Rational indicators for irrigation scheduling of cotton based on microchanges of stem diameter

Both pot experiment and field experiment were conducted by continuous measurements of stem diameter variations, predawn leaf water potential and soil relative water content for growing cotton(Gossypium hirustum L. "Meimian 99B") in 2004 and 2005. The suitability of using parameters derived from stem diameter measurements, including maximum daily stem shrinkage(MDS), daily increase(DI), and recovering time(RT) as indicators for irrigation scheduling of cotton was evaluated. The results show that the above three parameters are highly sensitive to the changes of plant water status. As predawn leaf water potential gradually decreased, MDS increased markedly, DI decreased slowly and slightly-varying under relatively high predawn leaf water potential, and RT increased sharply, then remained 24 h when the predawn leaf water potential was below a critical level of -0.1 MPa. Nevertheless, there was relatively high plant-to-plant variability in the MDS and DI measurements. Compared with MDS and DI, RT had the lowest variability(CV of 5.02% and 8.5% under high and low water content respectively). For rapidly growing cotton, there was a strong power relationship between RT and soil relative water content (r²=0.7101). Based on the high correlation between RT and predawn leaf water potential and soil relative water content, the RT range corresponding to mild water stress and severe water stress was determined, and RT was relatively constant in corresponding range of soil water content. It is suggested that RT is a comparatively ideal parameter with its high sensitivity to water deficit and low variability when utilizing the stem diameter-derived indicators for irrigation scheduling in growing cotton, but it is difficult to use MDS or DI as a single parameter for irrigation scheduling as a result of its high variability and difficulty of determining a critical value corresponding to water stress.