Learn Unit 4: Plant Physiology with Free Lessons & Tips

The five main groups of natural plant growth regulators are:

1. Auxins
2. Gibberellins
3. Cytokinins
4. Abscisic acid (ABA)
5. Ethylene

I'll focus on Auxins for further elaboration:

Auxins:

Discovery: The discovery of auxins dates back to the early 20th century, primarily credited to the experiments conducted by Charles Darwin and his son Francis. However, it was the work of Dutch scientist Frits Warmolt Went that provided substantial evidence for the existence and role of auxins. Went demonstrated that the bending of coleoptiles (the protective sheath covering the emerging shoot) in response to light was due to the migration of a growth-promoting substance from the tip of the coleoptile.

Physiological Functions: Auxins play crucial roles in various aspects of plant growth and development. Some of their key functions include:

• Promotion of cell elongation, particularly in stems and coleoptiles, which aids in tropisms such as phototropism and gravitropism.
• Inhibition of lateral bud growth, thereby promoting apical dominance.
• Stimulation of root initiation in stem cuttings and adventitious root formation.
• Regulation of vascular tissue differentiation.
• Fruit development and ripening.

Agricultural/Horticultural Applications: Auxins find extensive applications in agriculture and horticulture:

• Rooting Hormones: Synthetic auxins such as indole-3-butyric acid (IBA) and naphthaleneacetic acid (NAA) are commonly used as rooting hormones to promote root formation in cuttings. This application aids in vegetative propagation, allowing for the efficient cloning of desirable plant varieties.
• Weed Control: Synthetic auxins like 2,4-Dichlorophenoxyacetic acid (2,4-D) are used as herbicides to control broadleaf weeds in crops like cereals and turf grasses. These herbicides disrupt normal plant growth processes, leading to uncontrolled growth and eventual death of the target plants.
• Fruit Development: Auxin sprays are sometimes used to promote fruit set and development, particularly in seedless fruit varieties. By influencing fruit growth and development, auxins can enhance crop yields and quality.

In conclusion, auxins are fundamental plant growth regulators with diverse physiological functions and wide-ranging applications in agriculture and horticulture. Their discovery revolutionized our understanding of plant growth processes and continues to be instrumental in modern plant science and crop management practices.

Abscisic acid (ABA) is often referred to as the "stress hormone" in plants due to its critical role in mediating responses to various environmental stresses. Several factors contribute to this characterization:

1. Drought Stress: ABA levels in plants increase significantly in response to drought stress. It helps plants cope with water scarcity by promoting stomatal closure, reducing water loss through transpiration, and maintaining cellular water balance. Stomatal closure induced by ABA conserves water but also reduces CO2 uptake for photosynthesis, which is a trade-off mechanism to balance water conservation with the need for carbon assimilation.

2. Salt Stress: High soil salinity can disrupt the osmotic balance of plant cells, leading to water loss and cellular dehydration. ABA helps plants respond to salt stress by regulating ion uptake and transport, promoting ion homeostasis, and minimizing water loss through stomatal closure.

3. Cold Stress: Exposure to low temperatures can cause cellular damage and impair physiological processes in plants. ABA accumulation in response to cold stress helps plants acclimate to cold conditions by inducing the expression of genes involved in cold tolerance, adjusting membrane fluidity, and enhancing antioxidant defense mechanisms.

4. Heat Stress: High temperatures can also pose significant challenges to plant growth and survival. ABA plays a role in heat stress responses by regulating stomatal closure, preventing excessive water loss, and modulating the expression of heat shock proteins and other stress-responsive genes.

5. Oxidative Stress: Environmental factors such as pollutants, pathogens, and excess light can generate reactive oxygen species (ROS) in plant cells, causing oxidative damage to biomolecules. ABA helps plants mitigate oxidative stress by regulating antioxidant enzyme activities, scavenging ROS, and modulating the expression of stress-responsive genes involved in antioxidant defense pathways.

Overall, abscisic acid acts as a signaling molecule that integrates environmental cues and coordinates adaptive responses to various stresses encountered by plants. Its role in mediating stress responses has earned it the designation of "stress hormone" in the context of plant physiology.

The statement "Both growth and differentiation in higher plants are open" highlights two fundamental concepts in plant development: growth and differentiation. Let's break down each of these terms and discuss why they are considered "open" in higher plants:

1. Growth: Growth in plants refers to the increase in size or mass of plant tissues and organs over time. This process involves cell division, cell enlargement (cell elongation), and cell differentiation. Growth is considered "open" in higher plants because:

   • It occurs throughout the life of the plant: Unlike in animals where growth typically ceases after reaching a certain stage of development, plants exhibit indeterminate growth, meaning they continue to grow throughout their lifespan. Meristematic tissues, located at the tips of shoots and roots, continuously divide to produce new cells, allowing for ongoing growth and development.
   • It is influenced by environmental factors: Environmental conditions such as light, temperature, water availability, and nutrient availability can profoundly affect plant growth rates and patterns. Plants exhibit plasticity in their growth responses, adjusting their growth rates and directions in response to environmental cues. This ability to modulate growth in response to environmental signals reflects the "open" nature of growth in higher plants.

2. Differentiation: Differentiation refers to the process by which cells become specialized to perform specific functions within the organism. In plants, differentiation involves the transformation of meristematic cells into various specialized cell types, tissues, and organs. Differentiation is considered "open" in higher plants because:

   • It is flexible and reversible: Unlike in animals where cell differentiation is often irreversible, plant cells retain a degree of flexibility even after they have undergone differentiation. Under certain conditions, plant cells can dedifferentiate and revert to a meristematic state, allowing for tissue regeneration and wound healing. This plasticity in cell fate reflects the "open" nature of differentiation in higher plants.
   • It is influenced by external signals: Environmental cues such as hormones, light, and mechanical stimuli can influence the direction and extent of cell differentiation in plants. For example, the differential distribution of auxin hormone can trigger cell elongation in specific regions of the plant, leading to the formation of specialized tissues such as vascular bundles. The responsiveness of plant cells to external signals highlights the dynamic and adaptable nature of differentiation in higher plants.

In summary, both growth and differentiation in higher plants are considered "open" processes because they exhibit ongoing plasticity, responsiveness to environmental cues, and the potential for modulation and reversal. This flexibility allows plants to adapt to changing environmental conditions and optimize their growth and development throughout their lifespan.

The statement "Both a short-day plant and a long-day plant can produce flowers simultaneously in a given place" might seem counterintuitive at first, given that short-day (SDP) and long-day plants (LDP) typically have different requirements for flowering initiation based on day length. However, several factors can contribute to situations where both types of plants flower simultaneously in the same location:

1. Intermediate Day Lengths: In regions where day length fluctuates around the critical photoperiod for both short-day and long-day plants, there may be periods when the day length falls within the range suitable for flowering initiation in both types of plants. For example, in transitional seasons such as spring and autumn, day length may be intermediate between the critical thresholds for SDP and LDP, allowing both types of plants to initiate flowering concurrently.

2. Cultural Practices: Agricultural and horticultural practices, such as artificial lighting or light deprivation techniques, can manipulate day length to induce flowering in plants. In controlled environments such as greenhouses or growth chambers, it is possible to provide conditions conducive to flowering initiation for both short-day and long-day plants simultaneously, irrespective of natural day length conditions outside.

3. Genetic Variation: Within plant species, there may be genetic variation in flowering responses to day length. Some individuals or cultivars within a species may exhibit more flexibility in their flowering responses and be capable of flowering under a wider range of day lengths. Such genotypic variation can allow for the simultaneous flowering of short-day and long-day plants in the same location.

4. Environmental Cues Beyond Day Length: While day length is a primary determinant of flowering in many plants, other environmental factors such as temperature, light quality, humidity, and soil moisture also play roles in flowering initiation and development. If these additional environmental cues align favorably for both short-day and long-day plants, it can lead to synchronous flowering despite differences in day length requirements.

5. Physiological Plasticity: Plants possess a degree of plasticity in their flowering responses, allowing them to adjust their developmental processes in response to environmental cues. Under certain conditions, short-day plants may exhibit some degree of flowering under long-day conditions, and vice versa. This flexibility can contribute to the simultaneous flowering of short-day and long-day plants in specific circumstances.

In summary, while short-day and long-day plants typically have distinct day length requirements for flowering initiation, various factors such as intermediate day lengths, cultural practices, genetic variation, additional environmental cues, and physiological plasticity can contribute to situations where both types of plants produce flowers simultaneously in a given location.

To address each of the given scenarios, specific plant growth regulators can be utilized based on their physiological effects:

(a) Inducing rooting in a twig: To stimulate root formation in a twig, one would typically use auxins, particularly synthetic auxins such as indole-3-butyric acid (IBA) or naphthaleneacetic acid (NAA). These auxins promote adventitious root formation and enhance the rooting process in cuttings.

(b) Quickly ripening a fruit: Ethylene is the plant growth regulator commonly used to hasten fruit ripening. Applying ethylene gas or ethylene-releasing compounds can accelerate the ripening process by promoting the conversion of starches to sugars, softening of fruit tissues, and development of characteristic color and aroma.

(c) Delaying leaf senescence: Abscisic acid (ABA) is involved in regulating senescence processes in plants. While ABA typically promotes senescence, its precise role can vary depending on the plant species and specific physiological context. Manipulating ABA levels or applying ABA analogs may help delay leaf senescence in certain situations.

(d) Inducing growth in axillary buds: Cytokinins are plant growth regulators known for their role in promoting cell division and shoot growth. Applying cytokinins to axillary buds can stimulate their growth and branching, leading to the development of lateral shoots and branches.

(e) 'Bolting' a rosette plant: Bolting refers to the rapid elongation of the flowering stem in certain rosette plants, often in response to environmental cues such as changes in temperature or day length. Gibberellins are the plant growth regulators typically associated with promoting stem elongation and flowering. Applying gibberellins can induce bolting in rosette plants.

(f) Inducing immediate stomatal closure in leaves: Abscisic acid (ABA) is the primary plant growth regulator responsible for inducing stomatal closure in leaves. ABA levels increase in response to environmental stresses such as drought, triggering the closure of stomata to reduce water loss through transpiration. Applying ABA or ABA analogs can induce immediate stomatal closure in leaves, helping plants conserve water during periods of water stress.

In summary, the selection of plant growth regulators for specific applications depends on their physiological effects and desired outcomes in manipulating plant growth and development processes.

A defoliated plant, which has had its leaves removed, would still respond to the photoperiodic cycle. This is because the ability to perceive changes in day length is not solely dependent on the presence of leaves, but rather on specialized light-sensitive structures within the plant called photoreceptors, primarily phytochromes.

Phytochromes are photoreceptor proteins that detect changes in the quality and duration of light, including variations in day length. They are primarily located in the meristematic tissues of the plant, such as the shoot apical meristem, where they play a key role in regulating flowering and other developmental processes in response to photoperiodic cues.

Even in the absence of leaves, phytochromes in the remaining stem tissue or other parts of the plant are still capable of perceiving changes in day length. This information is then transmitted to the plant's internal signaling pathways, triggering appropriate physiological responses such as flowering initiation, dormancy induction, or growth regulation.

Therefore, while the absence of leaves may affect certain aspects of the plant's response to the photoperiodic cycle, such as photosynthesis and hormone production, it would not entirely negate the plant's ability to detect and respond to changes in day length through the activity of photoreceptors like phytochromes.