Using soil drying as a regulative tool to enhance crop water use efficiency
Jianhua Zhang 

School of Life Sciences and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, Hong Kong

Abstract
We have been using soil drying as a regulative tool to improve crop water use efficiency at two fronts. The first one is to exploit stomatal responses to soil drying when part of the plant root system is irrigated while the rest part is left drying. We have developed several irrigation practices such that partial root-zone drying is maintained in the field for long term such that plant transpiration efficiency, termed as plant physiological water use efficiency, can be improved. In an extreme case of cotton production where almost all watering is delivered by irrigation, water use can be effectively reduced by 30% while the seed cotton yield was reduced by only 5%. The partial root-zone soil drying also promoted early flowering and early cotton harvest, which actually increased the cotton market price and reduced harvesting cost.
At another front, we use soil drying to enhance rice grain filling such that plant harvest index can be substantially improved and crop water productivity, termed as agronomic crop water use efficiency, is enhanced. Harvest index has been shown as a variable factor in rice production, especially in cases where whole plant senescence is unfavourably delayed. Such delayed senescence can delay the remobilisation of pre-stored carbon reserves in the straw and results in lower harvest index. A controlled soil drying can enhance whole plant senescence and therefore improve the remobilisation of pre-stored carbon reserve. We have developed a water management scheme for the rice production, the so called alternate wetting and drying scheme. Grain filling period is shortened but better use of pre-stored food reserve is achieved. Rice harvest index and water productivity are effectively enhanced without reduction in grain yield.

Key Words: plant water stress, water-saving crop production, grain filling, water use efficiency