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XI. RELATIVE HUMIDITY

XI. RELATIVE HUMIDITY. A. RH range outside B. High RH conditions in greenhouse Apr--May & Aug--Sept bright cool days cool nights June--July bright warm days cooler but warm nights C. Low RH conditions in greenhouse Oct--Mar usually heating cooler air dry to very dry environment.

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XI. RELATIVE HUMIDITY

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  1. XI. RELATIVE HUMIDITY A. RH range outside B. High RH conditions in greenhouse • Apr--May & Aug--Sept • bright cool days • cool nights • June--July • bright warm days • cooler but warm nights C. Low RH conditions in greenhouse • Oct--Mar • usually heating cooler air • dry to very dry environment

  2. Psychrometric Chart 95% 75% 46%

  3. % Relative humidity Or vapor pressure deficit Water vapor Transpiration rate In g H2O/g leaf C. Object of control • High humidity • Lower transpiration • leaf temp increases • Disease potential greater • Low humidity • Greater transpiration • water stress possible • Stomata close • CO2 uptake less • Death Relative Humidity 0% 100% Vapor Pressure Deficit

  4. Object: • High enough not to cause stress or limit metabolism • 60% • Low enough to prevent disease • less than 85%

  5. Reducing humidity • Ventilation

  6. Reducing humidity • Circulation

  7. Reducing humidity • Increase temperature • Ventilate and heat • Turn off pump to cooling system

  8. E. Increasing RH • Shade roof • Evaporative cooling • Lower temperatures • Spray walks and foliage during day • Continue mist into night

  9. F. Special problems 1. wilting after dark weather • Rapid loss of water from tender foliage • Roots cannot supply enough water 2. edema - oedema • Cells expand; cell wall cracks • Warm soil; High humidity • Cause • rapid uptake of water - low transpiration

  10. Guttation • Small drops of sap on edge of leaf • Water evaporates • High concentration of salts • Cause • high R.H. • rapid water uptake; slow transpiration

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