Practice question

Growth index measures proliferative performance of callus cultures quantitatively allowing comparison across media treatments hormone combinations and genotypes. Most accurate method utilizes change in dry weight rather than fresh weight or visual scoring because fresh weight is confounded by variable water content due to hyperhydricity, extracellular polysaccharide secretion, and vacuolar water uptake that fluctuates with osmolarity of medium. Dry weight determination involves harvesting callus, washing to remove agar, oven drying at 60 to 80 degrees Celsius to constant mass eliminating moisture, weighing analytical balance. Index calculated as final dry weight minus initial inoculum dry weight divided by initial dry weight multiplied by 100 percent, or sometimes as ratio of final to initial. This metric reflects true biomass accumulation including cell number increase and deposition of cell walls proteins and starch. Cell number counting is impractical in clumpy callus requiring enzymatic dissociation inducing error. Monitoring dry weight growth curve shows lag exponential stationary phases and correlates with metabolic activity secondary metabolite yield and requirement for timely subculture every 3-4 weeks preventing nutrient depletion and phenolic browning.