Leaf Water Potential (Ψ_leaf)

Definition

Leaf water potential (Ψ_leaf) is the free energy of water in leaf tissue relative to pure water at the same temperature and elevation, expressed in megapascals (MPa) and almost always negative under transpiration. More-negative values mean greater tension and greater water stress. It is the thermodynamic anchor of the facility's water-status hierarchy. At GreenWorld Sector-7 it is measured by PhytoSense Mesh in-situ stem psychrometer/probe nodes and reported per cultivar cohort with a midday reference window.

Why it matters

Ψ_leaf is the definitive arbiter when fast channels disagree: stomatal conductance and turgor can be confounded by VPD or osmotic adjustment, but leaf water potential reports the actual tension the xylem is under and the proximity to cavitation. CHLORA uses midday Ψ_leaf as the irrigation-deficit set-point and as the gate for any deficit-irrigation campaign. It does not feed CVI directly but validates the turgor and conductance sub-signals that do.

Formula

Reported directly from the equilibrated psychrometer; the components are diagnostic:

Ψ_leaf = Ψ_solute + Ψ_pressure   (component identity)

measurement (PhytoSense Mesh stem psychrometer):
  chamber relative humidity at thermal equilibrium gives
  Ψ_leaf = (R·T / V_w) · ln(RH)

  R    = gas constant
  T    = tissue absolute temperature (K)
  V_w  = partial molar volume of water
  RH   = equilibrium chamber relative humidity

calibration:
  multi-point against NaCl/KCl osmotic standards each week;
  temperature-compensated to ±0.05 MPa.

Reported to 0.1 MPa. Midday (solar noon ±1 h) used as the
governing reference; predawn logged for recovery checks.

Inputs

Units & Scale

MPa, reported to 0.1 MPa, conventionally negative. The governing value is the midday median per cohort; predawn recovery is logged separately. Facility value is cohort-count-weighted. Confidence degrades when the psychrometer fails to reach thermal equilibrium within its window or when tissue temperature drifts outside 18–30 °C, where the thermocouple correction grows large.

Sampling & Source

• Sampled every 10 minutes by PhytoSense Mesh stem psychrometer nodes.
• Governing threshold evaluated on the midday window (solar noon ±1 h).
• Predawn values logged nightly for next-morning recovery confirmation.
• Stale handling: flagged STALE after 30 min without equilibrium.

Thresholds

Recent Trend

Facility cohort-weighted midday Ψ_leaf, last 14 windows:

Interpretation Guidance

Related Metrics

Related SOPs

• SOP Library — psychrometer weekly osmotic calibration.
• Hydraulic-stress WARN/CRIT triage & deficit-irrigation suspension — Operations SOP set in the SOP Library.
• Alarm escalation & irrigation linkage — Monitoring Systems.