Plant Physiology – Study Notes
(For BPT – Biology 2026, Physiotherapy Program)
1. Simple Explanation
Plant physiology is the study of how plants function and survive. It looks at the processes that keep a plant alive, from the way it makes food (photosynthesis) to how it moves water and nutrients, responds to light, temperature, and stress, and how it grows and develops. Understanding these mechanisms helps us predict plant responses to environmental changes, a skill that is useful in rehabilitation medicine when we consider how plants can be used for therapeutic purposes (e.g., herbal medicines, botanical therapies).
2. Key Points
| Topic | What It Is | Why It Matters | Key Terms |
|---|---|---|---|
| Photosynthesis | Conversion of light energy into chemical energy (glucose) in chloroplasts | Supplies food for the plant and oxygen for animals | Light Reactions, Calvin Cycle, ATP, NADPH, Rubisco |
| Transpiration | Water loss through stomata | Drives water transport, cooling, nutrient uptake | Stomata, Cuticle, Vapor Pressure Deficit |
| Water Transport | Movement of water from roots to leaves | Maintains turgor, nutrient transport | Xylem, Cohesion‑Tension Theory, Water potential |
| Nutrient Transport (Xylem & Phloem) | Movement of minerals and sugars | Supplies building blocks for growth | Phloem, Sap, Sap flow, Sugar transporters |
| Plant Hormones (Phytohormones) | Chemical messengers regulating growth | Control of development, stress response | Auxins, Gibberellins, Cytokinins, Abscisic Acid, Ethylene |
| Signal Transduction | How cells perceive and respond to stimuli | Enables adaptation to light, gravity, touch | Photoreceptors, Hormone receptors, Second messengers |
| Plant Growth & Development | Cell division, expansion, differentiation | Determines size, shape, organ identity | Meristems, Organogenesis |
| Environmental Responses | Phototropism, gravitropism, thigmotropism | Allows plants to orient themselves | Phototropin, Auxin Redistribution |
| Plant Cell Wall & Structure | Provides rigidity, protection | Determines mechanical properties | Cellulose, Hemicellulose, Pectin |
| Stress Physiology | Response to drought, salinity, temperature, pathogens | Survival mechanisms; implications for medicinal plant quality | Osmoregulation, Antioxidant enzymes, Secondary metabolites |
3. Worked Example
Calculating the Transpiration Rate of a Leaf
Problem:
A 10 cm² leaf is exposed to a light intensity of 800 µmol m⁻² s⁻¹. The leaf temperature is 25 °C, and the relative humidity in the surrounding air is 50 %. Estimate the transpiration rate (E) in mmol m⁻² h⁻¹ using the simplified Penman–Monteith equation:
[ E = \frac{ \Delta (R_n - G) + \rho_a c_p , \frac{D}{\gamma} }{ \lambda ( \Delta + \gamma(1 + r_s/r_a) ) } ]
Assumptions & Simplifications:
| Symbol | Value / Approximation | Explanation |
|---|---|---|
| ( \Delta ) | 0.009 kPa °C⁻¹ | Slope of saturation vapor pressure curve at 25 °C |
| ( R_n ) | 200 µmol m⁻² s⁻¹ | Net radiation (approx. 200 µmol m⁻² s⁻¹ at 800 µmol m⁻² s⁻¹ light) |
| ( G ) | 0 | Soil heat flux negligible for a leaf |
| ( \rho_a ) | 1.2 kg m⁻³ | Air density |
| ( c_p ) | 1.013 kJ kg⁻¹ K⁻¹ | Specific heat of air |
| ( D ) | 2.5 kPa | Vapor pressure deficit (difference between saturation at 25 °C (3.17 kPa) and actual (1.585 kPa)) |
| ( \gamma ) | 0.066 kPa °C⁻¹ | Psychrometric constant |
| ( \lambda ) | 44.0 MJ kg⁻¹ | Latent heat of vaporization |
| ( r_s ) | 150 s m⁻¹ | Stomatal resistance (typical value) |
| ( r_a ) | 70 s m⁻¹ | Aerodynamic resistance (typical value) |
Calculation:
-
Numerator
[ \Delta (R_n - G) = 0.009 \times 200 = 1.8 ] [ \rho_a c_p \frac{D}{\gamma} = 1.2 \times 1.013 \times \frac{2.5}{0.066} \approx 46.4 ] [ \text{Numerator} = 1.8 + 46.4 = 48.2 ] -
Denominator
[ \Delta + \gamma(1 + r_s/r_a) = 0.009 + 0.066(1 + 150/70) ] [ = 0.009 + 0.066(1 + 2.143) = 0.009 + 0.066 \times 3.143 \approx 0.009 + 0.2077 \approx 0.2167 ]