Introduction to Biology II - Chapter 36

Introduction to Biology II - Chapter 36: Plant Nutritional Requirements

• Plants acquire mineral nutrients for growth and reproduction.
• Key elements for plant growth include Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), and Sulfur (S).
• Essential nutrients are elements or compounds crucial for normal plant growth and reproduction. Most vascular plants have 17 essential elements. Carbon, Hydrogen, and Oxygen make up 96% of plant dry mass.
• Macronutrients are required in significant quantities. They are major components in nucleic acids, proteins, and phospholipids. Nitrogen, Phosphorus, and Potassium are key macronutrients that act as limiting nutrients.
• Micronutrients are needed in small quantities and often act as cofactors for specific enzymes. They are vital for plant health despite their small requirements.
• Mobile nutrients (like N, K, P) are transferred from older leaves to newer leaves. Their scarcity reflects in the deterioration of older leaves.
• Immobile nutrients (like Ca, Fe) remain tied up in older leaves. Their scarcity is reflected in deficiency symptoms in newer leaves.
• Soil is a dynamic mixture of living and non-living components. Soil formation starts with weathering of solid rock via rain, wind, and water.
• Humus, decomposed organic matter, serves as a nutrient reserve in the soil.
• Soil texture influences root penetration, water and nutrient uptake, structural support, water retention, and oxygen availability. Loam (a balanced mix of sand, silt, clay, and humus) is the optimal soil for plants.
• Mature soil is a complex mixture of organic and inorganic components. Plants absorb water and minerals from upper layers, and the soil contains diverse organisms like plants, worms, fungi, etc. It forms a fragile ecosystem.
• Soil erosion is the removal of soil by wind or water. Sustainable agriculture involves practices to prevent soil erosion like planting trees as windbreaks and following the contour of hillsides with crop planting.
• Anions (negative ions) interact with water via hydrogen bonding. They are readily available, but easily leached from the soil.
• Cations (positive ions) dissolve in water, but are not immediately available. These are electrostatically attached to negatively charged organic matter and clay particles.
• Soil pH influences the availability of essential elements. Acidic soils are common in conifer forest regions. While alkaline soils are rich in limestone (CaCO₃).
• Cation exchange, where protons bind to negatively charged soil particles releasing bound cations like magnesium and calcium, makes nutrients available for plant uptake.
• Most nutrient uptake takes place just above the root tip (zone of maturation) where root hairs increase the surface area for absorption. Root hairs create a region of nutrient depletion surrounding them, promoting further root growth.
• Nutrients pass through plant cell walls freely, but plasma membranes are selectively permeable. Root hairs, with abundant membrane proteins, facilitate the transport of nutrients into root cells.
• Anions are absorbed by co-transporters against electrochemical gradients, while cations are absorbed via channels.
• Mycorrhizae are symbiotic fungi and plant roots that live in physical contact, with a mutualistic relationship. Fungi obtain sugars from plants, and these fungi provide plant nutrients (like nitrogen). Mycorrhizal networks increase the surface area for nutrient absorption significantly.
• Not all ion uptake is beneficial. Natural soils and contaminated soils have other metals like cadmium, zinc, nickel, lead, or sodium that damage plants. Sodium poisoning is particularly common in environments like ocean coastlines, areas treated with de-icing salts, and irrigated farmlands.
• Plants use two methods for ion exclusion: passive exclusion (root cells may lack transporters for toxic ions) and active exclusion (mechanisms like metallothioneins and phytochelatins binding to metal ions and preventing them from causing damage).
• Toxins that enter plant roots often encounter the Casparian strip in the endodermis, blocking the apoplastic pathway. The symplastic pathway is used instead.
• Plant cells use the tonoplast for active exclusion via antiporters in the tonoplast membrane, removing toxic substances from the cytosol and storing them in cell vacuoles.
• Nitrogen gas (N₂) makes up 80% of the atmosphere but is unusable by plants directly. Nitrogen fixation involves bacteria (and some archaea) converting N₂ into forms like ammonium (NH₄⁺), nitrites (NO₂⁻), or nitrates (NO₃⁻) that plants can use.
• Nitrogen-fixing bacteria form nodules in the roots of legumes, where they fix nitrogen.
• Autotrophs like plants are photosynthetic, synthesizing their own sugar from carbon dioxide. Some plants obtain nutrients through other methods, like parasitism (obtaining water and nutrients from other plants), epiphytism (inhabiting trees and absorbing water/nutrients from rain, dust, or air particles), or carnivory (trapping and digesting insects for nitrogen) to supplement their nutritional requirements.