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.
Two field experiments were conducted to evaluate the effects of differential irrigation on plant growth, development, and water status of 2 snap bean cultivars, ‘Oregon 1604’ and ‘Galamor’ (Phaseolus vulgaris L.). Plants were grown at various irrigation levels ranging from a well-watered control to a dry treatment which received only one irrigation to establish plants. Measurements on plants sampled weekly at 6 times during the growing season showed that total plant dry weight, total leaf dry weight, total leaf area, average area per leaf, and number of leaves per plant were reduced by water deficits in both cultivars. Also, for both cultivars, total leaf area per plant was reduced more by a decrease in area per leaf than by a reduction in leaf number. Specific dry leaf weight was higher in the drier treatments. During each year, a significant difference between treatments occurred earlier in the season for total leaf area per plant than for total plant weight. At predawn, leaf water potential (ψ) always was more negative in the dry treatment than in the control. Early in the season, there was no significant difference in midday ψ between the control and dry treatment. Later, as soil water became limiting, the dry treatment had a more negative ψ than the control. Near the end of the season, after the dry treatment had been subjected to a long period of water stress, midday ψ was more negative in the control than in the dry treatment. Although some osmotic adjustment occurred in the dry treatment, leaf turgor potential (ψp) was generally lower than in the control throughout the day. As ψ decreased from early morning through midday, transpiration rates increased due to an increase in evaporative demand on the leaves. Leaf diffusive resistance also increased with decreasing ψ but a “threshold value” for stomatal closure was not demonstrated.
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.
, 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
efficiency but not activity. Relative activity of the roots may be evaluated in this study by TE, although more information is needed to assess root activity in turfgrasses. Osmotic adjustment Osmotic adjustment facilitates water uptake and limits water loss