Plant water relations and the effects of elevated CO2: a review and suggestions for future research

Increased ambient carbon dioxide (CO₂) has been found to ameliorate water stress in the majority of species studied. The results of many studies indicate that lower evaporative flux density is associated with high CO₂-induced stomatal closure. As a result of decreases in evaporative flux density and increases in net photosynthesis, also found to occur in high CO₂ environments, plants have often been shown to maintain higher water use efficiencies when grown at high CO₂ than when grown in normal, ambient air. Plants grown at high CO₂ have also been found to maintain higher total water potentials, to increase biomass production, have larger root-to-shoot ratios, and to be generally more drought resistant (through avoidance mechanisms) than those grown at ambient CO₂ levels. High CO₂-induced changes in plant structure (i.e., vessel or tracheid anatomy, leaf specific conductivity) may be associated with changes in vulnerability to xylem cavitation or in environmental conditions in which runaway embolism is likely to occur. Further study is needed to resolve these important issues. Methodology and other CO₂ effects on plant water relations are discussed.Abbreviations A net photosynthesis - C_a ambient [CO₂] - C_i internal [CO₂] - E evaporative flux density - g₁ leaf conductance - g_s stomatal conductance - LSC leaf specific conductivity - IRGA infrared gas analyzer - LAI leaf area index - PAR photosynthetically active radiation - total plant water potential - _soil soil water potential - _s solute potential - _pt turgor pressure potential - _px xylem pressure potential - RH relative humidity - R : S root to shoot ratio - RWC relative water content - SLA specific leaf area - SLW specific leaf weight - SPAC soil-plant-atmosphere-continuum - SWC soil water content - VPD vapor pressure deficit - WUE water use efficiency     

Water relations of a tropical vine-like bamboo (Rhipidocladum racemiflorum): root pressures, vulnerability to cavitation and seasonal changes in embolism

The occurrence of root pressure, the vulnerability of xylemvessels to drought-induced cavitation, and the seasonal changesin hydraulic conductivity due to embolism were studied in theculms of Rhipidocladum racemiflorum (Steud.) McClure, a tropicalvine-like bamboo from central Panama. Positive hydrostatic potentialsup to 120 kPa occurred only during the wet season when the transpirationrate of the plant was low, i.e. at night or during rain events.Although the xylem vessels were large and efficient for conductingwater, they were highly resistant to cavitation. Xylem waterpotentials lower than –4.5 MPa were required to induce50% loss of hydraulic conductivity in culms. The minimum waterpotential reached –3.75 MPa at the end of the 1993 dryseason, so loss of hydraulic conductivity due to embolism remained<10%. The species is adapted to drier habitats both by wayof a low vulnerability to xylem cavitation and by root pressuresin the wet season that could refill vessels that became embolizedduring a severe dry season. Key words: Rhipidocladum racemiflorum, root pressure, cavitation, embolism, water relations     

Comparative evolutionary analysis of rDNA ITS regions in Drosophila

The internal transcribed spacer (ITS) of the ribosomal DNA is generally considered to be under low functional constraint, and it is therefore often treated as a typical nonfunctional spacer sequence. We have analyzed the ITS regions of five species from the Drosophila melanogaster subgroup, two Drosophila species from outside this group (D. pseudoobscura and D. virilis), as well as from the more distantly related dipteran fly Musca domestica. The sequence comparisons show a distinctive conservation/divergence pattern, indicating that some regions are more conserved than others. Moreover, secondary-structure calculations indicate several conserved structural elements within the ITS regions. On the other hand, a statistical test that allows us to estimate the fraction of sites that are not under selective constraint suggests that more than half of the spacer is apparently free to diverge and evolves with a rate that is close to the neutral rate of sequence evolution in Drosophila. The ITS sequences can be used to derive a molecular phylogeny for the species under study. We find that the ITS tree is largely in line with the so-far-known phylogeny of this group of species, with one difference. The species most distant within the D. melanogaster subgroup is D. yakuba, rather than D. orena, as is normally assumed.      

High nucleotide sequence variation in a region of low recombination in Drosophila simulans is consistent with the background selection model

We surveyed nucleotide sequence variation at glucose dehydrogenase (Gld), in a region of low recombination on chromosome 3R, from a population sample of Drosophila simulans. The levels of nucleotide variation were surprisingly high. There was no departure from the expectation of a neutral model for the level of polymorphism, indicating no evidence of a selective sweep in this region. There was a significant deficiency of singleton polymorphisms according to the Fu and Li test, although Tajima and Hudson, Kreitman, and Aguade (HKA) tests do not provide evidence of a significant elevation of variation due to balancing selection. Genetic map data for the D. simulans third chromosome were used to calculate expected values of pi for Gld under a current model of background selection, varying the values for the parameter sh (selection coefficient against deleterious mutations). We show that the recombinational landscape of D. simulans is sufficiently different from that of D. melanogaster that we expect higher variation under the background selection model, even when effective population sizes are assumed to be equal. The data for Gld were tested against the predictions using computer simulations of the distribution of the number of segregating sites conditioned on pi. Background selection alone can explain our observations as long as sh is larger than 0.005 and species-level effective population size is assumed to be several- fold larger than in D. melanogaster. Alternatively, the deleterious mutation rate may be smaller in D. simulans, or balancing selection may be acting nearby, thereby reducing the effect of background selection.      

Hydraulic Conductivity Recovery versus Water Pressure in Xylem of Acer saccharum

Experiments were conducted to determine the influence of stem diameter, xylem pressure potential, and temperature on the rate of recovery of hydraulic conductivity in embolized stems of Acer saccharum Marsh. Recovery of conductivity was accompanied by an increase in stem water content as water replaced air bubbles and bubbles dissolved from vessels into the surrounding water. The time required for stems to go from less than 3 to 100% hydraulic conductivity increased approximately with the square of the stem diameter and increased with decreasing xylem pressure potential. Recovery was halted when xylem pressure potential decreased below −6 kPa. Increasing xylem pressure from 13 to 150 kPa reduced the time for recovery by a factor of 4. Temperature had little influence on the rate of recovery of hydraulic conductivity. All of these results are in accord with a theory of bubble dissolution in which it is assumed that: (a) the rate of bubble dissolution is rate limited by diffusion of air from the bubbles to the outer surface of the stems, (b) the equilibrium concentration of gases in liquid in stems is determined by Henry's law at all air-water interfaces, (c) the equilibrium solubility concentration is determined only by the partial pressure of the gas in the gas phase and not directly by the liquid-phase pressure, and (d) the gas pressure of an entrapped air bubble in the lumen of a cell can never be less than atmospheric pressure at equilibrium.     

Water Relations and Hydraulic Architecture of a Tropical Tree (Schefflera morototoni) : Data, Models, and a Comparison with Two Temperate Species (Acer saccharum and Thuja occidentalis)

The water relations and hydraulic architecture of a tropical tree (Schefflera morototoni) and of two temperate species (Acer saccharum and Thuja occidentalis) are reported. Among the water relations parameters measured were leaf and stem water storage capacity, leaf water potential, transpiration, and vulnerability of stems to cavitation and loss of hydraulic conductivity by embolisms. Among the hydraulic architecture parameters measured were hydraulic conductivity per unit pressure gradient, specific conductivity, leaf-specific conductivity, and Huber value. In terms of vulnerability of stems to cavitation, stem and leaf capacitances, and leaf-specific conductivity, all three species followed the same sequence: Schefflera > Acer > Thuja. It is argued here that the high stem capacitance and high leaf-specific conductivity of Schefflera are necessary to compensate for its high vulnerability to cavitation. Extractable water storage per unit leaf area in Schefflera stems is >100 times that of Acer and may permit the species to survive unusually long, dry seasons in Panama. Although Schefflera frequently grows >20 meters, the biggest resistance to water flow in the shoots resides in the leaves.     

Leaf cuticles behave as asymmetric membranes : evidence from the measurement of diffusion potentials

Cuticles were isolated enzymatically from the leaves of two maple species (Acer saccharum Marsh and A. platanoides L.) and from orange (Citrus aurantium L.). The cuticles were placed in a plastic cuvette and different concentrations of KCl were perfused over the physiological inner and outer surfaces while the electrical potential (E¹⁰) that developed across the cuticles and was caused by ion diffusion was measured. E¹⁰ was always positive, indicating that the permeability of K⁺ was always greater than that of Cl^-. Measured E¹⁰ in cuticles did not fit the Goldman equation, whereas, E¹⁰ measured during KCl diffusion across selected artificial membranes fit the equation. The magnitude of E¹⁰ in cuticles and artificial membranes also was dependent on ionic strength, decreasing as ionic strength increased. These observations are explained by combining classical transport equations with equations that describe the equilibrium ion distribution between ionic double layers in the cuticle or membranes and the bathing solution.     

Molecular evolution of chloroplast DNA sequences

Comparative data on the evolution of chloroplast genes are reviewed. The chloroplast genome has maintained a similar structural organization over most plant taxa so far examined. Comparisons of nucleotide sequence divergence among chloroplast genes reveals marked similarity across the plant kingdom and beyond to the cyanobacteria (blue-green algae). Estimates of rates of nucleotide substitution indicate a synonymous rate of 1.1 x 10(-9) substitutions per site per year. Noncoding regions also appear to be constrained in their evolution, although addition/deletion events are common. There have also been evolutionary changes in the distribution of introns in chloroplast encoded genes. Relative to mammalian mitochondrial DNA, the chloroplast genome evolves at a conservative rate.      

SEASONAL OCCURRENCE OF XYLEM EMBOLISM IN SUGAR MAPLE (ACER SACCHARUM)

Xylem embolism, the reduction of water flow by air-filled vessels, was measured in a stand of 5- to 8-year-old sugar maple (Acer saccharum Marsh.) saplings growing in a nursery bed in northwestern Vermont. Embolism was quantified as percentage loss in hydraulic conductivity of trunk and branch segments relative to maximum values obtained by removing air from vessels by repeated high pressure (173 kPa) perfusions. Ten segments per tree were cut from 6 trees for each of 11 measurement periods spaced at roughly monthly intervals from May 1986 to June 1987. During the 1986 growing season, embolism increased significantly from 11 to 31% in the larger branches and trunk (segment diameter #8805;0.5 cm), but remained at about 10% in twigs (segment diameter <0.5 cm). This was unexpected because the greatest water stess and thus potential for embolism occurs in twigs. During the winter, embolism increased throughout the trees and the trend with diameter was reversed; by February, small twigs were 84% embolized vs. 69% for larger branches and trunk. Dye perfusions showed that winter embolism in trunks was localized on the south side; this may have resulted from water loss by sublimation or evaporation in the absence of water uptake. Beginning in late March, embolism decreased throughout the trees to approximately 20% in June. This decrease was associated with positive xylem pressure of at least 16 kPa which may have originated in the roots, because weather conditions at the time were unfavorable for the generation of stem pressures characteristic of Acer species in early spring.     

Experimental evidence for negative turgor pressure in small leaf cells of Robinia pseudoacacia L versus large cells of Metasequoia glyptostroboides Hu et W.C.Cheng. 1. Evidence from pressure‐volume curve analysis of dead tissue.

This paper provides a mini‐review of evidence for negative turgor pressure in leaf cells starting with experimental evidence in the late 1950s and ending with biomechanical models published in 2014. In the present study, biomechanical models were used to predict how negative turgor pressure might be manifested in dead tissue, and experiments were conducted to test the predictions. The main findings were as follows: (i) Tissues killed by heating to 60 or 80 °C or by freezing in liquid nitrogen all became equally leaky to cell sap solutes and all seemed to pass freely through the cell walls. (ii) Once cell sap solutes could freely pass the cell walls, the shape of pressure‐volume curves was dramatically altered between living and dead cells. (iii) Pressure‐volume curves of dead tissue seem to measure negative turgor defined as negative when inside minus outside pressure is negative. (iv) Robinia pseudoacacia leaves with small palisade cells had more negative turgor than Metasequoia glyptostroboides with large cells. (v) The absolute difference in negative turgor between R. pseudoacacia and M. glyptostroboides approached as much as 1.0 MPa in some cases. The differences in the manifestation of negative turgor in living versus dead tissue are discussed.