ABSTRACIOsmotic adjustment in Rosa hybrida L. cv Samantha was characterized by the pressure-volume approach in drought-acclimated and unacclimated plants brought to the same level of drought strain, as assayed by stomatal closure. Plants were colonized by either of the vesicular-arbuscular mycorrhizal fungi Glomus deserticola Trappe, Bloss and Menge or G. intraradices Schenck and Smith, or were nonmycorrhizal. Both the acclimation and the mycorrhizal treatments decreased the osmotic potential (*,P) of leaves at full turgor and at the turgor loss point, with a corresponding increase in pressure potential at full turgor. Mycorrhizae enabled plants to maintain leaf turgor and conductance at greater tissue water deficits, and lower leaf and soil water potentials, when compared with nonmycorrhizal plants. As indicated by the *i',, at the turgor loss point, the active *I. depression which attended mycorrhizal colonization alone was 0.4 to 0.6 megapascals, and mycorrhizal colonization and acclimation in concert 0.6 to 0.9 megapascals, relative to unacclimated controls without mycorrhizae. Colonization levels and sporulation were higher in plants subjected to acclimation. In unacclimated hosts, leaf water potential, water saturation deficit, and soil water potential at a particular level of drought strain were affected most by G. intraradices. G. deserticola had the greater effect after drought preconditioning.Recent evidence suggests that colonization of root systems by VA2 mycorrhizal fungi affords host plants greater resistance to drought stress3 (2,22). Mycorrhizal plants may avoid drought to some extent through enhanced water uptake at low soil moisture levels (26). In onion the effect appears to be conferred through improved phosphorus nutrition (22) 'The terminology of Levitt (18) has been employed throughout this paper in distinguishing an environmental limitation ('stress') from the related plant response to the limitation ('strain).potential has been observed in wheat (2); however, definitive studies on osmotic adjustment in mycorrhizal plants are lacking. The influence of drought-acclimation and mycorrhizal colonization on tissue water relations and osmotic response in equally sized and adequately P-nourished rose plants is reported in this study. As drought may modify the partitioning of water into apoplastic and symplastic fractions (24), parameters for estimating these fractions were also calculated. MATERIALS AND METHODS
SUMMARYPlants of Rosa hybrida L. cv. ' Samantha', grown with high or low phosphorus (P) fertilization, were inoculated with Glomus intraradices Schenck & Smith, G. deserticola Trappe, Bloss & Menge, or left non-mycorrhizal. All plants except the low-P non-mycorrhizal group had similar tissue dry weights, leaf and root surface areas, growth ratios, stomatal characteristics and P contents. The higher rate of P fertilization increased plant dry weight three-fold and leaf area four-fold in non-mycorrhizal plants, but had no effect on growth characteristics of mycorrhiz^al plants except for a depression of colonization levels. Under well-watered conditions (75-90% and 45-50% relative soil water content, 6), low-P mycorrhizal plants displayed greater leaf conductance (g) than both high-P mycorrhizal and non-mycorrhizal plants. Leaf water potential {r/r) was similar in colonized and uncolonized roses. Fungal species did not differ significantly in their influence on ^ or g.
Shoot water relations and carbohydrate levels were compared for droughted nonmycorrhizal and vesicular-arbuscular (VA) mycorrhizal Rosa hybrida L. cv 'Samantha' plants grown with high and low phosphorus fertilization. Leaf diffusive conductance (gt) of plants colonized by Glomus intraradices Schenk and Smith and Glomus deserticola Trappe, Bloss and Menge were 2 • and 1.5 x greater, respectively, than in nonmycorrhizal plants. Regardless of P fertilization, leaf osmotic and bulk water potentials were 0.5 to 1.1 MPa higher in mycorrhizal than in nonmycorrhizal plants. Leaf starch, chlorophyll and water contents were higher in G. intraradices-colonized plants than .in the high-P nonmycorrhizal plants, while fructose, glucose and total soluble carbohydrates were lower. Level'of P fertilization had no effect on water relations or soluble carbohydrate content of nonmycorrhizal roses. The water status of droughted rose was improved more by G. intraradices than by G. deserticola.
Tissue elasticity can affect plant response to drought, in terms of turgor maintenance and water uptake from drying soils. The purpose of this study was to determine the effect of mycorrhizal colonization and drought acclimation on rose (Rosa hybrida L. cv. Samantha) leaf elasticity. Bulk elasticity was characterized by the pressurevolume method using plots of the elastic modulus as a function of leaf turgor pressure, total water potential and relative water content. The treatments, arranged in a 2 × 3 factorial design, included acclimated and unacclimated plants, and either Glomus irararadices Schenck and Smith, Glomus deserticola Trappe, Bloss and Menge, or a non‐mycorrhizal control. Plants with root mycorrhizal colonization showed reduced leaf elasticity (i.e. higher elastic moduli) over a broad range of leaf waler potential and water content. Both mycorrbizal colonization and acclimation facilitated the maintenance of positive values of turgor and elasticity at lower leaf water potential and water content than in controls. Mycorrhizal infections may aid plants in acclimating to water deficits through effects on leaf tissue elasticity.
Paclobutrazol was found to be an effective growth retardant for weeping fig (Ficus benjamina L.). Plants treated with a soil drench at rates of application from 0.125 mg a.i. to 8.0 mg a.i. per 10-cm pot exhibited retardation in plant height, leaf production, internode length, and leaf size. All measured responses were retarded linearly with increasing concentration, to the point of saturation at 0.5 mg a.i. per 10cm pot. Leaves developing under the influence of paclobutrazol also appeared darker green than in untreated plants. The growth retarding influence of a single application of PP333 persisted after transplanting. Chemical names used: (2RS,3RS)-l-(4-chlorophenyl)-4,4-dimethyl-2(l,2,4-triazole-l-yl)pentan-3-ol (paclobutrazol; ICI-PP333).
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