Harvest Index (HI), introduced by C.M. Donald in 1962, quantifies the efficiency of a crop in partitioning its total photosynthetic assimilates into harvested, economically valuable organs. It is calculated as the ratio of economic yield ($Y_e$, e.g., grain yield) to the total above-ground biological yield / biomass ($Y_b$, grain plus straw/vegetative biomass): $\text{Harvest Index} = \frac{\text{Economic Yield}}{\text{Biological Yield}} \times 100$.

Ernst Münch's Pressure-Flow (Mass-Flow) Hypothesis explains phloem translocation as a mass movement of water and dissolved solutes along a hydrostatic pressure gradient. At the source (e.g., mature photosynthesizing leaves), active loading of sucrose into sieve elements lowers the solute potential, drawing water inward from the xylem via osmosis and generating high turgor pressure. At the sink (e.g., roots, fruits, growing shoot tips), sucrose is unloaded and metabolized or stored as starch, causing water to leave the sieve elements and resulting in low turgor pressure. Consequently, sap flows en masse from the high-pressure source to the low-pressure sink.

• In vascular plants, photoassimilates synthesized in photosynthetic mesophyll cells (source) are loaded into phloem sieve tubes and translocated to heterotrophic tissues (sink).
• The overwhelming majority of translocated carbohydrate is sucrose, a non-reducing disaccharide composed of $\alpha$-D-glucose and $\beta$-D-fructose linked by an $\alpha(1\rightarrow 2)$ glycosidic bond.
• Sucrose is favored for long-distance transport because it is chemically non-reducing and unreactive, protecting it from premature enzymatic oxidation and degradation during bulk flow transit.
• Reducing monosaccharides like glucose and fructose are metabolically too reactive for long-distance phloem translocation.
• Thus, the commonly translocated compound in phloem is sucrose (Option 3).