Relationships between Root Temperature and the Transport of Ammonium and Nitrate Ions by Italian and Perennial Ryegrass (Lolium multiflorum and Lolium perenne).

At root temperature below 14 C the absorption of (15)N from NH(4) (+) greatly exceeded that from NO(2) (-) by tillers of Lolium multiflorum and Lolium perenne under conditions where pH, external concentration, plant N status, and pretreatment temperature were varied. There was a marked increase in the temperature sensitivity of NO(3) (-) transport below 14 C, irrespective of the temperature at which plants were grown previously. A marked increase in the temperature sensitivity was also seen for NH(4) (+) transport, but this occurred at the lower temperature of 10 C. Pretreatment of roots at 8 C lowered this still further to 5 C. Above and below these transition temperatures the Q(10) values for NO(3) (-) and NH(4) (+) transport were similar. Thus, the increased absorption of NH(4) (+) relative to NO(3) (-) at low temperatures seems to be related primarily to the difference in transition temperatures.It seems possible that NO(3) (-) and NH(4) (+) are absorbed through separate regions of the cell membrane differing in lipid composition and phase transition temperatures.