Dear Gaber,
Your question is very interesting.
The stomatal conductance is frequently used for evaluating the plant-water status. Instruments like infrared gas analyzers (IRGA) or porometers have been used to determine the stomatal opening, by measuring the transpiration rate on a small portion of leaves. Stomatal conductance and transpiration rate may provide a good indication about the root-to-shoot hydraulic conductance. That is O.K. if comparing plants subjected to similar environments.
On the other hand, the actual transpiration, and therefore, the sap flow of plants are also dependent on the microclimate conditions. For example, in a protected environment you can have a high stomatal conductance, but the plant transpiration can be very low, since the incoming solar radiation and the water vapor deficit are reduced. This means that a high hydraulic conductance is not required if the plant transpiration is reduced. An opposite condition can happen in open sky, when the atmospheric evaporative demand is very high. In a high evaporative demand the transpiration must be also intense - which requires a high root-to-shoot hydraulic conductance - while the leaf stomata may remain partly closed.
Anyways, hopefully this discussion may proceed.
Dear Gaber,
Yes it's true that root and shoot hydraulic conductance are important factors which determine transpiration. Additionally, the availability of moisture around root is one of the most important parameters that manages root and shoot hydraulic conductance and then the rate of transpiration and stomatal conductance as well.
Dear Gaber,
Your question is very interesting.
The stomatal conductance is frequently used for evaluating the plant-water status. Instruments like infrared gas analyzers (IRGA) or porometers have been used to determine the stomatal opening, by measuring the transpiration rate on a small portion of leaves. Stomatal conductance and transpiration rate may provide a good indication about the root-to-shoot hydraulic conductance. That is O.K. if comparing plants subjected to similar environments.
On the other hand, the actual transpiration, and therefore, the sap flow of plants are also dependent on the microclimate conditions. For example, in a protected environment you can have a high stomatal conductance, but the plant transpiration can be very low, since the incoming solar radiation and the water vapor deficit are reduced. This means that a high hydraulic conductance is not required if the plant transpiration is reduced. An opposite condition can happen in open sky, when the atmospheric evaporative demand is very high. In a high evaporative demand the transpiration must be also intense - which requires a high root-to-shoot hydraulic conductance - while the leaf stomata may remain partly closed.
Anyways, hopefully this discussion may proceed.
Thank you Homero for your informative comment
I was actually trying to find some way to measure hydraulic conductance of root and shoot under coditions of water deficit. I found that most procedures are destructive, i.e., measure the hydraulic conductance of deteached plant parts by placing them between two solutions of different water potentials I feel that this trend is not realistic in that the behaviour of detached plant parts is not necessarily similar to that in vivo. Second: the water availability during the procedure may not match that in vivo. For example, the low water potential may not match that of the air to which the plant is exposed either under controlled conditions or outdoors.
Altough the approach of using E and Gs data from IRGA machines does not give indictation of hydraulic conductance of the root or shoot in separate, I thought that it still give a direct indication of the overall plant hydraulic conductance.
Feed back from you and all colleagues is appreciated.
Dear Gaber,
The conventional method of measuring plant hydraulic conductance, known as the evaporative flux method, involves measurements of the steady-state evaporative flux densities (transpiration) and water potentials in the soil and leaf. The hydraulic conductance of the whole plant is the quotient between the transpiration flux density (in a steady-state condition) and the difference of water potentials in the soil-root boundary and in leaves.
There are many methodologies for measuring either the transpiration flux density and the water potentials in the soil and leaves. There are also many papers in the literature describing and discussing those methods. Some of them are focusing to the whole plant while others are driving to some part of the soil-plant system.
Go ahead.
Dear Gaber
The question you raised and the answers given by Dr. Homero are very interesting. For the studies related to whole plant hydraulic conductance and root and shoot relationship in hydraulic conductance a measure of transpiration flux through xylem sap movement will be more helpful in addition to regular stomatatal conductance and water potentials of leaf and soil. For the xylem sap measurement thermal dissipation probes are ideal for woody plants.
Hi Gaber, I am interesting in hydraulic redistribution now. I have read several articles relating to that. I agree with the answers given by Dr. Homero. For your problem, I suggest to directly measure sap flow using heat ratio method (Burgess et al 2001). Of cource, if the condition allowed, measuring E and gs using gas exchange system and water potential in leaf and soil simultaneously should be better.
When plants open their stomata to achieve a high stomatal conductance (gs) to capture CO2 for photosynthesis, water is lost by transpiration. Water evaporating from the air spaces is replaced from cell walls, in turn drawing water from the xylem of leaf veins, in turn drawing from xylem in the stems and roots. As water is pulled through the system, it experiences hydraulic resistance, creating tension throughout the system and a low leaf water potential (Ψleaf). The leaf itself is a critical bottleneck in the whole plant system, accounting for on average 30% of the plant hydraulic resistance.
Leaf hydraulic conductance (Kleaf = 1/ leaf hydraulic resistance) is the ratio of the water flow rate to the water potential gradient across the leaf, and summarizes the behavior of a complex system: water moves through the petiole and through several orders of veins, exits into the bundle sheath and passes through or around mesophyll cells before evaporating into the airspace and being transpired from the stomata. Kleaf is of strong interest as an important physiological trait to compare species, quantifying the effectiveness of the leaf structure and physiology for water transport, and a key variable to investigate for its relationship to variation in structure (e.g., in leaf venation architecture) and its impacts on photosynthetic gas exchange. Further, Kleaf responds strongly to the internal and external leaf environment. Kleaf can increase dramatically with irradiance apparently due to changes in the expression and activation of aquaporins, the proteins involved in water transport through membranes, and Kleaf declines strongly during drought, due to cavitation and/or collapse of xylem conduits, and/or loss of permeability in the extra-xylem tissues due to mesophyll and bundle sheath cell shrinkage or aquaporin deactivation.
There are many methodologies for measuring either the transpiration flux density and the water potentials in the soil and leaves. There are also many papers in the literature describing and discussing those methods. Some of them are focusing to the whole plant while others are driving to some part of the soil-plant system. Measuring E and gs using gas exchange system and water potential in leaf and soil simultaneously should be better.
Dear Gaber,
Please see attached file . You can find different methods for measuring
hydraulic conductance.
One of the challenges of using an IRGA like the LI-6400 is that you are only measuring a very small part of the plant's surface, and there can be a large amount of variance between leaves on a single plant.
Dear Gaber,
Yes, its fair in most cases. In most of my practical experiments I planted some Atriplex species. When I am recorded the survival %, It was increased month after month. So some shrubs appear die but with time it was a life and the first appearance seems to be a way of drought tolerance. Thus, it isn't a role or a relationship between the shoot and root.
Hallo Gaber,
yes, it is (to a certain extend) fair to measure hydraulic root-to-petiole conductance by leaf transpiration combined with measurements of xylem tension, e.g. if no other approaches are applicable (e.g. plant cannot be removed from soil etc.). Minimum requirements are a balance (to measure leaf transpiration; better is a porometer), a scholar pressure bomb and means to determine leaf areas involved. However, one has to be aware of certain inherent assumptions: 1) Is the so-called predawn "water potential" of leaves really obtained at zero transpiration? 2) Is at that time an equilibrium between soil and leaf "water potential" achieved? 3) Does the hydraulic conductance stay constant at all transpiration rates? - Most likely this is not the case! If not, is it essential here? One may also approach the hydraulic conductance by obtaining the affiliated xylem tensions for certain but different transpiration rates. 4) Do we get a good idea of xylem tension via the pressure bomb? - In fact, we do!). 5) Is the liquid water phase really in a soil-plant continuum? Most likely, yes, as long as no cavitation has occurred, or as long fine roots are in full contact with the soil or as long as no ice crystals have formed (in winter).
Reasons why we are allowed to obtain all these information by LOOKING AT LEAVES ONLY are:
a) The "force" lifting the water column from soil to leaf mesophyll is generated in the trillions of extremely narrow capillaries between near-parallel segments of cell walls and between corners of cell walls. Here the three prerequisites for lifting water come together: The dipole "water" generating strong cohesive forces, the "dipole"-like cellulose molecules resulting adhesive forces and the phase border between liquid and gas - this is the same effect as observed in glass capillaries lifting a water column. The smaller the radius of the capillary, the higher the tension to lift water. This happens physically WITHOUT any transpiration!!!! This tension we can directly measure with the pressure bomb! And the smallest capillaries in the plant are in the intercellulars of the mesophyll !!!
b) When-ever water vaporizes inside the intercellular air spaces of the mesophyll this increases xylem tension (the so-called "leaf water potential" gets more negative - but better use the term leaf (petiole) xylem tension), for immediately the film of liquid water withdraws into even smaller corners of the intercellular capillaries and the radii of the menisci of the film become smaller.
c) Now the effect of SOIL "water potential" and HYDRAULIC root-to-petiole RESISTANCE comes in: If soil water tension is high (very negative soil water potential) it is immediately seen at the leaf level: Because liquid water is under high tension, this tension "arrives" at the liquid film in the intercellular capillaries holding liquid water in narrow capillary sections - small radii of menisci - high tension. The same effect is imposed by high resistances in the root-to-shoot pathway.
In summary: In the leaf mesophyll all these effects from soil and axes are "visible". And by correct interpretation, measurements on the leaf level allow for a reasonable estimate of hydraulic conductances.
See e.g.
Nobel, P.S. (1991) Physicochemical and Env. Plant Physiol; chapter on capillary rise.
Katharina Munk (2001) Botanik. Spektrum Verlag. There p. 9-9 (Fig. 9-5(B). In German.
John B. Passioura (1984) Austral J Plant Phys 11:333-340 !!!! and 11:341-350 and 12:455-461.
Examples of application: Oecologia 64: 344-354.
Plant, Cell and Env. 10:27-37.
I think it is fair, but you will only get a relative rate of conductance. i.e. plants with high stomatal conductance will be expected to have higher hydraulic conductance, but you will not be able to put a actual value on it.
Many thanks for all contributiors, This discussion helped alot.
I agree with Jean Christophe Domec. The methodology gives an approximate value. The value gives the qualitative picture of what is happening in the field
Only indirectly -- if you examine van den Honert's (1948) paper on water flow in plants and his justification for the catenary theory (chain like process), then you will understand why, in particular, the instantaneous rate of transpiration or stomatal conductance may or may not bear any relationship to hydraulic conductivity through the rest of the system. There is new evidence for extraordinary water capacitance (transfusion tracheids in upper canopy foliage of coastal redwood) in the foliage of certain conifers which might mean that one could have high stomatal conductance and high rates of transpiration and very low hydraulic conductance through the rest of the flow pathway.
Dr. Hinckley,
Your observation is correct. The resistive model is not quite appropriate for many cases, due to the water capacitance into the soil-plant system. For this reason, a steady-state condition should be required for all the water pathway. And this is not easy to obtain, in particular when the evaporative demand is high. In the field we have observed significant displacements between the sap flow at the bottom of the steam and the transpiration rate in maize plants, during the morning. At the afternoon and evening we have the opposite situation, when the plants are recovering the diurnal water loss. Besides, in general, the transpiration flux is inconstant in the field, mainly because of clouds and wind. So, the steady-state condition may happen during short periods and therefore instantaneous determinations tend to be complicate, as you said.
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