[Response of water status indicators in apple saplings to drought stress under shelter from rain.]

The main tree water status indicators which sensitively responded to drought stress and related to tree water balance were investigated in treatment of progressive decrease of soil water potential under shelter from rain. The results showed that stem maximum daily shrinkage (MDS) and midday stem water potential (Ψ_stem) were most sensitive to drought stress among all the water status indicators. MDS not only significantly responded to reference crop evapotranspiration (ET₀), but also was sensitive to soil drought stress. MDS was significantly positively related to ET₀, and the correlation between relative stem daily maximum shrinkage (MDS_r) and relative soil water potential (Ψ_{r soil}) was highly significant. Moreover, the stems could be measured in succession and recorded automatically. Midday Ψ_stem was also sensitive to soil drought stress, and significantly negatively related to ET₀. The correlation between relative midday stem potential (Ψ_{r stem}) and Ψ_{r soil} was significant. But so far, it is difficult to automatically measure either leaf or stem water potential. Predawn leaf water potential (Ψ_pd), daily stem growth (DG) or stomatal conductance (g_s) also responded to drought stress to some extent under moderate or heavy drought stress, but they were not sensitive.