Plant Physiological Processes, Propagation and Crop Management

Physiological Processes

• Photosynthesis: The process of converting light energy into sugars.

  • Equation: $6CO_2 + 12H_2O \xrightarrow{\text{chlorophyll, light}} C_6H_{12}O_6 + 6O_2 + 6H_2O$
  • Organs: Leaf (main organ), chloroplastid (organelle containing thylakoids/grana and stroma).
  • Component Reactions: Light reaction (photochemical; producing $ATP$ and $NADPH_2$) and Dark (Blackman) reaction (thermochemical $CO_2$ fixation).
  • $CO_2$ Fixation Pathways:
    1. $C_3$ (Calvin-Benson): Ribulose biphosphate ($RuBP$ or $RuDP$) is the acceptor; 3-phosphoglyceric acid ($PGA$) is the first stable product.
    2. $C_4$ (Hatch-Slack): Phosphoenolpyruvic acid ($PEP$) is the acceptor; product is oxaloacetic acid (converted to malic/aspartic acids).
    3. CAM (Crassulacean acid metabolism): $CO_2$ fixed in the dark as malic acid; decarboxylated during the light period for the $C_3$ pathway.

• Respiration (Biological Oxidation): Transformation of organic substrates to release energy ($ATP$).

  • Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + E$
  • Sequences: Glycolysis, Krebs cycle, and Electron Transport System ($ETS$). Mitochondrial respiration includes Krebs and $ETS$.

• Transpiration: Loss of water vapor via hydathodes (Guttation), cuticle, lenticels, or stomata (accounting for $90\%$ of loss).

• Translocation: Uptake and transport of raw materials and photosynthates.

  • Tissues: Xylem (upward solution movement); Phloem (sucrose movement).
  • Mechanisms: Diffusion, cytoplasmic streaming, and mass/bulk flow.

• Mineral Nutrition:

  • Criteria for Essentiality: Proposed by Daniel Arnon and Perry Stout (1939).
  • Macronutrients: $N$, $P$, $K$, $S$, $Ca$, $Mg$.
  • Micronutrients: $Fe$, $Mn$, $Zn$, $Cu$, $Mo$, $B$, $Cl$.
  • Beneficial Nutrients: $Co$, $Al$, $Ni$, $Na$, $Si$, $I$.

Growth and Development

• Definitions:

  • Development: Growth, differentiation, and organization involving morphogenesis.
  • Growth: Irreversible change in size, weight, or mass.
  • Differentiation: Result of selective gene action.

• Phytohormones:

  • Auxin ($IAA$): Promotes cell elongation, apical dominance, and root initiation. Synthesized from tryptophan.
  • Gibberellins ($GA_1$, $GA_3$): Regulate stem growth, bolting, and seed germination. Synthesized from mevalonic acid.
  • Cytokinins (CKs/Zeatin): Induce cell division and delay leaf senescence. Synthesized from adenine.
  • Ethylene: Gas synthesized from methionine; regulates fruit ripening and senescence.
  • Abscisic acid ($ABA$): Induces stomatal closure and promotes seed storage protein synthesis; counteracts $GA$.

• Plant Movements:

  • Tropisms: Directional responses to stimuli (e.g., phototropism, geotropism, solar tracking).
  • Nastic Movements: Non-directional responses (e.g., nyctinasty, hydronasty, thigmotropism).

Plant Propagation

• Sexual Propagation: Reproduction via seeds (spermatophytes).

  • Seed Classes: Breeder, Foundation (white tag), Registered (green tag), and Certified (blue tag).
  • Germination Factors: Adequate water, temperature ($15^\circ C - 30^\circ C$), gases ($20\%\,O_2$, $0.03\%\,CO_2$), and light.

• Asexual Propagation: Produces genetically identical clones.

  • Methods: Apomictic embryos, cuttings, grafting (joining scion and rootstock), budding, layering (marcotting), runners, suckers, and tubers.
  • Structures: Rhizomes (pachymorph, leptomorph, mesomorph), stolons, bulbs, and corm (tunic-covered).

Soil and Water Management

• Soil Characteristics:

  • Lowland (Puddled) Soil: Develops an upper oxidized zone (brown) and a lower reduced zone (dark/blue-gray).
  • Chemicals: Flooding increases availability of Phosphorus ($P$), iron, and manganese.
  • Denitrification: Loss of nitrogen as $N_2$ and $N_2O$ gas from nitrates ($NO_3$).

• Tillage: Mechanical manipulation for weed control and soil structure. Includes plowing (moldboard/disk), harrowing, and cultivation (intertillage).

• Fertilizers:

  • Inorganic: Single analysis (Urea) or complete ($NPK$).
  • Organic: Farmyard manure ($FYM$ ratio $3:1$ solid to liquid), Guano ($9\%\,P$, $13\%\,N$), green manure, and compost (can be accelerated via Trichoderma).

• Liming: Adjusting soil pH using calcium carbonate or dolomite. Wait $3$ weeks after application before testing.

• Water Management:

  • Conservation: Mulching, terracing, contour ploughing, and strip cropping.
  • Irrigation: Gravity, sprinkler, and drip systems.
  • Cropping Systems: Monocropping, multiple cropping (succession, relay, intercropping, and multi-storey).

Pest Management

• Definitions: Epidemic, Pathogen, Vector (carrier), and Signs/Symptoms (external manifestation).
• Principles: Management to an economically acceptable level, not total elimination. Logistics and return on investment (aim for $3$ to $4$ pesos return per $1$ peso spent).
• Integrated Pest Management ($IPM$): Holistic selection of biological, chemical, and cultural tactics. Seeks to contain populations below economic loss levels.

Crop Improvement

• Varieties:

  • Landrace: Genetically diverse, highly adaptable mixtures.
  • Pure Line: Absolutley genotypic and phenotypic uniformity (Johannsen and Biffin).
  • Multi-line: Mechanically mixed lines with different disease resistance (Borlaug).
  • Hybrid: Utilizes heterosis (hybrid vigor); types include single, double, and three-way crosses.

• Breeding Techniques:

  • Selection: Pedigree (based on parent performance) or Mass (based on population performance).
  • Hybridization: Production of heterozygous populations.
  • Polyploid Breeding: Using colchicine or heat to create "gigas" plants (larger cells).
  • Mutation Breeding: Using radiation ($X$-rays, gamma rays) or chemicals ($EMS$).
  • Parasexual Breeding: Protoplast fusion (genetic engineering) and anther/pollen culture to yield haploid plants.