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C4 plants have higher photosynthetic efficiency (~60%) compared to C3 plants (~30%) because they concentrate CO2 around RuBisCO, reducing photorespiration. This adaptation makes them suited for hot climates - important agricultural knowledge.

Rhizobium bacteria form symbiotic associations with legume roots, fixing atmospheric N2 into ammonia. The bacteria live in root nodules. This symbiotic relationship is agriculturally significant and frequently tested in competitive exams.

Wood is composed of secondary xylem with thick-walled tracheids and vessel elements that provide mechanical support and water transport. Annual rings visible in wood represent secondary xylem produced by vascular cambium.

Chlorophyll absorbs blue and red light but reflects and transmits green light, making plants appear green. This is why plants cannot utilize green light efficiently for photosynthesis - a fundamental physics-biology concept.

ABA is a stress hormone that accumulates during drought, regulating stomatal closure and promoting seed dormancy. It antagonizes gibberellins and cytokinins. This is crucial for understanding plant responses to environmental stress.

Monocots typically have fibrous root systems with adventitious roots. They lack secondary growth (no cambium) and have scattered vascular bundles. Dicots have tap roots and show secondary growth.

CO2 saturation point is where further increase in CO2 doesn't increase photosynthesis rate because other factors (like enzyme activity) become limiting. Compensation point is where photosynthesis equals respiration.

Stomata generally close at night (except CAM plants) to reduce transpiration. Guard cells are rich in mitochondria and chloroplasts to provide ATP and regulate osmotic potential. This circadian regulation is fundamental to plant physiology.

The Casparian strip is a band of suberin (waterproof lipid polymer) that prevents passive transport across the endodermis, forcing minerals to undergo active transport. This ensures selective ion uptake - a key NCERT concept.

2,4-D is a synthetic auxin widely used as a selective herbicide. Natural auxin is IAA (Indole-3-acetic acid). This is application-based knowledge relevant for agricultural science in UPSC.