Abstract
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.
The regulation of NAD+-dependent sorbitol dehydrogenase (NAD-SDH, EC 1.1.1.14) by sugar was investigated by using sliced tissues of japanese pear (Pyrus serotina Nakai cv. Kousui) fruit in order to determine its role in the mechanism of sugar accumulation in fruit tissue. The results of the activities and steady-state levels of the protein and mRNA indicate that NAD-SDH in japanese pear fruit is among the sugar-inducible genes. By preincubating the sliced tissues for 16 hours in a medium without sugar, NAD-SDH activity declined and reached a stable level that was maintained for up to 40 hours. The washing procedure also reduced the sugar concentration in the apoplast and cytosol of the sliced tissues to low concentrations and enabled them to be manipulated by exogenous applications of carbohydrate solutions. Incubation of tissues in 50 or 100 mm sorbitol for 8 hours led to enhanced expression of the NAD-SDH gene as determined by increased mRNA and protein levels and enhanced enzyme activity. The presence of 100 mm glucose, sucrose, or mannitol also gave significant stimulation on the levels of activity, protein, and mRNA of NAD-SDH compared with those of control tissues bathed in media in which the osmotic potential had been adjusted to that of the sugar solutions by adding polyethylene glycol. However, fructose was ineffective in stimulating NAD-SDH activities and the level of the protein was not enhanced but the level of mRNA was increased. Therefore, it is suggested that NAD-SDH gene transcription is enhanced by each sugar investigated, and fructose appears to be unique as it also influences NAD-SDH at a post-transcriptional level.
, 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