Fig. 6
The schematic model of drought responses and xylem development of Catalpa bungei under diverse drought types as affected by nitrogen forms. C. bungei respond to severe drought (FRD) by sharply decreasing stomata conductance and increasing WUEi. Under FRD condition, the effect of N forms is covered up by the drastic drought effect. As compared with FRD, stomata conductance and gas exchanges were less inhibited by PRDs. Stomata regulation and WUEi was superior in V-PRD to H-PRD due to active induction of ABA signaling. Under both PRDs and FRD, nitrate is superior to ammonium in terms of antioxidant defense, stomata regulation and leaf WUEi. Under V-PRD, nitrate benefits WUEi via activating the signaling network of NO, Ca2+ and ABA which promotes stomata regulation. Under FRD, nitrate favors WUEi via enhancing NO signal which promotes antioxidant defense and elevates photosynthetic efficiency. Soluble protein plays a major role in maintaining xylem development under PRDs, while the activity of SOD and CAT play critical roles in improving WUEi. The toxicity effect of ammonium on xylem development is significant under H-PRD while it is less significant under V-PRD condition