Evapotranspiration question

[Ray]

Originally Posted by rbarata

This sugests that water absortion by the roots and lost by transpiration, for each plant have fixed rates.

I don't think that's true, because those processes do not function independently of each other or other biological processes.

[rbarata]

So, basically, bellow a certain value (min threshold) it doesn't make sense to water because the stomata closes to stop water loss.
This stops the creation of a negative pressure at higher levels of the plant tissue (leaves, etc) whichs stops the upward movement of water, and consequently stopping water absortion at root level.

Am I thinking correctly?

[Ray]

To put it bluntly, I don't know.

However, I think that losses through stomata are passive, not active, and there can be loss through surface cells directly. That's why some orchids have thick, waxy, cuticle layers (think phalaenopsis) that slow it.

[Gthumbz89]

I believe that if you are perfect in your media choice, watering habits, and temperatures then it is possible to lessen the effects of water loss from low humidity.

Also, allot of the common varieties of orchids are very tolerant to low humidity if they are properly carred for. I grow phals, cattleyas, oncidiums and some zygopetalum hybrids here in Arizona with less than 10 percent humidity.

If they have a bad root system then they wont bloom, but if i allow them to get a nice root mass and water on time then they bloom successfully.

I cant speak for other types that are more sensitive like masdevallias, or miltoniopsis though. I have killed the later easily or keept them limping along with little victories on the way.

[estación seca]

Many, many orchids open their stomata mainly at night, when it is cooler, and evapotranspiration is less. In this they are like most cacti and most succulents. They are able to take up water from their roots throughout the Sun's journey around the Earth, but it is more important for it to be available at night, when they are losing water.

For this reason most succulentists recommend watering in the evening. I would also suggest this for people growing orchids in low-humidity climates, though it contradicts most advice, which is to water in the morning. This advice, however, comes from people growing in wet greenhouses, which must be kept closed to retain heat. Plants in low-humidity environs are dry within an hour or two, even if the crowns are soaked.

[camille1585]

Originally Posted by estación seca

Many, many orchids open their stomata mainly at night, when it is cooler, and evapotranspiration is less. In this they are like most cacti and most succulents. They are able to take up water from their roots throughout the Sun's journey around the Earth, but it is more important for it to be available at night, when they are losing water.

For this reason most succulentists recommend watering in the evening. I would also suggest this for people growing orchids in low-humidity climates, though it contradicts most advice, which is to water in the morning. This advice, however, comes from people growing in wet greenhouses, which must be kept closed to retain heat. Plants in low-humidity environs are dry within an hour or two, even if the crowns are soaked.

I don't think it's that many. Thick leaved orchids, like Phals, Vandas, and Catt alliance, tend to be CAM photosynthesizers. But most of the thin leaved orchids are in large part C3. A handful of orchids are known to be C4 photosynthesizers, but I think that metabolic pathways of orchids are poorly understood. I do remember reading that some orchids are facultative CAM, and will switch to more efficient C3 when there is plenty of water available, and I think Phals fall into that category. Though I agree that in low humidity environments you can probably get away with watering in the evening as long as the plants are not wet.