We isolated a cDNA clone encoding limonoid UDPglucosyltransferase (limonoid GTase) from the albedo of Satsuma mandarin (Citrus unshiu Marc.) and investigated the contribution to limonoid glucoside accumulation in fruit. The isolated cDNA clone (CitLGT) was 1732 bp in length encoding 511 deduced amino acids with a predicted molecular mass of 57.5 kDa. The products of in vitro translation from an expression vector had the limonoid GTase activity. Southern blot analysis of genomic DNA indicated that CitLGT was present as a single copy gene in the Citrus genome. The amount of transcript corresponding to CitLGT mRNA changed the same way as the fluctuation of limonin glucoside content during fruit development of navel orange (Citrus sinensis Osb.). This indicates that the transcription of CitLGT regulates the conversion of limonoid aglycones to glucosides in citrus fruit.z 2000 Federation of European Biochemical Societies.

Application of 10–50 μM Al ions to a plasma membrane‐enriched microsomal fraction, isolated from roots of maize (Zea mays L.), resulted in decreased Mg2‐ATPase activity. This was probably caused by changes in membrane structure as detected by the use of spin probes. Both enzymatic activity and membrane structure could in part be protected from Al injury when organic acids, similar to those found in maize root tissue, were administered prior to the metal. When stressed by application of Al ions, the Al‐tolerant maize hybrid, W64, maintained higher concentrations of organic acids, especially malic and trans‐aconitic, than the Al‐sensitive maize hybrid, A632. We hypothesize that citric and malic acid, because of their high stability constants with Al and/or the acid's concentration reduce Al toxicity in maize root tissue, especially in the Al‐tolerant line.

Salinity-induced alterations in tomato (Lypersicon esculentum Mill. cv Heinz 1350) root plasma membrane properties were studied and characterized using a membrane vesicle system. ported by membrane-bound carriers at the expense of the electrochemical gradient (15). The biochemical properties (18) and the proton pump activity (26) of the plasma membranebound, ion-stimulated ATPase have been reported for a variety of plant species. These studies have focused mainly on the characterization of ATPase and proton pump activities on membrane fractions isolated from nonstressed tissues. The relationship between proton pump activity and cellular ion regulation is especially important in salt-stressed tissues. With this thought, we focused our investigation on salt-induced changes in plasma membrane function. Giannini et al. (10) describe a method for the isolation of plasma membrane-enriched vesicles that are transport competent and retain full activity for a prolonged time period when stored at -80°C. Using this plasma membrane vesicle system, we initiated a comparative study of proton transport, and membrane-bound ATPase activity in native vesicles isolated from control and salt-stressed tomato roots (Lycopersicon esculentum Mill. cv Heinz 1350). Characterization of salt stress effects on proton pump activity, ATPase kinetics, and the ion-stimulation of these processes provides essential information that is necessary to understand the role of the plasma membrane in the physiological mechanisms of salt tolerance. MATERIALS AND METHODS Plant MaterialTomato (Lypersicon esculentum Mill. cv Heinz 1350) seeds were germinated in vermiculite for 1 week and then were placed into aerated hydroponic solution culture (0.5x Hoagland solution [pH 6.0]) and maintained in a growth chamber. The plants were provided with 16 h daylight at a light intensity of 250 ,uE m-2 s-'. Day/night temperatures were maintained at 30°C/25°C. Salt stress was applied 10 d after transplanting at a rate of 0.1 MPa d-' (15 mm NaCl, 3 mM CaC12) for 4 consecutive days resulting in a 75 mm applied salt stress at a Na:Ca ratio of 5:1. The plants were 35-d-old at harvest. Plasma Membrane IsolationThe plasma membrane-enriched vesicle fractions that were used for ion transport measurements were isolated by the method of Giannini et al. (10); all solutions were kept at ice www.plantphysiol.org on May 12, 2018 -Published by Downloaded from

Foxtail barley (Hordeum jubatum L.), a wild species that can inhabit highly saline environments, may have more potent salt‐tolerance genes than commercial barley (H. vulgare L.). The objective of this study was to compare the growth and ion relations of the commercial cultivar Harrington with foxtail barley under saline conditions, to determine whether a differential response to salinity stress exists between these species. Seedling performance was evaluated as a function of increasing concentrations of Na2SO4, MgSO4, and CaCl2 (within the electrolyte conductivity range of 0.26 to 1.38 S m−1) and the Ca mole fraction (Camf) of salt treatments in solution culture experiments. The Camf of the salt treatments, defined as the molar ratio of Ca/(Ca + Na + Mg), was maintained at 0.02 or 0.09. Salt treatments at a Camfr of 0.02 significantly reduced leaf area, shoot growth, root growth, and the root‐to‐shoot ratio (R/S) of Harrington seedlings relative to foxtail barley. Growth of Harrington seedlings improved substantially when the Camf was increased from 0.02 to 0.09. The two genotypes differed significantly in root weight and R/S under saline conditions; these differences were independent of the Camf. Evaluation of root and shoot tissue ion contents revealed that H. jubatum seedlings accumulated less Na from the medium than Harrington and preferentially compartmentalized Na in root rather than shoot tissue. The wild species maintained higher levels of Ca, had more favorable K/Na ratios, and maintained higher R/S levels of SO4 than Harrington. Differences between H. jubatum and Harrington in root and shoot ion accumulation suggest that membrane permeability and selectivity properties are controlled by both genotype and Ca supply.