Fig. 8.
The schematic model of drought tolerance of Populus as affected by adequate- and low- N condition. The root proliferation under drought was suppressed by N deficiency and enhanced by adequate N application, which may benefit water uptake under drought. The degree of adaptive changes of xylem and the increment of vessel frequency under drought were enhanced by adequate-N application, which may reduce the water losses. The elevated IAA after N application contributed to the adaptive changes of xylem under drought, and promoted the secondary development of stem. N application increased the concentration of ABA in leaves, which contributed to the sensitive stomata adjustment and promoted WUEi and long-term water use efficiency under drought. The biosynthesis of ABA, IAA and JA were synergistically promoted by N application and they cooperatively participated in drought tolerance. Moreover, N application improved antioxidant defense in leaves via regulating the production of N-related antioxidants including proline and carotenoid, and thus lead to improved drought tolerance and better growth performance of poplars