Crop Physiology
Crop physiology explains how light, water, and nutrients become yield. The exam tests photosynthesis types, growth regulators, seed germination, and source–sink relationships.
Photosynthesis and water relations
- C3 plants fix CO2 into a 3-carbon compound; examples include rice, pulses, and cotton. Photorespiration reduces efficiency.
- C4 plants concentrate CO2 and avoid much photorespiration; examples include maize, sugarcane, and sorghum.
- CAM plants open stomata at night and store malic acid; examples include pineapple and many succulents.
- Transpiration pulls water through the xylem; stomata regulate water loss. Water potential moves water from high to low potential.
Growth, development, and regulators
Photoperiodism controls flowering in short-day and long-day plants; vernalization is low-temperature promotion of flowering. Seed germination begins with imbibition, followed by radicle and plumule emergence. Dormancy prevents premature germination. Auxin promotes rooting and apical dominance; gibberellin promotes stem elongation and bolting; cytokinin promotes cell division and delays senescence; abscisic acid closes stomata under stress; ethylene promotes ripening and abscission.
Worked example
A crop produces 5 t/ha grain and 5 t/ha straw. Biological yield = 5 + 5 = 10 t/ha. Harvest index = economic yield ÷ biological yield × 100 = 5 ÷ 10 × 100 = 50%. If straw is included in the economic product, the calculation changes; state the definition used.
Traps that cost marks
Do not use total biological yield as the harvest index denominator when the question defines economic yield differently. C4 does not mean no photorespiration. ABA is the stress hormone; ethylene is the ripening hormone. Photoperiodism is day length, not temperature.