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A pressure chamber technique was used to estimate hydraulic root resistance in rough lemon (Citrus jambhiri Lush.) seedlings. The effect of previous water stress on hydraulic root resistance was evaluated. A factorial 3 × 3 design with four replications was established with potted rough lemon seedlings in a growth chamber. Three water-stress treatments were applied by watering at intervals of 1, 2 and 3 days, and root resistance measurements were made after 6, 12 and 18 days of treatment. Plants that had experienced mild and severe water stress (irrigation interval of 2 and 3 days, respectively) had higher hydraulic root resistances after several drying cycles than those plants irrigated daily. Additional cycles had no significant effect. The increase in root resistance was not due to decreased root growth but apparently to changes in the permeability of the root cell membranes or to increased suberin deposition in the cell walls of the cortical cells. in a short-term experiment (1 h), temperature strongly affected water flow through rough lemon roots in the range 5 to 35°C. Temperature influenced root membrane permeability, since reduced flow could not be explained by changes in water viscosity. [ABSTRACT FROM AUTHOR]

A field study was conducted to determine how atmospheric and edaphic conditions influenced the water relations of avocado trees (Persea americana Mill. cv. Bacon). With high and low levels of incident photosynthetically active radiation (PAR, 400–700 nm wave length), and either wet or dry soil, leaf conductance decreased as the absolute humidity difference from leaf to air increased. For any water stress treatment, conductance was higher at high PAR than at low PAR. Both conductance and transpiration were higher in well‐watered trees than in stressed trees, and in prestressed trees levels were intermediate to unstressed and stressed trees. A model for water flux through the soil‐plant‐atmosphere continuum was used to examine the relationship of leaf xylem pressure potential to transpiration in well‐watered trees and in trees stressed by dry soil. There was a close linkage between leaf xylem pressure potential and transpiration in unstressed and previously stressed trees with high or low PAR, i.e. similar potentials occurred with equivalent transpiration regardless of previous treatment or time of day. In stressed trees, xylem pressure potential was lower than in unstressed trees both during the day and night, and at a given transpiration rate the potential was lower after 1400 h than before that time. The model indicated that in stressed trees xylem pressure potential was uncoupled from transpiration, presumably because of altered resistance in the soil‐root portion of the transport system. [ABSTRACT FROM AUTHOR]

Seven-month-old rough lemon (Citrus jambhiri Lush.) seedlings were subjected to high- and low-humidity treatments (vapor pressure deficits of 8.0 and 19.6 mbar) for 3 weeks. Half of each group was well supplied with water and half was subjected to a series of three drying cycles in which xylem pressure potential fell to below -25 bar. The relationship between leaf conductance and xylem pressure potential was similar during each drying phase and was unaffected by atmospheric humidity. Several days elapsed after rewatering before normal stomatal opening occurred. When all the plants were subsequently kept well watered, leaf conductances decreased as the leaf to air vapor pressure difference was increased. However, the conductances of previously stressed plants were lower than those of unstressed plants, and consequently previously stressed plants had lower transpiration rates. [ABSTRACT FROM AUTHOR]

This paper reports on further studies of a model for interpreting leaf water potential data for Citrus. Experimental data confirmed the assumption that the ratio of vapor pressure deficit to leaf diffusion resistance adequately estimates transpiration when leaf-to-air temperature differences are small. Data collected diurnally indicated that the relationship between leaf water potential and transpiration followed a sequence of steady states without hysteresis. No difference in water transport characteristics was found for Valencia orange on three rootstocks in well-watered soil, but the two rootstocks Cleopatra mandarin and Rangpur gave slightly greater leaf water stress in Valencia orange leaves than 'Troyer' citrange rootstock at high transpiration rates under mild soil water deficits. In laboratory studies, previously unstressed seedlings had higher leaf water potentials than field trees at equivalent transpiration rates. After several drying cycles, however, leaf water potentials were similar to those observed in the field. [ABSTRACT FROM AUTHOR]

The factors responsible for the low transpiration rates of citrus were investigated. Leaf resistance to water vapor exchange by orange seedlings (Citrus sinensis L. cv. Koethen) including a substantial boundary layer resistance, was as low as 1 s cm-1 in humid air. Leaf resistance of well watered plants increased to values as large as 5 S cm-1 when the difference in absolute humidity between leaf and air was increased. Leaf resistance was only slightly influenced by temperature between 20 and 30°C providing the humidity difference between leaf and air was kept constant. Leaf resistance increased when leaf temperature was increased between 20 and 30°C when the absolute humidity external to the leaf was kept constant. Increased humidity differences resulted in greater increases in leaf resistance during initial experiments than when the experiments were repeated with the same leaves indicating acclimation by the plant. It was concluded that the effects of humidity differences on leaf resistance are partially responsible for the low transpiration rates of citrus. [ABSTRACT FROM AUTHOR]

We report on the design and performance of a system that speeds measurement of radial tree-ring increments on tree stem disks; this method replaces the usual binocular microscope with a video image, and automates the measuring and recording processes. The system was used to measure bole sections cut from stems at various heights to determine volume growth of representative trees in an old-growth ponderosa pine stand. The objective of the measurement system was to speed acquisition of annual growth increments from a large number of disks. A personal computer controls the location of a video camera in a 3-axis positioning system. The operator views the sample on a video monitor and positions the camera over each ring by selecting it with a computer-driven mouse. The computer measures and records the distance that the camera moves between each ring. Task selection is facilitated by menu-driven software that also formats, checks and organizes data files. Measurements have a resolution of 0.026 mm; however, finer resolution could be obtained with a different camera lens. Tests of measurement variability (repeated measurements by individual operators on a single radius) indicated standard errors of 0.006 mm or less for the first measurement sets for four operators. Correlation coefficients among four radii per bole section were as low as 0.66 for a whole tree, suggesting that measurements on single radii may provide poor estimates of radial growth for old trees. This system also offers the potential for automatic ring detection and measurement. [ABSTRACT FROM PUBLISHER]

Old trees of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) and lodgepole pine (Pinus contorta var. latifolia Engelm.) were studied to determine volume growth patterns in relation to leaf area. Ponderosa pine trees varied in age from 166 to 432 years and were about 77 cm in diameter; lodgepole pine trees varied in age from 250 to 296 years and were about 31 cm in diameter. With the exception of several ponderosa pine trees less than 200 years old, trees of both species had flattened tops, heavy branches, and foliage distribution characteristically found only in the oldest trees. After trees were felled, annual volume increments were determined from crossdated radial increments measured on discs at 4-m height intervals, and leaf areas were determined based on leaf area/branch sapwood area ratios for 1/5 sections of the crown for each tree.In ponderosa pine, three distinct volume growth patterns occurred: (1) a gradual increase in annual volume growth until felling; (2) a more rapid increase in growth to a plateau that persisted for a century or more; and (3) a rapid increase in growth followed by a generally sudden decrease in growth to less than half the earlier rates, and persisting at these lower rates for as long as seven decades. In lodgepole pine, fewer trees exhibited the sudden growth decline observed in ponderosa pine. Most short-term growth variations in ponderosa pine were synchronized among all trees, suggesting a common climatic signal. In lodgepole pine, annual variations in volume growth were slight.Volume growth in the most recent years before felling was weakly correlated with leaf area at the time of felling (r2 = 0.45 for both species). However, in both species, trees having a high volume growth rate and leaf area at the time of felling had grown slowly when young, whereas trees having low volume growth rate and leaf area at felling grew rapidly when young. Thus a wide range of early and late growth patterns can lead to old-growth conditions in these species. Growth efficiencies (grams of dry matter per m2 total leaf area) were generally higher for trees having the lowest leaf areas, and in almost all cases were below 100 g m−2. [ABSTRACT FROM AUTHOR]

Changes in issues and advances in methodology have contributed to substantial progress in tree physiology research during the last several decades. Current research focuses on process interactions in complex systems and the integration of processes across multiple spatial and temporal scales. An increasingly important challenge for future research is assuring sustainability of production systems and forested ecosystems in the face of increased demands for natural resources and human disturbance of forests. Meeting this challenge requires significant shifts in research approach, including the study of limitations of productivity that may accompany achievement of system sustainability, and a focus on the biological capabilities of complex land bases altered by human activity. [ABSTRACT FROM AUTHOR]

A water flux model, which assumes that the dynamic functioning of the soil-plant-atmosphere continuum may be described by a series of steady states, was examined, as a means for interpreting leaf water potential measurements in 'Valencia' orange trees (Citrus sinensis (L) Osbeck). According to the model, leaf water potential should be related to transpirational flux, which in this experiment was estimated by the ratio of vapor pressure deficit of the atmosphere to leaf diffusion resistance (VPD/rleaf). Leaf water potentials decreased in a specific relationship with increasing values of VPD/rleaf provided that soil water was adequate and soil temperature was not too low, but regardless of season of the year or climatic or edaphic differences among 3 field locations. When soil water tensions exceeded 0.3 bar or when soil temperatures were lower than 15°C, deviations from the model occurred in the form of more negative leaf water potentials than predicted by VPD/rleaf. The model predicts from simple measurements made on intact plants that these differences were due to the modification of flow resistances by cool temperatures. and the modification of both resistances and the potential of water at the source in the case of soil water depletion. The model may be a useful tool for interpreting. plant water potential data under contrasting environmental conditions. [ABSTRACT FROM AUTHOR]