Plant extension, soil water extraction and water stress in sugarcane.

Stalk growth rate has been used extensively in the sugar industry as a stress index for the irrigation of sugarcane. Automatic soil water monitoring has recently become available in the sugar industry and some growers and extension staff are now using this information as well as stalk extension measurements to schedule irrigation. In many regions of the sugar industry, irrigation is insufficient to meet crop water requirements and yield loss inevitably occurs before growers can irrigate. There is a need to assist growers and extension staff to interpret both soil water and plant extension measurements in terms of yield loss to avoid using scarce water prematurely and to avoid unnecessary delays in growth recovery after water stress. It was shown that water stress in sugarcane can be detected early by regular automatic measurement of leaf extension, making use of the diurnal extension pattern to minimise confounding effects of temperature on leaf extension. A stress index based on leaf extension rate measured in the early morning and during the day was highly sensitive to changes in soil water content and climatic conditions. Soil water content 400 mm below the stool was correlated most with the stress index. The stress index reached a critical value of 0.5 when 75% to 80% of available water at this depth had been extracted. Total crop fresh biomass was reduced after 5 days of stress beyond the critical level, and dry biomass was reduced after 10 days. These relationships can be used to develop guidelines for irrigation based on automatic soil moisture monitoring.