The research explored the apparent contradiction between the cohesion‐tension theory and the lack of blockage following freezing in conifers. Five to ten cm lengths of the main stem of actively transpiring Thuja occidentalis, Picea glauca and Pinus sylvestris were frozen. On thaw, there was no evidence of blockage in the formerly frozen xylem. Microscopic examination of frozen wood and hydrodynamic considerations showed that air was present in the xylem water and that bubbles were probably present upon freezing. On freezing about 9 per cent of the water irreversibly migrated into the unfrozen plant parts. Lack of blockage in the presence of air bubbles was reconciled to the cohesion‐tension theory by a model in which only the largest bubble in a number of interconnected cells expands. As this bubble expands tension is released allowing all bubbles in adjoining tracheids to redissolve. Potential rate of resolution of unexpanded bubbles following release of tension was calculated to be less than 0. 1 second.

Landscape Architecture (E.J.S.), University ofMinnesota, St. Paul, Minnesota 55108 MATERIALS AND METHODSThis study was undertaken to quantify the effect of aluminum and calcium on membrane permeability. The influence of Ca2" (0.2-3.7 millimolar) and Al' (0-3.7 millimolar) on the permeability of root cortical cells of Quercus rubra was measured using three nonelectrolytes (urea, methyl urea, and ethyl urea) Aluminum, as Al3", can interfere with physiological processes and be toxic to plant cells (1,5,(8)(9)(10)15). Plant cell membranes may be a site of primary lesions from Al3" (1,10). Hofler (10) and Bohm-Tuchy (1) showed that Al3" changed the attachment of the plasmalemma to the cell wall causing the membrane to assume a convex shape on plasmolysis. Bohm-Tuchy (1) also suggested that Al3" solidifies the outer region of the protoplasmic layer. Vierstra and Haug (19) using EPR spectroscopy showed that Al3" decreased membrane fluidity in isolated membranes and intact cells of a thermophilic bacterium.Ca2" is another ion well known to influence the integrity and functions of membranes (13,14). Recent work (9) shows negative interactions between Al3+ and Ca2" with regard to calmodulin activity and other physiologic processes (5, 15).The capacity of Al3" and other cations to alter the biochemical and biophysical properties of both the lipid and protein portions of membranes is well reviewed by Haug (8). Changes in the lipid portion of the membrane could result in an alteration of permeability to nonelectrolytes and to water. A major objective of this study was to measure how Al3+ altered the combined permeability of the plasma and vacuolar membranes in the root cortex cells of red oak. A related objective was to determine whether the effects of Al3" vary with the concentration of Ca2". Permeability Measurements. The solute permeability of the plasma membrane and tonoplast in series was measured by the techniques described by Stadelmann (17). Immediately after sectioning, the tissue was sequentially exposed to increasing concentrations of sucrose. The sections were in 0.1 M sucrose for 1 h, in 0.2, 0.3, and 0.4 M for 30 min each, and 0.5 M for 1 h. At this osmolarity (0.59 osm) all intact cells were plasmolyzed. The tissue sections in 0.5 M sucrose were transferred in a droplet of the same solution into a perfusion chamber (21). In the chamber, the sucrose solution was replaced with isotonic solutions or urea family permeators (urea, methyl urea, or ethyl urea

No abstract

ABSTRACrThe fluorescent compound trisodium, 3-hydroxy-5,8,10-pyrenetrisulfonate (PTS) was used to quantify the apoplastic flux through red pine (Pings resinosa Ait.) root systems-that portion of the total water flux reaching the xylem without ever crossing a semipermeable membrane. Flow was induced by pressure through detopped root systems, and by transpiration through intact seedlings. Apoplastic flux was determined by multiplying total flux by the ratio of PT'S concentration in the xylem exudate to PTS concentration in the bathing medium.Under aeration, apoplastic flux was less than 1% of total flux. Under anaerobic conditions, up to 50% of total flux was apoplastic suggesting that anaerobic conditions change the pathway of water flow into root xylem. The change under anaerobic conditions was reversible. Detopped root systems under pressure and intact seedlings under transpiration gave similar results. In detopped root systems, the magnitude of the pressure gradient may alter the apoplastic contribution to total flux. component of water flux through root systems.The existence of an exclusively apoplastic pathway is demonstrated by the appearance in the xylem of materials totally excluded by membranes (15). In roots without secondary cambium, breaks in the endodermis in the emergence zone around lateral roots (15, 16), wounds, and the root tips where the casparian strip is incompletely developed (4) all provide potential apoplastic pathways. The pathway across the endodermis has been traced with electron opaque elements, radioactively labeled substances, and apoplastic dyes. PTS was used as the marker of apoplastic water flux in our study. PTS is fluorescent, highly water soluble, nontoxic, not adsorbed onto cell walls, and is totally excluded from the symplast (14, 15, 19). The proportion of uptake which was exclusively apoplastic was calculated using an equation modified from one used by Mees and Weatherley ( 11) to calculate leaks in their root system under pressure:Water flow through root systems may be expressed by the following relationship: Q4= Qa+Qs (1) where Q, is the total water flux; Qa is the exclusively apoplastic flux, involving water which reaches the xylem without ever crossing a semipermeable membrane; and Qa is the symplastic flux, involving water that travels most of its pathway either in the symplast or apoplast but which must cross a membrane at least once during its passage to the xylem. Apoplastic flux is considered relatively unimportant since Q, is severely inhibited by metabolic inhibitors, and ion movement into roots appears to function as if it were crossing a membrane (5, 13). However, quantitative estimates of the apoplastic contribution to total flux were not found. A need to quantify the components in flux models is frequently indicated (1, 2, 10). A major objective of this study was to quantify the apoplastic

Quaking aspen (Populus tremuloides Michx.) seedlings were grown in nutrient solutions to investigate their responses to a range of solution Ca levels (10-1000 µM) similar to those found in aspen stands of upper Lakes States. Growth increased significantly with increasing level of solution Ca. The level of solution Ca associated with 90% of maximum elongation (critical level) was 61 µM for shoots and 88 µM for roots. Critical Ca levels for biomass probably were lower. Concentrations of Ca in leaves and roots increased significantly with increasing solution Ca. Elongation of shoots and roots was also strongly and positively related to Ca concentrations in leaves and roots. Critical Ca concentrations (oven-dry mass) for shoot elongation were 0.46% in leaf tissue and 0.12% in root tissue, while critical Ca concentrations for root elongation were 0.54% in leaf tissue and 0.13% in root tissue. Solution Ca may have induced deficiencies of other elements, but direct Ca deficiency was the primary cause of growth reduction. These critical Ca levels in solutions or tissues cannot be used to diagnose Ca deficiency in aspen forests until it is known how other soil and plant factors affect the Ca requirement of aspen.