Water loss and the atmosphere

      Plants lose moisture by transpiration from their leaves into the surrounding atmosphere. The less moisture they lose, the more they like it. We tend to think that the higher the relative humidity, the moister the air and the better it is for plants up to a certain point. The point is that relative humidity does NOT relate directly to the rate at which transpiration of water from the plant occurs. Relative humidity is not proportional to the rate of moisture loss from the plant.  How come?

            The moisture holding capacity of air is measured in units of pressure, and there are two important measurements concerned with figuring out how much moisture a given block of air can potentially absorb.  First is the saturation vapor pressure (SVP): think of this as the maximum amount of water vapor a given block of air can hold.  Second measurement is the difference between the amount of water vapor actually in a given block of air and its SVP (i.e., the maximum amount of water it could absorb). This difference is called the vapor pressure deficit, or VPD.  Think of VPD as the water sucking power of the air, because it is actually the VPD that interests your plants, not the relative humidity.

            At face value, VPD (sucking power) seems to be the same as relative humidity - because relative humidity is the ratio of the actual vapor pressure in the air to the SVP.  Its not the same, because the SVP of a given block of air increases exponentially as the air temperature rises - the higher the temperature, the greater the amount of water vapor that air can hold. Rather than giving a physical explanation of why humidity and VPD are different measurements, because I'll get out of my depth in about two seconds, just look at how the VPD (sucking power) changes at various temperatures if the relative humidity stays the same at 75%:

Looking at the temperature and vpd on a graph, you can see how the vpd is increasing exponentially as the temperature rises, while the relative humidity remains constant:

  The work that plants do

  Consider your plant's average working day. They are really the world's primary producers of food, although they get bugger all credit for it. I mean, go into your average garden centre and what do you see? Rows and rows of little bottles and cartons of plant "food". These so-called foods are really just minerals the plant needs to help it in its food manufacturing efforts. Plants survive by manufacturing their energy requirements (stored as starch) from sunlight, carbon dioxide and water plus a number of mineral elements (N,P,K, Ca etc). This is done in a complex series of reactions, called the Calvin-Benson pathway, whereby carbon dioxide from the atmosphere is converted into carbohydrates for long-term energy storage. These Calvin-Benson reactions are light independent, and are driven by short-term energy in the form of ATP (Adenosine triphosphate) and NADPH (too long a name). ATP and NADPH are produced in the plant by light dependent activities originating from sunlight absorbed by the plant's chlorophyll.

In the majority of plants atmospheric carbon dioxide is captured through tiny pores in the epidermis (called stomata) that open during the daylight hours (called C3 respiration). Oxygen produced as a byproduct is expelled. If the outside air is dryer than the epidermal cells, which it usually is during sunlight hours, water vapor will diffuse out into the atmosphere through the open stomata. This moisture loss is a disadvantage for the plant, which usually has to take up compensatory water through its root system.

That's the "standard model" of plant behavior. Its not, in fact, how XTs normally operate.  Moisture loss is a huge problem for XTs, because they do not have access to root-derived water.  They have evolved with a different method of capturing atmospheric carbon dioxide. This enables them to minimize water loss by using a process called CAM respiration.  Unlike the "standard model plant," during the day, the XTs' leaf pores, the stomata, close up; thereby avoiding moisture loss to a considerable degree. Through the night hours, when the temperature drops and the VPD decreases (hereby reducing the potential for water vapor loss from the plant), the air has much less sucking power and the CAM plant opens its stomata and captures carbon dioxide. As the carbon dioxide is captured it is converted to organic compounds - mainly malic acid - which are stored until daylight. Around dawn, the stomata close up again. During the day, stored malic acid is converted back to carbon dioxide within the plant tissues and the Calvin Benson cycle proceeds to fix carbon and the plant produces its energy stores. [3]

  How VPD fits in

Remember the mechanics of vapor pressure deficit, VPD.  It is a measure of the capacity, or potential, of air to take up more moisture. The higher the VPD the greater the potential for sucking moisture out of the plant.  As we saw, when the temperature rises, air round your plant sucks moisture from open stomata faster and faster unless the relative humidity increases at a rate corresponding to the increasing temperature.  On the other hand, as the temperature drops the moisture-holding capacity decreases exponentially.  So a cool night equals a lowering of the VPD, much lower moisture loss for the XT while its stomata are open to receive carbon dioxide (remember that deserts generally have very cool nights. The only one I have spent the night in was the Makgadikigadi Pans in Botswana, and it was COLD.)

  How does all this relate to growing conditions? I have a machine that records the temperature and humidity every 30 minutes, and then I store the records on a computer. This way I can calculate the VPD every half-hour of the day in my greenhouses, shade houses, seedling incubator, in the house with air conditioning on or off. Based on these recordings, let's discuss some of the VPD readings I've calculated.  Please bear in mind that my recorder is a desktop weather station - it is not designed as a scientific instrument. The sensors are not aspirated, like they need to be for precise recordings.  To get results that are scientifically acceptable, I would need to use an aspirated hygrometer or a sling psychrometer (a thing like a rattle, with wet and dry bulb thermometers, which you spin for several minutes every time you take a reading.)  The figures I have are an indication only, and at best are useful for comparing environments. They are not a precise record of the actual VPD.

One of the places people have difficulty growing XTs is in the home. I think this is particularly true if you have an air-conditioned house. Look at what happens in our office.  The room is about 150 m3 and the air-conditioner is a Fujitsu reverse cycle unit.

Without the air-conditioner, the VPD is running between 5 and 6 at night, climbing to about 10 during the day. Once we turn on the air-conditioner the VPD jumps up to between 20 and 25 while the thing is on. At that sort of range plants are going to start sucking up a lot of water to keep up, and I doubt that XTs will cope for long. This is a good indicator of why watering requirements for XTs are going to be much different in the two types of situation. I don't grow XTs indoors, but one of our Wellington Tillandsia Study Group members, Beryl McKellar, does[4].   We can assume that her conditions were very similar to my house without air-conditioning. She grows her plants indoors, as I said, and mists them every morning in summer, every 3 to 4 days in winter.  But in the drier, high-deficit air of the air-conditioned room, you would surely have to soak the plants regularly to make up for the water loss they suffer. What I don't know yet is what VPD figure is optimal for XTs at night. Well, I suppose a VPD of 0 is optimal! But there must be a particular temperature range conducive to their maintaining transport at an efficient level to permit transpiration, at which level they can tolerate a certain VPD. Work needs to be done! Meantime, compare our indoor figures with a couple of summer greenhouses.

 
            The night-time VPD is still down at the 4 to 5 level, and I would say that this is probably around the maximum you would want to sustain for mature XTs.  Let's use it as a starting figure, anyway. I have no idea how sensitive XTs are to VPD range at night, but my experience shows that young seedlings are very sensitive. To recap, I start XT seeds off, and grow them for up to nine months, in my incubator. This is a solid timber box; 1200mm long, 700mm high and 700mm deep.  Two fluorescents are hung in the ceiling, immediately below them is a wire-netting screen from which the seeds are sown on hanging strips of fine plastic netting. In the bottom is a deep tray with wet sand in it covering a soil-warming cable. The front of the box was originally a solid wood panel that lifted up to allow access. This incubator was always very successful, and particularly I never lost any seedlings once they got started. However, about a year ago I replaced the solid front panel with a double thickness of plastic so my mother-in-law could get at the seeds to water while we were away. And since then, I have had a lot of problems including the loss of about 60 batches of seedlings last year. Previously, I only had to water the seedlings once or twice a day; now I have to water 4 times a day or I start losing them.  Given that the light and temperature are pretty much the same in both cases, look at how the VPD differs:

In the solid incubator, the relative humidity stayed at 100% while the lights were off—in the plastic fronted version it only gets to 75%. Whilst 75% relative humidity sounds pretty good, look at the difference in VPD rates! In effect, then, seedlings thrive when the VPD ranges between 0 and 5 Cps, but are very difficult and require heavy watering when the VPD stays between 5 and 10 Cps.