Calibration of sap flow techniques in citrus using the stem perfusion method

Abstract

The aim of this study was to calibrate and decide on the most appropriate sap flow technique for citrus species in the laboratory by pushing water through cut branches. Various sap flux density techniques, including heat pulse techniques (heat ratio and compensation heat pulse methods) and the heat dissipation technique were calibrated in four citrus species, namely Citrus sinensis (Oranges), Citrus reticulata (Soft citrus), Citrus paradise (Grapefruit) and Citrus limon (Lemons). Sap flux density, determined by these three techniques, was compared to that determined gravimetrically, which was calculated as the rate of change in the mass of water passing through the stem segment divided by the area of conducting wood. Results showed that the sap flux density was consistently underestimated by all techniques and across all citrus species/varieties. However, fairly good correlations (R2>0.7) between sap flux densities determined by a sap flow technique and gravimetric determinations were found for all techniques in some stems. Despite the good correlations found in the study, a single calibration factor for each technique could not be found for citrus using the stem perfusion method. Calibration factors were determined as the inverse of the slope of the linear relationship between sap flux density determined with a sap flow technique and that determined gravimetrically. These correction factors varied between citrus species and even within stems of the same species. Vessel dimensions (lumen diameter) and distance between groups of xylem vessels in citrus species was determined in order to try and explain the underestimation of sap flux density and the large variations in the calibration factors obtained during the calibration of the various sap flow techniques. The results revealed that the variation and underestimation were caused by contact of the probes with inactive xylem and due to differences in the nature of sapwood. The xylem vessels were unevenly distributed throughout the sapwood with large distance between the vessels, meaning that the sapwood of the studied species was considered inhomogeneous and therefore departed from the idealised theory of heat pulse measurements. The theory needs to be adapted to account for such sapwood and because of the large variation in the sapwood properties between different citrus species, calibration of these techniques is probably necessary for each new species and orchard in which measurements are to be made. Our analysis of the performance of sap flow techniques showed that the HR method should perhaps be considered before the CHP and TD methods.