Abstract
Water availability for plant growth is becoming increasingly limited, whereas rising atmospheric carbon dioxide concentration may have interactive effects with drought stress. The objectives of this study were to determine whether elevated CO2 would mitigate drought-induced water deficit and photosynthesis inhibition and enhance recovery from drought damages on rewatering and to determine whether the mitigating effects during drought stress and the recovery in photosynthesis during rewatering by elevated CO2 were the result of the regulation of stomatal movement or carboxylation activities in tall fescue (Festuca arundinacea Schreb. cv. Rembrandt). Plants were grown in controlled-environment chambers with ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1) and maintained well watered (control) or subjected to drought stress and subsequently rewatered. Elevated CO2 reduced stomatal conductance (gS) and transpiration rate of leaves during both drought stress and rewatering. Osmotic adjustment and soluble sugar content were enhanced by elevated CO2. Elevated CO2 enhanced net photosynthetic rate with lower gS but higher Rubisco and Rubisco activase activities during both drought and rewatering. The results demonstrated that elevated CO2 could improve leaf hydration status and photosynthesis during both drought stress and rewatering, and the recovery in photosynthesis from drought damages on rewatering was mainly the result of the elimination of metabolic limitation from drought damages associated with carboxylation enzyme activities.
Drought stress is one of the most detrimental abiotic stresses for plant growth. Water deficit in plants leads to stomatal closure and reduces photosynthesis resulting from restricted CO2 diffusion through leaf stomata (stomatal limitation) and inhibition in carboxylation activity (metabolic limitation) (Flexas et al., 2004). Therefore, minimizing cellular dehydration and maintaining active photosynthesis are key strategies for plant survival or persistence through dry-down periods (Nilsen and Orcutt, 1996). In addition, rapid recovery of damaged plant tissues or rehydration and resumption of photosynthesis after drought stress when water becomes available is particularly important for perennial plant species to ensure rapid regrowth and stand re-establishment in areas with alternating drought and rewatering events. Post-drought recovery is largely dependent on the existing leaves to be able to resume rapidly physiological activities such as photosynthesis; however, regulating factors for cellular rehydration and photosynthesis resumption from drought damage on rewatering have received limited attention.
The atmospheric CO2 concentration has been increasing, which has been shown to promote drought tolerance or mitigate drought damages in various plant species, particularly effective for C3 species (Kirkham, 2011), including perennial grass species such as tall fescue (Yu et al., 2012a, 2012b, 2014) and kentucky bluegrass (Poa pratensis L.) (Song et al., 2014). The positive effects of elevated CO2 on drought tolerance of plants are mainly the result of the reduction in water use as the result of CO2 induction of stomatal closure and promotion of photosynthesis (Kirkham, 2011; Wullschleger et al., 2002). The promotive effects of elevated CO2 on photosynthesis have been mainly associated with overcoming metabolic limitations from drought stress by reducing photorespiration and increasing ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) carboxylation (Leakey et al., 2006; Reddy et al., 2010). Many previous studies have examined effects of elevated CO2 on various physiological changes during drought stress; however, limited information is available for effects of elevated CO2 on physiological recovery of plants from drought stress on rewatering (Widodo et al., 2003). It is not clear whether effects of elevated CO2 would promote rehydration and resumption of photosynthesis when plants are rewatered and whether the recovery in photosynthesis is related to the elimination of stomatal limitation (i.e., stomatal reopening) and/or metabolic limitations (i.e., Rubisco activation) from drought damages.
Tall fescue is a widely used C3 perennial turfgrass species. It has superior drought avoidance compared with many other commonly used cool-season turfgrass species as a result of its deep extensive root system, but has limited capacity of dehydration tolerance and recuperative ability from drought damages (Fry and Huang, 2004; Qian et al., 1997). The physiological effects of CO2 and rewatering of drought responding on tall fescue provide further insights into mechanisms imparting of perennial grass species adapted to drought and rewatering in the environmental atmospheric CO2 concentrations and would also be important for developing practices of management to promote drought tolerance and recovery for widely used turfgrass species. Therefore, the objectives of the research were to study whether the elevated CO2 would mitigate drought-induced water deficit and photosynthesis inhibition and enhance recovery from drought damages on rewatering and to determine whether the mitigating effects during drought stress and the recovery in photosynthesis during rewatering by elevated CO2 were the result of the regulation of stomatal movement or carboxylation activities in tall fescue.
Materials and Methods
Plant material and growth conditions.
Tall fescue (‘Rembrandt’) sod (10 cm diameter) was obtained on 10 June 2011 from the turf research field at Rutgers University in North Brunswick, NJ, and were established in plastic pots (10 cm diameter and 60 cm deep) filled with a mixture of sand and topsoil (fine, montmorillonitic, mesic, aquic arqui-dolls) (1:3, v/v). Plants were maintained in a greenhouse for 2 months for establishing the canopy and roots with an average temperature at 23/16 °C (day/night) and 760 μmol·m−2·s−1 photosynthetically active radiation (PAR). Plants were moved to a growth chamber (GC15; Environmental Growth Chambers, Chagrin Falls, OH) for the acclimation to controlled-environment conditions for 14 d before imposing CO2 and watering treatments. The growth chambers were set at 11-h photoperiod with 680 μmol·m−2·s−1 PAR and day/night temperatures at 23/16 °C. During plant establishment in the greenhouse or growth chamber, plants were trimmed weekly to ≈10 cm, watered every 2 d, and fertilized weekly with half-strength Hoagland’s nutrient solution (Hoagland and Arnon, 1950).
Treatments and experimental design.
The experiment consisted of two CO2 concentrations and two watering treatments, which were arranged in a randomized complete block split-plot design with CO2 concentrations as main plots and watering treatments as subplots. Each CO2 treatment was replicated in four growth chambers. Each watering treatment was replicated in four pots placed inside each growth chamber under each CO2 treatment.
The CO2 treatments included ambient CO2 concentration (400 ± 10 μmol·mol−1) and elevated CO2 concentration (800 ± 10 μmol·mol−1). Plants were grown at the two CO2 levels in growth chambers for 14 d before and then during drought (15 d) and rewatering (6 d) treatments for a total of 35 d. The CO2 controlling setup of growth chambers followed the design as described in Yu et al. (2012a, 2012b). The concentration of CO2 inside each growth chamber was maintained through an automated, open-chamber CO2 control system connected to a gas tank containing 100% CO2 (Airgas, Radnor, PA). The CO2 levels were continuously monitored through an infrared gas analyzer (LI-820; LI-COR, Lincoln, NE) and controlled using an automatic system consisting of a programmable logic controller unit, solenoid valves, and a laptop computer with monitoring software accurate to within 10 μmol·mol−1.
After 14 d of pre-exposure to CO2 treatments, plants were then exposed to two levels of water treatments under either ambient or elevated CO2 concentration: 1) well-watered control by watering plants every 2 d until water drainage from the bottom of the pot to maintain soil volumetric water content (SWC) at the pot capacity (≈25%); 2) drought stress by withholding irrigation for 15 d when SWC decreased to 4.98%; and 3) rewatering for 6 d with irrigating previously drought-stressed plants and SWC resumed to the pot capacity.
Physiological measurements.
Leaf relative water content (RWC) of fully expanded leaves was determined at 0, 5, 10, and 15 d of drought stress and at 2 and 6 d of rewatering based on fresh weight (FW), turgid weight (TW), and dry weight (DW) using the following formula: RWC (%) = [(FW – DW)/(TW – DW)] × 100. Leaf fresh weight was immediately weighed after excised from the plants and then was soaked in deionized water for 6 h at 23 °C. Leaf samples were then blotted dry and immediately weighed for determination of TW. Samples were then dried in an oven at 80 °C for 72 h and weighed again for DW (Bagatta et al., 2008).
Leaf osmotic potential (ψS) was measured at 5, 10, and 15 d of drought stress following the method described by Fu et al. (2010). Briefly, leaf samples were soaked in deionized water for 12 h at 4 °C and blotted dry. Leaves were submerged in liquid nitrogen for rapid freezing and then stored at –20 °C until analyzed. Frozen samples were thawed for 30 min and cell sap was pressed using a laboratory press (Fred S. Carver, Wabash, IN). A 10-μL aliquot of the expressed sap was pipetted onto a filter paper disc that was placed in the sampling chamber of an osmometer (Wescor, Logan, UT) for analyzing solute concentration [C (millimoles per kilogram)]. Osmolarity of leaf sap was converted from millimoles per kilogram to megapascals using the formula: MPa = –C × 2.58 × 10−3 (Bajji et al., 2001)
Soluble sugar content of leaves was measured at 5, 10, and 15 d of drought stress using the spectrophotometric method described by Pei et al. (2010) with modifications. Briefly, 0.2 g fresh leaves were boiled in 10 mL distilled water for 30 min. The extract was filtered through two layers of cheesecloth. The filtrate (0.5 mL) was mixed with 1.5 mL distilled water and 1 mL of 9% phenol and 5 mL H2SO4. Tubes with this mixture were left at 23 °C for 30 min. The absorbance of the solution was measured with a spectrophotometer (Spectronic Genesys 2; Thermo Electron Corp., Madison, WI) at 485 nm. The soluble sugar concentration was determined using a standard curve.
Leaf net photosynthesis (Pn), transpiration rate (Tr), and gS were determined on the second fully expanded leaves at 0, 5, 10, and 15 d of drought and 2 and 6 d of rewatering using an infrared gas analyzer (LI-6400; LI-COR). Ten individual leaves attached to the plants were taken from each pot and were placed in the leaf chamber with a built-in red and blue light source of the LI-6400, and all measurements were taken on at the level of 800 μmol·m−2·s−1 photosynthetic photon flux. The CO2 concentration settings in leaf chamber were 400 (μmol·mol−1) for ambient CO2 treatments and 800 (μmol·mol−1) for elevated CO2 treatments, respectively. Leaf instantaneous water use efficiency (WUE) was calculated as the ratio of Pn to Tr. Stomatal opening and closure in response to elevated CO2 concentration, drought stress, and rewatering was examined on the second fully expanded from the top of plants from each treatment and photographs were taken using a camera fitted to a microscope (H600L; Nikon Instruments, Tokyo, Japan).
Rubisco extraction and activity assays were conducted using methods described by Hu et al. (2010) with modifications. Samples of fresh leaves (0.2 g) collected at 10 and 15 d of drought and 2 d of rewatering were immediately frozen in liquid nitrogen and kept at –80 °C before extraction. For extraction, the leaf tissue was ground in liquid nitrogen with 3 mL extraction buffer containing 50 mm Hepes-KOH (pH = 7.5 at 25 °C), 10 mm MgCl2, 2 mm ethylene diamine tetraacetic acid, 10 mm dithiothreitol, 10% glycerol (v/v), 1% bovine serum albumin (w/v), and 1% Triton X-100 (v/v) (Sigma, St. Louis, MO). The supernatant was isolated by centrifugation at 14,000 gn for 10 min at 4 °C and used immediately for Rubisco activity assays. Rubisco activity was measured by adding RuBP to the assay solution [100 mm Bicine (pH = 8.0), 25 mm KHCO3, 20 mm MgCl2, 3.5 mm adenosine-5′-triphosphate, 5 mm phosphocreatine, 5 units glyceraldehyde-3-phosphate dehydrogenase, 5 units 3-phosphoglyceric phosphokinase, 17.5 units creatine phosphokinase, and 0.25 mm NADH] and absorbance measured at 340 nm with a spectrophotometer (Helios Alpha; Thermospectronic, Rochester, NY). Initial activity was measured immediately after adding RuBP and total activity was measured after incubating samples at 25 °C for 5 min. Rubisco activation state was determined by the ratio of initial to total activity. Fifty microliters of leaf extract was used to determine soluble protein concentration (Bradford, 1976). Total soluble protein was estimated from standard curves prepared with bovine serum albumin and used to calculate Rubisco activity expressed as micromoles CO2 per second per milligram protein.
Statistical analysis.
All data were analyzed using SAS statistics software (Version 9.0; SAS Institute, Cary, NC). The analysis of variance with a fixed model was used to determine differences among treatments in RWC, Pn, gS, Tr, WUE, ψS, soluble sugar content, Rubisco activity, and Rubisco activation state. Means of different treatments were tested with least significant difference at a probability level of 0.05.
Results and Discussion
Effects of elevated CO2 concentration on leaf internal water status and water use efficiency under drought stress and rewatering.
Leaf water retention capacity, water use rate, and WUE are important factors controlling plant water status during drought stress and ultimately whole-plant drought resistance. RWC is a good indicator of leaf hydration status and the level of drought tolerance (Flexas and Medrano, 2002). In this study, RWC of tall fescue was maintained at an average of 89.87% in well-watered plants during the experimental period, regardless of the CO2 treatments, but it declined below the well-watered control level after 10 d of drought stress in both ambient and elevated CO2 treatments (Fig. 1). The decline in RWC was less severe in the elevated CO2 with drought treatment than in the ambient CO2 with drought treatment. A significantly higher leaf RWC in tall fescue was observed in elevated CO2 with drought-stressed plants than those in ambient CO2 with drought treated at 10 and 15 d of drought stress. After 2 d of rewatering, plants treated with elevated CO2 under drought recovered more rapidly than those treated with ambient CO2 under drought. These results suggested that leaves treated with elevated CO2 had a greater capacity of retaining water or resistance to dehydration during drought stress as well as rapid rehydration or recovery potential on rewatering.
Changes in leaf relative water content (RWC) of tall fescue in response to 15 d of drought stress and 6 d of rewatering under ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1). 800-D = drought-stressed plants under elevated CO2 concentration; 800-W = well-watered plants under elevated CO2 concentration; 400-D = drought-stressed plants under ambient CO2 concentration; 400-W = well-watered plants under ambient CO2 concentration. Vertical bars indicate least significant different values (P ≤ 0.05) for the comparison among treatments at a given day of drought or rewatering treatment.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Changes in leaf relative water content (RWC) of tall fescue in response to 15 d of drought stress and 6 d of rewatering under ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1). 800-D = drought-stressed plants under elevated CO2 concentration; 800-W = well-watered plants under elevated CO2 concentration; 400-D = drought-stressed plants under ambient CO2 concentration; 400-W = well-watered plants under ambient CO2 concentration. Vertical bars indicate least significant different values (P ≤ 0.05) for the comparison among treatments at a given day of drought or rewatering treatment.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Changes in leaf relative water content (RWC) of tall fescue in response to 15 d of drought stress and 6 d of rewatering under ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1). 800-D = drought-stressed plants under elevated CO2 concentration; 800-W = well-watered plants under elevated CO2 concentration; 400-D = drought-stressed plants under ambient CO2 concentration; 400-W = well-watered plants under ambient CO2 concentration. Vertical bars indicate least significant different values (P ≤ 0.05) for the comparison among treatments at a given day of drought or rewatering treatment.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Cellular hydration status of leaves during drought stress depends on the balance between water retention and water loss, which is governed by stomatal behaviors and osmotic adjustment among various other physiological factors (Farooq et al., 2009; Kramer and Boyer, 1995). Osmotic adjustment helps to maintain the cell water balance with active accumulation of solutes in the cytoplasm, thereby minimizing the harmful effects of drought (Clifford et al., 1998). Elevated CO2 could alleviate leaf dehydration through osmotic adjustment under drought stress. In this study, elevated CO2 treatment promoted osmotic adjustment, as shown by the decreases in the level of ψS under drought stress conditions (Fig. 2). Osmotic adjustment is associated with the accumulation of some osmoregulation substances such as water-soluble carbohydrates (Geissler et al., 2009; Pérez-López et al., 2010; Yoshiba et al., 1997). Soluble sugars are the predominant forms of osmoregulants in C3 perennial grasses during the early phase or a moderate level of drought stress (DaCosta and Huang, 2006) and decreases in ψS during drought stress are accompanied by increases in sucrose content in tall fescue (Fu et al., 2010). In the present study, soluble sugar content increased along with lowering ψS under elevated CO2 concentration, particularly under drought conditions (Fig. 3). These results suggested that elevated CO2 could facilitate osmotic adjustment in association with soluble sugar accumulation and consequently retained more water in leaves of tall fescue. Other solutes such as proline and glycine betaine also play roles in osmotic adjustment. How elevated CO2 may affect osmotic adjustment in relation to the accumulation of those solutes is yet to be determined. Several previous studies also reported enhanced osmotic adjustment by elevated CO2 in different plant species (Ferris and Taylor, 1994; Pérez-López et al., 2010). Some other studies have demonstrated that the elevated CO2 concentration had minimal effect on ψS during drought stress (Aranda et al., 2008; Polley et al., 1999; Robredo et al., 2007), and a few studies even showed decreased osmotic adjustment in response to drought under elevated CO2 concentration (Tschaplinski, et al., 1993, 1995). The variable results of elevated CO2 effects on osmotic adjustment under drought stress from different studies may be the result of different plant species, growth stages, drought severity levels or duration as well as CO2 concentrations examined in different experiments. Nevertheless, our results suggested that the enhanced osmotic adjustment in association with sugar accumulation by elevated CO2 concentration could play roles in maintaining greater RWC in tall fescue leaves under drought stress in this study.
Osmotic potential of leaves in tall fescue at 15 d of drought stress under ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1). 800-D = drought-stressed plants under elevated CO2 concentration; 800-W = well-watered plants under elevated CO2 concentration; 400-D = drought-stressed plants under ambient CO2 concentration; 400-W = well-watered plants under ambient CO2 concentration. Columns marked with different letters were significantly different among treatments at a given day of treatment based on a least significant difference test at P ≤ 0.05.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Osmotic potential of leaves in tall fescue at 15 d of drought stress under ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1). 800-D = drought-stressed plants under elevated CO2 concentration; 800-W = well-watered plants under elevated CO2 concentration; 400-D = drought-stressed plants under ambient CO2 concentration; 400-W = well-watered plants under ambient CO2 concentration. Columns marked with different letters were significantly different among treatments at a given day of treatment based on a least significant difference test at P ≤ 0.05.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Osmotic potential of leaves in tall fescue at 15 d of drought stress under ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1). 800-D = drought-stressed plants under elevated CO2 concentration; 800-W = well-watered plants under elevated CO2 concentration; 400-D = drought-stressed plants under ambient CO2 concentration; 400-W = well-watered plants under ambient CO2 concentration. Columns marked with different letters were significantly different among treatments at a given day of treatment based on a least significant difference test at P ≤ 0.05.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Total soluble sugar content of leaves in tall fescue at 15 d of drought stress under ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1). 800-D = drought-stressed plants under elevated CO2 concentration; 800-W = well-watered plants under elevated CO2 concentration; 400-D = drought-stressed plants under ambient CO2 concentration; 400-W = well-watered plants under ambient CO2 concentration. Columns marked with different letters were significantly different among treatments at a given day of treatment based on a least significant difference test at P ≤ 0.05.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Total soluble sugar content of leaves in tall fescue at 15 d of drought stress under ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1). 800-D = drought-stressed plants under elevated CO2 concentration; 800-W = well-watered plants under elevated CO2 concentration; 400-D = drought-stressed plants under ambient CO2 concentration; 400-W = well-watered plants under ambient CO2 concentration. Columns marked with different letters were significantly different among treatments at a given day of treatment based on a least significant difference test at P ≤ 0.05.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Total soluble sugar content of leaves in tall fescue at 15 d of drought stress under ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1). 800-D = drought-stressed plants under elevated CO2 concentration; 800-W = well-watered plants under elevated CO2 concentration; 400-D = drought-stressed plants under ambient CO2 concentration; 400-W = well-watered plants under ambient CO2 concentration. Columns marked with different letters were significantly different among treatments at a given day of treatment based on a least significant difference test at P ≤ 0.05.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Elevated CO2 concentration could also alleviate leaf dehydration through lowering the rate of water loss. In this study, Tr (Fig. 4A) was suppressed by elevated CO2 under both well-watered conditions and drought stress. Drought stress led to a rapid decline in Tr under both CO2 regimes. Plants under the elevated CO2 treatment had significantly lower Tr than those under the ambient CO2 treatment during the first 10 d of drought stress and by 15 d of drought stress, Tr declined to the lowest level and there were no significant differences between the two CO2 treatments. The reduction in Tr by elevated CO2 concentration also has been reported by others (Assmann, 1993; Morison and Gifford, 1983; Tyree and Alexander, 1993). Along with Tr decline, gS decreased in plants exposed to both drought and elevated CO2 but increased during rewatering (Fig. 4B).
Changes in transpiration rate [Tr (A)], stomatal conductance [gS (B)], and water use efficiency [WUE (C)] of tall fescue in response to 15 d of drought stress and 6 d of rewatering under ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1). 800-D = drought-stressed plants under elevated CO2 concentration; 800-W = well-watered plants under elevated CO2 concentration; 400-D = drought-stressed plants under ambient CO2 concentration; 400-W = well-watered plants under ambient CO2 concentration. Vertical bars indicate least significant different values (P ≤ 0.05) for the comparison among treatments at a given day of drought or rewatering treatment.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Changes in transpiration rate [Tr (A)], stomatal conductance [gS (B)], and water use efficiency [WUE (C)] of tall fescue in response to 15 d of drought stress and 6 d of rewatering under ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1). 800-D = drought-stressed plants under elevated CO2 concentration; 800-W = well-watered plants under elevated CO2 concentration; 400-D = drought-stressed plants under ambient CO2 concentration; 400-W = well-watered plants under ambient CO2 concentration. Vertical bars indicate least significant different values (P ≤ 0.05) for the comparison among treatments at a given day of drought or rewatering treatment.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Changes in transpiration rate [Tr (A)], stomatal conductance [gS (B)], and water use efficiency [WUE (C)] of tall fescue in response to 15 d of drought stress and 6 d of rewatering under ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1). 800-D = drought-stressed plants under elevated CO2 concentration; 800-W = well-watered plants under elevated CO2 concentration; 400-D = drought-stressed plants under ambient CO2 concentration; 400-W = well-watered plants under ambient CO2 concentration. Vertical bars indicate least significant different values (P ≤ 0.05) for the comparison among treatments at a given day of drought or rewatering treatment.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
The partial closure of stomata induced by elevated CO2 may contribute to the increases in leaf instantaneous WUE in tall fescue. In this study, WUE of tall fescue increased with drought stress under both CO2 treatments, and it was significantly enhanced by elevated CO2 under drought stress (Fig. 4C). The WUE in plants treated with the elevated CO2 concentration was 1.54-, 2.09-, and 1.85-fold greater than those plants under the ambient CO2 treatment at 5, 10, and 15 d of drought stress, respectively. Under rewatering conditions, no differences in WUE were detected in the two CO2 treatments. Improved WUE by elevated CO2 concentration has also been reported in various plant species exposed to drought stress (Centritto et al., 2002; Chun et al., 2011; Ghannoum et al., 2002; Laila and Adel, 2002; Retuerto and Woodward, 1993; Tschaplinski et al., 1995). WUE has been suggested as an effective criterion to select for superior drought-performing turfgrasses (Ebdon and Kopp, 2004). The improvement in WUE with elevated CO2 has a great potential for water conservation and reducing irrigation in turfgrass management.
Stomata remained open in well watered and excessive CO2 conditions (Fig. 5A) but closed under drought stress with ambient CO2 (Fig. 5B) or under elevated CO2 with adequate water (Fig. 5C) or under drought stress with elevated CO2 (Fig. 5D). On rewatering, stomata of drought-stressed leaves reopened under ambient CO2 (Fig. 5E) but remained at least partially closed under elevated CO2 (Fig. 5F). These results suggested that elevated CO2 concentration could induce stomata closure at least partially, leading to the reduction in the rate of water loss through transpiration either under well-watered conditions or drought stress. However, CO2-induced stomatal closure may inhibit photosynthetic capacity under drought stress and photosynthetic recovery after rewatering, as discussed subsequently.
Microscopic images of leaf epidermis showing stomatal aperture of tall fescue leaves in well-watered plants with ambient CO2 concentration (400 μmol·mol−1) (A), drought-stressed plants (10 d) with ambient CO2 concentration (B), well-watered plants with elevated CO2 concentration (800 μmol·mol−1) (C), drought-stressed plants (10 d) with elevated CO2 concentration (D), rewatered plants (6 d) with ambient CO2 concentration (E), and rewatered plants (6 d) with elevated CO2 concentration (F).
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Microscopic images of leaf epidermis showing stomatal aperture of tall fescue leaves in well-watered plants with ambient CO2 concentration (400 μmol·mol−1) (A), drought-stressed plants (10 d) with ambient CO2 concentration (B), well-watered plants with elevated CO2 concentration (800 μmol·mol−1) (C), drought-stressed plants (10 d) with elevated CO2 concentration (D), rewatered plants (6 d) with ambient CO2 concentration (E), and rewatered plants (6 d) with elevated CO2 concentration (F).
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Microscopic images of leaf epidermis showing stomatal aperture of tall fescue leaves in well-watered plants with ambient CO2 concentration (400 μmol·mol−1) (A), drought-stressed plants (10 d) with ambient CO2 concentration (B), well-watered plants with elevated CO2 concentration (800 μmol·mol−1) (C), drought-stressed plants (10 d) with elevated CO2 concentration (D), rewatered plants (6 d) with ambient CO2 concentration (E), and rewatered plants (6 d) with elevated CO2 concentration (F).
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Effects of elevated CO2 concentration on photosynthetic rate and carboxylation enzyme activity under drought stress and rewatering.
Leaf Pn of tall fescue was enhanced by elevated CO2 under both well-watered conditions and drought stress (Fig. 6A). Leaf Pn under both CO2 regimes declined during 15-d drought stress, but the decline was less pronounced (36% reduction from 8.21 to 5.23 μmol·m−2·s−1) under the elevated CO2 treatment than that under the ambient CO2 treatment (58% reduction from 7.42 to 3.12 μmol·m−2·s−1). Elevated CO2-enhanced Pn has been reported under non-stress and stress conditions in other plant species (Albert et al., 2011; Campbell et al., 1988; Sage et al., 1989; Vu et al., 1997), but few reported effects of elevated CO2 on recovery in photosynthesis in response to rewatering (Widodo et al., 2003). In this study, on rewatering, leaf Pn with the elevated CO2 treatment recovered more rapidly than that did with the ambient CO2 treatment, and at 2 d of rewatering, Pn of elevated CO2-treated plants was ≈37% greater than that under ambient CO2 treatment. However, leaves under elevated CO2 still maintained lower gS compared with those exposed to ambient CO2 after rewatering, which could have limited the recovery potential in Pn (Fig. 6A) even in fully rehydrated leaves (Fig. 1) during rewatering. Factors regulating the recovery in Pn under elevated CO2 concentration on rewatering after prolonged periods of drought stress were not well understood. The results in this study indicated that greater Pn during rewatering in CO2-enriched plants relative to that under ambient CO2 was not the result of the elimination in stomatal limitation, but could be the result of other factors such as metabolic activity of carboxylation.
Changes in net photosynthetic rate (Pn) of tall fescue leaves in response to 15 d of drought stress and 6 d of rewatering under ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1) (A), and effects of elevated CO2 concentration on Rubisco activity (B) and Rubisco activation state (C) at 10 and 15 d of drought and 2 d of rewatering. 800-D = drought-stressed plants under elevated CO2 concentration; 800-W = well-watered plants under elevated CO2 concentration; 400-D = drought-stressed plants under ambient CO2 concentration; 400-W = well-watered plants under ambient CO2 concentration. Vertical bars in (A) indicate least significant different (lsd) values (P ≤ 0.05) for the comparison among treatments at a given day of drought or rewatering treatment. Columns in (B) and (C) marked with different letters were significantly different among treatments at a given day of treatment based on a lsd test (P ≤ 0.05).
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Changes in net photosynthetic rate (Pn) of tall fescue leaves in response to 15 d of drought stress and 6 d of rewatering under ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1) (A), and effects of elevated CO2 concentration on Rubisco activity (B) and Rubisco activation state (C) at 10 and 15 d of drought and 2 d of rewatering. 800-D = drought-stressed plants under elevated CO2 concentration; 800-W = well-watered plants under elevated CO2 concentration; 400-D = drought-stressed plants under ambient CO2 concentration; 400-W = well-watered plants under ambient CO2 concentration. Vertical bars in (A) indicate least significant different (lsd) values (P ≤ 0.05) for the comparison among treatments at a given day of drought or rewatering treatment. Columns in (B) and (C) marked with different letters were significantly different among treatments at a given day of treatment based on a lsd test (P ≤ 0.05).
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Changes in net photosynthetic rate (Pn) of tall fescue leaves in response to 15 d of drought stress and 6 d of rewatering under ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1) (A), and effects of elevated CO2 concentration on Rubisco activity (B) and Rubisco activation state (C) at 10 and 15 d of drought and 2 d of rewatering. 800-D = drought-stressed plants under elevated CO2 concentration; 800-W = well-watered plants under elevated CO2 concentration; 400-D = drought-stressed plants under ambient CO2 concentration; 400-W = well-watered plants under ambient CO2 concentration. Vertical bars in (A) indicate least significant different (lsd) values (P ≤ 0.05) for the comparison among treatments at a given day of drought or rewatering treatment. Columns in (B) and (C) marked with different letters were significantly different among treatments at a given day of treatment based on a lsd test (P ≤ 0.05).
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 140, 1; 10.21273/JASHS.140.1.19
Rubisco is the key enzyme for carbon fixation of photosynthesis, and its activity is controlled by another enzyme, Rubisco activase. Rubisco activity showed a significant decline compared with the well-watered control at 10 and 15 d of drought stress under both CO2 treatments (Fig. 6B). No significant differences were observed in Rubisco activity between ambient CO2 and elevated CO2 treatments under well-watered conditions, but Rubisco activity was significantly higher in CO2-enriched plants than those with ambient CO2 at 10 and 15 d of drought stress (Fig. 6B). Rubisco activation state decreased to a significantly lower level at 10 and 15 d of drought stress than that of the well-watered control plants under both ambient and elevated CO2 treatments (Fig. 6C). The drought-induced decline in Rubisco activation state was to a lesser extent under the elevated CO2 (reduction by 19%) than that under ambient CO2 treatment (reduction by 35%). Drought-induced decline in Rubisco activity and activation state has been reported in other plant species (Erice et al., 2006; Vu et al., 1998). At 10 and 15 d of drought stress, Rubisco activation state was significantly greater under the elevated CO2 treatment than that under the ambient CO2 treatment, suggesting that increasing CO2 concentration could enhance the photosynthetic capacity of tall fescue by stimulating Rubisco carboxylation activities under drought stress. Previous studies reported enhancement in leaf photosynthesis by elevated CO2 under heat stress was associated with the up-regulation of Rubisco (Prasad et al., 2009). After 2 d of rewatering, both Rubisco activity and activation state increased to a significantly higher level than that at 15 d of drought but did not fully recover to the well-watered control level under either ambient or elevated CO2 treatment. Both Rubisco activity and activation state were significantly greater in plants exposed to elevated CO2 than those under ambient CO2 concentration at 2 d of rewatering. These results indicated that the resumption of Rubisco activity and activation state could contribute to the greater recovery of Pn on rewatering after drought stress, particularly under elevated CO2.
In summary, elevated CO2 improved water status in tall fescue during drought stress, which could be the result of CO2-induced stomatal closure with limited transpirational water loss and enhanced osmotic adjustment in association with soluble sugar accumulation. Elevated CO2 enhanced net photosynthetic rate in tall fescue leaves even with lower gS but enhanced Rubisco and Rubisco activase activity during both drought and rewatering. The mitigating effects of elevated CO2 on drought inhibition of photosynthesis and the enhanced recovery in photosynthesis on rewatering were mainly the result of the elimination of metabolic limitation from drought damages associated with increased enzyme activities for carboxylation.
Literature Cited
Albert, K.R., Ro-Poulsen, H., Mikkelsen, T.N., Michelsen, A., van der Linden, L. & Beier, C. 2011 Interactive effects of elevated CO2, warming, and drought on photosynthesis of Deschampsia flexuosa in a temperate heath ecosystem J. Expt. Bot. 62 4253 4266
Aranda, I., Puértolas, J., Pardos, M. & Pardos, J.A. 2008 Elevated atmospheric CO2 does not modify osmotic adjustment to light and drought in the Mediterranean oak Quercus suber L For. Systems 17 3 9
Assmann, S.M. 1993 Signal transduction in guard cells Annu. Rev. Cell Biol. 9 345 375
Bagatta, M., Pacifico, D. & Mandolino, G. 2008 Evaluation of the osmotic adjustment response within the genus Beta J. Sugar Beet Res. 45 119 133
Bajji, M., Lutts, S. & Kinet, J.M. 2001 Water deficit effects on solute contribution to osmotic adjustment as a function of leaf ageing in three durum wheat (Triticum durum Desf.) cultivars performing differently in arid conditions Plant Sci. 160 669 681
Bradford, M.M. 1976 A rapid and sensitive method for quantization of microgram quantities of protein utilizing the principle of protein-dye binding Anal. Biochem. 72 248 254
Campbell, W.J., Allen, L.H. Jr & Bowes, G. 1988 Effects of CO2 concentration on Rubisco activity, amount, and photosynthesis in soybean leaves Plant Physiol. 88 1310 1316
Centritto, M., Lucas, M.E. & Jarvis, P.G. 2002 Gas exchange, biomass, whole-plant water-use efficiency and water uptake of peach (Prunus persica) seedlings in response to elevated carbon dioxide concentration and water availability Tree Physiol. 22 699 706
Chun, J.A., Wang, Q.G., Timlin, D., Fleisher, D. & Reddy, V.R. 2011 Effect of elevated carbon dioxide and water stress on gas exchange and water use efficiency in corn Agr. For. Meteorol. 151 378 384
Clifford, S.C., Arndt, S.K., Corlett, J.E., Joshi, S., Sankhla, N., Popp, M. & Jones, H.G. 1998 The role of solute accumulation, osmotic adjustment and changes in cell wall elasticity in drought tolerance in Ziziphus mauritiana (Lamk.) J. Expt. Bot. 49 967 977
DaCosta, M. & Huang, B. 2006 Changes in carbon partitioning and accumulation patterns during drought and recovery for colonial bentgrass, creeping bentgrass, and velvet bentgrass J. Amer. Soc. Hort. Sci. 131 484 490
Ebdon, J.S. & Kopp, K.L. 2004 Relationships between water use efficiency, carbon isotope discrimination, and turf performance in genotypes of kentucky bluegrass during drought Crop Sci. 44 1754 1762
Erice, G., Irigoyen, J.J., Perez, P., Martinez-Carrasco, R. & Sanchez-Diaz, M. 2006 Effect of elevated CO2, temperature and drought on photosynthesis of nodulated alfalfa during a cutting regrowth cycle Physiol. Plant. 126 458 468
Farooq, M., Wahid, A., Kobayashi, N., Fujita, D. & Basra, S.M.A. 2009 Plant drought stress: Effects, mechanisms and management Agron. Sustainable Dev. 29 185 212
Ferris, R. & Taylor, G. 1994 Elevated CO2, water relations and biophysics of leaf extension in four chalk grassland herbs New Phytol. 127 297 307
Flexas, J., Bota, J., Loreto, F., Cornic, G. & Sharkey, T.D. 2004 Diffusive and metabolic limitations to photosynthesis under drought and salinity in C3 plants Plant Biol. 6 269 279
Flexas, J. & Medrano, H. 2002 Drought-inhibition of photosynthesis in C3 plants: Stomatal and non-stomatal limitations revisited Ann. Bot. (Lond.) 89 183 189
Fry, J. & Huang, B. 2004 Applied turfgrass science and physiology. Wiley, Hoboken, NJ
Fu, J.M., Huang, B. & Fry, J. 2010 Osmotic potential, sucrose level, and activity of sucrose metabolic enzymes in tall fescue in response to deficit irrigation J. Amer. Soc. Hort. Sci. 135 506 510
Geissler, N., Hussin, S. & Koyro, H. 2009 Interactive effects of NaCl salinity and elevated atmospheric CO2 concentration on growth, photosynthesis, water relations and chemical composition of the potential cash crop halophyte Aster tripolium L Environ. Expt. Bot. 65 220 231
Ghannoum, O., von Caemmererand, S. & Conroy, J.P. 2002 The effect of drought on plant water use efficiency of nine NAD–ME and nine NADP–ME Australian C4 grasses Funct. Plant Biol. 29 1337 1348
Hoagland, D.R. & Arnon, D.I. 1950 The water-culture method for growing plans without soil. California Agr. Expt. Sta. Circ. 347
Hu, L.X., Wang, Z.L. & Huang, B. 2010 Diffusion limitations and metabolic factors associated with inhibition and recovery of photosynthesis from drought stress in a C3 perennial grass species Physiol. Plant. 139 93 106
Kirkham, M.B. 2011 Elevated carbon dioxide impact on soil and plant water relations. CRC Press, Boca Raton, FL
Kramer, P.J. & Boyer, J. 1995 Water relations of plants and soils. Academic Press, San Diego, CA
Laila, E.A. & Adel, E.A. 2002 Effect of elevated CO2 and drought on proline metabolism and growth of saf-flower (Carthamus mareoticus L.) seedlings without improving water status Pak. J. Biol. Sci. 5 523 528
Leakey, A.D.B., Uribelarrea, M., Ainsworth, E.A., Naidu, S.L., Rogers, A., Ort, D.R. & Long, S.P. 2006 Photosynthesis, productivity, and yield of maize are not affected by open-air elevation of CO2 concentration in the absence of drought Plant Physiol. 140 779 790
Morison, J.I.L. & Gifford, R.M. 1983 Stomatal sensitivity to carbon dioxide and humidity Plant Physiol. 71 789 796
Nilsen, E.T. & Orcutt, D.M. 1996 The physiology of plants under stress. Wiley, New York, NY
Pei, Z.F., Ming, D.F., Liu, D., Wan, G.L., Geng, X.X., Gong, H.J. & Zhou, W.J. 2010 Silicon improves the tolerance to water-deficit stress induced by polyethylene glycol in wheat (Triticum aestivum L.) seedlings J. Plant Growth Regul. 29 106 115
Pérez-López, U., Robredo, A., Lacuesta, M., Muñoz-Rueda, A. & Mena-Petite, A. 2010 Atmospheric CO2 concentration influences the contributions of osmolyte accumulation and cell wall elasticity to salt tolerance in barley cultivars J. Plant Physiol. 167 15 22
Polley, H.W., Tischler, C.R., Johnson, H.B. & Pennington, R.E. 1999 Growth, water relations, and survival of drought exposed seedlings from six maternal families of honey mesquite (Prosopis glandulosa): Response to CO2 enrichment Tree Physiol. 19 359 366
Prasad, P.V.V., Vu, J.C.V., Boote, K.J. & Allen, L.H. Jr 2009 Enhancement in leaf photosynthesis and upregulation of Rubisco in the C4 sorghum plant at elevated growth carbon dioxide and temperature occur at early stages of leaf ontogeny Funct. Plant Biol. 36 761 769
Qian, Y.L., Fry, J.D. & Upham, W.S. 1997 Rooting and drought avoidance of warm-season turfgrasses and tall fescue in Kansas Crop Sci. 37 905 910
Reddy, A.R., Rasineni, G.K. & Raghavendra, A.S. 2010 The impact of global elevated CO2 concentration on photosynthesis and plant productivity Curr. Sci. 99 46 57
Retuerto, R. & Woodward, F.I. 1993 The influences of increased CO2 and water supply on growth, biomass allocation and water use efficiency of Sinapis alba L. grown under different wind speeds Oecologia 94 415 427
Robredo, A., Pérez-López, U., Sainz de la Maza, H., González-Moro, B., Lacuesta, M., Mena-Petite, A. & Muñoz-Rueda, A. 2007 Elevated CO2 alleviates the impact of drought on barley improving water status by lowing stomatal conductance and delaying its effects on photosynthesis Environ. Expt. Bot. 59 252 263
Sage, R.F., Sharkey, T.D. & Seemann, J.R. 1989 Acclimation of photosynthesis to elevated CO2 in five C3 species Plant Physiol. 89 590 596
Song, Y., Yu, J. & Huang, B. 2014 Elevated CO2-mitigation of high temperature stress associated with maintenance of positive carbon balance and carbohydrate accumulation in kentucky bluegrass PLoS One 9 e89725
Tschaplinski, T.J., Norby, R.J. & Wullschleger, S.D. 1993 Responses of loblolly pine seedlings to elevated CO2 and fluctuating water supply Tree Physiol. 13 283 296
Tschaplinski, T.J., Stewart, D.B. & Norby, R.J. 1995 Interactions between drought and elevated CO2 on osmotic adjustment and solute concentrations of tree seedlings New Phytol. 131 169 177
Tyree, M.T. & Alexander, J.D. 1993 Plant water relations and the effects of elevated CO2: A review and suggestions for future research Vegetatio 104 47 62
Vu, J.C., Allen, L.H. Jr, Boote, K.J. & Bowes, G. 1997 Effects of elevated CO2 and temperature on photosynthesis and Rubisco in rice and soybean Plant Cell Environ. 20 68 76
Vu, J.C.V., Baker, J.T., Pennanen, A.H., Allen, L.H. Jr, Bowes, G. & Boote, K.J. 1998 Elevated CO2 and water deficit effects on photosynthesis, ribulose bisphosphate carboxylase-oxygenase, and carbohydrate metabolism in rice Physiol. Plant. 103 327 339
Widodo, W., Vu, J.C.V., Boote, K.J., Baker, J.T. & Allen, L.H. Jr 2003 Elevated growth CO2 delays drought stress and accelerates recovery of rice leaf photosynthesis Environ. Expt. Bot. 49 259 272
Wullschleger, S.D., Tschaplinski, T.J. & Norby, R.J. 2002 Plant water relations at elevated CO2-implications for water-limited environment Plant Cell Environ. 25 319 331
Yoshiba, Y., Kiyosue, T., Nakashima, K., Yamaguchi-Shinozaki, K. & Shinozaki, K. 1997 Regulation of levels of proline as an osmolyte in plants under water stress Plant Cell Physiol. 38 1095 1102
Yu, J., Chen, L., Xu, M. & Huang, B. 2012a Effects of elevated CO2 on physiological responses of tall fescue (Festuca arundinacea) to elevated temperature, drought stress, and the combined stresses Crop Sci. 52 1848 1858
Yu, J., Du, H., Xu, M. & Huang, B. 2012b Metabolic responses to heat stress under elevated atmospheric CO2 concentration in a cool-season grass species J. Amer. Soc. Hort. Sci. 137 221 228
Yu, J., Yang, Z., Jespersen, D. & Huang, B. 2014 Photosynthesis and protein metabolism associated with elevated CO2-mitigation of heat stress damages in tall fescue Environ. Expt. Bot. 99 75 85
