Water uptake rates

[ScottMcC]

I had another thought related to the discussion yesterday about root rot.

I've noticed that of my plants in s/h, some tend to suck up water far faster than others, even of similar size. Since water is available essentially as fast as the plant wants it, this would seem to indicate that some of the plants are indeed able to take up water more quickly.

now obviously there are several explanations for this phenomenon. for instance, a plant with more roots is going to be able to absorb more water, just because there's more surface area. and clearly, a plant that loses a lot of water through its leaves is going to need to take up more water to make up for this. same goes for a plant that is rapidly growing.

anyway, I think that plants of different genera are also able to take up water at different rates, solely based on intrinsic properties of the roots themselves. for instance, my oncidiums suck up water like it's nobody's business, and need to be watered every 3 days or so. I think the fine, filamentous roots are just adapted to have a very high rate of water uptake. my phals, on the other hand, can make it 1.5 weeks, and I think that this reflects a slower maximum uptake rate.

again, no data to back up these thoughts, but I'd love to hear peoples opinions, and if they do have data, even better.

[Ross]

Man... you must have a lot of time on your hands

I, seriously, have never thought about this and if I did I would think - so what - as long as I can keep the liquid flowing, it shouldn't matter. What is driving this observation?

The extent of my concern about watering is how long can I get between waterings? My drive for S/H and for Orchidarium is solely based on this issue. Cacti were great for me except they were prone to bugs - something I haven't had so far with orchids.

[Tindomul]

I would think, on first observation, that finer roots are less able to store water and therefore need to work overtime to get enough water to the plant body.
Also, I would think that the more leaves a plant has, the more water it drinks. And pseudobulbs would affect the plant in the opposite direction. Maybe thats why Oncies have big fat pseudobulbs, but then what about Catt types. They got relatively small pseudobulbs, and roots that are thick, but nothing like that of a Phal.
Masdies have thin roots, and no Pseudobulbs. Hmm there goes that theory. But they have much thicker leaves than Oncidiums.

Interesting question.

[Ross]

The point seems to be, however, that water availability has to do with how fast orchids suck the stuff up. I'm not sure I can add to that So uptake rates may, in fact, have more to do with availability and health of the roots than anything else? Let's not forget a "rest period" (IE. a period of oxygen regen) between soakings.

[ScottMcC]

well here's the extension of that thought from earlier today. if you could design a medium that would support optimum water uptake, without making the roots prone to edema, ie at the maximum tolerated rate of absorption, then that would in turn lead to better growth, right?

but, since water uptake rates appear to vary across genera, I would think this would be kinda hard to do, and thus I see this as one of hte potential limitations of current orchid culture techniques.

[Tindomul]

Snip"well here's the extension of that thought from earlier today. if you could design a medium that would support optimum water uptake, without making the roots prone to edema, ie at the maximum tolerated rate of absorption, then that would in turn lead to better growth, right?
"end snip.

isn't that medium and method called SemiHydroponics?

[ScottMcC]

well that's Ray's goal, as I understand it.

however, there are some limitations. the biggest one I see is that the older roots of many plants, especially cattleyas, tend to get soggy and eventually die. The new ones do fine, but it can definitely shock the plant at least a little, and you may even lose a few older pseudobulbs. sub-optimal.

here's another refinement of my thinking...

there are two rates at play here: water delivery into the root, and water transport from the root to the rest of the plant. obviously there must be a balance between the two in order to keep the root well hydrated, but not edematous. my thought is that both of these rates are different for each species, which explains may of the differences in water requirements for different plants. furthermore, plants which have a high uptake rate into the roots, but a slow transport rate out of the roots, are more prone to root edema and subsequent rot. as such, these plants are less suited to culture methods in which the roots have constant water exposure.

anyway, let me know what you think.

[Ross]

I'm also wondering why a given spp can have thin roots with one plant and very thick roots for another (same spp.?) I have 2 specimens of Haraella, both mounted so roots are totally exposed. One plant has very thin roots and small leaves (maybe resulting from decreased water/nutrient uptake?) other has roots 4-5 times as thick and leaves 3 times the size of the other specimen. Both are growing and blooming. I wonder if genetics has a large role in this discussion?

[Ross]

Another thought, why does a Phalenopsis plant (even a small one) have roots many times larger than, say, a specimen sized Oncidium? The Oncidium has massive water storage facilities and wouldn't need constant source of water, yet when water is presented it needs to drink quickly? Phalenopsis on the other hand has no water storage so perhaps the roots serve part of the storage function?

[ospylac]

Hey Ross, Are your two Haraellas with wildly differing root sizes the same age and parentage? The older the orchid, the bigger the roots and leaves would be.
Genetics plays a role here to be sure. Different orchids employ a variety of strategies for water uptake and storage. Phalaenopsis definately use their roots, and their thick leaves for water storage. Orchids rather uniquely use all portions of the plant (except the flower!)for water storage. So mass is storage whether its leaf, root or swollen stem (pseudobulb) does not really matter; it's a water savings account if you're an orchid. The larger the proportional mass of the plant, the more able it is to survive a drought.