Global increase in ambient temperature is a critical factor for plant growth. In order to study the changes in growth over short intervals, various primary and secondary metabolites, and their relationships with thermotolerance, 1-month-old sugarcane (Saccharum officinarum) sprouts were grown under control conditions (28 degrees C) or under heat-stress conditions (40 degrees C), and measurements were made at six 12-h intervals. Heat stress greatly reduced dry matter and leaf area of sprouts initially but only nominally later on. Changes in the rates of relative growth and net assimilation were greater than relative leaf expansion, indicating an adverse effect of heat on assimilation of nutrients and CO(2) in producing dry matter. Although reduction in leaf water potential was an immediate response to heat, this effect was offset by early synthesis of free proline, glycinebetaine and soluble sugars (primary metabolites). Among secondary metabolites, anthocyanin synthesis was similar to primary metabolites; carotenoids and soluble phenolics accumulated later while chlorophyll remained unaffected. Relationships of growth attributes and metabolite levels, not seen in the controls, were evident under heat stress. In summary, observed changes in metabolite levels were spread over time and space and were crucial in improving net assimilation and heat tolerance of sugarcane.