Chapter References

S., Snedden, W. A., and Blumwald, E. (1999) Salt tolerance conferred by over expression of vacuolar Na+ H+ antiport in Arabidopsis. Science 285 1256-1258. Becwar, M. R., Rajashekar, C., Bristow, K. J. H., and Burke, M. J. (1981) Deep undercooling of tissue water and winter hardiness limitations in timberline flora. Plant Physiol. 68 111-114. Binzel, M. L., Hess, F. D., Bressan, R. A., and Hasegawa, P. M. (1988) Intracellular compartmentation of ions in salt adapted...

Acclimation to O2 Deficit Involves Synthesis of Anaerobic Stress Proteins

When maize roots are made anoxic, protein synthesis ceases except for the continued production of about 20 polypeptides (Sachs and Ho 1986). Most of these anaerobic stress proteins have been identified as enzymes of the gly-colytic and fermentation pathways. The mechanism for sensing reduced oxygen levels under hypoxic or anoxic conditions is not completely clear. However, one of the earliest events to occur following lowering of O2 levels is an elevation of the intracellular Ca2+. Evidence...

Submerged Organs Can Acquire O2 through Specialized Structures

In contrast to flooding-sensitive and flooding-tolerant species, wetland vegetation is well adapted to grow for extended periods in water-saturated soil. Even when shoots are partly submerged, they grow vigorously and show no signs of stress. In some wetland species, such as the water lily (Nymphoides peltata), submergence traps endogenous eth-ylene, and the hormone stimulates cell elongation of the petiole, extending it quickly to the water surface so that the leaf is able to reach the air....

Roots Are Damaged in Anoxic Environments

Root respiration rate and metabolism are affected even before O2 is completely depleted from the root environment. The critical oxygen pressure (COP) is the oxygen pressure at which the respiration rate is first slowed by O2 deficiency. The COP for a maize root tip growing in a well-stirred nutrient solution at 25 C, is about 0.20 atmosphere (20 kPa, or 20 O2 by volume), almost the concentration in ambient air. At this oxygen partial pressure (for a discussion of partial pressures, see Web...

Ion Exclusion Is Critical for Acclimation and Adaptation to Salinity Stress

In terms of metabolic energy, use of ions to balance tissue water potential in a saline environment clearly has a lower energy cost for the plant than use of carbohydrates or amino acids, the production of which has a significantly higher energy cost. On the other hand, high ion concentrations are toxic to many cytosolic enzymes, so ions must be accumulated in the vacuole to minimize toxic concentrations in the cytosol. Because NaCl is the most abundant salt encountered by plants under salinity...

Plants Use Different Strategies to Avoid Salt Injury

Plants minimize salt injury by excluding salt from meristems, particularly in the shoot, and from leaves that are actively expanding and photosynthesizing. In plants that are salt sensitive, resistance to moderate levels of salinity in the soil depends in part on the ability of the roots to prevent potentially harmful ions from reaching the shoots. Recall from Chapter 4 that the Casparian strip imposes a restriction to the movements of ions into the xylem. To bypass the Casparian strips, ions...

Ice Crystal Formation and Protoplast Dehydration Kill Cells

The ability to tolerate freezing temperatures under natural conditions varies greatly among tissues. Seeds, other partly dehydrated tissues, and fungal spores can be kept indefinitely at temperatures near absolute zero (0 K, or -273 C), indicating that these very low temperatures are not intrinsically harmful. Fully hydrated, vegetative cells can also retain viability if they are cooled very quickly to avoid the formation of large, slow-growing ice crystals that would puncture and destroy...

Chilling And Freezing

Chilling temperatures are too low for normal growth but not low enough for ice to form. Typically, tropical and subtropical species are susceptible to chilling injury. Among crops, maize, Phaseolus bean, rice, tomato, cucumber, sweet potato, and cotton are chilling sensitive. Passiflora, Coleus, and Gloxinia are examples of susceptible ornamentals. When plants growing at relatively warm temperatures (25 to 35 C) are cooled to 10 to 15 C, chilling injury occurs Growth is slowed, discoloration or...

Adaptation to Heat Stress Is Mediated by Cytosolic Calcium

Enzymes participating in metabolic pathways can have different temperature responses, and such differential ther-mostability may affect specific steps in metabolism before HSPs can restore activity by their molecular chaperone capacity. Heat stress can therefore cause changes in metabolism leading to the accumulation of some metabolites and the reduction of others. Such changes can dramatically alter the function of metabolic pathways and lead to imbalances that can be difficult to correct. In...

HSPs Mediate Thermotolerance

Heat Shock Factor And Hsp70 Cycle Ppt

Conditions that induce thermal tolerance in plants closely match those that induce the accumulation of HSPs, but that correlation alone does not prove that HSPs play an essential role in acclimation to heat stress. More conclusive experiments show that expression of an activated HSF induces constitutive synthesis of HSPs and increases the thermotol-erance of Arabidopsis. Studies with Arabidopsis plants containing an antisense DNA sequence that reduces HSP70 synthesis showed that the...

At High Temperatures Photosynthesis Is Inhibited before Respiration

Both photosynthesis and respiration are inhibited at high temperatures, but as temperature increases, photosynthetic rates drop before respiratory rates (Figure 25.10A and B). The temperature at which the amount of CO2 fixed by photosynthesis, equals the amount of CO2 released by respiration, in a given time interval is called the temperature compensation point. At temperatures above the temperature compensation point, photosynthesis cannot replace the carbon used as a substrate for...

Stomata Close during Water Deficit in Response to Abscisic Acid

Water Stress Stomata

The preceding sections focused on changes in plant development during slow, long-term dehydration. When the onset of stress is more rapid or the plant has reached its full leaf area before initiation of stress, other responses protect the plant against immediate desiccation. Under these conditions, stomata closure reduces evaporation from the existing leaf area. Thus, stomatal closure can be considered a third line of defense against drought. Uptake and loss of water in guard cells changes...

Water Deficit Limits Photosynthesis within the Chloroplast

The photosynthetic rate of the leaf (expressed per unit leaf area) is seldom as responsive to mild water stress as leaf expansion is (Figure 25.4) because photosynthesis is much less sensitive to turgor than is leaf expansion. However, mild water stress does usually affect both leaf photosynthesis and stomatal conductance. As stomata close during early stages of water stress, water-use efficiency (see Chapters 4 and 9) may increase (i.e., more CO2 may be taken up per unit of water transpired)...

Osmotic Adjustment of Cells Helps Maintain Plant Water Balance

As the soil dries, its matric potential (see Web Topic 3.3) becomes more negative. Plants can continue to absorb water only as long as their water potential (Yw) is lower (more negative) than that of the soil water. Osmotic adjustment, or accumulation of solutes by cells, is a process by which water potential can be decreased without an accompanying decrease in turgor or decrease in cell volume. Recall Equation 3.6 from Chapter 3 Yw Ys + Yp. The change in cell water potential results simply...

Water Deficit Increases Resistance to Liquid Phase Water Flow

When a soil dries, its resistance to the flow of water increases very sharply, particularly near the permanent wilting point. Recall from Chapter 4 that at the permanent wilting point (usually about -1.5 MPa), plants cannot regain turgor pressure even if all transpiration stops (for more details on the relationship between soil hydraulic conductivity and soil water potential, see Figure 4.2.A in Web Topic 4.2). Because of the very large soil resistance to water flow, water delivery to the roots...

Heat Stress And Heat Shock

Most tissues of higher plants are unable to survive extended exposure to temperatures above 45 C. Non-growing cells or dehydrated tissues (e.g., seeds and pollen) can survive much higher temperatures than hydrated, vegetative, growing cells (Table 25.3). Actively growing tissues rarely survive temperatures above 45 C, but dry seeds can endure 120 C, and pollen grains of some species can endure 70 C. In general, only single-celled eukaryotes can complete their life cycle at temperatures above 50...

Membrane Properties Change in Response to Chilling Injury

Leaves from plants injured by chilling show inhibition of photosynthesis, slower carbohydrate translocation, lower respiration rates, inhibition of protein synthesis, and increased degradation of existing proteins. All of these responses appear to depend on a common primary mechanism involving loss of membrane function during chilling. For instance, solutes leak from the leaves of chilling-sensitive Passiflora maliformis (conch apple) floated on water at 0 C, but not from those of...

Salt Injury Involves Both Osmotic Effects and Specific Ion Effects

Dissolved solutes in the rooting zone generate a low (more negative) osmotic potential that lowers the soil water potential. The general water balance of plants is thus affected because leaves need to develop an even lower water potential to maintain a downhill gradient of water potential between the soil and the leaves (see Chapter 4). This effect of dissolved solutes is similar to that of a soil water deficit (as discussed earlier in this chapter), and most plants respond to excessive levels...

Osmotic Stress Induces Crassulacean Acid Metabolism in Some Plants

Crassulacean acid metabolism CAM is a plant adaptation in which stomata open at night and close during the day see Chapters 8 and 9 . The leaf-to-air vapor pressure difference that drives transpiration is much reduced at night, when both leaf and air are cool. As a result, the water-use efficiencies of CAM plants are among the highest measured. A CAM plant may gain 1 g of dry matter for only 125 g of water used a ratio that is three to five times greater than the ratio for a typical C3 plant...

Osmotic Stress Changes Gene Expression

As noted earlier, the accumulation of compatible solutes in response to osmotic stress requires the activation of the metabolic pathways that biosynthesize these solutes. Several genes coding for enzymes associated with osmotic adjustment are turned on up-regulated by osmotic stress and or salinity, and cold stress. These genes encode enzymes such as the following Buchanan et al. 2000 A'1-Pyrroline-5-carboxylate synthase, a key enzyme in the proline biosynthetic pathway Betaine aldehyde...

At Higher Temperatures Plants Produce Heat Shock Proteins

In response to sudden, 5 to 10 C rises in temperature, plants produce a unique set of proteins referred to as heat shock proteins HSPs . Most HSPs function to help cells withstand heat stress by acting as molecular chaperones. Heat stress causes many cell proteins that function as enzymes or structural components to become unfolded or misfolded, thereby leading to loss of proper enzyme structure and activity. Such misfolded proteins often aggregate and precipitate, creating serious problems...