Interpreting Leaf Water Potential Measurements with a Model of the Soil-Plant-Atmosphere Continuum

A water flux model, which assumes that the dynamic functioning of the soil-plant-atmosphere continuum may be described by a series of steady states, was examined as a means for interpreting leaf water potential measurements in ‘Valencia’ orange trees (Citrus sinensis (L.) Osbeck). According to the model, leaf water potential should be related to transpirational flux, which in this experiment was estimated by the ratio of vapor pressure deficit of the atmosphere to leaf diffusion resistance (VPD/r_leaf). Leaf water potentials decreased in a specific relationship with increasing values of VPD/r_leaf provided that soil water was adequate and soil temperature was not too low, but regardless of season of the year or climatic or edaphic differences among 3 field locations. When soil water tensions exceeded 0.3 bar or when soil temperatures were lower than 15°C, deviations from the model occurred in the form of more negative leaf water potentials than predicted by VPD/r_leaf. The model predicts from simple measurements made on intact plants that these differences were due to the modification of flow resistances by cool temperatures and the modification of both resistances and the potential of water at the source in the case of soil water depletion. The model may be a useful tool for interpreting plant water potential data under contrasting environmental conditions.     

Metabolism of dulcitol in Euonymus japonica

Dulcitol-1(6)-¹⁴C was administered to leaves of E. japonica and samples were taken for time periods ranging from 0·5 to 24 hr. For each time period the absolute activity of the glucose, galactose and dulcitol pools was determined. Such studies demonstrated that dulcitol is converted to glucose and galactose. The initial product was glucose, some of which was converted to galactose, glacturonic acid and glucuronic acid. Fractionation of a leaf sample into its pectin, lignin, hemicellulose and α-cellulose components, with subsequent hydrolysis, showed that the dulcitol pool is used in the synthesis of structural carbohydrates. The activity of these fractions was shown to reside in dulcitol, glucose, galactose, galacturonic acid and glucuronic acid residues.     

Bisulfite compounds as metabolic inhibitors: nonspecific effects on membranes

Bisulfite compounds are shown to be nonspecific inhibitors ofphotosynthetic processes and of ion transport in green tissues.CO₂ fixation and light-dependent transient changes in externalpH are inhibited about 50% by 5x10^–4 M glyoxal-Na-bisulfite.Chloride uptake in the light and in the dark is inhibited tothe same extent at this concentration. At 5x10^–3 M theinhibitor reduces ATP levels in the light and in the dark, andeffects on glycolate oxidase and PEP carboxylase are observed.The extent of inhibition is dependent on time of treatment withglyoxal-Na-bisulfite and freshly prepared NaHSO₃ is equallyas effective as the addition compound. Possible explanations of the bisulfite effects and the relationshipsto SO₂ effects on photosynthesis are discussed. (Received September 1, 1971; )     

Growth Responses of Dwarf Bean to Infection by Pseudomonas phaseolicola (Burk) Dows

At an early growth stage inoculation of both unifoliates ofdwarf beans with Pseudomonas phaseolicola (Burk) Dows. Race1 inhibited trifoliate production, but inoculation of one unifoliateonly reduced subsequent leaf expansion, and the opposite uninfectedunifoliate became significantly larger than unifoliates on uninfectedplants. Inoculation of the first or second trifoliates or thecotyledonary node at a later developmental stage all reducedprimary-shoot growth by 25 to 30 per cent, but there were markeddifferences in the extent of secondary-shoot development inleaf axils below the inoculation point. The growth effects were related to the distribution and activityof the bacterial toxin within the plant, particularly to differencesin the rate of invasion of apical regions. The primary toxineffects appear to be on apical function and leaf expansion.