Thomas M. Hinckley

The Pressure Chamber as an Instrument for Ecological Research

Publisher Summary This chapter discusses the pressure chamber as an instrument for ecological research. It provides the workers with a thorough and useful discussion of the effective use of the pressure chamber technique. It offers a uniform terminology. To assemble, interpret, and evaluate published ecological studies the pressure chamber has been employed. And suggest areas of research where the technique is potentially useful. The chapter discusses some unknowns and problems associated with the pressure chamber. Assembly of the device and its associated components is briefly discussed. The chapter reviews the status of the pressure chamber technique in current ecological and eco-physiological research and suggests what appear to be promising new directions and applications. Four strategies for expressing and interpreting P data were explored. The pressure chamber may also be used effectively in studies of plant pathology, entomology, and pollution effects. Activities of pathogenic agents which disrupt vascular function or cause membrane damage can often be detected. Productive use of the pressure chamber technique in forestry, agriculture, and horticulture in such operations as irrigation, transplanting, and fertilization has proved successful.

A Severe Drought: Impact on Tree Growth, Phenology, Net Photosynthetic Rate and Water Relations

An unusually severe drought occurred in central Missouri during the summer of 1976. The drought resulted in an average soil water potential of ?26.1 bars in the upper 45 cm of soil profile in spite of the addition of 4.9 cm of irrigation water. It enabled us to examine its effects on phenology, growth and physiological processes, as well as the water relations of white oak (Quercus alba L. ), and eight other species found in this oak-hickory forest. The drought had a dramatic impact on base (presunrise) xylem pressure potential of white oak in both irrigated ( ? 19.6 to ?34.3 bars) and nonirrigated specimens ( ? 27.8 to ?45.2 bars). Growth was reduced, die-back increased, net photosynthetic rate was depressed to near the compensation point and phenological patterns in the following year (1977) were altered. An estimation of the number of days on which stornata were closed for most of the photoperiod was compared to the number of days when mature leaves were present. The following ranking of species based on this index of stomatal control was possible (from most to least time spent with stornata closed) : sunflower > flowering dogwood = hlack walnut > sugar maple > northern red oak > white oak =? eastern red cedar = black oak. Various drought avoidance mechanisms are also presented and discussed in regard to these eight species. The long periods of low soil water potentials and base xylem pressure potentials experienced during the drought of 1976 did not prevent the recovery of the growth processes that autumn or the subsequent spring. All study species seemed well-adapted to survival and to continued functioning during this severe drought.

Seasonal Changes in Tissue Water Relations of Three Woody Species of the Quercus-Carya Forest Type

Tissue water relations of white and northern red oaks and mockernut hickory were studied during the growing season of 1979. In all species, osmotic potentials at full saturation and turgor loss point decreased during the period of leaf maturation; subsequently, osmotic potential responded to soil moisture availability. Estimates of the bulk modulus of elasticity and the relative water content at the turgor loss point revealed that white oak possessed less elastic leaf tissue than did either northern red oak or hickory. The bulk leaf pressure potential associated with the initiation of stomatal closure was lower in white oak and hickory (0.2 MPa) than in northern red oak (0.4 MPa) and remained seasonally constant, while total leaf water potential associated with stomatal closure was lower during periods of drought as osmotic potential decreased. The drought-tolerating behavior of white oak is consistent with its frequent occurrence and success in xeric habitats. Northern red oak and mockernut hickory exhibited responses more typical of drought-avoiding species, which would result in sustained turgor-mediated processes essential for growth and high competitive ability at moderate moisture stresses characteristic of more mesic habitats.

A Theoretical Model for Calculation of Xylem Sap Pressure from Climatological Data

A theoretical model is presented for the calculation of stem-xylem sap pressure from climatological data in an open-grown stand of Abies procera and A. amabilis located at 1206 m elevation in the Washington Cascades. Stem-xylem sap pressure is divided into two components: (1) a base level which is a function of soil moisture, and (2) a diurnal depression in sap pressure from this base level which is a function of current atmospheric evaporative demand. Predicted values of stem-xylem sap pressure were compared to observed values and no sig- nificant difference was observed. Other plant processes such as stomatal aperture were predicted from the model. Specific deficiencies in the model were observed and are discussed. Substantial apparent nighttime transpiration occurred within these two species under laboratory and field conditions, leading to a discrepancy between base level sap pressure and soil moisture. I NTRODUCTION The integrated abiotic and biotic factors affecting plants contribute to their water status (Fritts, 1966). Since growth frequently is limited by even mild foliar desiccation (Slatyer, 1967), the understanding of plant water status is essential to evaluating plant-environment inter- actions and the resulting ecological distribution of plants. Our effort to understand plant water status has led to the formulation of a model which predicts water status of forest trees from climatological data.

Evidence for Error in Pressure-Bomb Estimates of Stem Xylem Potentials

The hydrostatic component of water potential was measured concurrently in stems and needles of five species of conifers (Pinus contorta, P. jeffreyi, Pseudotsuga menziesii, Abies amabilis, and A. procera). In Douglas—fir and the true firs pressure—chamber measurements of potentials were up to 4 bars more negative in stems than in needles. The difference appears to be due to filling of non—vascular xylem tissue with fluid during measurement. This tissue seems to function as a water reservoir, enabling needles to maintain relatively high turgor levels during periods of rapid transpiration. See full-text article at JSTOR

Effect of Vertical and Temporal Variations in Stand Microclimate and Soil Moisture on Water Status of Several Species in an Oak-Hickory Forest

Xylem pressure potential (P) and leaf surface resistance (R!) were measured for three different species which occupied different vertical positions in an oak-hickory forest. No rain fell during the 19day, late summer study period, and diurnal patterns of ? and R2 for white oak {Quercus alba L.), sugar maple (Acer saccharum Marsh) and sunflower (Helianthus strumosus L.) were greatly modified by decreasing soil moisture availability. Microclimatic factors varied vertically within the forest, and plant water status varied vertically in response to such factors. The most important of these was solar radiation, which probably caused higher leaf temperatures in the upper canopy and, thus, a steeper gradient of vapor pressure from the leaf to the air than was observed for the more shaded understory and ground vegetation.

Xylem pressure potential and chlorophyll fluorescence as indicators of freezing survival in black locust and Western hemlock seedlings

Abstract Xylem pressure potential was determined using the Scholander pressure chamber on stems of cold hardened and non-hardened black locust ( Robinia pseudoacacia L.) seedlings following freezing to various nonlethal and lethal temperatures and subsequent thawing. Correlation was found between immediate xylem pressure potential and long-term seedling survival. Chlorophyll fluorescence transients were monitored using needles of western hemlock ( Tsuga heterophylla (Raf.) Sarg.) seedlings following freezing to various non-lethal and lethal temperatures and subsequent thawing. Immediate and repeatable differences in fluorescence transients correlated with long-term seedling survival. Methodology is described and correlations discussed relative to using either chlorophyll fluorescence or xylem pressure potential as an immediate indicator of long-term freezing survival in woody plant seedlings.