Plant Nutrition: Process, Phases, and Photosynthesis

Posted on Dec 5, 2024 in Biology

Plant Nutrition

Nutrition is the process that plants use to manufacture their own organic matter and perform other vital functions. To carry out this process, they need solar energy and other essential elements; 15 are from the land and 1 from the atmosphere.

Phase 1: Absorption

The first phase of nutrition is the absorption of water and mineral salts into the xylem vessels. The water and dissolved minerals are absorbed by the root hairs, across the different tissues, down to the wood vessels. This process can be accomplished in two ways:

  1. The water enters by osmosis from the root, and mineral salts by active transport through a protein called carriers or transporters.
  2. The water and mineral salts penetrate the intercellular spaces of the cortex to reach the endodermis, which has no intercellular spaces as it is occupied by a substance called suberin, forming the Casparian strip. From here, they penetrate the xylem.

Phase 2: Transport

Transport of Water and Minerals into the Xylem

Water and mineral salts (crude sap) reach the plant’s leaves, and this transport occurs without energy expenditure since they perform two processes:

  1. Root Pressure Theory: It states that water and dissolved minerals are pushed up by positive pressure at the root, where there is more pressure than in leaves. Proof of this is guttation, the presence of droplets on the edge of the leaves.
  2. Cohesion-Tension Theory: Transpiration is caused by water loss from the leaves. The water vapor leaves through the stomata, causing the suction of individual water molecules through hydrogen bonds, forming a “chain.” Capillary action is also involved in this process. The salts are dissolved in water, and therefore their transport is passive.

Phase 3: Gas Exchange

This phase takes place at the stomata and lenticels.

Stomata

Stomata are the result of two kidney-shaped cells with a hole in the center called the ostiole. There are stomata on both the upper and lower surfaces of the leaves, being more abundant on the underside.

CO2 enters the intercellular spaces, through the cells, and reaches the chloroplasts. O2 goes the opposite way. Not all the CO2 for photosynthesis comes from the atmosphere; some is generated in cellular respiration. Not all the oxygen used in cellular respiration and photosynthesis comes from that process; some comes from the atmosphere.

The stomata open and close based on the plant’s needs. If CO2 is needed, they open. If the plant is losing water, they close, a process involving a hormone called abscisic acid (ABA). Light and temperature also influence stomata opening and closing. Light activates photosynthesis and therefore consumes CO2, increasing stomatal opening. Higher temperatures stimulate cellular respiration, increasing CO2 concentration, causing the stomata to close, thus avoiding water loss by transpiration.

Lenticels

Lenticels are protuberances occurring in the epidermis of woody stems and branches. They are composed of cells that are separated from each other, creating large intercellular spaces.

Phase 4: Photosynthesis

Photosynthesis is the process by which plants convert inorganic materials into organic matter using solar energy. It takes place in chloroplasts, and the reaction is:

6CO2 + 6H2O → C6H12O6 + 6O2

This process is conducted in two phases: the light-dependent phase and the light-independent (dark) phase.

Light-Dependent Phase

Occurs in the thylakoid membrane of chloroplasts. Solar energy strikes the chlorophyll molecules in photosystems I and II. Photosystem I includes chlorophyll, electron transport chain enzymes, and ATP synthases.

The electrons in the chlorophyll are excited by solar energy to a higher energy level. These electrons are captured by chemical elements and transferred from one element to another, forming the electron transport chain. As electrons pass from one element to another, energy is released and used by ATP synthase to make ATP. The final electron acceptor is NADP+, which becomes NADPH + H+.

In photosynthesis, water provides the electrons, protons (to NADP+), and oxygen that is released into the atmosphere.

Light-Independent Phase (Dark Phase)

This phase does not directly require solar energy. It occurs in the stroma of chloroplasts and consists of a series of metabolic reactions known as the Calvin cycle. In this process, the energy from ATP and NADPH formed in the light-dependent phase is used. For each molecule of C6H12O6, 18 ATP and 12 NADPH are needed.

The incorporation of CO2 into the plant requires the presence of a protein called RuBisCO, the most abundant protein on Earth.

Depending on the first organic compound produced, plants are divided into C3, C4, and CAM categories. 89% of plants are C3, 10% are CAM, and 1% are C4. C3 plants include arboreal species like wheat and barley. C4 plants include corn and sugarcane, and CAM plants include cacti. This classification is based on climate, with C3 plants in more benign climates and CAM plants in deserts.