Photosynthetic, Antioxidant and Leaf Ultrastructural Responses to Drought In Two Spinach Cultivars of Contrasting Morphology

Understanding leaf physiological and structural responses to drought is essential for developing drought resistant crops and maximizing water use efficiency in agroecosystems worldwide.  A greenhouse experiment was conducted to evaluate drought acclimation of gas exchange, photosystem II function, antioxidant properties and leaf ultrastructure of two spinach cultivars with contrasting morphology, Bloomsdale (larger leaves and stature) and Hybrid Tyee.  Time to severe wilting that decreased net photosynthesis (P_net) to <2 µmol m^-2 s^-1 in half of the replications during a progressive drought was 10 days in Bloomsdale but 16 days in Tyee.  Four days after rewatering upon completion of progressive drought, leaf biomass had decreased 22% in Bloomsdale and 29% in Tyee, compared with respective well-watered controls.  At wilting, stomatal conductance (g_s) and transpiration (E) were 83% and 141% greater in Bloomsdale than Tyee, respectively.  Photosystem II efficiency and electron transport rate were 16% and 32% greater in Bloomsdale at wilting.  ACi curve analysis revealed that maximum Rubisco carboxylation efficiency (V_cmax) and RuBP regeneration capacity (J_max) of well-watered Bloomsdale plants were 22% and 34% greater, respectively, but CO₂ compensation point (CCP) was 22% less than Tyee.  A severely wilted Bloomsdale plant had 100%, and 70% greater V_cmax and J_max, respectively, but with a 44% higher CCP compared with Tyee.  Furthermore, severe drought stress caused 52% increase in CCP in Bloomsdale but 18% decrease in Tyee, probably indicating greater dependence of Bloomsdale on photorespiration as an electron sink than Tyee under stress.  Although gas exchange characteristics including P_net were still greater in Bloomsdale than Tyee one day after rewatering, both cultivars had the same gas exchange on the fourth day of rewatering.  While leaf ultrastructural and antioxidant investigations are currently underway, the choice between Bloomsdale and Tyee under drought appears to warrant considerations of the intensity of water stress.  Tyee is more likely to withstand a prolonged drought causing severe wilting than Bloomsdale.