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Agronomy QUESTION #2035
Question 1
The optimum harvest index (HI) of modern high-yielding wheat varieties compared to traditional tall varieties is higher because:
  • Modern varieties produce more total biomass per unit area
  • Modern varieties partition a greater proportion of total biological yield into the economic yield (grain), with HI values of 0.45–0.55 vs 0.30–0.35 in traditional varieties✔️
  • Modern varieties have shorter grain-filling periods, reducing respiration losses
  • Modern varieties have thicker stems that store more assimilates
Correct Answer Explanation
Harvest Index = Economic yield / Biological yield. Green Revolution breeding shifted assimilate partitioning toward grain by reducing stem height (dwarfing genes Rht1, Rht2), increasing HI from ~0.30 in tall varieties to 0.45–0.55 in semi-dwarf modern varieties — the primary mechanism behind yield gains.