What is the major mechanism for moving xylem sap from the roots to the leaves?

Sap flow

Sap-flow sensors measure transpiration flow as the ascent of sap within xylem tissue; measurements can be made in stems, trunks, branches, or tillers. Given that transpiration is sensitive to plant water status, with the effect being mediated by stomatal opening (see Section Stomatal Responses, below), sap flow can be used as an indicator of plant water status.

Sap flow rates can be out of phase with transpiration because of capacitance effects in stems or branches arising from the storage of water. Continuous data recording enables the time course of transpiration to be followed.

Two different techniques are used to measure sap flow; both use heat as a tracer. One is the stem (or trunk) sector heat-balance method, in which a section of the entire stem circumference is electrically heated, and the axial and radial heat-loss measured. The mass flow rate of sap is calculated as a function of the heat dissipated by the ascending sap. The other method is the heat-pulse method in which the heater and temperature sensor probes are placed inside the trunk in a radial direction. The sap velocity is calculated as a function of the time required by the flowing sap to transport heat to a particular location.

Sap Flow in Intact Plants

Sap flow rates in intact plants can be described using the Ohm's law, in analogy to electrical theory, as follows:

(1)E=ΔΨR

where E, ΔΨ, and R indicate the water flow rate (in units of mass of water per second), the water potential difference driving the flow between two points conveniently chosen (for instance the inside of the root stele and the leaf xylem), and the xylem hydraulic resistance, respectively (see also Figure 1). The xylem hydraulic resistance represents the inevitable energy losses of the unit mass of flowing water from roots to leaves per unit time, owing to the cohesion (i.e., friction) between adjacent water molecules and the adhesion between water molecules and cell walls in these tiny capillaries. The above equation implies that a plant can maximize the flow of sap through its xylem by two means: by maximizing the water potential difference between roots and leaves; and by minimizing the hydraulic resistance between these two points (i.e., a more efficient xylem). This fundamental distinction gives rise to a number of very important hypotheses about plant behavior in relation to xylem physiology and ecology, which will be developed below. However, we first need to understand the concept of xylem vulnerability.

Maple Sap Flow

Maple sap flow deserves special attention both because it forms the basis of an important industry in the northeastern United States and because it is interesting physiologically. There are several reasons for believing that it occurs quite independently of root pressure, including the fact that pressure gauges attached to roots of maple often show negative pressure when stems show positive pressure (see Fig. 11.18). More convincing is the fact that segments of stems and branches removed from maple trees show sap flow if supplied with water and subjected to temperatures that rise above and fall below freezing (Stevens and Eggert, 1945; Marvin and Greene, 1951; O'Malley and Milburn, 1983; Tyree, 1983).

The extensive observations on maple sap flow made by Clark (1874, 1875) are still applicable. In Massachusetts, maple sap flow can occur any time from October to April if freezing nights are followed by warm days. Sap flow ceases if temperatures are continuously above or below freezing; it stops in the spring when night temperatures no longer fall below freezing, and it usually ceases in the afternoon and does not start again until the temperature rises above freezing the next morning. Failure to understand that sap pressure in the stems of trees often undergoes daily variations from positive to negative has led to unfortunate errors in the interpretation of experimental data (Kramer, 1940). In contrast to the situation in maple, the root-pressure-generated flow of birch and grape sap increases as the soil warms until increased transpiration caused by opening of leaves brings an end to root pressure.

Because of its dependence on weather, maple sap flow usually is intermittent, and from 2 or 3 to 10 or 12 runs may occur in a single spring. Many producers of maple syrup use vacuum to increase the sap flow up to three times the normal amount (Koelling et al., 1968). The sugar content of maple sap varies from 0.5 to 7.0 or even 10.0%, but it usually is 2.0 or 3.0%, much lower than the sugar content of palm sap (Chapter 7).

Trees on infertile or dry soil will yield less than those growing on fertile, moist soil. The sugar yield obviously is related to photosynthesis, and large, well-exposed crowns are advantageous. Fertilization also is said to increase the yield. Trees grown for sap production should be more widely spaced than those grown for timber, and roadside trees are said to produce large quantities of sap. Jones et al. (1903) reported that defoliation during the summer greatly reduced maple syrup yield the next spring.

There has been significant progress in our understanding of sap flow. Sap flow is caused by stem pressure produced during alternating diurnal cycles of below- and above-freezing temperatures (Fig. 11.19). Stem pressure does not develop during the day unless the temperature regime permits both freezing and thawing. During freezing, maple twigs take up solutions, and on thawing, they exude a slightly smaller amount than was absorbed. This cycle can be repeated a few times as twigs hydrate, but at very high twig water content, freezing-induced absorption disappears (Milburn and O'Malley, 1984; Johnson and Tyree, 1992).

Milburn and O'Malley (1984) proposed a cellular mechanism to explain these observations (Fig. 11.20). Xylem of sugar maple stems has abundant gas-filled fibers with liquid-filled vessels and few intercellular spaces. As stem xylem cools, internal gas space declines in conformance with gas laws and with increased dissolution of gases into the liquid phase. When ice-nucleating temperatures are reached during a freeze-thaw cycle, liquid water moving to the interior surface of fiber cells begins to freeze on the walls, accumulating by vapor distillation and decreasing the air space within the fiber lumen and substantially elevating internal pressure. On thawing of the ice, the high gas pressure within the fibers forces liquid water from the lumen and, in bulk, to any region of lower pressure. If the moisture content is elevated to the point at which fibers fill with water, the absorption–exudation cycle disappears. Most experimental results are consistent with this hypothesis, but a reported requirement for sucrose (or other di- or oligosaccharides) in the sap (Johnson et al., 1987; Johnson and Tyree, 1992) has not been adequately reconciled to this simple physical model.

Velocity of Sap Flow

Several investigators have attempted to estimate the rate of sap flow through tree stems. Two general approaches have been employed: (1) measurement of movement of radioactive or stable isotopic tracers such as 2H, 3H, and 32P supplied to the xylem stream (Kuntz and Riker, 1955; Owston et al., 1972; Heine and Farr, 1973; Waring and Roberts, 1979; Calder et al., 1986; Dye et al., 1992) and (2) detection of convected heat flow associated with external or internal stem heating (Huber and Schmidt, 1937; Sakuratani, 1981). Although both methods are theoretically and technically feasible, heat flow applications have been more common. One such method involves a pulse of heat applied by probes or microwave radiation. By this technique, sap is spot heated and the time of travel required for convective movement upward to a temperature sensor is compared with that required for conductive movement of the heat pulse downward. Sap flow also has been measured by means of the energy balance of stems encircled by heating bands supplied with constant power (Sakuratani, 1981, 1984; Baker and van Bavel, 1987; Baker and Nieber, 1989). Estimates of sap movement based on heat flow measurements generally correlate well with transpiration rates of individual trees (Fig. 12.11). Sap flow velocity at the base of large trees frequently lags behind crown transpiration in the morning and exceeds it in the evening, reflecting the capacitance of stem and crown portions of the soil–plant–atmosphere continuum (Cohen et al., 1985) (Fig. 12.12).

Maximum rates of sap flow in trees are reported to vary between 1 and 2 m hr–1 in conifers, 1 to 6 m hr–1 in diffuse-porous trees, and 4 to 40 m hr–1 in ring-porous trees (Zimmermann and Brown, 1971). The velocity of flow in conifers and diffuse-porous trees is low, as water moves through conducting elements in a number of annual rings of sapwood, whereas in ring-porous broad-leaved trees it moves rapidly through relatively few vessels located in only one or two annual rings (Chapter 11). The unusually low rate of sap movement in conifers in 1977 (Fig. 12.13) was attributed to a low winter snowpack followed by a dry spring (Lopushinsky, 1986). When soil moisture was not limiting, seasonal patterns of sap movement in Douglas fir and ponderosa pine stems were regulated by air temperature, solar radiation, and vapor pressure deficit. As soil moisture decreased during the summer, the rate of sap movement no longer followed evaporative demand. Early in the summer, the rate of daily sap movement was highest near midday; in the autumn maximum rates occurred later in the day (Lopushinsky, 1986). Changes in xylem conductivity may also affect the velocity of sap movement. As the water content of the sapwood of Douglas fir decreased, stem conductivity also decreased (Waring and Running, 1978). Miller et al. (1980) found that sap flow in black and white oaks was most responsive to solar radiation up to 0.6 cal cm–2 min–1 flux density; thereafter, it was more responsive to changes in vapor pressure deficit of the air. Sap flow in oaks also was quite variable around the trunk, with sections below well-lit portions of the crown having far higher flow rates than shaded portions (Fig. 12.14).

There have been numerous attempts to estimate whole-plant transpiration rates from sap flow measurements. While some success has been achieved (e.g., Steinberg et al., 1990a; Heilman and Ham, 1990; Jarvis, 1993), there are conceptual and practical problems in scaling up one or a few estimates of sap flow to obtain a valid calculation of the water loss of an entire plant, especially a large tree. Heat pulse measurements generally underestimate true sap flow rates because insertions of the probes presumably cause local injury and flow disruption to the xylem pathway (Doley and Grieve, 1966; Cohen et al., 1981, 1985; Green and Clothier, 1988) (Fig. 12.11). Most success has been obtained with herbaceous or small woody plants (Cohen et al., 1988; Valancogne and Nasr, 1989; Steinberg et al., 1990b; Heilman and Ham, 1990). Large trees provide a formidable sampling problem because of variability in sap flow rates and water content both around the stem and radially within the xylem (Lassoie et al., 1977; Miller et al., 1980; Cohen et al., 1981). Olbrich (1991) found that whereas four sampling probes were needed to adequately estimate stem sap flow in small eucalyptus trees, at least eight probes were required for trees greater than 20 cm in diameter. In the future nuclear magnetic resonance (NMR) techniques may allow noninvasive monitoring of sap flow in stems (e.g., Reinders et al., 1988), but the instrumentation and magnets necessary for these measurements at present suggest that only small plants can be accommodated and that the utility of NMR will be limited to laboratory situations. Kaufmann and Kelliher (1991) discussed and evaluated methods for sap flow estimation.

Environmental Effects on Phloem Transport

After decades of controversy, mass flow has generally been accepted as the long-distance phloem transport mechanism. Velocities of sap flow measured by heat propagation, fluorochrome movement, and nuclear magnetic resonance matched those of mass flow rates calculated by theoretical models. Proteinaceous material deposited onto the sieve plates does not appear to entirely block the sieve pores in vivo, but leaves free corridors for mass flow. Environmental factors affecting the turgor difference between source and sink are mainly responsible for the mass flow rate, but mass flow is also dependent on physical resistance within the system (Figure 3). First of all, temperature may impact on the viscosity of the phloem sap. With sucrose, sieve tube sap is calculated to be less viscous at low temperatures than with raffinose-related sugars. It is not excluded that several rare sugars and sugar alcohols in the sieve tube sap function as fluidizers or as a protection against increased viscosity at low temperatures.

The size of the transport corridors within sieve pores, the bottlenecks in the transport pipes (Figure 3), may be regulated by the degree of protein deposition which may in turn depend on environmental factors. The Hagen–Poiseuille law predicts that hydraulic conductivity of sieve tubes is a function of the radii of the sieve pores and viscosity of the transported sap. Sieve pores interconnecting adjacent sieve elements determine the limiting radius of sieve tubes. Sieve pore radius is reversibly influenced by callose formation and protein deposition, both of which are responsive to temperature. Inefficiency of phloem transport at low temperatures may reflect clogging of sieve pores with phloem-specific proteins.

At high temperatures, large deposits of callose on sieve plate pores have been correlated with slowing of phloem translocation through localized regions of heat-treated stems. In addition, seasonal variation in callose deposition in deciduous perennials is correlated with phloem transport rates. Collectively these temperature impacts on hydraulic conductance of sieve tubes partially account for differences in chilling sensitivity of phloem transport between species. There is, however, a physiological component manifested in the continuous reloading of photoassimilates leaked to the phloem apoplasm (Figure 1). Indeed, recovery of transport rates through a stem section exposed to low temperatures is attributed to upstream reloading enhancing the pressure difference across the chilled stem portion to restore transport through the region of lowered hydraulic conductance.

Balance between leakage from and recovery into sieve tubes along the transport phloem determines whether there is a net unloading or loading of this pathway. These processes are influenced by prevailing environmental conditions. Net unloading and subsequent storage occurs under sink-limited conditions that can be imposed by environmental impacts such as water stress leading to downregulation of sink demand. A direct effect on unloading from the transport phloem is observed in stems colonized by holoparasites and bacteria that induce galls. In both cases, enhanced unloading is linked to a pronounced induction of a symplasmic pathway of unloading from the sieve tubes of the host plant. Conversely, environmental reduction in photoassimilate export from source leaves can lead to the onset of remobilization of stored reserves loading the transport phloem and subsequent delivery to nearby sinks. Thus, unloading from, and reloading into, transport phloem functions as a buffer to balance environmental impacts on source and sink processes.

The Aggregation Model

Many insects exploit resources that are patchy, consisting of small, separate units, and that are ephemeral in the sense that they persist for only one or two generations. Such resources can include fruit, fungi, sap flows, decaying leaves, flowers, dung, carrion, seeds, dead wood, and small bodies of water held in parts of terrestrial plants (phytotelmata). This general view of insect ecology inspired the aggregation model of competition (Shorrocks et al., 1984; Atkinson and Shorrocks, 1981), which allows a competitively inferior species to survive in probability refuges. These are patches of resource (a single fungus, fruit, etc.) with no or a few superior competitors that arise because the competing stages (usually larvae) have an aggregated distribution across the patches. These probability refuges are a permanent feature of such systems because patches, such as fungi, are ephemeral and aggregation increases mean crowding. Regional population density is limited by strong intraspecific competition in patches with high local density whereas low-density patches still exist (e.g., population size within a wood is limited by high density in some fungi, whereas other fungi still contain no or a few individuals). As with resource partitioning, coexistence is promoted because aggregation of the superior species increases its intraspecific competition and reduces interspecific competition.

In the aggregation model, the eggs of both insect species are independently distributed over the patches according to a negative binomial distribution, which has an exponent, k, inversely related to the degree of intraspecific aggregation. The use of the negative binomial and the assumption of independence have been justified for drosophilid flies. In the first version of the model, the parameter k (level of aggregation) was constant and independent of density. This is not valid for real populations, but relaxing this assumption does not prevent coexistence. Within each patch, competition is modeled by a difference form of the Lotka–Volterra equations.

The predictions of the aggregation model are that with k of the negative binomial <1 (strong aggregation), it is virtually impossible for the competitively superior species to eliminate the competitively inferior species. Figure 7 shows the model′s results as a graph of “critical α” against k of the negative binomial. Critical α is the competition coefficient that the superior species must have in order to exclude the inferior species. Also shown on the graph are distributions of α and k for drosophilid flies. For these flies it is clear that k of the negative binomial is usually <1 and that competition coefficients are not sufficiently large to prevent competitive exclusion. For many animals exploiting ephemeral and patchy resources, this model therefore provides a viable alternative to traditional resource partitioning as an explanation for the coexistence of species. The two-species model has been extended to a many-species model (Shorrocks and Rosewell, 1987) and predicts average group sizes of about seven species coexisting on identical resources.

Budding and Grafting of Scions

The majority of fruit cultivars are propagated by budding or grafting the scion cultivar onto a rootstock. Budding is undertaken in summer, at a time when the buds have ripened sufficiently to survive their excision and transfer to the rootstock and when the sap flow within the rootstock is optimal for bud insertion. The buds used are lateral buds formed on shoots developed in the current season. These are nongrowing quiescent buds found in the leaf axils of the mid-portions of the extension shoots. They are cut individually from these shoots together with a small sliver of the surrounding stem tissues, including epidermis, phloem, cambium, and xylem tissues. A notch or T-shaped cut of similar dimension to the bud sliver is then cut out of or into the side of the rootstock stem at approximately 10–30 cm above soil level. The bud is then inserted into the cut on the rootstock and the two parts bound together with degradable tapes, to prevent desiccation and aid healing. Where necessary (if nondegradable), the bud ties are cut, once the union between the bud and the rootstock has healed. Efficient healing will only occur if the cambia of the two components (bud and rootstock) are brought into close contact with each other and if desiccation is prevented.

In the first summer and autumn, the bud remains quiescent and no outgrowth is made. In the subsequent winter, the rootstock is cut off just above the position of the bud and in the spring the bud of the scion begins its growth into a young tree. The trees may be lifted and sold following 1 or 2 years growth in the nursery.

The principles of grafting are similar to those of budding, but the size of the scion component is larger and the timing of the operation differs. Scion grafts comprise pieces of stem cut from 1-year-old shoots during the winter months; these usually contain two or three lateral buds. These are attached to the rootstocks using various types of grafts, the most common of which is called a whip and tongue graft. As with budding, the main objectives are to join the cambia of the two components and to prevent desiccation. Occasionally, a technique known as framework grafting is used where fruit growers wish to change an orchard from one scion cultivar to another, without the need to plant new young trees.

Transport in the Xylem

The xylem is composed of dead cells with thick, lignified secondary cell walls. These form a low resistance pathway for the movement of solution. It comprises both narrow tracheids and vessel elements. The hollow vessel elements are joined in files and their adjoining end walls are perforated (perforation plates). The diameter of vessel elements can vary considerably, between 8 and 500 μm. The vessel elements are closely associated with living xylem parenchyma cells, which store carbohydrates and may regulate the fluxes of solutes into and out of the vessel elements.

Xylem sap is quite dilute, containing only about 10 mmol l−1 inorganic nutrients plus some organic nitrogen metabolites. The driving force for the movement of xylem sap is hydrostatic pressure and xylem sap flows according to Poiseuille's Law: Jv = −r2ΔP / 8ηl, where Jv is the volume of solution with viscosity η flowing per unit area in a cylinder of radius r and with a difference in (hydrostatic) pressure ΔP over a distance l. Measurements of ΔP and estimates of the ΔP required to attain the solution flows observed both indicate that a ΔP of about −0.03 MPa m−1 exists in the xylem of rapidly transpiring plants. This can be compared with the pressure gradient required for an equivalent solution flow through the small interstices of cell walls, which approximates −3×105 MPa m−1. This is the reason why the xylem is the preferred pathway for the transport of solution from the root to the shoot.

The rate and direction of transport of xylem sap is governed primarily by transpiration, which manifests itself in marked diurnal changes in ΔP in the xylem. Plants transpiring rapidly generate large tensions (negative hydrostatic pressures) in the xylem, which are maintained by the cohesion of water molecules resulting from hydrogen bonding. By contrast, the tension in the xylem diminishes, and hydrostatic pressures may become positive, under conditions of low transpiration. In some circumstances this draws water from the surrounding cells into the xylem and results in guttation.

Xylem sap is distributed within the shoot through the veins in the leaves. The subsequent movement of ions may be apoplastic, following the transpiration stream, or symplastic. Some ions are selectively accumulated by leaf cells and, as a consequence of evaporation, ions accompanying the transpiration stream may accumulate in the apoplast. These processes generate the characteristic ionic composition of the different cell types within the leaf. For example, P accumulates in bundle sheath and mesophyll cells, whereas Ca, Na, and Cl all accumulate in epidermal cells, especially those closest to the veins. Such differences between cell types in ion accumulation may be important in the adaptation of plants to saline or metalliferous soils.

11.3 Cold stress

Long term exposure to chilling stress (~4–8 °C) declines Lpr and sap flow in the roots. Chilling-tolerant plants may recover the effect, whereas sensitive plants not able to overcome and subsequently dry out and die. Transcriptome analysis of root and leaf in Arabidopsis, rice and maize revealed significant reduction in expression of most of the aquaporin genes in response to chilling stress (Jang et al., 2004; Aroca et al., 2005; Sakurai et al., 2005; Yu et al., 2006). However, during the recovery period, the expression of root-specific PIP genes has shown to be upregulated. Relative expression of OsPIP2;5 was enhanced in the long-term low root temperature (LRT) treated plants and impart in cold acclimatization in rice (Ahamed et al., 2012). The early cold response triggered the elevated expression of some PIPs and TIPs, including MaPIP1;1, MaPIP1;2, MaPIP2;4, MaPIP2;6, MaTIP1;3 in cold-tolerant banana species Musa spp. Dajiao, which indulged in the maintenance of leaf water potential and cold adaptation (He et al., 2018).