A mass-balanced, finite-difference solution to Munch's osmotically generated pressure-flow hypothesis is developed for the study of non-steady-state sucrose transport in the phloem tissue of plants. Major improvements over previous modeling efforts are the inclusion of wall elasticity, nonlinear functions of viscosity and solute potential, an enhanced calculation of sieve pore resistance, and the introduction of a slope-limiting total variation diminishing method for determining the concentration of sucrose at node boundaries. The numerical properties of the model are discussed, as is the history of the modeling of pressure-driven phloem transport. Idealized results are presented for a sharp, fast-moving concentration front, and the effect of changing sieve tube length on the transport of sucrose in both the steady-state and non-steady-state cases is examined. Most of the resistance to transport is found to be axial, rather than radial (via membrane transport), and most of the axial resistance is due to the sieve plates. Because of the sieve plates, sieve tube elasticity does not provide a significant enhancement to conductivity at high pressure, as previously suspected. The transit time of sucrose through a sieve tube is found to be inversely proportional to the square of the sieve tube's length; following that observation, it is suggested that 20 1-m-long sieve tubes could transport sucrose 20 times faster than a single 20 m sieve tube. Short sieve tubes would be highly sensitive to differentials between loading and unloading rate, and would require close cooperation with adjacent companion cells for proper function.
Ions can enhance water flow through the xylem via changes in the hydraulic resistance at border pit membranes. Because flow between adjacent xylem vessels occurs primarily via bordered pit fields, it is hypothesized that xylem sap ion concentrations would affect lateral movement of water more than longitudinal flow. Using tomato as a model system, evidence is presented for ion-mediated changes in xylem hydraulic resistance and the lateral transport of water. Water flow between adjacent xylem bundles increased by approximately 50% in the presence of ions while longitudinal flow only increased by approximately 20%. However, the enhancement of lateral exchange due to ions was magnified by the presence of a pressure difference between vascular bundles. These results indicate that the degree of nutrient-sharing among sectors of a plant may depend on both nutrient concentration and the availability of water in the root zone.
Many leaves survive the severing of their major veins in apparently excellent health. According to the classical explanation, the leaf minor veins provide "conductive overload," an excess of parallel conductive paths, rendering the major veins hydraulically dispensable. Whether such an excess of conductive paths exists has important implications for vascular design and for leaf response to vascular damage. We subjected leaves of Acer saccharum and Quercus rubra to cutting treatments that disrupted the major vein system and determined leaf survival, stomatal conductance (g), quantum yield of photosystem II (Phi(PSII)), and leaf hydraulic conductance (K(leaf)). For A. saccharum, the cuts led to the death of distal lamina. For Q. rubra, however, the treated leaves typically remained apparently healthy. Despite their appearance, the treated Q. rubra leaves had a strongly reduced K(leaf), relative to control leaves, and g and Phi(PSII) were reduced distal to the cuts, respectively, by 75-97% and 48-76%. Gas exchange proximal to the cuts was unaffected, indicating the independence of lamina regions and their local stomata. Analogous results were obtained with excised Q. rubra leaves. These studies demonstrate an indispensable, vital role of the major veins in conducting water throughout the lamina.
We studied the levels of genetic diversity of Swietenia macrophylla (big leaf mahogany) in five successional plots in the Santa Rosa National Park, Guanacaste, Costa Rica. We selected sites with different lengths of time since the last major disturbance (typically fire): 6, 9, 15 and 20 years. In addition, we also included a patch of mature forest that had experienced selective logging and other human activity in the past 100 years. Genetic diversity was assessed using five polymorphic DNA microsatellite loci. We found a total of 21 alleles in the five loci examined, in which the number of alleles present varied among the five sites studied. Allelic diversity varied between sites ranging from 20 to 14 alleles, and our data revealed that earlier successional sites have more alleles than older sites. There was significant heterogeneity in allele frequencies between sites; however, genetic differentiation between populations was low (FST = 0.063) indicating that most of the variation was found within sites and extensive gene flow between sites. In addition, our analysis also showed that genetic diversity of adult trees does not solely determine the diversity of seedlings and saplings found around them, also supporting the existence of extensive gene flow. The impact of these findings for the design of conservation strategies for tropical dry forests trees is discussed.
The question as to what triggers stomatal closure during leaf desiccation remains controversial. This paper examines characteristics of the vascular and photosynthetic functions of the leaf to determine which responds most similarly to stomata during desiccation. Leaf hydraulic conductance (K(leaf)) was measured from the relaxation kinetics of leaf water potential (Psi(l)), and a novel application of this technique allowed the response of K(leaf) to Psi(l) to be determined. These "vulnerability curves" show that K(leaf) is highly sensitive to Psi(l) and that the response of stomatal conductance to Psi(l) is closely correlated with the response of K(leaf) to Psi(l). The turgor loss point of leaves was also correlated with K(leaf) and stomatal closure, whereas the decline in PSII quantum yield during leaf drying occurred at a lower Psi(l) than stomatal closure. These results indicate that stomatal closure is primarily coordinated with K(leaf). However, the close proximity of Psi(l) at initial stomatal closure and initial loss of K(leaf) suggest that partial loss of K(leaf) might occur regularly, presumably necessitating repair of embolisms.
The relation between xylem vessel age and vulnerability to cavitation of sugar maple (Acer saccharum Marsh.) was quantified by measuring the pressure required to force air across bordered pit membranes separating individual xylem vessels. We found that the bordered pit membranes of vessels located in current year xylem could withstand greater applied gas pressures (3.8 MPa) compared with bordered pit membranes in vessels located in older annular rings (2.0 MPa). A longitudinal transect along 6-year-old branches indicated that the pressure required to push gas across bordered pit membranes of current year xylem did not vary with distance from the growing tip. To understand the contribution of age-related changes in vulnerability to the overall resistance to cavitation, we combined data on the pressure thresholds of individual xylem vessels with measurements of the relative flow rate through each annual ring. The annual ring of the current year contributed only 16% of the total flow measured on 10-cm-long segments cut from 6-year-old branches, but it contributed more than 70% of the total flow when measured through 6-year-old branches to the point of leaf attachment. The vulnerability curve calculated using relative flow rates measured on branch segments were similar to vulnerability curves measured on 6-year-old branches (pressure that reduces hydraulic conductance by 50% = 1.6-2.4 MPa), whereas the vulnerability curve calculated using relative flow rates measured on 6-year-old branches were similar to ones measured on the extension growth of the current year (pressure that reduces hydraulic conductance by 50% = 3.8 MPa). These data suggest that, in sugar maple, the xylem of the current year can withstand larger xylem tensions than older wood and dominates water delivery to leaves.