Search Results
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.
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 (ĻĻ) 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 (Ļ) 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 ĻĻ at full turgor. Tissue elasticity decreased somewhat, as the elastic modulus increased ā2 to 3 MPa in each species through the autumn. Water potential isotherms changed most dramatically through the autumn in magnolia. Initially, Ļ was ā1 MPa at 82% RWC and, by December, leaves were able to withstand Ļs of ā3 MPa before RWC dropped to 82%. These changes are similar to those commonly reported as responses to drought or salinity.
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 2Ć2Ć4 factorial design had two levels of mycorrhizae (Ā± 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 (ĪØ) 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 ĪØ 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 ĪØ, 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 ĪØ 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 2Ć3Ć2 factorial design had 2 levels of mycorrhizae (Ā± 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 Ć water and P Ć 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.
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.
Mycorrhizal colonization can alter stomatal behavior of host leaves during drought. This may be related to an altered production or reception of a chemical signal of soil drying. We tested whether intact root systems were required to observe a mycorrhizal effect on leaf transpiration (E), or whether some residual mycorrhizal influence on leaves could affect E of foliage detached from root systems. Transpiration assays were performed in the presence of several possible candidates for a chemical signal of soil drying. In detached leaves of Vigna unguiculata (cowpea), colonization interacted significantly with ABA and pH in regulating transpiration. Colonization affected E of detached Rosa hybrida (rose) leaves but had no effect on E of detached leaves of Pelargonium hortorum (geranium). In each species tested, increasing the ABA concentration decreased E. In cowpea, calcium appeared to alter stomatal sensitivity to ABA, as well as regulate stomatal activity directly. The pH of the feeding solution affected E in rose, but did not change E independently in cowpea or geranium. Adding phosphorus to the feeding solution did not alter E or the apparent sensitivity of stomata to ABA in any of the test species. Colonization of roots by mycorrhizal fungi can result in residual effects in detached leaves, that can alter the stomatal reception of chemical signals in both rose and cowpea.
The influence of irradiance and drought on osmotic and turgor adjustment was examined in leaves of rose (Rosa hybrida L. `Samantha'). Plants cultured under full ambient light in the greenhouse were placed in shade chambers and, after 2 weeks of acclimation, exposed to drought for 21 days. Treatments consisted of a water stress factor (well-watered and drought-stressed) and an irradiance factor (100%, 70%, and 30% of ambient irradiance). Pressure-volume analyses of leaves indicated that osmotic potentials at full turgor were decreased 0.42, 0.36, and 0.23 MPa by drought in the 100%, 70%, and 30% irradiance treatments, respectively. Plants stressed under 100% and 70% irradiance exhibited similar osmotic adjustments. Plants under 30% irradiance had higher osmotic potentials at full turgor under well-watered conditions than plants in the other two irradiance treatments and showed only 55% as much adjustment to drought. In each irradiance treatment, drought induced an increase in elastic modulus and a decrease in relative water content at zero turgor. Turgor pressures were higher across a range of relative water contents in plants in the two higher irradiance treatments under both soil moisture treatments. Turgor also was higher at any particular water potential at 100% and 70% irradiance than 30% irradiance, within each soil moisture treatment. Heavy, but not mild, shading inhibited osmotic and turgor adjustments in leaves during drought.