Leaf water potential measurements using the pressure chamber: Synthetic testing of assumptions towards best practices for precision and accuracy

Leaf water potential (psi(leaf)), typically measured using the pressure chamber, is the most important metric of plant water status, providing high theoretical value and information content for multiple applications in quantifying critical physiological processes including drought responses. Pressure chamber measurements of psi(leaf) (psi(leafPC)) are most typical, yet, the practical complexity of the technique and of the underlying theory has led to ambiguous understanding of the conditions to optimize measurements. Consequently, specific techniques and precautions diversified across the global research community, raising questions of reliability and repeatability. Here, we surveyed specific methods of psi(leafPC) from multiple laboratories, and synthesized experiments testing common assumptions and practices in psi(leafPC) for diverse species: (i) the need for equilibration of previously transpiring leaves; (ii) leaf storage before measurement; (iii) the equilibration of psi(leaf) for leaves on bagged branches of a range of dehydration; (iv) the equilibration of psi(leaf) across the lamina for bagged leaves, and the accuracy of measuring leaves with artificially 'elongated petioles'; (v) the need in psi(leaf) measurements for bagging leaves and high humidity within the chamber; (vi) the need to avoid liquid water on leaf surfaces; (vii) the use of 'pulse' pressurization versus gradual pressurization; and (viii) variation among experimenters in psi(leafPC) determination. Based on our findings we provide a best practice protocol to maximise accuracy, and provide recommendations for ongoing species-specific tests of important assumptions in future studies.

Automated summary

Leaf water potential is a crucial metric for measuring plant water status, but its measurement technique is complex and the underlying theory is not well understood. This study surveyed different methods of measuring leaf water potential and conducted experiments to test common assumptions and practices. Based on the findings, the study provides a best practice protocol for maximizing accuracy and recommends ongoing species-specific tests for important assumptions in future studies.