Main 44described by the standard biochemical model of photosynthesis (Farquhar et al. 1980), 126 the instantaneous rate of photosynthesis by C 3 plants is limited either by the capacity 127 of the enzyme Ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) 128 for the carboxylation of RuBP (V cmax ), or by the rate of electron transport for the 129 regeneration of RuBP, which depends on absorbed light and the electron transport 130 capacity (J max ). R d is used to support diverse metabolic processes including protein 131 turnover, phloem loading, the maintenance of ion gradients between cellular 132 compartments, nitrate reduction, and the turnover of phospholipid membranes. 133Among these, protein turnover is the largest contributor to R d variation and is 134 expected to scale closely with V cmax , which sets the daily maximum photosynthetic 135 rate achieved by leaves under natural growing conditions (Amthor 2000; Atkin et al.

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