NH4 assimilation in roots (and pH regulation) protein etc light sucrose cO2 shoot Xylem phloem root Excess cation uptake amino-N NH4 Raven& Smith (1976); modified pH protein etcNH4 + assimilation in roots (and pH regulation) amino-N NH4 + H+ protein etc protein etc sucrose CO2 shoot root xylem phloem pH  light ‘Excess’ cation uptake Raven & Smith (1976); modified