Carbohydrates are the energy source for most root activities, including membrane maintenance and osmotic adjustment. Yet, the relationship between root carbohydrate status and selective sodium chloride uptake remains unknown. The following study examined the effects of root carbohydrate starvation due to girdling on sodium and chloride uptake in mature citrus trees. Trees were girdled during the spring or during the autumn, when girdling is known to have more dramatic affects. In spring-girdled trees, 4 days after girdling, root total carbohydrate and starch decreased by 25% and 30%, respectively. The decrease in root carbohydrates was followed by a 20% reduction in root respiration rate. Based on root mineral analysis, spring-girdled trees were characterized by having 42% more sodium and 30% more chloride. The effects of girdling on shoot xylem sap mineral concentration were similar to trends in root mineral status; xylem sap from spring-girdled trees had 43% more sodium and 22% more chloride. Leaf chloride concentration measured 6 months after girdling was 74% higher in girdled trees and reached toxicity levels (0.65% vs. 0.37% dry mass, for girdled and nongirdled trees, respectively). The differences in leaf sodium, however, were nonsignificant (0.14% vs. 0.13% dry mass, for girdled and nongirdled trees, respectively). In autumn-girdled trees, the effects on leaf sodium and chloride concentration were more dramatic. Leaves from autumn-girdled trees (sampled 10 months later) had about two times more sodium and about five times more chloride in comparison to nongirdled trees (0.39 % vs. 0.20% dry mass sodium and 1.02% vs. 0.22% dry mass chloride, respectively). The above results link root carbohydrate status and selective sodium or chloride uptake in citrus trees.
The tensile properties of european pear (Pyrus communis L. `Beurre Bosc') and asian pear (Pyrus pyrifolia Nakai `Choguro') were examined using a microscope-mounted apparatus that allowed direct observation and recording of cell and tissue changes during testing. To manipulate turgor potential, tissue slices from fruit of different firmness (ripeness) were incubated in sucrose solutions of differing water potential. Solution water potentials were adjusted for individual fruit, and varied between -2.5 and 1 MPa from the water potential of the expressed juice. Fruit firmness declined from 100 to 20 N and from 60 to 25 N during ripening of european and asian pears, respectively. For both european and asian pears the relationship between fruit firmness and tensile strength of tissue soaked in isotonic solutions was sigmoidal, with the major mechanism of tissue failure being cell wall failure and cell fracture at high firmness and intercellular debonding at low firmness. In the intermediate zone, where fruit firmness and tissue tensile strength decreased simultaneously, a mixture of cell wall rupture and intercellular debonding could be observed. Tissue and cell extension at maximum force both declined similarly as fruit softened. Tensile strength of tissue from firm pears (>50 N firmness, >0.8 N tensile strength) decreased by as much as 0.6 N during incubation in solutions that were more concentrated than the cell sap (hypertonic solutions). When similar tissue slices were incubated in solutions that were less concentrated than the cell sap (hypotonic solutions), the tensile strength increased by up to 0.4 N. This is interpreted as stress-hardening of the cell wall in response to an increase in cell turgor. Tensile strength of tissue from soft pears was not affected by osmotic changes, as the mechanism of tissue failure is cell-to-cell debonding rather than cell wall failure.
Alnus maritima [Marsh.] Nutt. (seaside alder) is a rare species that occurs naturally only on soils that are frequently or constantly saturated with fresh water. The objective of our first experiment was to determine effects of drought and flooding treatments of differing severity on foliar gas exchange, water relations, and development of plants grown in containers in a greenhouse. In a second experiment we examined how the rate of water loss from soil during drought episodes affected the gas exchange and survival of leaves. In the first experiment, changes in soil moisture content, which ranged from saturation to 10% or less by volume across treatments, were associated with altered stem water potential and net photosynthesis. Analysis of the osmolarity of liquid extracted from leaves indicated that osmotic adjustment did not occur in response to drought. Shoot dry weight per plant ranged from over 7 g (only the lower portion of the soil profile kept saturated) to less than 3 g (entire soil profile constantly saturated). Episodes of drought of different severity led to plants with shoots that weighed between these two extremes, and exposure to soils with 10% water or less by volume did not elicit leaf desiccation or abscission. Results of the second experiment suggest that leaf desiccation can result from exposing plants to 10% water or less by volume if the drought develops rapidly in a small volume of soil. We conclude that, despite the niche it occupies in nature, seaside alder may have the potential to be used in managed landscapes with soils that vary in moisture content.
, 2007 ). High tolerance in GB-primed plants may contribute to cellular osmotic adjustment, protection of membrane integrity, stabilization of antioxidant enzymes that scavenge reactive oxygen species, or GB may play an indirect role in signal
. Metabolism and Osmotic Adjustment Osmotic adjustment is one of the most important strategies adopted by many plants to help them overcome salt stress ( Bernstein, 1961 , 1963 ). Kirkham et al. (1969) observed that highly salt-tolerant barley ( Hordeum
deficit and salt stresses activate common mechanisms of cellular response, such as osmotic adjustment and antioxidant activity, and trigger similar physiological changes to withstand stresses, such as stomatal closure ( Bartels and Sunkar, 2005 ; Wang et
carbohydrates provide energy and solutes for osmotic adjustment. Sucrose, an important component of TNC, is the dominant form of carbohydrate transported to developing plant organs and is one of the sugars stored in higher plants ( Khayat and Zieslin, 1987
conditions may provide a storage form of nitrogen that is reused when stress is over ( Singh et al., 1987 ) and may play a role in osmotic adjustment. Proteins may be synthesized de novo in response to salt stress or may be present constitutively at a low
; 2) quantify and characterize the ability of these taxa to osmotically adjust throughout the growing season, and 3) determine if predicted drought tolerance is variable within populations as well as among trees that represent different provenances
). However, active osmotic adjustment was also reported to develop only in situations in which water stress was imposed gradually for an extended period of time ( Arndt et al., 2000 ). The relationship between water stress and changes in concentrations of