If a plant is said to have an abiotic problem, it is not being affected by something living like pests, fungi, bacteria or viruses.
Common factors causing abiotic injuries include water, sun, temperature, wind, soil, chemicals, contact and nutrients. For example, two branches can cross on a tree. If one branch moves, it may start rubbing away at the other branch and cause a wound. This can be corrected by pruning away the damaged branch, which will prevent further problems. Many of these problems can happen at each end of their respective spectrums. Some trees and shrubs cannot handle wet feet (a common name for the condition when the soil is continuously wet, as can happen with clay soils) and may languish or die. Many others will not survive in bouts of drought.
Temperature Stress can also wreak havoc on a plant. As with any living organism, a plant has an optimal temperature range at which is grows and performs best. If the temperature is too cold for the plant, it can lead to cold stress, also called chilling stress. Extreme forms of cold stress can lead to freezing stress. Cold temperatures can affect the amount and rate of uptake of water and nutrients, leading to cell desiccation and starvation. Under extremely cold conditions, the cell liquids can freeze outright, causing plant death.
Hot weather too can affect plants adversely. Intense heat can cause plant cell proteins to break down, a process called denaturation. Cell walls and membranes can also “melt” under extremely high temperatures and the permeability of the membranes is affected.
Temperature stress includes both high temperature and low temperature stresses
- High temperature (Heat)
- Low temperature (Cold)
High Temperature Stress
Plant life exists between the temperatures of -89°C to +58°C. However most of the plants are adapted to limited range of temperatures. If the temperature drops below 15°C, plants experience low temperature stress and if it is above 45°C, plants are subjected to high temperature stress. Increase of 15 – 20°C above normal temperature causes deeper modification of growth without being necessarily lethal viz., protein denaturation, enzyme inactivation and reduction in chloroplast’s photosynthetic activity.
Heat stress on growth and development
- Seedling establishment is hampered
- Pollen development is affected
- Grain and fruit development and quality is affected
- Cellular changes during heat stress When plants are exposed to temperatures higher than 45°C, it experiences heat stress. The cellular changes due to heat stress are
- Disruption of cytoskeleton and microtubules
- Fragmentation of golgi complex
- Increase in number of lysosomes
- Swelling of mitochondria thereby resulting in decreased respiration and oxidative phosphorylation
- Disruption of normal protein synthesis
- Disappearance of polysomes
- Disruption of splicing of mRNA precursors
- Cessation of pre-RNA processing
- Decline in transcription by RNA polymerase I
- Inhibition of chromatin assembly
- Decline in DNA synthesis
- Heat Stress and Heat Shock
- Levels of heat tolerance
Most intact higher plants cannot tolerate exposure to temperatures higher than 45°C, though dry seeds and pollen can survive considerably higher. Only single celled eukaryotes can complete their life cycle at about 50°C and only certain Archaea can divide above 60°C. Brief exposures to heat can induce acquired thermotolerance in some plants. Heat stress is greater when water stress exists, but it may be high on emerging seedlings even in moist soil due to the higher sunlight absorption of darker moist soil. Mechanisms of heat dissipation and tolerance High leaf temperature and minimal evaporative cooling lead to heat stress. Succulents with CAM such as Opuntia (prickly pear cacti) and Sempervivum (houseleeks or live-forever of family Crassulaceae) can tolerate tissue temperature of 60 to 65°C. In CAM plants with closed stomata C3 plants experiencing water stress reduce stomatal conductance and water transpiration, causing increased leaf temperature while sun exposed. High leaf temperatures can also occur when humidity is high, preventing evapotranspiration (as in tropical environments or greenhouses). Irrigation increases crop yields in cotton, corn and sorghum. Effects of heat stress As temperature increases, photosystem rates drop before respiration decreases. Above the temperature compensation point, the plant evolves more CO2 from mitochondrial respiration than it assimilates and fixes by photosystem (i.e. CO2 assimilation is negative). In this state, CHO reserves decline and fruits and vegetables lose their sweetness.
Role of membranes in plant adaptation and acclimation to high temperatures Adaptation or acclimation to high temperatures varies. For example, Atriplex sabulosa (a cool weather species) shows a more rapid falloff of PS with rising temperatures around 40 to 48°C compared to Tidestromia oblongifolia (a hot weather species), whereas growth rates at much lower temperatures (16°C) favors A. sabulosa. Crop species tend to grow best at the temperatures in which they evolved and adapted, but the underlying mechanisms that would explain these differences are complex.
The declines of PS seen at higher temperature are mainly due to instability of membrane bound electron transport, a phenomenon that dominates over denaturation of enzymes that can occur at even higher temperatures. Membrane stability is affected by temperature. High temperature causes excess membrane fluidity, especially in the presence of greater unsaturation of fatty acids in the membrane lipids. Greater membrane lipid unsaturation plays a role in improving chill tolerance by making membranes more liquid and flexible when cold. In oleander (Nerium oleander), high temperature acclimation is accompanied by a greater degree of saturated fatty acids in membrane lipids, which makes them less fluid and a similar effect is induced in Arabidopsis mutants that make less omega-3 (unsaturated) fatty acids. High temperatures lead to membrane disruption and ion leakage, as well as inhibition of PS and respiration. Although high temperatures also cause denaturation of proteins such as RuBisCO and could thereby affect enzyme stability, this generally occurs at higher temperatures than those initially affecting PS.
Leaf adaptations against high temperatures Plants in high light and heat environments must reduce their exposure to solar radiation or improve heat dissipation. They do this with increased leaf hairs (pubescence), more reflective surface waxes, paraheliotropic tracking, wilting, leaf rolling, as well as with smaller and/or highly dissected leaves to reduce boundary layer thickness. Some desert plants, such as Encelia farinosa (white brittlebush) adapt with with seasonally dimorphic leaves: green and hairless in winter, white and pubescent in summer.
Protective heat shock proteins and other heat tolerance agents At higher temperatures, plant produces heat shock proteins. These act as molecular chaperones, preventing deleterious unfolding or misfolding of enzymes and structural protein components. HSPs are induced by sudden as well as by gradually rising temperatures and play a critical role in mediating heat tolerance. These are also found in animals and were first discovered in Drosophila. In addition, ABA and salicyclic acid as well as ethylene play a role in increasing tolerance to heat stress.
There are several signaling pathways involved in heat stress. γ- aminobutyric acid (GABA), which is synthesized by glutamate decarboxylase (GAD), accumulates in response to heat stress and appears to play an important role in integrating metabolic responses to stress.