Unit 8: Plant Physiology — Interactive Notes
All autotrophic organisms need CO₂ and water, supplying carbon, oxygen and hydrogen — predominant nutrients for synthesizing organic molecules. ★ Fertilizers supply macro/micronutrients to crops.
Macronutrients — needed in relatively larger amounts. There are nine: carbon, hydrogen, oxygen, nitrogen, potassium, calcium, phosphorus, magnesium and sulphur.
| Nutrient | Role | Deficiency Effect |
|---|---|---|
| Nitrogen | Energy metabolism, protein production | Leaf loss, stunted growth |
| Phosphorus | Part of ATP; root growth, flowering | Delayed flowering, browning/wrinkling of leaves |
| Potassium | Water regulation, transport, photosynthesis | Dark patches on leaves |
| Calcium | Cell wall stability, seed development | Yellow/brown patches on leaves |
| Magnesium | Core of chlorophyll, phosphorus transport | Weak stalks, yellow/brown spots, loss of greenness |
| Sulfur | Nitrogen metabolism | Lighter plant colour |
Micronutrients are needed in very small amounts. Seven: iron, manganese, zinc, molybdenum, copper, chlorine and boron.
| Nutrient | Role | Deficiency Effect |
|---|---|---|
| Iron | Chlorophyll synthesis, enzyme cofactor | Interveinal chlorosis |
| Manganese | Photosynthesis, N/carbohydrate metabolism, antioxidant enzymes (SOD) | Premature leaf fall, delayed maturity |
| Zinc | Chlorophyll synthesis, root development | Stunted growth |
| Molybdenum | N fixation/reduction, S/P metabolism, Fe utilization | Chlorosis of older leaves, stunted growth |
| Copper | Lignin synthesis, N metabolism, enzyme cofactor | Chlorosis, twisted leaves, stunted growth |
| Chlorine | Stomatal regulation, osmotic adjustment | Affects overall health/growth |
Insectivorous plants supplement organic molecules from captured insects (nitrogenous compounds benefit them) while remaining true autotrophs. Enzymes released by leaves break down captured insects.
Pitcher plant (Sarracenia purpurea): leaves modified into a sac/pitcher, partly filled with water; hooded terminal portion; stiff hairs prevent insect escape.
Venus-fly trap (Dionaea muscipula): two hinged lobes; inner trichomes trigger rapid closure on contact with prey; interlocking bristles prevent escape; enzymes digest trapped insects.
Sundew (Drosera intermedia): catches prey with shiny "dew" drops on tentacle-like hairs; insects attracted by odour are trapped and digested.
Stomatal opening is regulated by guard cells. At night, stomata close to conserve water (respiration continues: O₂ in, CO₂ out).
Multisensory Hydraulic Valves: guard cells open/close in response to light, CO₂ and humidity.
Starch Sugar Hypothesis (H. Van Mohl, 1856): guard cells perform photosynthesis, producing sugars during the day — sugar increases, water potential drops, water moves in, guard cells become turgid, stomata open. At night, sugars convert to starch (or are respired), water moves out, guard cells become flaccid, stomata close. ★ This doesn't fully explain rapid turgor changes.
Influx of K⁺ ion: active transport of K⁺ into guard cells reduces osmotic potential, water enters via osmosis, guard cells become turgid, stomata open.
Role of blue light: enhances this by acidifying the surrounding environment, promoting K⁺ uptake. At night, K⁺ passively diffuses out, guard cells become flaccid, stomata close.
★ Hormonal involvement: at high temperature, wilting leaf cells release abscisic acid, which stops active K⁺ transport into guard cells — overriding light/CO₂ effects, stopping K⁺ pumping and closing stomata.
Palisade tissue is located just beneath the upper epidermis — elongated, tightly packed, chlorophyll-rich cells, organized to maximize light absorption.
CO₂ diffuses through stomata, through spongy mesophyll air spaces, into palisade cells for photosynthesis. O₂ produced diffuses back out through spongy mesophyll and stomata.
1. Parenchyma: cells of epidermis, cortex, pith absorb water, creating turgor pressure (from elevated osmotic pressure in the vacuole, via the tonoplast) that maintains cell rigidity.
2. Collenchyma: found beneath epidermis of young stems/leaf stalks/veins; lack secondary walls; primary walls thickened at corners (extra cellulose); elongate with growth, support young parts before secondary growth.
3. Sclerenchyma: thick, lignin-saturated secondary cell walls; mostly non-living; provides structural support.
Types of sclerenchyma: (i) Fibres (Tracheids) — elongated, cylindrical, in bundles/bundle caps. (ii) Sclereids — smaller, in seed coats/nut shells, protective. (iii) Vessels (Tracheae) — long tubular water-conducting structures in xylem.
Water potential (Ψw) reflects the kinetic energy of water molecules in a system — pure water has maximum Ψw (defined as zero). Water moves from higher to lower Ψw (osmosis = movement across a partially permeable membrane along this gradient).
1. Solute Potential (Ψs): the change in Ψw due to solute molecules — always negative; more solute = more negative Ψs.
2. Pressure Potential (Ψp): due to pressure exerted by water — increases when pressure greater than atmospheric is applied; builds up as water enters cells by osmosis, causing turgidity.
Total water potential: Ψw = Ψs + Ψp. The steeper the potential gradient, the faster the water flow.
Roots provide large surface area via extensive branching and root hairs (epidermal cell extensions) for water/mineral uptake. Water and minerals enter root epidermal cells (active/passive transport), move to cortex, then into xylem via three pathways:
1. Apoplast Pathway: continuous system of adjacent cell walls; becomes discontinuous at the endodermis due to Casparian strips.
2. Symplast Pathway: through interconnected protoplasts via plasmodesmata; less important except for minerals near the endodermis.
3. Vacuolar Pathway: through cell membranes, cytoplasm, tonoplast and vacuoles; movement is negligible.
Xylem carries water and dissolved minerals (sap) from roots to stem/leaves and provides mechanical support.
a. Tracheids: elongated, thin, thick lignified walls, tapered ends, linked by bordered pits (lateral water movement).
b. Vessels: shorter, broader, linear arrangement forming continuous channels; perforation plates enable efficient water movement.
c. Xylem fibres: elongated, thickened lignified walls, dead at maturity, add structural integrity.
d. Xylem parenchyma: living, thin-walled, retain nutrients/water, participate in lateral translocation and xylem healing.
TACT (Transpiration, Adhesion, Cohesion, Tension) — also called the cohesion-tension theory — explains how water moves against gravity through xylem.
1. Transpiration: water evaporation from leaves via stomata creates negative pressure, pulling water upward — the primary driving force.
2. Adhesion: attraction between water molecules and xylem walls — prevents breaks in the water column (especially in tall plants).
3. Cohesion: attraction between water molecules (H-bonding) — forms an unbroken column from roots to leaves, extending the transpirational pull.
4. Tension: negative pressure from transpiration's pull — vital for continuous ascent of water.
Phloem transports organic nutrients (products of photosynthesis) from leaves to other plant parts — found on the outer side of vascular tissue, constituting the inner bark.
1. Sieve Tube Elements: transport sugars/organic material; 'sieve areas' interconnect elements, forming a sieve tube.
2. Companion Cells: associated with sieve tube elements via plasmodesmata; supply ATP/proteins (sieve elements lack a nucleus).
3. Phloem Parenchyma: stores sugars, resins, latex, mucilage — plant defence and moisture retention.
4. Phloem Fibres: specialized sclerenchyma, elongated spindle-shaped, provide mechanical support.
Diffusion is too slow (1m/8 years) to explain phloem sugar movement (~1m/hour). The pressure-flow theory (mass-flow hypothesis), proposed by Ernst Munch in 1930, is the most widely accepted — relying on osmotic pressure differences between source and sink.
Steps: 1. Excess glucose (from photosynthesis) converts to sucrose. 2. Sucrose is actively transported to companion cells, then diffuses into sieve tubes, raising sucrose concentration there. 3. Water moves into sieve tubes by osmosis (from nearby xylem), increasing water potential at the source. 4. At the sink, sugar is actively unloaded, water follows, lowering water potential there. 5. The water potential difference drives flow from source to sink, carrying sucrose along.
Growth is a permanent increase in size (height, width, mass). In higher plants, growth is limited to growing points — groups of cells called meristems, capable of continuous division.
1. Apical meristems: at tips of roots/shoots; perpetual growth zones; responsible for primary growth (increase in cell number).
2. Intercalary meristems: separated from the apex by permanent tissue; at bases of internodes (e.g., grasses); role in leaf/flower production; temporary.
3. Lateral meristems: cylinders of dividing cells along peripheral regions; responsible for growth in thickness (secondary growth) in woody plants — vascular cambium (produces secondary xylem/phloem) and cork cambium (produces cork/bark).
Primary growth increases plant length, facilitated by apical meristems (more prominent in herbaceous plants).
1. Cell Division: at apical meristem tips (zone of cell division) — cells non-vacuolated, small, with central spherical nuclei.
2. Cell Elongation: new cells increase volume via water uptake (zone of cell elongation) — cells vacuolated, large, nuclei peripheral; different cells elongate to different final sizes (e.g., xylem cells elongate more lengthwise).
3. Cell Differentiation: elongation stops, cells specialize, walls thicken (zone of cell differentiation) — each cell type performs a specific function.
Secondary growth is the increase in thickness/girth of stems and roots, due to vascular cambium and cork cambium activity — more prominent in woody perennial plants.
Vascular cambium cells divide, producing secondary phloem (outer margin) and secondary xylem (inner margin) — secondary xylem drives thickness increase. Cork cambium division produces new cork on both sides. Bark = region outside vascular cambium (secondary phloem, cork cambium, cork).
Spring: cambium highly active, wide-vessel light spring wood. Autumn: cambium activity decreases, narrower/denser dark autumn wood. Together they form one annual ring — used in dendrochronology to determine tree age and study climate variability.
Plant growth regulators (hormones): five major groups — auxins, cytokinins, gibberellins, abscisic acid, ethylene.
| Auxins (IAA) | Gibberellins |
|---|---|
| Promote cell division in cambium | Produced in apical portions of roots/shoots |
| In roots: promote growth at low conc., inhibit at high conc. (e.g. geotropism) | Promote cell enlargement (with auxins), cell division in apical meristem/cambium |
| Promote bud initiation in shoots (sometimes antagonistic to cytokinins) | Promote 'bolting' of rosette plants, bud initiation |
| Promote apical dominance and fruit growth; may induce parthenocarpy | Substitute for red light — promote flowering in long-day plants, inhibit in short-day |
| Delay leaf senescence; inhibit abscission | Break bud/seed dormancy; delay senescence |
Produced in roots, young fruits, seeds. Promote cytokinesis during cell division; increase DNA replication and RNA/protein synthesis rates.
Functions: promote stem growth (cell division in apical meristem/cambium); inhibit primary root growth; promote lateral root growth; promote bud initiation and leaf growth; promote fruit growth (rarely parthenocarpy); promote lateral bud growth, break bud dormancy; delay leaf senescence; promote stomatal opening.
Abscisic acid (ABA): synthesized mainly in mature green leaves, fruits, root caps.
• Inhibits stem/root growth during stress (drought, waterlogging).
• Promotes bud/seed dormancy.
• Promotes flowering in short-day plants, inhibits in long-day plants (antagonistic to gibberellins).
• Promotes leaf senescence and abscission.
• Promotes stomatal closing under water stress.
Ethylene: a natural metabolic product.
• Inhibits stem/root growth during stress.
• Breaks bud dormancy.
• Promotes flowering in pineapple.
• Promotes fruit ripening.
Osmoregulation: maintaining stable internal water/solute equilibrium regardless of environment. Marine organisms (osmoconformers) match seawater's osmotic pressure; others (osmoregulators) actively regulate.
1. Hypotonic: low external solute — water enters, cell swells (turgid).
2. Hypertonic: high external solute — water exits, cell shrinks (plasmolysis).
3. Isotonic: equal solute concentration — no net water movement, no effect on cell size/shape.
Hydrophytes — adapted to aquatic environments:
• Marine hydrophytes: thrive in saline (hypertonic) conditions; excrete excess salts via salt glands, synthesize organic solutes (proline, glycine, betaine, sugars) to retain water; thick cuticles; e.g. seaweeds, seagrass, mangroves.
• Freshwater hydrophytes: grow in hypotonic environments; expel excess water via hydathodes/vacuoles; actively absorb K⁺/Ca²⁺ for osmotic balance; thin/absent cuticles, reduced root systems; e.g. water lilies, lotus, tape grass.
Mesophytes: live in moderate environments (not too dry/wet); well-developed roots/shoots, full vascular systems; no special adaptations needed; flat, broad, green leaves with surface stomata; e.g. rose, tomatoes, daisies.
Xerophytes: minimize water loss and store water (succulents have fleshy water-storing stems); waxy leaf coatings, leaf dropping in dry periods, leaf folding/repositioning. Examples: thorn trees, desert marigold, cacti, blue agave.
Halophytes: inhabit saline soil (NaCl, MgCl₂, MgSO₄); succulent leaves/stems, sometimes spine-modified leaves. Examples: sea arrowgrass, sea lavender.
Thermoregulation: maintaining internal temperature despite environmental variation.
1. High Temperature: denatures enzymes, damages metabolism. Plants use evaporative cooling (hot/dry weather causes stomatal closure from water deficiency). Plants facing 40°C+ synthesize heat-shock proteins that protect enzymes/proteins from denaturation.
2. Low Temperature:
Non-freezing: alters membrane fluidity (lipids lock into crystalline structures, affecting solute transport and membrane proteins). Adaptation: increasing unsaturated fatty acid proportion (maintains membrane structure, prevents crystal formation — requires time, so rapid chilling is more stressful than gradual cooling).
Freezing: causes ice crystal formation (confined to cell wall = survivable; within protoplasm = lethal, perforating membranes/organelles). Adaptation: cold-region plants (oaks, maples, roses) change solute composition, causing cytosol to super-cool without ice formation.
Tropic movements (tropisms): growth movements triggered by external stimuli, occurring as curvature of a whole organ towards/away from the stimulus.
1. Phototropism: response to light; shoot tips show positive phototropism, roots negative.
2. Geotropism: response to gravity; roots positive, shoots negative.
3. Thigmotropism: response to touch (e.g. climbing vines coiling around a support).
4. Chemotropism: response to chemicals (e.g. pollen tube growth toward the ovule via stigma's sugary environment).
Photoperiodism: the response to changes in day length enabling plants to adapt to seasonal changes.
Garner and Allard (1920) found tobacco plants flower only after exposure to a series of short days — naturally in autumn, but inducible artificially.
Classification (later refined to dark period length):
| Type | Requirement | Examples |
|---|---|---|
| Short-day plants (SDPs) | Dark period longer than critical length e.g. cocklebur 8.5h | Cocklebur, chrysanthemum, soyabean, tobacco |
| Long-day plants (LDPs) | Dark period shorter than critical length e.g. henbane 13h | Henbane, snapdragon, cabbage, spring wheat, spring barley |
| Day-neutral plants (DNPs) | Flowering independent of photoperiod | Cucumber, tomato, garden pea, maize, cotton |
Phytochrome, a photoreceptor protein, exists in two forms: P660 (quiescent — absorbs red light 660nm, converts to P730) and P730 (active — absorbs far-red light 730nm, converts to P660).
P660→P730 occurs in daylight; P730→P660 occurs in the dark. The conversion rate provides a "clock" for measuring darkness duration. Red light inhibits flowering in short-day plants but promotes it in long-day plants. Since P730 accumulates in day and diminishes at night, SDPs can only flower if the night is long enough for P730 to be sufficiently inactivated. ★ Now it's agreed this interconversion isn't the sole time-measuring mechanism.
Florigen: once the biological clock is stimulated, florigen hormone is produced in leaves, travels through phloem to floral buds, initiating flowering.
Vernalisation: induction of flowering by exposure to low (chilling) temperature — effective in biennial/perennial plants. The stimulus is received by the shoot apex (mature stem or seed embryo), not the leaves (unlike photoperiodism). Duration: 4 days to 3 months (~4°C most effective).
Significance: stimulates production of hormone "vernalin". Photoperiodism and vernalisation synchronize reproductive behaviour with the environment (favourable-time reproduction) and ensure same-species members flower together, encouraging cross-pollination for genetic variability.
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