Whole-tree water use efficiency is decreased by ambient ozone and not affected by O3-induced stomatal sluggishness

Abstract

Steady-state and dynamic gas exchange responses to ozone visible injury were investigated in an ozone-sensitive poplar clone under field conditions. The results were translated into whole tree water loss and carbon assimilation by comparing trees exposed to ambient ozone and trees treated with the ozone-protectant ethylenediurea (EDU). Steady-state stomatal conductance and photosynthesis linearly decreased with increasing ozone visible injury. Dynamic responses simulated by severing of a leaf revealed that stomatal sluggishness increased until a threshold of 5% injury and was then fairly constant. Sluggishness resulted from longer time to respond to the closing signal and slower rate of closing. Changes in photosynthesis were driven by the dynamics of stomata. Whole-tree carbon assimilation and water loss were lower in trees exposed to ambient O(3) than in trees protected by EDU, both under steady-state and dynamic conditions. Although stomatal sluggishness is expected to increase water loss, lower stomatal conductance and premature leaf shedding of injured leaves aggravated O(3) effects on whole tree carbon gain, while compensating for water loss. On average, WUE of trees exposed to ambient ozone was 2-4% lower than that of EDU-protected control trees in September and 6-8% lower in October.

Figure 1. Examples of dynamic response of g_s and A_max after detachment of the leaf (A: calculation of the dynamic parameters in a leaf with 0% visible injury, B: time courses of absolute values in g_s, C: time courses of absolute values in A).

Δg_s and ΔA show the range of g_s and A_max variation, respectively, over 30 min from the leaf severing. T_resp (g_s) and T_resp(A) show the time to start decrease of g_s and A_max, respectively. Slope(g_s) and Slope(A) show the rate of decrease for g_s and A_max, respectively, over 30 min.

Figure 2. Total number of leaves (A) and percentage of ozone injured leaves (more than 5% of injured surface) (B) per tree (+SE) (WAT: water treated plants; EDU: EDU treated plants).

* and *** denote significance at the 5% and 0.1% level, respectively; n.s. indicates no significance. Different letters above the bars indicate significant differences among bars (Tukey HSD test, P<0.05, n = 5 trees).

Figure 3. Relationships between steady-state leaf gas exchange (A: stomatal conductance (g_s), B: light-saturated photosynthesis (A_max)) and visible ozone foliar injury.

Figure 4. Relationships between visible ozone foliar injury and dynamic response of stomatal conductance (g_s) and photosynthesis (A_max) over 30 min after leaf severing (A: Δg_s at 30 min; B: Slope(g_s); C: T_resp (g_s); D: ΔA; E: Slope(A); F: T_resp (A)).

Figure 5. Estimated steady-state carbon assimilation (A: A_tree), water loss (B: W_loss) and instantaneous water use efficiency expressed as A_tree/W_loss (C: WUE) at tree level (+SE) (WAT: water treated plants; EDU: EDU treated plants).

* and *** denote significance at the 5% and 0.1% level, respectively; n.s. indicates no significance. Different letters above the bars indicate significant differences among bars (Tukey HSD test, P<0.05, n = 5 trees).

Figure 6. Estimated carbon assimilation (A: A_{tree_st}), water loss (B: W_{loss_st}) and instantaneous water use efficiency expressed as A_{tree_st}/W_{loss_st} (C: WUE_{_st}) at tree level under severe water stress imposed by severing a leaf (+SE) (WAT: water treated plants; EDU: EDU treated plants).

* and *** denote significance at the 5% and 0.1% level, respectively; n.s. indicates no significance. Different letters above the bars indicate significant differences among bars (Tukey HSD test, P<0.05, n = 5 trees).