Relatively little is known about the physiological basis for variation in sucrose content among sugarcane clones despite substantial research at the molecular and biochemical levels. We used irrigation and continuous monitoring of photosynthesis and plant extension rate to modify dry matter partitioning in four clones differing widely in sucrose content. Three pot experiments were conducted on two low sucrose content clones, KQ97-2599 and KQ97-2835, and two high sucrose content clones, Q117 and KQ97-5080, in a temperature-controlled glasshouse. As expected, sucrose content on a dry mass basis of whole stalks was greater in high (55% maximum) than in low sucrose clones (40% maximum), but sucrose content in the two clones selected for low sucrose reached 55% in some internodes. Differences between clones in whole-plant net photosynthesis and aerial biomass accumulation were small. However, biomass was distributed over fewer stalks in the high sucrose clones (4–7 stalks per pot) than in the low sucrose clones (9–11 stalks per pot). The high sucrose clones also allocated a considerably greater proportion of dry matter to the stalk (70% maximum) than the low sucrose clones (60% maximum). It is suggested that the relatively large amount of new leaf tissue produced by the high tillering, low sucrose clones placed an additional demand for structural photo-assimilate in these clones and delayed the accumulation of sucrose in the stalk. The results indicated that there is little direct genetic control on the maximum amount of sucrose that can accumulate in stalk tissue and that genetic contrasts in sucrose content reside more in the morphology of the plant and responses to ripening stimuli such as mild water stress, and how these traits influence supply and demand for photo-assimilate.