Performance of various water stress indicators for prediction of fruit size response to deficit irrigation in plum

The performance of several continuous and discontinuous measurements of soil and plant water status was studied in a drip-irrigation experiment on plum (Prunus salicina Lindl. ‘Black Gold’) where different levels of water restriction during the fruit growth period were imposed, and compared to a more irrigated (control treatment) over 2 years. Soil matric potential (Ψm) was measured with granular matrix sensors (Watermark®) and short period variations of trunk diameter (TDV) with lineal variable differential transformer sensors. From TDV measurements, maximum daily shrinkage (MDS) and trunk growth rate (TGR) were calculated. Besides, predawn leaf water potential (Ψpd), midday stem water potential (Ψstem), and midday stomatal conductance (gs) were measured along both seasons. The seasonal variation of all indicators showed that, under mild water restrictions that reduced fruit weight and yield, Ψm and TGR did not gave a clear indication of a decrease in soil or plant water status. Instead, the evolution of the rest of indicators was in reasonable agreement with timing and severity of the water deficit applied. The degree of correlation between the various water status indicators and fruit weight at harvest was as follows: (Ψstem ≅ Ψpd ≅ gs > MDS > TGR > Ψm), but gs was much more variable than Ψpd and Ψstem. Based on these results we conclude that, although MDS, TGR and Ψm provide continuous measurement of plant and soil water status, both Ψpd and Ψstem should be preferable as water stress indicators in plum.