Photon costs of shoot and root NO 3 - , and root NH 4 + , assimilation in terrestrial vascular plants considering associated pH regulation, osmotic and ontogenetic effects.

The photon costs of photoreduction/assimilation of nitrate (NO 3 - ) into organic nitrogen in shoots and respiratory driven NO 3 - and NH 4 + assimilation in roots are compared for terrestrial vascular plants, considering associated pH regulation, osmotic and ontogenetic effects. Different mechanisms of neutralisation of the hydroxyl (OH - ) ion necessarily generated in shoot NO 3 - assimilation are considered. Photoreduction/assimilation of NO 3 - in shoots with malic acid synthesis and either accumulation of malate in leaf vacuoles or transport of malate to roots and catabolism there have a similar cost which is around 35% less than that for root NO 3 - assimilation and around 20% less than that for photoreduction/assimilation of NO 3 - , oxalate production and storage of Ca oxalate in leaf vacuoles. The photon cost of root NH 4 + assimilation with H + efflux to the root medium is around 70% less than that of root NO 3 - assimilation. These differences in photon cost must be considered in the context of the use of a combination of locations of NO 3 - assimilation and mechanisms of acid-base regulation, and a maximum of 4.9-9.1% of total photon absorption needed for growth and maintenance that is devoted to NO 3 - assimilation and acid-base regulation.