Using Real-Time Observation of the Soil-Plant-Atmosphere-Continuum to Predict Daily Water-Use in Polytunnel Grown Raspberry Rubus Idaeus L. Plants.

In this study we present a method for the real-time observation of the soil-plant-atmosphere-continuum (SPAC) using vapour pressure deficit (VPD), substrate moisture and irrigation water volume measurements. The experimental arrangement consisted of 8 raspberry plants (Glen Ample) grown in 4 pots of coir under a polytunnel. Coir substrate specific sources of measurement variance have been addressed by using a substrate temperature correction algorithm for the capacitance-based moisture measurements. Precision irrigation, using substrate moisture closed-loop control, was employed to maintain ‘stable’ substrate water status conditions. Coir moisture was set at levels to avoid run-off conditions. Two irrigation controllers were arranged so that 4 plants were maintained at a coir moisture of 40% whilst 4 plants had the coir moisture stepped from 40% to 35% (for several days) and then again to 30%. We collected daily water-use (DWU) and daily average VPD (DAVPD) data and obtained a linear relationship with DWU/DAVPD over a 29-day period. Extrapolation of this relationship for the next 10-days enabled DWU predictions to be derived from DAVPD data. These predictions resulted in a linear relationship with measured DWU, achieving an R² of 0.83 - during this time the substrate moisture was reduced to 30%. Our predictive DWU method broke down on 3 consecutive days when the peak polytunnel VPD ranged from 4kPa to 8kPa and air temperatures exceeded 40°C, indicating stomatal closure and plant stress. We fitted a Blackman relative growth rate equation through the initial 39-days of DWI/DAVPD data from the 4 plants maintained at 40% substrate moisture. Variances from the fitted model identified the period of more extreme polytunnel environmental conditions and indicated a 3-5-day pause in plant growth.