The impact of water stress is simulated through the regulation of different processes (carbon assimilation, expansive growth − both aerial and root organs − , tiller development, and sucrose accumulation) according to an approach proposed by Singels & Bezuidenhout (2002). Different water stress factors (SWDFn), which represent the rate of the n-th process relative to the unstressed rate, are determined by potential water supply and demand ratio (0 is fully stress and 1 no stress conditions), using the approach implemented in CERES (Jones and Kiniry, 1986; Ritchie et al., 1986):

where fi is a dimensionless factor, inverse value of a species parameter (RWUEPi, the threshold below which the n-th process is limited), WSp is potential root water uptake from the soil (mm day-1) and TRmax is maximum transpiration rate (mm day-1). SWDF1 is the factor influencing carbon assimilation and root depth and SWDF2 is the factor influencing the plant elongation rate, the leaf are index, and the sucrose accumulation.

Once WSp/TRmax drops below a severe water stress threshold (defined by Critsw, a species parameter), photosynthesis only gradually recover from water stress. In this case SWDF1 is linearly increased as a function of the accumulated thermal time since the since the day of severe water stress event, with thermal time for full recovery defined by a species parameter (HuRecover, °C-day). The linear recovery is stopped when SWDF1 drops again below the Critsw value before the specified period, thermal time is reset to zero and the gradual recovery restarts.

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