Effect of nitrogen stress and abscisic acid on nitrate absorption and transport in barley and tomato

The potential of barley (Hordeum vulgare L.) and tomato (Lycopersicon esculentum Mill.) roots for net NO₃^-absorption increased two-to five fold within 2 d of being deprived of NO₃^-supply. Nitrogen-starved barley roots continued to maintain a high potential for NO₃^-absorption, whereas NO₃^-absorption by tomato roots declined below control levels after 10 d of N starvation. When placed in a 0.2 mM NO₃^-solution, roots of both species transported more NO₃^-and total solutes to the xylem after 2 d of N starvation than did N-sufficient controls. However, replenishment of root NO₃^-stores took precedence over NO₃^-transport to the xylem. Consequently, as N stress became more severe, transport of NO₃^-and total solutes to the xylem declined, relative to controls. Nitrogen stress caused an increase in hydraulic conductance (L_p) and exudate volume (J_v) in barley but decrased these parameters in tomato. Nitrogen stress had no significant effect upon abscisic acid (ABA) levels in roots of barley or flacca (a low-ABA mutant) tomato, but prevented an agerelated decline in ABA in wild-type tomato roots. Applied ABA had the same effect upon barley and upon the wild type and flacca tomatoes: L_p and J_v were increased, but NO₃^-absorption and NO₃^-flux to the xylem were either unaffected or sometimes inhibited. We conclude that ABA is not directly involved in the normal changes in NO₃^-absorption and transport that occur with N stress in barley and tomato, because (1) the root ABA level was either unaffected by N stress (barley and flacca tomato) or changed, after the greatest changes in NO₃^-absorption and transport and L_p had been observed (wild-type tomato); (2) changes in NO₃^-absorption/transport characteristics either did not respond to applied ABA, or, if they did, they changed in the direction opposite to that predicted from changes in root ABA with N stress; and (3) the flacca tomato (which produces very little ABA in response to N stress) responded to N stress with very similar changes in NO₃^-transport to those observed in the wild type.Abbreviation and symbols ABA abscisic acid - J_v exudate volume - L_p root hydraulic conductance