*4.5. Increasing Ascorbic Acid Prolongs Leaf Function*

Increasing Asc through increasing DHAR expression resulted in higher levels of ribulose bisphosphate carboxylase/oxygenase large subunit (RbcL), chlorophyll, and CO2 assimilation but this had no effect on plant growth under normal conditions [57]. In contrast, reducing Asc content through reduced biosynthesis resulted in slower shoot growth, smaller leaves, and reduced shoot fresh weight and dry weight [80] while plants with lower Asc content following suppression of DHAR expression exhibited a slower rate of leaf expansion, slower shoot growth, delayed flowering time, and reduced foliar dry weight [57]. These phenotypes correlated with reduced leaf function as measured by a disproportionate loss in chlorophyll a, a reduction in RbcL, and a lower rate of CO2 assimilation [57]. The lower rate of CO2 assimilation was not due to a limitation in CO2 diffusion into DHAR-suppressed leaves as the sub-stomatal CO2 concentration was actually higher [57]. Rather, the reduced growth likely resulted from a premature loss of leaf function and early onset of senescence in mature leaves that may have reduced photosynthate available to young leaves.
