and another environmental factor is VPD.
Maybe it's not so much the temp. alone as it is the combination of temp. and humidity. I've since stopped fighting against humidity in the grow room unless it gets way up to 75% which is extremely rare here. In fact, I bought a little Humidifer for the veg room.
Relative Humidity in Greenhouse Tomato Production
Relative Humidity in Greenhouse Tomato Production
It is unclear to what extent high vpd is deleterious to the plant, assuming that adequate water is available, but vpd's over 1.0 are considered to be too high. In Northern Europe and the cooler areas of North America, vpds over 1 kPa are rarely seen but in most US greenhouses and some summer production situations in northern latitudes with continental climates, vpds will sometimes exceed this range. A greenhouse temperature of 26°C and relative humidity of 60% would result in a vpd of 1.35, for example. High moisture deficit results in small, thick, 'hard' leaves. If plants transpire more water than can be supplied through the roots, blossom-end rot may develop in the fruit and stomates may close, resulting in poor growth.
these two sources use different numbers to express VPD, but the chart below makes it easy to see what they're both talking about.
Argus Controls
Argus Controls Environmental Control Systems Greenhouse Control Systems Climate Control Systems
VPD Application Note (PDF: 702 KB) - Full version of our paper on understanding and using VPD
VPD is a good indicator of plant stress brought about by either excessive transpiration (high VPD values) or the inability to transpire adequately (low VPD values). While plants are affected by changes in the surrounding air mass, they can also actively affect this same air mass. Whenever plants increase their own rate of transpiration a local decrease in the VPD will occur as the surrounding air absorbs this evaporated moisture.
There is also an associated cooling effect as the process of water evaporation absorbs a lot of heat. This cooling will further reduce the water holding capacity of the air mass and thus lower the VPD value.
VPD Does Not Measure Plant Water Use!
VPD can only tell you about the potential for water to evaporate from the leaves. There are several other factors that affect water transport including salinity in the rooting media, root health, and whether the leaf stomata are opened or closed. Although the actual rate of water loss is not directly proportional to VPD, there is a general relationship. It can tell you whether the crop is experiencing drying conditions and you can then make some assumptions based on this.
When the VPD is too low (humidity too high) plants are unable to evaporate enough water to enable the transport of minerals (such as calcium) to growing plant cells, even though the stomata may be fully open. Therefore, a VPD target threshold can be used to influence ventilation and/or heating equipment used to increase the VPD by reducing the air moisture level.
When the VPD is too high (humidity too low) the rate of evaporation from the leaves can exceed the supply of water into the roots. This in turn will cause the stomata to close, and photosynthesis to slow or stop. Once the stomata close, the leaves are at risk of high temperature injury since evaporative cooling is reduced due to the lack of water to evaporate
To avoid injury and death from wilting, many plant species will either curl their leaves or orient them downward in an attempt to expose less surface area to the sun. This can significantly downgrade the quality of potted and foliage plants and can also reduce the growth rate and quality of vegetable crops.
The ideal VPD range varies with the crop species and the stage of growth. The table below indicates VPD values in millibars at various temperatures and humidity levels. Although the values do not change, the interpretation of the numbers will vary for each crop species, the stage of growth, cultivation methods, crop acclimatization, and local conditions. We have arbitrarily selected the green shaded area (approximately 5.0 to 12.0 mb) as being ideal for an imaginary crop. The yellow areas indicate an acceptable but marginal VPD range and the red areas are either too high or too low. Again, you need to make your own interpretations for your crops.
another non technical way to think about this is look at the amount of plant growth in a desert as a compared to a rain forest.
