Using sap flow sensors for estimating tomato transpiration in greenhouse soilless culture under different salinity conditions

In greenhouse soilless culture, precision irrigation depends on accurate measurement of crop transpiration (T). Stem sap flow sensors (SFS) could be applied for estimating crop transpiration over short time intervals (minutes) in vegetable crops with vertical growth and a uniform stem diameter such as tomato. In this work conducted in 2021 with tomato (Solanum lycopersicum L. ‘Pisanello’) grown under greenhouse in a closed-loop substrate culture, the effect of nutrient solution salinity on crop water use was evaluated and the relationship between T measured in individual plants with an SFS or an electronic balance and crop water uptake was analysed. The plants were irrigated with different salinities of the nutrient solution: 3.0 (control), 6.0 (S1), and 9.0 (S2) dS m-1. The sap flow rates of individual tomato plants were measured using the stem heat balance method to estimate plant T. A climatic station continuously recorded the climate parameters inside the greenhouse during the experiment and the reference evapotranspiration (ET0) was calculated using the FAO-24 Penman equation. Each growing system hosted 30 plants and was equipped with a water meter positioned on the electrovalve used to refill the mixing tank. Daily plant water uptake (WU) was determined with a water meter by recording the amount of nutrient used to refill the mixing tank. The daily values of T measured on individual plants with a SFS (TSFS), or an electronic balance (TEB) were compared with each other and with WU. Crop WU and T did not differ significantly between the control and S1 treatment while they were significantly reduced in S2 plants. The linear regression between TSFS and WU and between TSFS and TEB was analysed separately for the three salinity treatments. Since the equations were not significantly different, neither in the intercept nor in the slope, a unique regression equation was calculated for the relationships between TSFS and WU, and between TSFS and TEB. There was a close correspondence between TSFS and WU and between TSFS and TEB, with the slope close to one and the intercept not significantly different from zero for both regression equations. These results suggest that SFS could be used to estimate plant T and then crop water requirements in greenhouse tomato culture under different salinity conditions.