Photosynthetic activity in C4 sugarcane has been suggested to be regulated by the demand for photoassimilate from sink tissues, including culm storage of sucrose and other components of the plant (e.g. roots and shoot primordia). This study examined the extent to which sink-demand influences leaf photosynthetic activity and controls leaf turnover in field-grown sugarcane. To increase sink-demand on selected leaves, plants were defoliated apart from the immature leaf before the first fully expanded leaf (2nd leaf) and the mature 8th leaf. Changes in leaf gas exchange and fluorescence characteristics were recorded for both leaves over a 28 d period. Variations in leaf and culm sucrose and hexose concentrations and allocation of 14C-labelled photosynthate were also measured. A decrease in culm internodal sucrose concentrations in partially defoliated plants was associated with significant increases in assimilation (A) and electron transport rates (ETR) for both leaves 2 and 8. Conversely, accumulation of sucrose in the culms of control plants was related to a decline in photosynthetic rates in leaf 8 during the treatment period. Leaves 2 and 8 of defoliated plants (27 d) were characterised by an increase in partitioning of 14C to mature and immature internodes, respectively, compared to control plants. In addition, hexose levels in leaves of defoliated plants decreased significantly (36% and 48% decrease in leaves 2 and 8, respectively) compared to corresponding leaves of untreated controls over the duration of the experiment, indicating that the signaling mechanisms regulating the decline in leaf photosynthetic activity are likely hexose-mediated. It was concluded that leaf physiological ageing in sugarcane was promoted by sucrose accumulation during culm maturation as a consequence of decreased sink-demand for photosynthate.