An important factor is under what conditions does C4 photosynthesis give benefit. Clearly, under light saturation at high temperature, significant gains in photosynthesis will result with the provisos described by Edwards et al (this volume). However, the benefit at limiting light is uncertain. Enhanced quantum yield for C4 has been observed at limiting light, provided temperature exceeds 25 °C as is the norm for tropical rice throughout the day. Important in this consideration, however, is the degree of coupling of the C4 and C, pathways. For example, significant leakage of fixed CO, can remove the already marginal quantum yield advantage at low light. Evolution has of course perfected the cellular differentiation and enzyme localization necessary to confer maximum growth advantage from the C4 pathway. The complexity involved represents an enormous challenge in the genetic manipulation of rice. Without such perfection, introduction of C4 may lead to a loss of quantum yield and a yield disadvantage under some conditions, such as the tropical wet season when solar radiation is low. Moreover, significant amounts of photosynthesis are carried out at limiting light intensity even under conditions of high solar radiation, and yield losses could be significant under these conditions.
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