Q.The principle pathway of water translocation in angiosperms is ..........

Because most soil minerals are charged ions that cannot cross the cell membrane freely, root uptake occurs by two routes. Passive absorption moves ions into root cells by simple diffusion, from higher to lower concentration, without energy expenditure; a special case is Donnan equilibrium, where certain anions become fixed inside the cell membrane and additional mobile cations accumulate to electrically neutralise them, even against their own concentration gradient. Active absorption moves ions against their concentration gradient and requires ATP energy from respiration -- oxygen-deprived roots show a sharp drop in active mineral uptake. Hoagland and Davis (1923) proposed the carrier hypothesis to explain active transport: specific membrane proteins ('carriers') actively pump cations and anions from the soil into root epidermal cells, with Bennet and Clarke (1956) suggesting the carrier is conjugated with lecithin. Mineral ion absorption is independent of water absorption, but once inside the root, minerals pass through the cortex to the xylem and travel with water along the transpiration stream to the rest of the plant, before being assimilated and redistributed via the phloem. A mineral becomes toxic once it reduces tissue dry weight by 10% or more; the toxic effect of one ion can be offset by, or interfere with the uptake of, another -- a phenomenon called antagonism (e.g. excess manganese disrupts calcium, magnesium and iron uptake).