Stress Physiology Chapter 25 Abiotic stress: Water availability (drought, flooding) Temperature (hot, cold) Salinity O2 concentration Nutrient limitation (N, P, micro nutrients) Pollution (air, soil) Radiation (high, low) Wind Biotic: Herbivory Disease (fungi, bacteria, virus) etc

Economic importance The yield of field-grown crops in the U.S. is only 22% of the genetic potential yield (Boyer 1982). Ecological importance Stress factors limit the distribution of plant species

Stress - a disadvantageous influence on the plant exerted by an external factor. Disadvantageous = reduced growth & reproduction (sometimes also reduced process rates, e.g. photosynthesis) Growth after 1 month High T Low T

Stress tolerance - the ability to maintain functioning when exposed to a wide range of conditions. Usually a relative term based on comparisons among species or genotypes of their responses to different levels of some factor (temp., moisture, etc.). Growth after 1 month High T Low T RED has a greater stress tolerance than BLUE

Acclimation - an increase in stress tolerance of an individual organism following exposure to stress. Growth after 1 month Adequate Water moisture limitation RED: no previous exposure to drought: no stress tolerance BLUE: previous exposure to drought: increased stress tolerance

Adaptation - a genetically-determined increase in stress tolerance as a result of selection over generations. Growth after 1 month High T Low T RED has a greater stress tolerance than BLUE

Stress Stress tolerance Acclimation Adaptation Older literature Stress avoidance: for example: early seed-set to avoid drought

Water stress – drought tolerance Heat stress and heat shock Chilling and freezing Salinity O2 deficiency Much research is directed towards discovering the mechanisms of stress tolerance, acclimation etc.

Water stress – drought tolerance Heat stress and heat shock Chilling and freezing Salinity O2 deficiency Much research is directed towards discovering the mechanisms of stress tolerance, acclimation etc. First some general remarks about drought stress

Precipitation and productivity of global ecosystems Fig. 3.2

Water Stress Fig. 3.1

Rice (Oryza sativa L.) is the staple food for more than two-third of the world's population (Dowling et al, 1998). About 7.5 % of total rice production comes from irrigated lowland production (Bouman and Tung 2001). Drought stress is a major constraint for about 50% of the world production area of rice.

The timing of water stress is very important.

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Drought stress and consequences for natural vegetation

Dealing with water stress Three general ecological strategies Postponement of desiccation Ability to prevent desiccation despite reduced water availability. 2. Tolerance of desiccation Ability to maintain function while dehydrated 3. Drought escape Complete life cycle before the onset of drought.

Effects of water stress that reduce growth Reduction in cell and leaf expansion Reduction in photosynthesis, due first to decreased stomatal conductance, then to inhibition of chloroplast metabolism. 3. Altered allocation - greater investment in non- photosynthetic tissues such as roots & mycorrhizae

Fig. 3.12 Responses to deal with stress

Leaf expansion is very sensitive to water deficit. Fig. 25.4 PP25040.jpg

Why is leaf expansion so sensitive to drought? YW = YS + YP Leaf expansion is slowed by water stress because turgor pressure declines.

Acclimation to drought stress

Additional strategies for adapting leaf area to drought Loss of leaves Wilting Morphology - Vertical leaves Reduction of radiation load results in less evaporative demand

A very important drought response: stomatal closure Advantage: less loss of water Disadvantage: less transport of CO2. Mechanism: 1- loss of water from stomatal cells, turgor drops, stoma closes 2- cell actively decrease solute concentration YW = YS + YP Solute potential rises (less negative), turgor drops, stoma closes Long-distance action: via hormones: Abscisic acid (ABA) Split-root experiment

Effects of drought on photosynthesis are generally minor 1- early effect: mostly via stomatal closure 2- late effect: metabolic breakdown

Phloem translocation seems to be less sensitive to water stress than photosynthesis.

Water uptake from the soil happens when soil potential is higher than plant water potential Osmotic adjustment helps plants cope with water stress. YW = YS + YP A decrease in YS helps maintain turgor, YP, even as total water potential decreases. Osmotic adjustment is a net increase in solute content per cell. Many solutes contribute to osmotic adjustment. K+, sugars, organic acids, amino acids Osmotic adjustment may occur over a period days. Costs of osmotic adjustment: synthesis of organic solutes, maintenance of solute gradients, and “opportunity costs”, energy the could be used for other functions

Responses to water stress Osmotic adjustment Stomatal closure hydropassive - guard cell dehydration hydroactive - guard cell metabolism; ABA, solutes, etc. Leaf abscision and reduced leaf growth reduces surface area for water loss Smaller leaves lose more heat via convective heat loss Increased root growth with reduced leaf expansion, more C translocated to roots increases water supply Increased wax deposition on leaf surface reduces cuticular transpiration, increases reflection Induction of CAM in facultative CAM plants in response to water or osmotic stress

Also many responses at the cellular level: Proteins increase and decrease in response to water stress One special group of proteins: LEA-proteins (late embryogenesis abundant) Accumulate in dehydrating leaves, and during seed ripening Function: protection of membranes (hydrophylic proteins) prevention of random crystallization of proteins

Table 25.3 2. Heat Stress And Thermotolerance

Photosynthesis declines before respiration Ion leakage is a sign of membrane damage due to high temps. (or freezing.) Fig. 25.10

What happens when plant tissues reach harmful temperatures? Membranes lose function because they become too fluid. Soluble proteins may denature, degrading function Membrane-bound proteins may become dysfunctional because of denaturation or excessive membrane fluidity. These effects can be seen in the changes in photosynthesis, respiration, and ion leakage of membranes. Fig. 1.5

Adaptive or acclimation responses to high temperatures Vertical leaf orientation Leaf pubescence Altered membrane fatty acids more saturated fatty acids that don’t melt as readily 4. Production of heat shock proteins (HSPs) in response to rapid heat stress “molecular chaperones”, increase enzymes resistance to denaturation; help maintain proper protein folding 5. Increased synthesis of gamma-aminobutyric acid (GABA)