Limited research has been conducted and uncertainty exists regarding sugarcane response to water stress during different development phases. This information is necessary to optimize the allocation of limited irrigation water for sugarcane production. The objective of this study was to understand and quantify the response of crop water use (CWU), canopy development, stalk elongation, biomass accumulation and partitioning, and sugarcane yield to mild water stress, imposed through deficit drip irrigation, during different development phases. A field experiment consisting of a plant and first ratoon crop of cultivar N49 was conducted near Komatipoort. For the three water stress treatments, available soil water (ASW) was maintained between 30 and 60% of capacity during the tillering phase (TP), stalk elongation phase (SEP) and through both phases. ASW was maintained above 60% of capacity in the well-watered control and during periods when stress was not intended. Rainfall prevented water stress from developing in the TP of the plant crop. In the ratoon crop, 72% less irrigation was applied in the TP, resulting in 50 days of stress (ASW<50%). This did not affect stalk population but reduced CWU by 13%, shortened stalks by 21% and affected the canopy by reducing green leaf number (GLN) and green leaf area index (GLAI). Relieving the stress during SEP allowed the crop to re-establish its canopy, capture adequate photosynthetically active radiation (PAR) and restore rates of photo-assimilation (as suggested by CWU) and stalk elongation to support rapid biomass production. This restoration of plant processes allowed the ratoon crop to attain a cane and stalk dry biomass (SDM) yield that was only 9 and 11% lower (statistically insignificant), respectively, than the well-watered control at lodging (crop age of 286 days). During the SEP of the plant and ratoon crop, 42 and 85% less irrigation was applied, resulting in the crops experiencing 74 and 39 days of stress and using 7 and 8% less water, respectively. This did not affect stalk population or the crop canopy, but reduced stalk height by about 6 and 14% in the plant and ratoon crops, respectively. In both crops, shorter stalks and a negatively affected CWU which reduced photo-assimilate production, reduced cane yield by 14 and 10% (statically insignificant) and SDM yield by 15 and 5% (statistically insignificant), in the plant and ratoon crops respectively. © University of Pretoria iv Deficit irrigation throughout the TP and SEP of the ratoon crop reduced irrigation amount by 74%, resulting in 110 days of stress and reducing CWU by 16% and stalk height by 14%. PAR capture was reduced through reduced GLAI. This resulted in a significant reduction of 15% in cane yield. SDM yield was reduced by 17%, although this was not statistically significant. Stalk sucrose content was not influenced by deficit irrigation but was rather dependent on the duration of the drying-off period prior to harvest. Sucrose yields were therefore largely determined by SDM. Results suggest that the soil water potential (SWP) measured at 0.25 and 0.40 m depths, halfway between drip emitters within a plant or ratoon crop, can drop to about -40 kPa before irrigation is applied, without sacrificing cane or sucrose yield. Lastly, a ratoon crop can rapidly recover from stress during the TP, provided that the SWP during SEP is maintained above -40 kPa. / Dissertation (MSc Agric)--University of Pretoria, 2013. / gm2014 / Plant Production and Soil Science / unrestricted