Hardiness Strategy

Plant hardiness refers to a plant's ability to endure challenging growing conditions, typically focusing on climatic factors. This includes a plant's capacity to withstand extreme temperatures, drought, flooding, and wind. Hardiness is often associated with a plant’s native geographic location—its longitude, latitude, and elevation—and is commonly represented through hardiness zones.

In temperate regions, hardiness is frequently used to indicate a plant’s resistance to cold, known as 'cold-hardiness', and is measured by the lowest temperatures a plant can tolerate. Plants are generally categorized into three hardiness groups: tender, half-hardy, and hardy. The tolerance of plants to various growing conditions can differ significantly. Selective breeding for varieties that can endure specific climates plays a crucial role in agriculture and horticulture. Plants can also adapt to some degree to changing climates. Nursery growers often engage in cold hardening or "hardening off" processes to prepare plants for the conditions they will encounter in their later environments.

US Hardiness Zones:

Key avoidance strategies

Mesophytes 

Mesophytes are plants adapted to temperate zones that thrive under a closed forest canopy. Many wildflowers and non-woody plants emerge early in the spring before the trees have fully leafed out. This timing takes advantage of the more sheltered ground, reducing frost risk and giving them a head start. Some, like winter annuals, germinate in the fall, grow through winter, and complete their life cycle by spring.

Shade tolerance varies among herbaceous plants. Some can survive after the canopy closes, while others quickly die back. Their survival depends on environmental factors such as water availability and light levels. Tropical vines, like Monstera deliciosa and various Philodendron species, often grow away from light initially. This behavior helps them locate and climb tree trunks to reach brighter light regions.

Xerophytes

Xerophytes are plants adapted to dry conditions, with drought tolerance varying from moderate to extreme desiccation tolerance. These plants utilize specialized photosynthesis processes like C4 carbon fixation or crassulacean acid metabolism (CAM). CAM, for instance, allows carbon dioxide uptake at night, keeping stomata closed during the hot day to minimize water loss.

Structural adaptations for dry conditions include:

• Modified stomata to reduce water loss, such as fewer numbers or waxy surfaces.

• Water storage in succulent tissues or tubers.

• Enhanced root systems for better water absorption.

• Trichomes (small leaf hairs) that capture atmospheric moisture.

Halophytes 

Halophytes are plants that thrive in saline environments, such as saline semi-deserts, mangrove swamps, marshes, and coastal areas. Examples include salt marsh grass (Spartina alterniflora). They are relatively rare, making up only about 2% of plant species, with most plants being "glycophytes," which are more sensitive to salinity.

Salt tolerance in plants is measured by their ability to withstand dissolved salts. Sea water contains about 40 grams per liter of dissolved salts, while glycophytes like beans and rice tolerate 1-3 g/l. In contrast, Salicornia bigelovii (dwarf glasswort) can thrive in up to 70 g/l, making it a potential halophyte crop. Plants like barley (Hordeum vulgare) and date palms (Phoenix dactylifera) tolerate about 5 g/l and are considered marginal halophytes.

Halophyte adaptations can be either salt tolerance or avoidance. Facultative halophytes, for example, may avoid high salt effects by timing their life cycles to low salinity periods. True halophytes may excrete excess salts through leaves or concentrate them in leaves that later fall off.

Carbon gain and water loss

Carbon dioxide (CO₂), essential for photosynthesis, is present in the atmosphere at about 384 ppm. During the day, most plants need their stomata to be open to absorb CO₂. However, this also allows water vapor, which saturates the leaf’s air spaces, to escape. As a result, plants face a trade-off between capturing CO₂ and losing water vapor.

Alternative approaches

Typically, CO₂ is fixed to ribulose-1,5-bisphosphate (RuBP) by the enzyme RuBisCO in mesophyll cells. This process worsens the carbon/water trade-off for two reasons: RuBisCO has a relatively low affinity for CO₂ and also fixes oxygen to RuBP, causing a wasteful process called photorespiration. To mitigate these issues, RuBisCO requires high CO₂ concentrations, leading to larger stomatal openings and more water loss.

Plants have an alternative enzyme, PEP carboxylase, which also fixes CO₂. This enzyme has a higher affinity for CO₂, allowing for effective carbon fixation with smaller stomatal openings and less water loss. However, the products of PEP carboxylase fixation require an energy-intensive conversion process to continue with photosynthesis. Thus, this alternative is beneficial in environments where water is scarce but light is abundant, or where high temperatures increase oxygen’s solubility relative to CO₂, exacerbating RuBisCO’s oxygenation issue.

Crassulacean Acid Metabolism (CAM) Plants

CAM plants, primarily desert-dwelling, adapt by opening their stomata at night to fix CO₂ using PEP carboxylase. They store the fixed carbon in large vacuoles overnight. During the day, they close their stomata and release the stored CO₂ into RuBisCO, minimizing photorespiration. This method is advantageous in extremely arid conditions but is limited by the storage capacity of vacuoles. Most plants lack CAM adaptations and must regulate their stomata throughout the day based on factors such as light intensity, humidity, and CO₂ concentration. The stomatal response involves osmotic regulation:

Opening Stomata

When conditions are favorable, a proton pump moves protons (H⁺) out of guard cells, making their internal electrical potential more negative. This negative potential opens potassium (K⁺) channels, leading to K⁺ uptake. To balance this, negative ions like chloride (Cl⁻) or organic ions such as malate enter the guard cells, increasing osmotic pressure. This influx of water causes the cells to swell, increasing turgor pressure and causing the guard cells to bow apart, thus opening the stomatal pore.

Closing Stomata

In response to water stress, roots release abscisic acid (ABA), which binds to receptor proteins in guard cells. This raises cytosolic pH and increases free calcium ions (Ca²⁺) through external influx and internal release. Increased Ca²⁺ causes chloride (Cl⁻) and other inorganic ions to exit the cells and halts K⁺ uptake, reducing osmotic pressure. This causes the guard cells to become flaccid and close the stomata. Interestingly, guard cells have more chloroplasts than surrounding epidermal cells, though their precise role remains a topic of debate.

Inferring Stomatal Behavior from Gas Exchange

To assess stomatal opening, researchers often measure leaf gas exchange. This involves enclosing a leaf in a sealed chamber and driving air through it. By analyzing the concentrations of carbon dioxide (CO₂) and water vapor before and after the air passes through the chamber, the rates of carbon uptake and water loss by the leaf can be determined.

Since water loss occurs through diffusion, the transpiration rate is influenced by two main factors: the humidity gradient between the leaf's internal air spaces and the external air, and the resistance to diffusion provided by the stomatal pores. By calculating the transpiration rate and measuring the humidity gradient (the difference between the humidity inside the leaf, inferred from leaf temperature assuming the air spaces are saturated, and the ambient humidity), scientists can estimate stomatal resistance. This information helps researchers understand how stomata respond to various environmental conditions, such as light intensity and the concentrations of gases like water vapor, CO₂, and ozone.

The fossil record offers limited insight into the evolution of stomata, but it is believed that they evolved from modifications of conceptacles in ancestral algae-like plants. The development of stomata likely coincided with the evolution of the waxy cuticle, a combination that provided a significant advantage to early terrestrial plants.

Three major types of epidermal cells, all derived from the protodermal cells of the L1 tissue layer of the shoot apical meristem, include trichomes, pavement cells, and guard cells. These cells are arranged non-randomly. Asymmetrical cell division in protodermal cells results in one large cell, destined to become a pavement cell, and a smaller cell called a meristemoid. The meristemoid undergoes one to three additional asymmetrical divisions to become a guard mother cell, which then divides symmetrically to form a pair of guard cells surrounding each stoma.

Stomata were once thought to be entry points for pathogens. However, recent research indicates that stomata can sense the presence of certain pathogens. For example, in experiments with Arabidopsis leaves, virulent bacteria produced a chemical called coronatine, which caused the stomata to reopen within hours.

Adapting to Different pH Levels

Plants exhibit varying tolerances to pH levels, with only a few species thriving across a wide range of acidity. Consequently, plants are often categorized as acidophiles (acid-loving) or acidophobes (acid-fearing). Another related classification is calcicoles, which are "lime-loving" plants that prefer alkaline conditions.

In gardening, soil pH measures acidity or alkalinity, with a pH of 7 indicating neutral soil. Acidophobes prefer soil with a pH above 7. To mitigate acid intolerance in plants, gardeners often add lime, calcium, and nitrogen fertilizers to raise the soil pH.

Acidophilic plants are valuable for monitoring soil and water contamination levels. A decline in acidophilic species can signal increased acid rain in an area. This approach is also applied to monitor aquatic environments.

For example, white worms (Enchytraeus albidus), a common live food for aquariums, are acidophobes. Additionally, acidophobic compounds are those that are unstable in acidic conditions. Some acidophobic crops include alfalfa and clover.

Disease, disorder, and pest tolerance

Effective management of plant diseases and pests is crucial for ensuring reliable food production and significantly reduces the need for agricultural resources such as fuel, land, and water. Plant diseases can have devastating impacts, with recurring severe issues affecting crop yields. On average, diseases reduce plant yields by approximately 10% annually in more developed regions, while in less developed areas, the yield loss can exceed 20%.

Disease resistance in plants arises from both pre-existing defenses and responses activated upon infection, mediated by the plant’s immune system. Disease resistance generally refers to a reduction in pathogen growth on or inside the plant, whereas disease tolerance describes plants that sustain less damage despite similar levels of pathogen presence. The outcome of a disease is determined by the interplay between the pathogen, the plant, and environmental conditions, a concept known as the disease triangle.

Plant defense mechanisms include a range of structures and compounds:

• Structural Defenses: Plant cuticles and cell walls serve as physical barriers.

• Chemical Defenses: Antimicrobial chemicals such as glucosides and saponins, antimicrobial proteins, and enzyme inhibitors help protect against pathogens. Detoxifying enzymes break down toxins produced by pathogens.

• Receptors: These detect pathogens and activate plant defenses.

Inducible defenses, which are activated after infection, include:

• Cell Wall Reinforcement: The production of callose, lignin, suberin, and specialized cell wall proteins.

• Chemical Defenses: Antimicrobial chemicals like reactive oxygen species (e.g., hydrogen peroxide) and complex phytoalexins (e.g., genistein, camalexin).

• Antimicrobial Proteins and Enzymes: Proteins such as defensins and thionins, and enzymes like chitinases, beta-glucanases, and peroxidases.

• Hypersensitive Response: A rapid cell death response at infection sites, associated with resistance genes.

An example of adaptation in lower plants like mosses and ferns involves a genetic program that ensures survival only during the most favorable seasons. This strategy, however, presents challenges in balancing vegetative growth and reproduction within a limited lifespan. Desert-dwelling plants are a prime example of this strategy, adapting to extreme conditions by timing their growth and reproduction to the most suitable periods of the year.

Back to top