The Impact of Heat Stress and Water Deficit on the Photosynthetic and Stomatal Physiology of Olive (Olea europaea L.)-A Case Study of the 2017 Heat Wave

Abstract

Heat waves are predicted to increase in frequency and duration in many regions as global temperatures rise. These transient increases in temperature above normal average values will have pronounced impacts upon the photosynthetic and stomatal physiology of plants. During the summer of 2017, much of the Mediterranean experienced a severe heat wave. Here, we report photosynthetic leaf gas exchange and chlorophyll fluorescence parameters of olive (Olea europaea cv. Leccino) grown under water deficit and full irrigation over the course of the heat wave as midday temperatures rose over 40 °C in Central Italy. Heat stress induced a decline in the photosynthetic capacity of the olives consistent with reduced ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) activity. Damage to photosystem II was more apparent in plants subject to water deficit. In contrast to previous studies, higher temperatures induced reductions in stomatal conductance. Heat stress adversely affected the carbon efficiency of olive. The selection of olive varieties with enhanced tolerance to heat stress and/or strategies to mitigate the impact of higher temperatures will become increasingly important in developing sustainable agriculture in the Mediterranean as global temperatures rise.

Keywords: Mediterranean agriculture; OJIP; drought; heat stress; stomatal conductance; water use efficiency.

Figure 1

(a) Daily mean (solid black line), maximum and minimum (dashed lines either side of the mean) temperature during the experiment coinciding with the heat wave of summer 2017 (marked in light grey shading) which occurred from day 213 to 221. The darker grey shading marks the point at which the water deficit plants were ‘re-watered’ to receive full irrigation. Point measurements of photosynthesis (P_N) (b), stomatal conductance (G_{s H2O}) (c) and the maximum quantum efficiency of PSII (F_v/F_m) (d) of olive plants subject to full irrigation (white fill symbols, solid line) and water deficit (black fill symbols, broken line) were recorded at intervals during the experimental treatment. Error bars indicate one standard deviation either side of the mean. Time periods t0 (day 206), t1 (day 215) and t2 (day 219) marked above panel a refer to OJIP analysis in Figure 4.

Figure 2

The relationship between photosynthesis (P_N) and stomatal conductance to water vapor (G_{s H2O}) (a), and stomatal (G_{s CO2}) (b), mesophyll (G_{m CO2}) (c) and total (G_{tot CO2}), (d) conductance to CO₂ of olive plants during (square symbols, dashed best fit line) and after (circle symbols, solid best fit line) the heat wave (marked in light grey shading in Figure 1) subject to full irrigation (white fill symbols) and water deficit (grey fill symbols) treatment.

Figure 3

The response of photosynthesis (P_N) to increasing [CO₂] within the internal leaf air-space (C_i) (a) and physiological parameters calculated from the P_N-C_i response curve utilizing the method of Ethier and Livingston [33] (b) in well-watered olive plants during (grey fill symbols) and after (white fill symbols) the heat wave. Y-axis units for Vc_max, J_max, and P_{N max} are μmol m⁻² s⁻¹, and G_{m CO2} is measured in mmol m⁻² s⁻¹. * indicates significant difference between measurements conducted during and after the heat wave using a one-way ANOVA: Vc_max (F_1,7 = 11.0; P = 0.013), J_max (F_1,7 = 199.1; P = 2.1 × 10⁻⁶), G_{m CO2} (F_1,7 = 7.3; P = 0.031) and P_{N max} (F_1,7 = 151.1; P = 1.8 × 10⁻⁵). Error bars indicate one standard deviation either side of the mean.

Figure 4

Analysis of the chlorophyll fluorescence transient of olive plants at t₀ (circle symbol), t₁ (square symbol) and t₂ (triangle symbol) (see Figure 1 for sampling intervals) subject to full irrigation (white fill symbols) and water deficit (grey fill symbols) treatments: (a) average OJIP induction curves; (b) spider plot of parameters (see Materials and Methods for definitions and descriptions) extrapolated from the OJIP transient expressed in relation to values of plants receiving full irrigation at t₀. Error bars indicate one standard deviation either side of the mean.

Figure 5

Response of photosynthesis (P_N) (a), stomatal conductance (G_{s H2O}) (b), intercellular sub-stomatal air-space [CO₂] (C_i) (c), and the actual quantum efficiency of PSII under steady state conditions in the light (ΦPSII) (d) to an instantaneous increase in leaf temperature over a range of 20 to 41.5 °C of well-watered olive plants. Error bars indicate one standard deviation either side of the mean.