Influence of rewatering and time of sampling on solute accumulation of two Populus deltoides clones.

We investigated effects of rehydration and time of sampling on solute accumulation in plants of two greenhouse-grown Populus deltoides Bartr. clones (Ohio Red and Platte) subjected to multiple cycles of water stress. Osmotic potential of leaves at full turgor was measured at predawn on well-watered (control) and water-stressed (conditioned) plants that had been rewatered the previous afternoon. Water-soluble organic solutes (carbohydrates, phenolic glucosides and organic acids) were determined at both predawn and midday, before and after rewatering. Conditioning resulted in solute accumulation; however, rewatering of conditioned plants decreased the predawn concentration of glucose by 19-35% and the total solute concentration by 14-15%, relative to values before rewatering. There was a 52% increase in salicin concentration in response to rewatering in conditioned plants of the Platte clone. In conditioned plants of both clones, the concentration of glucose was generally lower at midday than at predawn (16-47%), whereas the concentration of sucrose was higher at midday than at predawn (46-133%). Time of sampling was an important factor in determining whether conditioning resulted in accumulation of glucose and fructose. Compared with control plants, there was a significant accumulation of glucose and fructose at predawn and either no accumulation or a significant reduction of these solutes at midday both before and after rewatering of conditioned plants. Sampling time also affected the amount of solute that accumulated in response to conditioning.

Organic solute accumulation and dehydration tolerance in three water-stressed Populus deltoides clones.

The relationship between osmotic potential, water-soluble organic solutes and electrolyte leakage was investigated in three greenhouse-grown eastern cottonwood (Populus deltoides Bartr.) clones. The Ohio Red (from southern Ohio) and Platte (from eastern Nebraska) clones were selected because they differ in dehydration tolerance, gas exchange and osmotic potential. A third clone, Wildcat (from western Nebraska) was included because of its origin in a very dry area. Plants were either watered daily (control) or watered every 4-6 days (preconditioned). A third group of plants was watered daily until it was water-stressed at the end of the experiment (nonconditioned). Leaves for osmotic potential and water-soluble solute determinations were sampled at predawn after plants were rewatered. All clones showed osmotic adjustment ranging from 0.23 to 0.48 MPa. Organic solutes contributed up to 48% of the total osmotic adjustment. The solutes that contributed at least -0.05 MPa each to leaf osmotic potential in any clone were sucrose, malic acid, glucose, fructose, myoinositol and salicin. Fructose and glucose concentrations were unchanged or declined in preconditioned Ohio Red and Platte but increased in Wildcat. The Ohio Red clone had the highest sucrose concentration in both well-watered and preconditioned plants, and a lower injury index (more membrane stability) than Wildcat in the preconditioned plants, whereas nonconditioned Platte displayed the greatest electrolyte leakage of all clones and treatments. Cell membrane stability was correlated to osmotic potential and sucrose concentration only in Ohio Red.

Net Photosynthesis, Electron Transport Capacity, and Ultrastructure of Pisum sativum L. Exposed to Ultraviolet-B Radiation.

Pisum sativum L. was exposed to ultraviolet-B (UV-B) radiation (280-315 nm) in greenhouse and controlled environment chambers to examine the effect of this radiation on photosynthetic processes. Net photosynthetic rates of intact leaves were reduced by UV-B irradiation. Stable leaf diffusion resistances indicated that the impairment of photosynthesis did not involve the simple limitation of CO(2) diffusion into the leaf. Dark respiration rates were increased by previous exposure to this radiation. Electron transport capacity as indicated by methylviologen reduction was also sensitive to UV-B irradiation. The ability of ascorbate-reduced 2,6-dichlorophenolindophenol to restore much of the electron transport capacity of the UV-B-irradiated plant material suggested that inhibition by this radiation was more closely associated with photosystem II than with photosystem I. Electron micrographs indicated structural damage to chloroplasts as well as other organelles. Plant tissue irradiated for only 15 minutes exhibited dilation of thylakoid membranes of the chloroplast in some cells. Some reduction in Hill reaction activity was also evidenced in these plant materials which had been irradiated for periods as short as 15 minutes.

Rapid field measurement of the assimilation rate versus internal CO(2) concentration relationship in green ash (Fraxinus pennsylvanica Marsh.): the influence of light intensity.

Assimilation rate (A) versus intercellular CO(2) concentration (C(i)) relationships for leaflets of five-year-old green ash (Fraxinus pennsylvanica Marsh.) trees were computed from gas exchange measurements obtained in the field with a closed-circuit, portable photosynthesis measurement system comprising an LI-6200 gas analyzer and an LI-6000 computer, (Li-Cor, Inc., Lincoln, Nebraska, USA). Observations were made over a range of light intensities achieved by attenuating direct sunlight with neutral density filters, and over a range of ambient CO(2) concentrations achieved by breathing into the assimilation chamber and then lowering the CO(2) concentration to the desired level with the LI-6200's soda-lime scrubber. Boundary layer conductance was determined by use of a leaf replica made of moist filter paper. Typically, A-C(i) curves at four light intensities were obtained in three to four hours. The initial slope (when A = 0) of the A-C(i) curve obtained at a light intensity of 1750 micromol m(-2) s(-1) (full sunlight) was similar to that obtained at a light intensity of 840 micromol m(-2) s(-1). However, when light intensity was reduced further (to 370 and 160 micromol m(-2) s(-1)), the initial slope of the A-C(i) curve also decreased, indicating that at these light intensities, assimilation was limited by photochemical energy supply, as well as CO(2) concentration.

Inhibition of Photosystem II in Isolated Chloroplasts by Lead.

Inhibition of photosynthetic electron transport in isolated chloroplasts by lead salts has been demonstrated. Photosystem I activity, as measured by electron transfer from dichlorophenol indophenol to methylviologen, was not reduced by such treatment. However, photosystem II was inhibited by lead salts when electron flow was measured from water to methylviologen and Hill reaction or by chlorophyll fluorescence. Fluorescence induction curves indicated the primary site of inhibition was on the oxidizing side of photosystem II. That this site was between the primary electron donor of photosystem II and the site of water oxidation could be demonstrated by hydroxylamine restoration of normal fluorescence following lead inhibition.