Environmental and Stress Physiology

1. Photoperiodism is the response of plants to the relative length of day and night.
2. Garner and Allard (1920) first reported photoperiodism in Nicotiana tabacum and Xanthium.
3. Plants are classified as short-day, long-day, or day-neutral based on flowering response.
4. Rice, cotton, and soybean are short-day plants.
5. Wheat, oat, and spinach are long-day plants.
6. Tomato and cucumber are day-neutral plants.
7. Phytochrome is the photoreceptor pigment responsible for photoperiodic response.
8. Vernalization is the induction of flowering by exposure to low temperature.
9. Lysenko (1938) coined the term vernalization.
10. Gibberellins can substitute the vernalization effect in some plants.

Photoperiodism and Flowering

1. Short-day plants (SDPs) flower when day length is less than the critical period.
2. Long-day plants (LDPs) flower when day length is greater than the critical period.
3. Day-neutral plants (DNPs) flower irrespective of day length.
4. Critical photoperiod is the day length at which a plant starts flowering.
5. Phytochrome exists in two forms: Pr (red) and Pfr (far-red).
6. Pfr is the active form that promotes flowering in LDPs.
7. Pr is converted to Pfr under red light (660 nm).
8. Pfr is converted back to Pr under far-red light (730 nm).
9. Florigen is the hypothetical flowering hormone transported from leaves to shoot apex.
10. Vernalization requirement is found mainly in winter wheat and barley.

Respiration

1. Respiration is the oxidative breakdown of food to release energy as ATP.
2. Aerobic respiration requires oxygen and produces CO₂, H₂O, and energy.
3. Anaerobic respiration occurs in absence of oxygen and produces ethanol or lactic acid.
4. Glycolysis occurs in the cytoplasm.
5. Krebs cycle (TCA cycle) occurs in the mitochondrial matrix.
6. Electron transport chain (ETC) occurs in the inner mitochondrial membrane.
7. ATP yield per glucose molecule in aerobic respiration = ~36–38 ATP.
8. Pasteur effect – anaerobic respiration rate increases in the absence of oxygen.
9. Respiratory quotient (RQ) = CO₂ released / O₂ consumed.
10. RQ for carbohydrates = 1; fats = 0.7; proteins = 0.8.

Water Relations and Stress Physiology

1. Drought stress causes stomatal closure and reduces photosynthesis.
2. Flood or waterlogging stress causes anaerobic respiration in roots.
3. Salt stress causes ion toxicity and osmotic stress in plants.
4. Heat stress denatures proteins and enzymes, reducing growth.
5. Cold stress reduces membrane fluidity and chlorophyll content.
6. ABA (abscisic acid) increases under drought and salt stress.
7. Proline accumulation is a common stress tolerance indicator.
8. Osmotic adjustment helps plants survive water deficit.
9. Antioxidant enzymes like superoxide dismutase and catalase protect plants from oxidative stress.
10. Stress-induced leaf senescence is accelerated by ethylene.

Seed Dormancy and Germination

1. Seed dormancy is the failure of a viable seed to germinate under favorable conditions.
2. Primary dormancy occurs during seed formation.
3. Secondary dormancy develops in mature seeds under adverse conditions.
4. Physiological dormancy is caused by hormonal imbalance (ABA/GA).
5. Morphological dormancy occurs due to immature embryo.
6. Physical dormancy is due to hard seed coat.
7. Scarification breaks physical dormancy (mechanical or chemical).
8. Stratification breaks physiological dormancy (chilling treatment).
9. Gibberellins can substitute for cold treatment to break dormancy in some seeds.
10. Seed germination begins with imbibition of water.
11. Seed viability is the ability to germinate under favorable conditions.
12. Seed vigor indicates the growth potential of seedling.
13. Tetrazolium test is used to test seed viability.
14. Vigour index = Germination (%) × Seedling length.
15. Orthodox seeds can tolerate drying and low temperature.
16. Recalcitrant seeds cannot survive drying or freezing (e.g., coconut, mango).
17. Moisture content for safe seed storage = 8–10%.
18. Respiration rate increases during seed germination.
19. Seed priming improves germination and seedling vigor.
20. Hydration and enzyme activation are essential for seed germination.

Plant Growth Indices

1. Relative growth rate (RGR) = Increase in plant biomass per unit biomass per unit time.
2. Net assimilation rate (NAR) = Increase in dry matter per unit leaf area per unit time.
3. Leaf area ratio (LAR) = Leaf area / Plant biomass.
4. Specific leaf area (SLA) = Leaf area / Leaf dry weight.
5. Crop growth rate (CGR) = Increase in plant dry matter per unit ground area per unit time.
6. Radiation use efficiency (RUE) = Dry matter produced per unit intercepted radiation.
7. LAI (Leaf Area Index) = Leaf area per unit ground area.
8. Harvest index (HI) indicates partitioning efficiency of dry matter to economic yield.
9. Source limitation reduces yield due to insufficient photosynthates.
10. Sink limitation reduces yield due to inability to store photosynthates efficiently.