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Vapor pressure deficit in the greenhouse

What is VPD? The leaf to air humidity gradient controls crop growth just as much as light and temperature. We grow most of our ornamental plants in greenhouses so that our plants are protected from extreme cold, wind and unwanted moisture in the form of rain, snow, or fog. With modern greenhouse temperature controls and glazing we can do a pretty good job protecting our plants from the extremes. Unfortunately, we soon forget how the plan has adapted to the outside environment over the last few million years and how these controlled environments affects its physiology.

by Paul Thomas - pathomas@uga.edu

Plants outdoors need wind, sunlight and a low humidity level outside the leaf to transpire efficiently. This transpiration helps cool the leaf and also brings up nutrients to the leaf tissue as water evaporates from the leaf stomates. No one has proven that low transpiration reduces growth, but we do know is that with adequate transpiration, nutrients are readily brought up from the roots. According to one of the laws of physics, things (molecules) always move by passive diffusion from a place of high concentration to a place of low concentration. Happily, it is no different a situation in a leaf. The spongy tissue in the leaf mesophyll has pockets that contain water vapor saturated air. This is where gas exchange occurs in the leaf as CO2 uptake, O2 release occurs, and as water escapes the leaf tissue through evapotranspiration.

Outside the leaf, the water vapor is much, much lower. This makes for a large, steep water vapor gradient between the leaf and the outside air, facilitating the movement of the water vapor out of the leaf and good water uptake from the roots along with nutrient uptake. Leaf temperature (sunlight on the leaf), as well as the air temperature and its capacity to hold water vapor (relative humidity), and the presence of air movement (wind) all affect this process dramatically.

When the relative humidity is low, the deficit between the leaf and the outside dry air is “high.” Hence we often measure “high” vapor pressure deficit (VPD) on bright sunny days with moderately low humidity. (High = a value of 1.15 kPa or above). This can generate rapidly wilting plants, hard leaf tissue, deformed leaves and marginal edge burn. Conversely, in winter, on a cloudy day, in a closed-up greenhouse, the humidity levels can be very high in the greenhouse, and thus the deficit between the leaf and the greenhouse air is very low. A “low” VPD (a VPD of 0.1 to 0.45 kPa), meaning very high humidity, increases the potential for disease infection and generates very soft leaf tissue. New growers are often surprised that when maintaining high can actually cause soft growth, irregular crop maturation and even crops loss if the HAF fans are not run all day. Plant disease thrives when leaf surfaces are wet, and the external leaf environment is very moist.

Read the full e-Gro Alert here.

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