Plant Physiology

Water movement and plant transport

Water movement is described using water potential: water moves from higher to lower water potential when a pathway is available. Adding dissolved solute lowers solute potential. Pressure potential can oppose or support movement, and turgor pressure helps keep non-woody tissues firm.

Water enters roots and moves through apoplastic routes along cell walls and symplastic routes through connected cytoplasm. The endodermal Casparian strip restricts uncontrolled apoplastic entry into the vascular cylinder. Xylem carries water and mineral ions; phloem transports organic solutes between sources and sinks.

• Transpiration generates tension that helps draw a continuous water column through xylem. Cohesion among water molecules and adhesion to walls support this column.
• Guard-cell turgor regulates stomatal aperture. Stomata link gas exchange with water loss.
• Phloem transport follows source–sink relationships. A storage organ can act as a sink at one time and a source at another.

Mineral nutrition

Plants need macronutrients in relatively large amounts and micronutrients in smaller amounts. Both groups are essential; the distinction does not describe importance. Nitrogen contributes to amino acids and nucleic acids, magnesium is part of chlorophyll, and phosphorus occurs in ATP, nucleic acids and phospholipids.

A mineral deficiency can affect chlorophyll formation, growth or tissue survival. Symptoms depend partly on nutrient mobility: a mobile nutrient may be moved from older leaves to new growth, so deficiency signs often appear first in older tissues. Diagnose patterns using several features rather than colour alone.

• Biological nitrogen fixation converts atmospheric nitrogen into forms that can enter biological molecules. Nitrogenase is sensitive to oxygen.
• In legume root nodules, leghaemoglobin helps maintain conditions compatible with nitrogen fixation.
• Nitrogen fixation, nitrification and denitrification are different transformations in the nitrogen cycle.

Photosynthetic pigments and the light reactions

Photosynthesis converts light energy into chemical energy. Pigments absorb selected wavelengths and transfer excitation energy to reaction centres. The light-dependent reactions occur in thylakoid membranes. Water supplies electrons to photosystem II, releasing oxygen, while electron transport contributes to a proton gradient.

ATP synthase uses the proton gradient to produce ATP. In non-cyclic electron flow, electrons pass through photosystem II and photosystem I, and NADP⁺ is reduced to NADPH. Cyclic electron flow around photosystem I contributes ATP without net NADPH production or oxygen release.

• The oxygen released by photosynthesis originates from water, not from carbon dioxide.
• Photosystem numbering reflects discovery order; non-cyclic flow begins at photosystem II.

Carbon fixation: C₃ and C₄ pathways

The Calvin cycle takes place in the chloroplast stroma. RuBisCO attaches carbon dioxide to RuBP, producing an unstable intermediate that gives rise to two molecules of 3-phosphoglycerate. ATP and NADPH support reduction reactions, and much of the resulting carbon is used to regenerate RuBP.

C₄ plants first fix inorganic carbon using PEP carboxylase in mesophyll cells. Four-carbon compounds move to bundle-sheath cells, where carbon dioxide is released near RuBisCO. This concentrates carbon dioxide and reduces photorespiration, though the concentrating mechanism requires additional energy.

• Photorespiration begins when RuBisCO acts as an oxygenase. It consumes resources and does not produce a net carbohydrate gain.
• The term “light-independent” does not mean the Calvin cycle normally runs only at night. It depends on products and regulation associated with light reactions.

Follow the energy requirement

For a net gain of one three-carbon triose phosphate through the Calvin cycle, fixation of 3 CO₂ requires 9 ATP and 6 NADPH. Two net triose phosphates provide the carbon equivalent of a hexose. These totals describe the Calvin cycle, not the extra cost of a C₄ concentrating mechanism.

Respiration in plants

Plant cells respire as well as photosynthesise. Glycolysis occurs in the cytosol, splitting glucose into pyruvate with a net gain of ATP and reduced NAD. Under aerobic conditions, pyruvate oxidation and the citric acid cycle occur in the mitochondrial matrix.

The electron transport chain in the inner mitochondrial membrane transfers electrons toward oxygen, the final electron acceptor. Proton pumping creates the gradient used by ATP synthase. Fermentation regenerates NAD⁺ when oxidative pathways cannot meet the need, but it does not supply the large ATP yield associated with aerobic respiration.

• Glycolysis yields a net 2 ATP per glucose by substrate-level phosphorylation.
• Oxidative phosphorylation couples ATP synthesis to an electron-transport-generated gradient.
• Exact total ATP yields depend on shuttle systems, coupling and cellular conditions; avoid applying one fixed total to every cell.

Growth and plant growth regulators

Growth involves irreversible increases in size, often associated with cell division, elongation and differentiation. Meristems retain dividing cells. A sigmoid growth curve includes a slow initial phase, rapid growth and a plateau as constraints increase. Development includes growth, differentiation and maturation.

Plant growth regulators act in interacting networks. Auxins influence cell elongation, apical dominance and rooting; gibberellins promote stem elongation and can help break dormancy. Cytokinins promote cell division and can delay senescence. Abscisic acid contributes to stress responses and dormancy, while ethylene promotes fruit ripening and several senescence responses.

• Responses depend on concentration, tissue, developmental stage and interactions among regulators.
• Photoperiodism is a developmental response to day/night length. In many flowering responses, the uninterrupted dark period is especially important.
• Vernalisation is the promotion of flowering by suitable cold exposure in responsive plants.