Using a pressure bomb

Pressure Bomb
Measures midday water potential of the cells in leaf. Measured by applying pressure to match the negative water potential of leaf cells. Expressed in bars. (1 bar = 0.1 MPa).

  • Procedure:
    Measure midday (noon to 3 pm window is ok), or one hour after fog lifts
    Mark vine as sampled.
    Select exposed, fully expanded, mid-shoot leaf with long stem.
    Enclose leaf in plastic bag before cutting (optional: blow air into bag first)
    Cut with razor and drop bag into chamber with only cut end exposed
    Select second leaf from same vine. If reading within 1 bar, log the average. If difference two bar or greater, read third leaf and discard outlier.
  • Sample by block size:
    < 5 acres = 1 vine / acre
    < 100 acres = 1 vine / 5 acres

Midday Water Potential

Rating

Visual assessment

> -10 bars

No stress

No stress

-10 to -12 bars

Mild stress

Mild stress

-12 to -14 bars

Moderate stress

Moderate stress

-14 to -16 bars

High stress

High stress

< -16 bars

Severe stress

Severe stress

Common mistakes:
Measuring on cloudy, partially cloudy, windy, or hot and dry days produces meaningless readings. Not enclosing the leaf in a bag prior to cutting. High stress conditions can be misread since water potentials artificially high when stomata are closed. Porometer can confirm by determining if stomata are closed.

Some general info: Evaporative demand from the atmosphere drives the removal of water from the vine, and produces a gradient of water potentials in the plant. The most negative is the leaf. MLWP is the reference point most commonly used in scientific literature. It can only be measured on a fully transpiring leaf, hence the sunlit leaf on an average not cloudy or windy day. Plant water potential changes diurnally. At night when stomata close and the plant equilibrates with the soil water potential.

Useful conversions for amount of water to apply
hours = (inches*27154) / (emitter_rate_in_gph*emitters_per_vine*vines_per_acre)
inches = (hours*emitter_rate_in_gph*emitters_per_vine*vines_per_acre) / 27154
acre-inch = 27154 gallons

Further reading:
Greenspan, M. 2005. “Integrated Irrigation” Practical Winery and Vineyard March/April pp. 21-34
Williams, LE 2001. “Irrigation of Winegrapes in California”. Practical Winery and Vineyard. pp. 42-45

credit : Tom Shapland 101C lectures

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