Xylem cavitation in the leaf of Prunus laurocerasus and its impact on leaf hydraulics

This paper reports how water stress correlates with changes in hydraulic conductivity of stems, leaf midrib, and whole leaves of Prunus laurocerasus. Water stress caused cavitation-induced dysfunction in vessels of P. laurocerasus. Cavitation was detected acoustically by counts of ultrasonic acoustic emissions and by the loss of hydraulic conductivity measured by a vacuum chamber method. Stems and midribs were approximately equally vulnerable to cavitations. Although midribs suffered a 70% loss of hydraulic conductance at leaf water potentials of -1.5 MPa, there was less than a 10% loss of hydraulic conductance in whole leaves. Cutting and sealing the midrib 20 mm from the leaf base caused only a 30% loss of conduction of the whole leaf. A high-pressure flow meter was used to measure conductance of whole leaves and as the leaf was progressively cut back from tip to base. These data were fitted to a model of hydraulic conductance of leaves that explained the above results, i.e. redundancy in hydraulic pathways whereby water can flow around embolized regions in the leaf, makes whole leaves relatively insensitive to significant changes in conductance of the midrib. The onset of cavitation events in P. laurocerasus leaves correlated with the onset of stomatal closure as found recently in studies of other species in our laboratory.     

Maple sap uptake, exudation, and pressure changes correlated with freezing exotherms and thawing endotherms

Sap flow rates and sap pressure changes were measured in dormant sugar maple trees (Acer saccharum Marsh.). In the forest, sap flow rates and pressure changes were measured from tap holes drilled into tree trunks in mature trees and sap flow rates were measured from the base of excised branches. Excised branches were also brought into the laboratory where air temperature could be carefully controlled in a refrigerated box and sap flow rates and sap pressures were measured from the cut base of the branches.

Under both forest and laboratory conditions, sap uptake occurred as the wood temperature declined but much more rapid sap uptake correlated with the onset of the freezing exotherm. When sap pressures were measured under conditions of negligible volume displacement, the sap pressure rapidly fell to −60 to −80 kilopascals at the start of the freezing exotherm. The volume of water uptake and the rate of uptake depended on the rate of freezing. A slow freezing rate correlated with a large volume of water uptake, a fast freezing rate induced a smaller volume of water uptake. The volume of water uptake ranged from 0.02 to 0.055 grams water per gram dry weight of sapwood. The volume of water exuded after thawing was usually less than the volume of uptake so that after several freezing and thawing cycles the sapwood water content increased from 0.7 to 0.8 grams water per gram dry weight.

These results are discussed in terms of a physical model of the mechanism of maple sap uptake and exudation first proposed by P. E. R. O'Malley. The proposed mechanism of sap uptake is by vapor distillation in air filled wood fiber lumina during the freezing of minor branches. Gravity and pressurized air bubbles (compressed during freezing) cause sap flow from the canopy down the tree after the thaw.

     

Identification of the precursor protein to basement membrane heparan sulfate proteoglycans

The precursor protein of a basement membrane specific heparan sulfate proteoglycan has been identified as a 400,000 Mr polypeptide. Antibodies against large and small forms of this proteoglycan, isolated from a basement membrane (Engelbreth-Holm-Swarm, EHS) tumor, immunoprecipitated the same 400,000 protein from pulse-labeled EHS cells. The proteoglycan precursor protein was not recognized by antibodies against other basement membrane components or by antibodies to the cartilage proteoglycan. Furthermore, heparan sulfate proteoglycan purified from the EHS tumor blocked the immunoprecipitation of the precursor protein. Pulse-chase studies with [35S]methionine showed the precursor protein was converted to a proteoglycan. Pulse-chase studies with 35SO4 showed the large, low density proteoglycan appeared first and was degraded to a smaller, high density proteoglycan. We propose that the precursor protein is used after very little or no modification in the assembly of a large, low density heparan sulfate proteoglycan and that a portion of the population of these macromolecules are subsequently degraded to a smaller form.     

Dynamic measurements of roots hydraulic conductance using a high-pressure flowmeter in the laboratory and field

A new high-pressure flowmeter(HPFM)is described which is capableof rapid water-flow measurements. The HPFM permits dynamic determinationof hydraulic conductance of roots, K_r, and can be used in tehlaboratory or field. The base of a root is connected to theHPFM and water is perfused into the root system opposite tothe normal direction of flow during trnaspiration. The perfusionpressure is changed at a constant rate of 3–7 kPa s^–1while measuring the flow into the root every 2–4 s. Theslope of the plot of flow versus applied pressure is K_r. This paper describes the HPFM, presnents the theory of dynamicflow measurements, discusses sources of error, presnets evidencethat dynamic measurements of K_r in Ficus maclellandi (and sixother tropical species from Panama) yield the correct result,and demonstrates the use of the method under field conditionsin Panama on Cecropia obtusifolia and Palicourea guianensis. Key words: High-pressure flowmeter, root and shoot hydraulic conductance, Ficus maclellandi, Cecropia obtusifolia, Palicourea guianensis     

Tree Leaf Form in Brunei: A Heath Forest and a Mixed Dipterocarp Forest Compared1

Canopy‐top leaves of the dominant tree species from two 0.96‐ha plots in Brunei, northern Borneo, were sampled for structural and chemical analysis. Thirteen species from the mixed dipterocarp forest at Andulau and 14 from the lowland heath forest at Badas were studied. The heath‐forest species had significantly thicker leaves and were lower in nitrogen and ash concentration than those from the mixed dipterocarp forest. There were no significant differences between the two species groups in leaf mass per unit area (LMA), leaf fracture toughness, carbon concentration, 8¹³C, neutral detergent fiber concentration, sclerophylly index, and stomatal density. A significant negative correlation between %C and 8¹³C was found for the species from the mixed dipterocarp forest, but not those from the heath forest. The degree of sclerophylly measured in physical terms overlapped between the two sites to a considerable degree; however, all six species tested that were present in both plots had higher leaf fracture toughness in the heath forest. The possible reasons for the marked sclerophylly in the mixed dipterocarp forest are discussed.     

Collapse of Water-Stress Emboli in the Tracheids of Thuja occidentalis L

We report the kinetics of embolus formation and collapse in the tracheids of Thuja occidentalis L. stem segments. Radial wood sections were trimmed to 4 mm long paralleling the tracheids by 1 mm wide and 0.1 mm thick. They were observed under a dissecting microscope at 128x while sections were dehydrated and rehydrated. During dehydration, cavitations resulted in the formation of emboli in tracheids, but we concluded that the cavitated tracheids did not immediately fill with air at atmospheric pressure. This conclusion was based on the time required for the emboli to collapse after the rewetting of the dehydrated segment. By hypothesis, the time for the emboli to collapse should be proportional to the amount of air in the emboli. The time for all the emboli to collapse was a linear function of the dehydration time for times up to 15 min. For dehydrations greater than 80 min, the time for collapse after rewetting was constant, and we concluded that the tracheids have saturated with air by 80 min of dehydration. The kinetics of embolus formation is discussed in terms of the air-seeding hypothesis for cavitation, and collapse is discussed in terms of the physics of gas dissolution and diffusion. Embolus formation and dissolution in intact herbaceous and woody plants should follow the same physical laws.     

Measuring vessel length in vascular plants: can we divine the truth? History,theory, methods,and contrasting models

Key message

The Cohen method of measuring vessel-length distributions is much more accurate than the DD algorithm on integer values, which should be abandoned. More research is needed to get the real distribution of vessel length.

Abstract

Scientists have been measuring the vessel length of plants for more than 50 years. The method involves infusing stem or segments with a visible substance that completely fills vessels cut open at the infusion surface. The number of infused vessels is then quantified versus distance from the infusion surface. A theoretical model is then used to convert the counts of infused vessels to a vessel length distribution. Over the years the methods and theory have changed greatly. The purpose of this review is to give the reader an understanding of why vessel length is important and to provide a theoretical basis for selection of the best method and theory to arrive at vessel length data.