mediated repression of photosynthetic gene expression (Jang et al., 1997; Dai et al., 1999; Moore et al., 2003). Other recent study shows that protein kinases (KIN10 and KIN11) may be involved in governing the entirety of carbohydrate metabolism, growth and development in response to carbohydrates in plants (Baena-Gonzalez et al., 2007). Data from a study investigating the effect of chilling stress on leaf photosynthetic rate suggest that H2O2, a reactive oxygen species can induce deactivation of Rubisco (Zhou et al., 2006). As described in the Introduction, inorganic phosphate has been found to promote activation of Rubisco by enhancing the affinity of uncarbamylated inactive Rubisco to CO2 (Bhagwat, 1981; McCurry et al., 1981; Anwaruzzaman et al., 1995). Data from a more recent study suggest that pH within the chloroplasts can be an important factor affecting leaf photosynthetic rate, since the study has demonstrated that pH can affect distribution of Rubisco activase within the chloroplasts by affecting binding of the enzyme to the thylakoid membranes (Chen et al., 2010). Distribution of Rubisco activase within the chloroplasts can affect activation state of Rubisco, since Rubisco activase plays a role in promoting the activation of Rubisco by dissociating RuBP from uncarbamylated inactive Rubisco (Crafts-Brandner & Salvucci, 2000), which tightly binds RuBP (Jordan & Chollet, 1983). Since ATP is needed for the catalytic action of Rubisco activase (Crafts-Brandner & Salvucci, 2000) and it is well known that ATP is needed for regeneration of RuBP, a substrate for Rubisco in Calvin cycle (see Kasai, 2008), it is evident that ATP is also an important factor affecting leaf photosynthetic rate. However, the precise mechanism of how hexokinase and protein kinases exercise regulation of photosynthetic carbohydrate metabolism including the carbohydrate-mediated repression of photosynthetic gene expression is not yet clear. In addition, effects of excessive photosynthetic source capacity on the levels of H2O2, inorganic phosphate, pH and ATP within the chloroplasts in which central photosynthesis is performed have not been analyzed in intact plants at real times under light. A main reason seems to be the lack of appropriate methods. Therefore, further researches including those following the development of new methods are important to elucidate further the regulatory mechanism for leaf photosynthesis under excessive

of continuous exposure to light.

photosynthetic source capacity.

Studies using single-rooted soybean leaves, each of which is constituted from a primary leaf, a short petiole and roots developed from the petiole, have implicated that there is a regulation of leaf photosynthesis through deactivation of Rubisco, which is associated with accumulation of photosynthetic carbohydrates in leaf under excessive photosynthetic source capacity. The present study using intact soybean plants from which single-rooted soybean leaves can be made has also suggested the same regulatory mechanism for leaf photosynthesis under excessive photosynthetic source capacity. It is therefore concluded that for efficient improvement of plant matter (biomass) production, well-balanced improvement of source and sink would be essential. Further studies are desired for more complete understanding of the regulatory mechanism for leaf photosynthesis under excessive photosynthetic source capacity and its application.
