Seedlings of Acacia auriculiformis A. Cunn. ex. Benth., Albizia lebbeck (L.) Benth., Gliricidia sepium (Jac.) Walp and Leucaena leucocephala (Lam.) de Wit. were inoculated with an ectomycorrhizal (Boletus suillus (L. ex. Ft.) or indigenous vesicular-arbuscular mycorrhizal (VAM) fungi in a low P soil.The plants were subjected to unstressed (well-watered) and drought-stressed (water-stressed) conditions. In Gliricidia and Leucaena, both mycorrhizal inoculations stimulated greater plant growth, P and N uptake compared to their non-mycorrhizal (NM) plants under both watering regimes. However, in Acacia and AIbizia, these parameters were only stimulated by either ectomycorrhiza (Acacia) or VA mycorrhiza (Albizia). Growth reduction occurred as a result of inoculation with the other type of mycorrhiza. This was attributed to competition for carbon between Acacia and VA mycorrhizas and parasitic association between Albizia and ectomycorrhiza.Drought-stressed mycorrhizal and NM Leucaena, and drought-stressed mycorrhizal Acacia tolerated lower xylem pressure potentials and larger water losses than the drought-stressed mycorrhizal and NM Albizia and Gliricidia. These latter plants avoided drought by maintaining higher xylem pressure potentials and leaf relative water content (RWC). All the four leguminous plants were mycorrhizal dependent. The higher the mycorrhizal dependency (MD), the lower the drought tolerance expressed in terms of drought response index (DRI). The DRI may be a useful determinant of MD, as they are inversely related.
Young seedlings of English Oak, Quercus robur L., and Silver Birch, Betula verrucosa Ehrl., were subjected to a number of consecutive periods during which water was withheld. During one 14-day period leaf-and soil-water potentials and leaf- and root-solute potentials of two groups of plants were sampled at noon of each day. One group of plants was watered every day while water was withheld from the other group. Solute accumulation in roots and leaves of oak seedlings subjected to water stress resulted in maintenance of turgor and high leaf conductance as the soil dried. In birch seedlings turgor was only maintained by stomatal closure at high soil water potential.Fourteen consecutive water stress cycles greatly reduced the growth of birch seedlings but had little effect on oak seedlings other than to alter root morphology. Water stress treatment resulted in the production of long thin roots in this plant. Stomatal behaviour in oak and birch seedlings during the 14-week stress period was consistent with observed changes in leaf water and solute potentials. Daily solute accumulation in oak leaves was presumably responsible for the maintenance of plant growth as water potentials fell.
A greenhouse investigation was conducted to determine the effect of arbuscular mycorrhiza and drought on the growth of two tropical hedgerow legume trees (Gliricidia sepium and Leucaena leucocephala) under simulated eroded soil conditions. It was a factorial design with two levels of watering regime (adequate watering and drought), inoculation with Glomus deserticola
First year seedlings of English oak (Quercus Cobur) and silver birch (Betula pendula) were subjected to pressure-volume analysis to investigate the water potential components and cell wall properties of single leaves. It was hoped that this rapid-drying technique would differentiate between reductions in plant solute potential resulting from dehydration and the effects of solute accumulation.Comparison of results from these experiments with those of slow drying treatments (over a number of days) with plants growing in tubes of soil, indicated that some solute accumulation may have occurred in drying oak leaves. High leaf turgor and leaf conductance were maintained for a significant period of the drying cycle. Roots of well-watered oak plants extended deep into the soil profile, and possibly as a result of solute regulation and therefore turgor maintenance, root growth of unwatered plants was greater than that of their well-watered counterparts. This was particularly the case deep in the profile. As a result of deep root penetration, water deep in the soil core was used by oak plants to maintain plant turgor, and quite low soil water potentials were recorded in the lower soil segments.Root growth of well-watered birch seedlings was prolific but roots of both well-watered and unwatered plants were restricted to the upper part of the profile. Root growth of unwatered plants was reduced despite the existence of high soil water potentials deep in the profile. Shallow rooting birch seedlings were unable to use this water.Pressure-volume analysis indicated that significant reductions of water potential, which are required for water uptake from drying soil, would occur in oak with only a small reduction in plant water content compared to the situation in birch. This was a result of the low solute potential in oak leaves combined with a high modulus of elasticity of cell walls. Deep rooting of oak seedlings, combined with these characteristics, which will be particularly important when soil deep in the profile begins to dry, mean that this species may be comparatively successful when growing on dry sites.
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