Hydraulic conductivity (K) in the soil and xylem declines as water potential (psi) declines. This results in a maximum rate of steady-state transpiration (E(crit)) and corresponding minimum leaf psi (psi(crit)) at which K has approached zero somewhere in the soil-leaf continuum. Exceeding these limits causes water transport to cease. A model determined whether the point of hydraulic failure (where K = 0) occurred in the rhizosphere or xylem components of the continuum. Below a threshold of root:leaf area (A(R):A(L)), the loss of rhizosphere K limited E(crit) and psi(crit). Above the threshold, loss of xylem K from cavitation was limiting. The A(R):A(L) threshold ranged from > 40 for coarse soils and/or cavitation-resistant xylem to < 0.20 in fine soils and/or cavitation-susceptible xylem. Comparison of model results with drought experiments in sunflower and water birch indicated that stomatal regulation of E reflected the species' hydraulic potential for extracting soil water, and that the more sensitive stomatal response of water birch to drought was necessary to avoid hydraulic failure. The results suggest that plants should be xylem-limited and near their A(R):A(L) threshold. Corollary predictions are (1) within a soil type the A(R):A(L) should increase with increasing cavitation resistance and drought tolerance, and (2) across soil types from fine to coarse the A(R):A(L) should increase and maximum cavitation resistance should decrease.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.1998.00287.x

Limitation of plant water use by rhizosphere and xylem conductance: results from a model

Weighing lysimeters, large-tree potometers, ventilated chambers, radioisotopes, stable isotopes and an array of heat balance/heat dissipation methods have been used to provide quantitative estimates of whole-tree water use. A survey of 52 studies conducted since 1970 indicated that rates of water use ranged from 10 kg day(-1) for trees in a 32-year-old plantation of Quercus petraea L. ex Liebl. in eastern France to 1,180 kg day(-1) for an overstory Euperua purpurea Bth. tree growing in the Amazonian rainforest. The studies included in this survey reported whole-tree estimates of water use for 67 species in over 35 genera. Almost 90% of the observations indicated maximum rates of daily water use between 10 and 200 kg day(-1) for trees that averaged 21 m in height. The thermal techniques that made many of these estimates possible have gained widespread acceptance, and energy-balance, heat dissipation and heat-pulse systems are now routinely used with leaf-level measurements to investigate the relative importance of stomatal and boundary layer conductances in controlling canopy transpiration, whole-tree hydraulic conductance, coordinated control of whole-plant water transport, movement of water to and from sapwood storage, and whole-plant vulnerability of water transport to xylem cavitation. Techniques for estimating whole-tree water use complement existing approaches to calculating catchment water balance and provide the forest hydrologist with another tool for managing water resources. Energy-balance, heat dissipation and heat-pulse methods can be used to compare transpiration in different parts of a watershed or between adjacent trees, or to assess the contribution of transpiration from overstory and understory trees. Such studies often require that rates of water use be extrapolated from individual trees to that of stands and plantations. The ultimate success of this extrapolation depends in part on whether data covering short time sequences can be applied to longer periods of time. We conclude that techniques for estimating whole-tree water use have provided valuable tools for conducting basic and applied research. Future studies that emphasize the use of these techniques by both tree physiologists and forest hydrologists should be encouraged.

/pdf/a-review-of-whole-plant-water-use-studies-in-tree-cx4nvivue8.pdf

A review of whole-plant water use studies in tree.

Our objective is to describe a multi-layer model of C 3canopy processes that effectively simulates hourly CO 2 and latent energy(LE) fluxes in a mixed deciduous Quercus-Acer (oak-maple) stand in central Massachusetts, USA . The key hypothesis governing the biological component of the model is that stomatal conductance (g s) is varied so that daily carbon uptake per unit of foliar nitrogen is maximized within the limitations of canopy water availability . The hydraulic system is modelled as an analogue to simple electrical circuits in parallel, including a separate soil hydraulic resistance, plant resistance and plant capacitance for each canopy layer . Stomatal opening is initially controlled to conserve plant water stores and delay the onset of water stress . Stomatal closure at a threshold minimum leaf water potential prevents xylem cavitation and controls the maximum rate of water flux through the hydraulic system . We show a strong correlation between predicted hourly CO 2 exchange rate (r2= 0.86) andLE (r2= 0.87) with independent whole-forest measurements made by the eddy correlation method during the summer of 1992. Our theoretical derivation shows that observed relationships between CO 2assimilation andLE flux can be explained on the basis of stomatal behaviour optimizing carbon gain, and provides an explicit link between canopy structure, soil properties, atmospheric conditions and stomatal conductance .

/pdf/modelling-the-soil-plant-atmosphere-continuum-in-a-quercus-486u98ks74.pdf

Modelling the soil-plant-atmosphere continuum in a Quercus-Acer stand at Harvard Forest : the regulation of stomatal conductance by light, nitrogen and soil/plant hydraulic properties

As already explained in the previous chapter, generally the water potential, Ψ, of a plant cell is expressed as the sum of three components, as follows (Dainty 1976): 

$$psi = P + \pi + \tau$$

(2.1)

where P, π and τ are the pressure, osmotic and matric potentials, respectively. In a tissue or organ, Eq. (2.1) applies to each cell individually but if different cells have different solute concentrations, moduli of elasticity or volumes of apoplasmic water, then even at equilibrium the cellular values of P, π and τ may differ from cell to cell. Although measurements of π have frequently been made on individual cells using plasmolytic methods and measurements of P have recently been made on individual cells of higher plants using the pressure probe (Steudle et al. 1975; Husken et al. 1978, 1980), it is in general not practical to make a large number of such measurements in the tissues and organs of higher plants. In spite of this problem Tyree and Hammel (1972) have argued that it is meaningful to define bulk values of the water relations parameters to characterize the water relations properties of tissues and organs.

Water in Tissues and Cells

Leaf gas exchange, transpiration, water potential and xylem water flow measurements were used in order to investigate the daily water balance of intact, naturally growing, adult Larix and Picea trees without major injury. The total daily water use of the tree was very similar when measured as xylem water flow at breast height or at the trunk top below the shade branches, or as canopy transpiration by a porometer or gas exchange chamber at different crown positions. The average canopy transpiration is about 12% lower than the transpiration of a single twig in the sun crown of Larix and Picea. Despite the similarity in daily total water flows there are larger differences in the actual daily course. Transpiration started 2 to 3 h earlier than the xylem water flow and decreased at noon before the maximum xylem water flow was reached, and stopped in the evening 2 to 3 h earlier than the water flow though the stem. The daily course of the xylem water flow was very similar at the trunk base and top below the lowest branches with shade needles. The difference in water efflux from the crown via transpiration and the water influx from the trunk is caused by the use of stored water. The specific capacitance of the crown wood was estimated to be 4.7 x 10-8 and 6.3 x 10-8 kg kg-1 Pa-1 and the total amount of available water storage was 17.8 and 8.7 kg, which is 24% and 14% of the total daily transpiration in Larix and Picea respectively. Very little water was used from the main tree trunk. With increasing transpiration and use of stored water from wood in the crown, the water potential in the foliage decreases. Plant water status recovers with the decrease of transpiration and the refilling of the water storage sites. The liquid flow conductance in the trunk was 0.45 x 10-9 and 0.36 x 10-9 mol m-2s-1 Pa-1 in Larix and Picea respectively. The role of stomata and their control by environmental and internal plant factors is discussed.

Canopy transpiration and water fluxes in the xylem of the trunk of Larix and Picea trees - a comparison of xylem flow, porometer and cuvette measurements

Resistances to the flow of water through young potted apple trees were estimated by measuring the transpiration rate of trees with and without root systems. Root system resistances were obtained by difference. Whole-plant resistances were of the order 10 x 1013 Pa s m-3 and there was some evidence that root resistances (Rr) varied with transpiration rate; the ratio liT■ Rx (where Rx is resistance to water flow in the stem system) altered from 2:1 at relatively high transpiration rates to 1:1 at lower rates. The trunk of a 9-year-old orchard tree (trunk diameter ~7 cm, height ~2-5 m) was cut under water and estimates of the flow resistances in this tree were obtained. These were much lower than the resistances to flow in the potted trees. Capacitance (defined as the change in stored water content per unit change in plant water potential) values were calculated for the small trees and the large tree from measurements of weight and water potential changes after the trees were removed from water. They were very similar on a weight basis (approx. 2-0 x 10~8 kg kg-1 Pa-1). Leaf capacitance values (~1 x 10~8 kg Pa-1 m-2) were also obtained. Stomatal conductances decreased with water potential and increased with short-wave radiation, but the relationships were not definitive. Estimates of boundary layer conductance in a greenhouse (very low wind speeds) were of the same order (~ 5 mm s-1) as values obtained previously.

J. J. Landsberg

Papers

Studies on the Movement of Water Through Apple Trees

Studies on the Movement of Water Through