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The determination of tissue water potential components is important for understanding plant growth and response to the environment. Pressure-volume (PV) analysis is often considered to give the most accurate estimate of symplastic osmotic potential. Additional information about tissue water relations can also be computed from PV curves estimates of bulk cell wall elasticity, symplastic water volume, and turgor potential at various states of tissue water content. The generation of PV curves is a time-consuming procedure, however, and involves considerable computation. This presentation describes a computer spreadsheet template for traditional evaluation of a PV curve through linear regression of the zero turgor segment. The template allows real-time plotting of the inverse ψ/ water loss relating, provides estimates of most commonly calculated PV characteristics and permits instant graphic visualizations of changes in water potential components and elasticity with changes in water potential, total tissue water and symplastic water content. The advantages of spreadsheet analysis of PV curves are simplicity, consistency, thoroughness and speed. A fleeting acquaintance with spreadsheet software and a thorough understanding of pressure-volume theory on the part of the user is assumed.
Using psychrometric pressure-volume analysis, root water relations following drought were characterized in Rosa hybrida L. plants colonized by the vesicular-arbuscular mycorrhizal fungus Glomus intraradices Schenck & Smith. Measurements were also made on uncolonized plants of similar size and adequate phosphorus nutrition. Under well-watered conditions mycorrhizal colonization resulted in lower solute concentrations in root symplasm, and hence lower root turgors. Following drought, however, mycorrhizal roots maintained greater turgor across a range of tissue hydration. This effect was apparently not due to increased osmotic adjustment (full turgor osmotic potentials were similar in mycorrhizal and nonmycorrhizal roots after drought) or to altered elasticity but to an increased partitioning of water into the symplast. Symplast osmolality at full turgor was equivalent in mycorrhizal and nonmycorrhizal roots but because of higher symplastic water percentages mycorrhizal roots had greater absolute numbers of osmotic (symplastic) solutes. Drought-induced osmotic potential changes were observed only in mycorrhizal roots, where a 0.4 megapascal decrease (relative to well-watered controls) brought full turgor osmotic potential of mycorrhizae to the same level as nonmycorrhizal roots under either moisture treatment.
Understanding physiological drought resistance mechanisms in ornamentals may help growers and landscapers minimize plant water stress after wholesale production. We characterized the drought resistance of four potted, native, ornamental perennials: purple coneflower [Echinacea purpurea (L.) Moench], orange coneflower [Rudbeckia fulgida var. Sullivantii (Beadle & Boynt.) Cronq.], beebalm (Monarda didyma L.), and swamp sunflower (Helianthus angustifolius L.). We measured a) stomatal conductance of leaves of drying plants, b) lethal water potential and relative water content, and c) leaf osmotic adjustment during the lethal drying period. Maintenance of stomatal opening as leaves dry, low lethal water status values, and ability to osmotically adjust indicate relative drought tolerance, with the reverse indicating drought avoidance. Echinacea purpurea had low leaf water potential (ψL) and relative water content (RWC) at stomatal closure and low lethal ψL and RWC, results indicating high dehydration tolerance, relative to the other three species. Rudbeckia fulgida var. Sullivantii had a similar low ψL at stomatal closure and low lethal ψL and displayed relatively large osmotic adjustment. Monarda didyma had the highest ψL and RWC at stomatal closure and an intermediate lethal ψL, yet displayed a relatively large osmotic adjustment. Helianthus angustifolius became desiccated more rapidly than the other species, despite having a high ψL at stomatal closure; it had a high lethal ψL and displayed very little osmotic adjustment, results indicating relatively low dehydration tolerance. Despite differences in stomatal sensitivity, dehydration tolerance, and osmotic adjustment, all four perennials fall predominantly in the drought-avoidance category, relative to the dehydration tolerance previously reported for a wide range of plant species.
We characterized the drought resistance of four native ornamental perennials: purple coneflower (Echinacea purpurea), orange coneflower (Rudbeckia fidgida var. Sullivantii), beebalm (Monarda didyma) and swamp sunflower (Helianthus angustifolius). We measured (a) stomatal conductance as leaves dried, (b) lethal water status values and (c) leaf osmotic adjustment during the lethal drying period. Maintenance of stomatal opening as leaves dry, low lethal water status values and ability to osmotically adjust indicate relative drought tolerance, with the reverse indicating drought avoidance. E. purpurea had low leaf water potential (ΨL) and relative content (RWC) at stomatal closure, and low lethal ΨL and RWC. R. fulgida var. Sullivantii had a similar low ΨL at stomatal closure, low lethal ΨL and displayed relatively large osmotic adjustment. M. didyma had the highest ΨL and RWC at stomatal closure and an intermediate lethal ΨL, yet displayed a relatively large osmotic adjustment. H. angustifolius became desiccated more rapidly than the other species despite having a high ΨL at stomatal closure; it had a high lethal ΨL and displayed very little osmotic adjustment. Despite differences in stomatal sensitivity, dehydration tolerance and osmotic adjustment, all four perennials fall predominantly into the drought avoidance category.
Our objectives were to determine (a) if mycorrhizal (VAM) fungi can alter drought-induced, nonhydraulic regulation of shoot growth, and (b) how much of a root system can be dried or severed before hydraulic effects on shoot responses become evident. Sorghum was grown with roots equally divided among four pots. The 2×2×4 factorial design had two levels of mycorrhizae (± Glomus intraradix), two levels of root treatment (dried or severed) and four levels of amount of roots treated (0, 1, 2 or 3 pots dried or severed). Neither leaf water potential (Ψ) nor Cs were affected by drying 1 or 2 pots, and reductions in leaf area in these plants were therefore attributed to nonhydraulic signalling. When 2 pots were dried, leaf growth was reduced less in VAM than in nonVAM plants, despite lower P in VAM leaves and despite quicker soil drying by VAM roots. Drying or severing roots of 3 pots did result in drops in leaf Ψ and Cs, indicating a likely hydraulic effect on leaf water status in those plants. Leaf P declined progressively as more roots were dried or severed, possibly also affecting growth in plants with roots in 3 pots dried or severed. Leaf extension rates (LER) declined with only slight drops in soil Ψ, and LER declines were related to volume of soil drying. In VAM plants, leaf area reductions were correlated with length of time roots were exposed to soil Ψ between -0.02 and -0.50 MPa.
In Zea mays L. plants grown with roots divided between two pots, we tested (a) if leaf P concentration can affect nonhydraulic root to shoot signalling of soil drying, and (b) if a mycorrhizal (VAM) effect on signalling can occur independently of a VAM effect on leaf P. The 2×3×2 factorial design had 2 levels of mycorrhizae (± Glomus intraradix Schenck & Smith), 3 levels of P fertilization and 2 levels of water (both pots watered, or one pot watered while the other was allowed to dry). Total leaf length and shoot dry weight were not reduced in half-dried VAM plants, but each measure was ultimately reduced about 10% in half-dried nonVAM plants. Stomatal conductance (Cs), unaffected by VAM, was lower in half-dried, high-P plants than in high-P controls a few times during the latter half of the experiment, by as much as 65%. Leaf water potentials were not affected by partial soil drying, and reductions in leaf growth preceded reductions in Cs; hence, growth reductions were attributed to nonhydraulic signals coming from roots in drying pots. VAM × water and P × water interactions indicated that mycorrhizae influenced signal effects on final plant leaf length and that P fertilization influenced signal effects on Cs. Soil water potential, measured every 2 h with heat dissipation sensors, showed that soil drying was not affectd by VAM or P treatment.
Abstract
Osmotic adjustment in response to onset of winter dormancy was characterized in well-watered, potted sweetgum (Liquidambar styraciflua L.) and southern magnolia (Magnolia grandiflora L.) growing outdoors in Knoxville, Tenn. Analyses of water potential isotherms indicated that adjustment occurred in both species, with osmotic potential (ψπ) at full turgor decreasing 0.8 MPa in sweetgum (by the time of first color, 27 Oct.) and 1.0 MPa in magnolia (by 1 Dec.). Osmotic adjustment occurred despite the fact that plants did not suffer osmotic stress; morning and afternoon leaf relative water content (RWC) and leaf water potential (ψ) remained high throughout the fall. Leaf conductance was halved in sweetgum and doubled in magnolia as the autumn progressed. A correlation was found in magnolia between turgid : dry weight ratio and ψπ at full turgor. Tissue elasticity decreased somewhat, as the elastic modulus increased ≈2 to 3 MPa in each species through the autumn. Water potential isotherms changed most dramatically through the autumn in magnolia. Initially, ψ was −1 MPa at 82% RWC and, by December, leaves were able to withstand ψs of −3 MPa before RWC dropped to 82%. These changes are similar to those commonly reported as responses to drought or salinity.
Environmental factors regulating spread of dogwood anthracnose remain largely unstudied, so we conducted a two-year experiment to determine if light intensity or drought can affect this disease. After leaf emergence in 1990, two-year-old potted dogwood trees (Cornus florida L.) were placed outdoors in shade huts giving light treatments of 100%, 50%, 10% or 2% ambient light. One year later, trees were removed from huts to inoculate them (artificially or naturally) with Discula destructiva Redlin sp. Nov. After inoculation, trees were returned to their former light treatments and some of the trees were subjected to drought. Disease progression, quantified as increasing percentage of leaves with lesions, was unaffected by inoculation procedure. Light did affect the disease; by the end of the experiment, disease percentages in well-watered trees were 30% at 10% light, 15% at 2% light and below 5% at 100% and at 50% light. Drought increased disease progression on all shaded trees, ultimately 8x at 50% light, 1.4x at 10% light and 2x at 2% light.
Colonization of roots by arbuscular mycorrhizal (AM) fungi can increase host resistance to drought stress, although the effect is unpredictable. Since AM symbiosis also frequently increases host resistance to salt stress, and since drought and salt stress are often linked in drying soils, we speculated that the AM influence on plant drought response may be linked to AM influence on salt stress. We tested the hypothesis that AM-induced effects on drought responses would be more pronounced when plants of comparable size are exposed to drought in salinized soils. In two greenhouse experiments, several water relations characteristics were measured in sorghum plants colonized by Glomus intraradices, Gigaspora margarita, or a mixture of AM species during a sustained drought following exposure to salt treatments (NaCl stress, osmotic stress, or soil leaching). The presence of excess salt in soils widened the difference in drought responses between AM and non-AM plants in just two instances: days needed for plants to reach stomatal closure, and promotion of stomatal conductance. In other instances, the addition of salt tended to nullify an AM-induced change in drought response; e.g., an AM effect on the decline in leaf or soil water potential required to cause stomatal closure disappeared when soils were salinized. Our findings confirm that AM fungi can alter host response to drought but do not lend much support to the idea that AM-induced salt resistance might help explain why AM plants can be more resilient to drought stress than their non-AM counterparts.