Interaction of CO 2 concentrations and water stress in 1 semi-arid plants causes diverging response in instantaneous 2 water use efficiency and carbon isotope composition 3

In the context of global warming attributable to the increasing levels of CO2, severe drought 9 may be more frequent in areas with chronic water shortages (semi-arid areas). This necessitates 10 research on the interactions between increased levels of CO2 and drought on plant photosynthesis. It is 11 commonly reported that C fractionation occurred as CO2-gas diffuses from the atmosphere to the 12 sub-stomatal cavity. Few researchers have investigated C fractionation at the site of carboxylation to 13 cytoplasm before sugars are exported outward from the leaf. This process typically progresses in 14 response to variations in environmental conditions (i.e., CO2 concentrations and water stress), 15 including in their interaction. Therefore, saplings of two typical plant species (Platycladus orientalis 16 and Quercus variabilis) from semi-arid areas of Northern China were selected and cultivated in growth 17 chambers with orthogonal treatments (four CO2 concentrations ([CO2]) × five soil volumetric water 18 contents (SWC)). The δC of water-soluble compounds extracted from leaves of saplings was 19 determined for instantaneous water use efficiency (WUEcp) after cultivation. Instantaneous water use 20 efficiency derived from gas exchange (WUEge) was integrated to estimate differences in δC signal 21 variation before leaf-exported translocation of primary assimilates. The WUEge of Platycladus 22 orientalis and Quercus variabilis both decreased with increased soil moisture at 35%–80% of field 23 capacity (FC), and increased with elevated [CO2] by increasing photosynthetic capacity and reducing 24 transpiration. Instantaneous water use efficiency (iWUE) according to environmental changes, differed 25 between the two species. The WUEge in P. orientalis was significantly greater than that in Q. variabilis, 26 while an opposite trend was observed when comparing WUEcp between the two species. Total C 27 fractionation at the site of carboxylation to cytoplasm before sugar export (total C fractionation) was 28 species-specific, as demonstrated in the interaction of [CO2] and SWC. Rising [CO2] coupled with 29 moistened soil generated increasing disparities in δC between water-soluble compounds (δCWSC) 30 and estimates based on gas-exchange observations (δCobs) in P. orientalis, ranging between 31 0.0328‰–0.0472‰. Differences between δCWSC and δCobs in Q. variabilis increased as [CO2] and 32 SWC increased (0.0384‰–0.0466‰). The C fractionations from mesophyll conductance (gm) and 33 post-carboxylation both contributed to the total C fractionation that was determined by δC of 34 water-soluble compounds and gas-exchange measurement. Total C fractionation was linearly 35 dependent on stomatal conductance, indicating post-carboxylation fractionation could be attributed to 36 environmental variation. The magnitude and environmental dependence of apparent post-carboxylation 37 fractionation is worth our attention when addressing photosynthetic fractionation. 38

Abstract.In the context of global warming attributable to the increasing levels of CO 2 , severe drought may be more frequent in areas that already experience chronic water shortages (semiarid areas).This necessitates research on the interactions between increased levels of CO 2 and drought and their effect on plant photosynthesis.It is commonly reported that 13 C fractionation occurs as CO 2 gas diffuses from the atmosphere to the substomatal cavity.Few researchers have investigated 13 C fractionation at the site of carboxylation to cytoplasm before sugars are exported outward from the leaf.This process typically progresses in response to variations in environmental conditions (i.e., CO 2 concentrations and water stress), including in their interaction.Therefore, saplings of two typical plant species (Platycladus orientalis and Quercus variabilis) from semiarid areas of northern China were selected and cultivated in growth chambers with orthogonal treatments (four CO 2 concentration ([CO 2 ]) × five soil volumetric water content (SWC)).The δ 13 C of water-soluble compounds extracted from leaves of saplings was determined for an assessment of instantaneous water use efficiency (WUE cp ) after cultivation.Instantaneous water use efficiency derived from gas-exchange measurements (WUE ge ) was integrated to estimate differences in δ 13 C signal variation before leaf-level translocation of primary assimilates.The WUE ge values in P. orientalis and Q. variabilis both decreased with increased soil moisture at 35-80 % of field capacity (FC) and increased with elevated [CO 2 ] by increasing photosynthetic capacity and reducing transpiration.Instanta-neous water use efficiency (iWUE) according to environmental changes differed between the two species.The WUE ge in P. orientalis was significantly greater than that in Q. variabilis, while an opposite tendency was observed when comparing WUE cp between the two species.Total 13 C fractionation at the site of carboxylation to cytoplasm before sugar export (total 13 C fractionation) was species-specific, as demonstrated in the interaction of [CO 2 ] and SWC.Rising [CO 2 ] coupled with moistened soil generated increasing disparities in δ 13 C between water-soluble compounds (δ 13 C WSC ) and estimates based on gas-exchange observations (δ 13 C obs ) in P. orientalis, ranging between 0.0328 and 0.0472 ‰.Differences between δ 13 C WSC and δ 13 C obs in Q. variabilis increased as [CO 2 ] and SWC increased (0.0384-0.0466 ‰).The 13 C fractionation from mesophyll conductance (g m ) and post-carboxylation both contributed to the total 13 C fractionation that was determined by δ 13 C of water-soluble compounds and gas-exchange measurements.Total 13 C fractionation was linearly dependent on stomatal conductance, indicating that post-carboxylation fractionation could be attributed to environmental variation.The magnitude and environmental dependence of apparent post-carboxylation fractionation is worth our attention when addressing photosynthetic fractionation.

Introduction
Since the industrial revolution, atmospheric CO 2 concentration has increased at an annual rate of 0.4 % and is expected to increase to 700 µmol mol −1 , culminating in more frequent periods of dryness (IPCC, 2014).Increasing atmospheric CO 2 concentrations that exacerbate the greenhouse effect will increase fluctuations in global precipitation patterns, which will probably amplify drought frequency in arid regions and lead to more frequent extreme flooding events in humid regions (Lobell et al., 2014).Accompanying the increasing concentration of CO 2 , mean δ 13 C of atmospheric CO 2 is currently being depleted by 0.02-0.03‰ yr −1 (CU-INSTAAR/NOAA CMDL network for atmospheric CO 2 ; http://www.esrl.noaa.gov/gmd/).
The current carbon isotopic composition may respond to environmental change and its influence on diffusion via plant physiological and metabolic processes (Gessler et al., 2014;Streit et al., 2013).While depletion of δ 13 C CO 2 occurs in the atmosphere, variations in CO 2 concentration ([CO 2 ]) may affect δ 13 C of plant organs, which in turn respond physiologically to changes in climate (Gessler et al., 2014).The carbon discrimination ( 13) in leaves could also provide timely feedback to the availability of soil moisture and atmospheric vapor pressure deficit (Cernusak et al., 2013).Discrimination of 13 C in leaves relies mainly on environmental factors that affect the ratio of intercellular to ambient [CO 2 ] (C i / C a ).Rubisco activities and the mesophyll conductance derived from the difference of [CO 2 ] between intercellular sites and chloroplasts are also involved (Farquhar et al., 1982;Cano et al., 2014).Changes in environmental conditions affect photosynthetic discrimination, recording differentially in the δ 13 C of water-soluble compounds (δ 13 C WSC ) in different plant organs.Several processes during photosynthesis alter the δ 13 C of carbon transported within plants.Carbon fractionation during photosynthetic CO 2 fixation has been reviewed elsewhere (Farquhar et al., 1982;Farquhar and Sharkey, 1982).
Post-photosynthetic fractionation is derived from equilibrium and kinetic isotopic effects that determine isotopic differences between metabolites and intramolecular reaction positions.These are defined as post-photosynthetic or postcarboxylation fractionation (Jäggi et al., 2002;Badeck et al., 2005;Gessler et al., 2008).Post-carboxylation fractionation in plants includes the carbon discrimination that follows carboxylation of ribulose-1, 5-bisphosphate, and internal diffusion (RuBP, 27 ‰), as well as related transitory starch metabolism.(Gessler et al., 2008(Gessler et al., , 2014)), fractionationassociated phloem transport, remobilization or storage of soluble carbohydrates, and starch metabolism fractionation in sink tissue (tree rings).In the synthesis of soluble sugars, 13 C depletions of triose phosphates occur during export from the cytoplasm and during production of fructose-1, as does 6bisphosphate by aldolase in transitory starch synthesis (Rossmann et al., 1991;Gleixner and Schmidt, 1997).Synthesis of sugars before transportation to the twig is associated with the post-carboxylation fractionation generated in leaves.Although these are likely to play a role, another consideration is [CO 2 ] in the chloroplast (C c ), not in the intercellular space.In the simplified equation of Farquhar's model (Evans et al., 1986;Farquhar et al., 1989), this is actually defined as carbon isotope discrimination (δ 13 C).Differences between gasexchange-derived values and online measurements of δ 13 C have often been used to estimate C i -C c and mesophyll conductance for CO 2 (Le Roux et al., 2001;Warren and Adams, 2006;Flexas et al., 2006Flexas et al., , 2012;;Evans et al., 2009;Evans and von Caemmerer, 2013).In this regard, changes in mesophyll conductance could be partly responsible for the differences in the two measurements, as mesophyll generally increases in the short term in response to elevated CO 2 (Flexas et al., 2014), but tends to decrease under drought (Hommel et al., 2014;Théroux-Rancourt et al., 2014).Therefore, it is necessary to avoid confusion between carbon isotope discrimination derived from synthesis of soluble sugars and/or mesophyll conductance.The degree to which carbon fractionation is related to environmental variation has yet to be fully investigated.
The simultaneous isotopic analysis of leaves allows the determination of temporal variation in isotopic fractionation (Rinne et al., 2016).This will aid in an accurate recording of environmental conditions.Newly assimilated carbohydrates can be extracted, and these are termed the water-soluble compounds (WSCs) in leaves (Brandes et al., 2006;Gessler et al., 2009).WSCs can also be associated with an assimilationweighted mean of C i / C a (and C c / C a ) photosynthesized over periods ranging from a few hours to 1-2 days (Pons et al., 2009).However, there is disagreement as to whether fractionation caused by post-carboxylation and/or mesophyll resistance can alter the stable signatures of leaf carbon and thence influence instantaneous water use efficiency (iWUE).In addition, the manner in which iWUE derived from isotopic fractionation responds to environmental factors, such as elevated [CO 2 ] and/or soil water gradients, is largely unknown.
Consequently, we investigated the δ 13 C of the fastturnover carbohydrate pool in sapling leaves of two tree species, Platycladus orientalis (L.) Franco and Quercus variabilis Bl., native to semiarid areas of China.We conducted gas-exchange measurements in controlledenvironment growth chambers.One goal is to differentiate the 13 C fractionation from the site of carboxylation to cytoplasm prior to sugar transportation in P. orientalis and Q. variabilis, that is, the total 13 C fractionation determined from the δ 13 C of WSCs and gas-exchange measurements.Another goal is to discuss the potential causes for the observed divergence, estimate contributions of postphotosynthesis and mesophyll conductance on these differences, and describe how carbon isotopic fractionation responds to the interactive effects of elevated [CO 2 ] and water stress.
2 Material and methods 2.1 Study site and design P. orientalis and Q. variabilis saplings, selected as experimental material, were obtained from the Capital Circle forest ecosystem station, a part of the Chinese Forest Ecosystem Research Network (CFERN), 40 • 03 45 N, 116 • 5 45 E, Beijing, China.This region is forested by P. orientalis and Q. variabilis.We chose saplings of similar basal diameters, heights, and growth class.Each sapling was placed into an individual pot (22 cm diameter × 22 cm high).Undisturbed soil samples were collected from the field, sieved (with particles > 10 mm removed), and placed into the pots.The soil bulk density in the pots was maintained at 1.337-1.447g cm −3 .After a 30-day transplant recovery period, the saplings were placed into growth chambers for orthogonal cultivation.
The controlled experiment was conducted in growth chambers (FH-230, Taiwan Hipoint Corporation, Kaohsiung City, Taiwan).To reproduce the meteorological conditions of different growing seasons in the research region, daytime and nighttime temperatures in the chambers were set to 25 ± 0.5 • from 07:00 to 17:00 GMT+8 and 18 ± 0.5 • from 17:00 to 07:00 GMT+8.Relative humidity was maintained at 60 and 80 % during the daytime and nighttime, respectively.The mean daytime light intensity was 200-240 µmol m −2 s −1 .The chamber system was designed to control and monitor [CO 2 ].Two growth chambers (A and B) were used in this study.Chamber A maintained [CO 2 ] at 400 (C 400 ) and 500 ppm (C 500 ).Chamber B maintained [CO 2 ] at 600 (C 600 ) and 800 ppm (C 600 ).The target [CO 2 ] in each chamber had a standard deviation of ±50 ppm during plant cultivation and testing.
An automatic watering device was used to irrigate the potted saplings to avoid heterogeneity when scheduled watering was not performed (Fig. 1).The watering device consisted of a water storage tank, holder, controller, soil moisture sensors, and a drip irrigation component.Prior to use, the tank was filled with water, and the soil moisture sensor was inserted to a uniform depth in the soil.After connecting the controller to an AC power supply, target soil volumetric water content (SWC) was set and monitored by soil moisture sensors.Since changes in SWC could be sensed by the sensors, this automatic watering device could be regulated to begin or stop watering the plants.One irrigation device was installed per chamber.Based on mean field capacity (FC) of potted soil (30.70 %), we established orthogonal treatments of four [CO 2 ] values × five SWCs (Table 1).In Table 1, A 1 -A 4 denotes [CO 2 ] of 400 (C 400 ), 500 (C 500 ), 600 (C 600 ), and 800 ppm (C 800 ) in the chambers; B 1 -B 5 denotes 35-45 % (10.74-13.81 %), 50-60 % (15.35-18.42 %), 60-70 % (18.42-21.49 %), 70-80 % (21.49-24.56 %), and 100 % of FC (CK,.Each orthogonal treatment of [CO 2 ] × SWC for two saplings per species was re-peated twice.Each treatment lasted 7 days.One pot was exposed in each of the [CO 2 ] × SWC treatments.Pots in the chambers were rearranged every 2 days to promote uniform illumination.

Foliar gas-exchange measurement
Fully expanded primary annual leaves of the saplings were measured with a portable infrared gas photosynthesis system (LI-6400, LI-COR, Lincoln, US) before and after the 7-day cultivation.Two saplings per species were replicated per treatment (SWC × [CO 2 ]).For each sapling, four leaves were sampled and four measurements were conducted on each leaf.Main photosynthetic parameters, such as net photosynthetic rate (P n ) and transpiration rate (T r ), were measured.Based on theoretical considerations of Von Caemmerer and Farquhar (1981), stomatal conductance (g s ) and intercellular [CO 2 ] (C i ) were calculated using the LI-COR software.Instantaneous water use efficiency via gas exchange (WUE ge ) was calculated as the ratio P n / T r .

Plant material collection and leaf water-soluble compound extraction
Eight recently expanded sun leaves were selected per sapling and homogenized in liquid nitrogen after gas-exchange measurements were finished.For extraction of WSCs from the leaves (Gessler et al., 2004), 50 mg of ground leaves and 100 mg of PVPP (polyvinylpolypyrrolidone) were mixed and incubated in 1 mL distilled water for 60 min at 5 • in a centrifuge tube.Each leaf sample was replicated twice.
The tubes containing the mixture were heated in 100 • water for 3 min.After cooling to room temperature, the supernatant of the mixture was centrifuged (12 000 × g for 5 min) and 10 µL of supernatant was transferred into a tin capsule and dried at 70 • .Folded capsules were used for δ 13 C analysis of WSCs.The samples of WSCs from leaves were combusted in an elemental analyzer (Euro EA, HEKAtech GmbH, Wegberg, Germany) and analyzed with a massspectrometer (DELTA plus XP, Thermo Finnigan).Carbon isotope signatures were expressed in δ notation (parts per thousand), relative to the international Pee Dee Belemnite (PDB) standard: where δ 13 C is the heavy isotope and R sample and R standard refer to the isotope ratio between the particular substance and the corresponding standard, respectively.The precision of repeated measurements was 0.1 ‰.Table 1.Orthogonal treatments applied to P. orientalis and Q. variabilis.

P. orientalis
2.4 Isotopic calculation 2.4.1 13 C fractionation from the site of carboxylation to cytoplasm prior to sugar transportation Based on the linear model of Farquhar and Sharkey (1982), the isotope discrimination, , was calculated as where δ 13 C a and δ 13 C WSC are the isotope signatures of ambient [CO 2 ] in chambers and WSCs extracted from leaves, respectively.The C i : C a was determined by where C i and C a are the [CO 2 ] within substomatal cavities and in growth chambers, respectively; a is the fractionationoccurring CO 2 diffusion in still air (4 ‰); and b refers to the discrimination during CO 2 fixation by ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), and internal diffusion (30 ‰).The instantaneous water use efficiency by gas-exchange measurement (WUE ge ) was calculated as where 1.6 is the diffusion ratio of stomatal conductance for water vapor to CO 2 in chambers and e is the difference between e lf and e atm , representing the extra-and intracellular water vapor pressure, respectively: e = e lf − e atm = 0.611 × e 17.502T (240.97+T ) where T and RH are the leaf-surface temperature and relative humidity, respectively.Combining Eqs.(2, 3, and 4), the instantaneous water use efficiency was determined by the δ 13 C WSC of leaves, defined as where ϕ is the respiratory ratio of leaf carbohydrates to other organs at night (0.3).Then the 13 C fractionation from the site of carboxylation to cytoplasm prior to sugar transportation (defined as the total 13 C fractionation) was estimated by the observed δ 13 C of WSCs from leaves (δ 13 C WSC ) and the modeled δ 13 C calculated from gas-exchange measurements (δ 13 C model ).The δ 13 C model was calculated by model from Eq. (2); model was determined by combining Eqs.(3,4)

Method of estimating mesophyll conductance and the contribution of post-carboxylation fractionation
CO 2 diffusion into photosynthetic sites includes two main processes.CO 2 first moves from ambient air surrounding the leaf (C a ) through stomata to the substomatal cavities (C i ).
From substomatal cavities, CO 2 then moves to the sites of carboxylation within the chloroplast stroma (C c ) of the leaf mesophyll.The latter procedure of diffusion is termed mesophyll conductance (g m ; Flexas et al., 2008).The carbon isotope discrimination was generated from the relative contribution of diffusion and carboxylation, reflected by C c to C a .
The carbon isotopic discrimination ( ) can be presented as (Farquhar et al., 1982 where C a , C s , C i , and C c are the [CO 2 ] in the ambient air, at the boundary layer of the leaf, in the substomatal cavities, and at the sites of carboxylation, respectively; a b is the CO 2 diffusional fractionation at the boundary layer (2.9 ‰); e s is the discrimination for CO 2 diffusion when CO 2 enters in solution (1.1 ‰, at 25 • ); a l is the CO 2 diffusional fractionation in the liquid phase (0.7 ‰); e and f are carbon discriminations derived in dark respiration (R D ) and photorespiration, respectively; k is the carboxylation efficiency, and * is the CO 2 compensation point in the absence of dark respiration (Brooks and Farquhar, 1985).
When gas in the cuvette is well stirred during gasexchange measurements, diffusion across the boundary layer is negligible and Eq. ( 10) can be written as There is no consensus about the value of e, although recent measurements estimate it as ranging from 0 to 4 ‰.The value of f has been estimated to range from 8 to 12 ‰ (Gillon and Griffiths, 1997;Igamberdiev et al., 2004;Lanigan et al., 2008).As the most direct factor, b influences the calculation of g m , which is thought to be approximately 30 ‰ in higher plants (Guy et al., 1993).
The difference of [CO 2 ] between substomatal cavities and chloroplasts is omitted, while diffusion related to darkrespiration and photorespiration are negligible and Eq. ( 11) may be simplified to Equation ( 12) denotes the linear relationship between carbon discrimination and C i / C a .This underlines subsequent comparison between expected (originating from gas-exchange, i , and measured obs ), which can be used to evaluate the differences of [CO 2 ] between intercellular air and sites of carboxylation associated with 13 C fractionation from mesophyll conductance.Consequently, g m is calculated by subtracting the obs of Eq. ( 11) from i [Eq.12]: and P n from Fick's first law relates www.biogeosciences.net/14/3431/2017/Biogeosciences, 14, 3431-3444, 2017 and In the calculation of g m , terms of respiration and photorespiration can be ignored and e and f are assumed to be zero or canceled in the calculation of g m .Then Eq. ( 16) can be rewritten as

Foliar gas-exchange measurements
When SWC increased between the treatments, P n , g s , and T r in P. orientalis and Q. variabilis peaked at 70-80 % of FC and 100 % of FC (Fig. 2).The C i in P. orientalis rose as SWC increased.It peaked at 60-70 % of FC and declined thereafter with increased SWC in Q. variabilis.The carbon uptake and C i were significantly improved by elevated [CO 2 ] at all SWCs for the two species (p < 0.05).Greater increases in P n in P. orientalis were found at 50-70 % of FC from C 400 to C 800 , which was at 35-45 % of FC in Q. variabilis.As water stress was reduced (at 70-80 and 100 % of FC), reduction of g s in P. orientalis was more pronounced with elevated [CO 2 ] at a given SWC (p < 0.01).Nevertheless, g s in Q. variabilis for C 400 , C 500 , and C 600 was significantly higher than that for C 800 at 50-80 % of FC (p < 0.01).Coordinated with g s , T r of the two species for C 400 and C 500 was significantly higher than that for C 600 and C 800 , except at 35-60 % of FC (p < 0.01, Fig. 2g and h).P n , g s , C i , and T r in Q. variabilis was significantly greater than the corresponding values in P. orientalis (p < 0.01, Fig. 2).

δ 13 C of water-soluble compounds in leaves
After observations of photosynthetic traits in leaves of the two species, the same leaves were immediately frozen and WSCs were extracted for all orthogonal treatments.The carbon isotope composition of WSCs (δ 13 C WSC ) of both species increased as SWC increased (Fig. 3a  for C 400 at 50-100 % of FC, the δ 13 C WSC in P. orientalis was significantly higher than that in Q. variabilis for most [CO 2 ] × SWC treatments (p < 0.01, Fig. 3).

Estimations of WUE ge and WUE cp
Figure 4a shows that increments of WUE ge in P. orientalis under severe drought (i.e., 35-45 % of FC) were highest for most [CO 2 ], ranging from 90.7 to 564.7 %.The WUE ge in P. orientalis decreased as SWC increased and increased as [CO 2 ] elevated.Differing from variation in WUE ge in P. orientalis with moistened soil, WUE ge in Q. variabilis increased slightly at 100 % of FC for C 600 or C 800 (Fig. 4b).The maximum WUE ge occurred at 35-45 % of FC for C 800 among all orthogonal treatments associated with both species.Elevated [CO 2 ] enhanced the WUE ge in Q. variabilis at all SWCs, except at 60-80 % of FC.The 32 saplings of P. orientalis had greater WUE ge than Q. variabilis did for the same [CO 2 ] × SWC treatments (p < 0.05).As illustrated in Fig. 5a, WUE cp in P. orientalis for C 600 or C 800 increased as water stress was alleviated beyond 50-60 % of FC, as well as that for C 400 or C 500 , while SWC exceeded 60-70 % of FC.Q. variabilis showed variable WUE cp with increasing SWC (Fig. 5b).Except for C 400 , WUE cp in Q. variabilis decreased abruptly at 50-60 % of FC and then increased as SWC increased for C 500 , C 600 , and C 800 .In contrast to the results for WUE ge , WUE cp in Q. variabilis was more pronounced than in P. orientalis among all orthogonal treatments.

Contribution of post-carboxylation fractionation
We evaluated the difference between i and obs in 13 C fractionation derived from mesophyll conductance.The postphotosynthetic fractionation after carboxylation can be calculated by subtracting g m -sourced fractionation from the total 13 C fractionation (Table 2).The g m -sourced fractionation provided a smaller contribution to the total 13 C fractionation than did post-carboxylation fractionation irrespective of treatment (Table 2).The g m -sourced fractionation in the two species illustrated different variations with increasing SWC, which declined at 50-80 % of FC and increased at 100 % of  FC in P. orientalis.However, in Q. variabilis, it increased with water stress alleviation at 50-80 % of FC and then decreased at 100 % of FC.Nevertheless, in the two species, post-carboxylation fractionation in leaves all increased as SWC increased.The g m -sourced fractionation in P. orientalis and Q. variabilis reached its peaks under C 600 and C 800 , respectively.Post-carboxylation fractionation was magnified with increases in [CO 2 ] in P. orientalis and reached a maximum under C 600 and then declined under C 800 .
3.7 Relationship between g s , g m , and total 13 C fractionation Total 13 C fractionation may be correlated with resistances associated with stomata and mesophyll cells.We performed linear regressions between g s / g m and total 13 C fractionation in P. orientalis and Q. variabilis (Figs. 7 and 8).The total 13 C fractionation was correlated to g s (p < 0.01).The positive linear relationships between g m and total 13 C fractionation (p < 0.01) indicated that the variation in [CO 2 ] through the chloroplast was correlated with carbon discrimination following leaf photosynthesis.

Photosynthetic traits
The exchange of CO 2 and water vapor via stomata can be modulated by the soil and/or leaf water potential (Robredo et al., 2010).Saplings of P. orientalis reached maximum P n and g s at 70-80 % of FC irrespective of [CO 2 ] treatments.
As SWC exceeded this soil water threshold, elevated CO 2  caused a greater reduction in g s , as was previously reported for barley and wheat (Wall et al., 2011).The decrease in g s responding to elevated [CO 2 ] could be mitigated with increases in SWC.The C i in Q. variabilis peaked at 60-70 % of FC and then declined as soil moisture increased (Wall et al., 2006(Wall et al., , 2011)).This may be because stomata tend to maintain a constant C i or C i / C a when ambient [CO 2 ] is increased, which would determine the amount of CO 2 directly used in the chloroplast (Yu et al., 2010).This result could be explained as stomatal limitation (Farquhar and Sharkey, 1982;Xu, 1997).However, C i in P. orientalis increased considerably, while SWC exceeded 70-80 % of FC, as found by Mielke et al. (2000).One possible contributing factor is that plants close their stomata to reduce water loss during organic matter synthesis while simultaneously decreasing the availability of CO 2 and generating respiration of organic matter (Robredo et al., 2007).Another possible explanation is that the limited root volume of potted plants may be unable to absorb sufficient water to support the full growth of shoots (Leakey et al., 2009;Wall et al., 2011).In the present study, increasing [CO 2 ] may cause nonstomatal limitations when SWC exceeds a soil moisture threshold of 70-80 % of FC.
The accumulation of nonstructural carbohydrates in leaf tissue may induce mesophyll-based and/or biochemical-based transient inhibition of photosynthetic capacity (Farquhar and Sharkey, 1982).Xu and Zhou (2011) developed a five-level SWC gradient to examine the effect of water on the physiology of a perennial, Leymus chinensis, and demonstrated that there was a clear maximum in SWC, below which the plant could adjust to changing environmental conditions.Micanda Apodaca et al. (2014) also concluded that in suitable water conditions, elevated CO 2 levels augmented CO 2 assimilation in herbaceous plants.
The P n of the two woody plant species increased with elevated [CO 2 ] similar to results seen with other C 3 woody plants (Kgope et al., 2010).Increasing [CO 2 ] alleviated severe drought and the need for heavy irrigation, suggesting that photosynthetic inhibition produced by a lack or excess of water may be mediated by increased [CO 2 ] (Robredo et al., 2007(Robredo et al., , 2010) ) and ameliorate the effects of drought stress by reducing plant transpiration (Kirkham, 2016;Kadam et al., 2014;Micanda Apodaca et al., 2014;Tausz Posch et al., 2013).

Differences between WUE ge and WUE cp
Increases in WUE ge in P. orientalis and Q. variabilis that resulted from the combination of P n increase and g s decrease were followed by a reduction in T r (Fig. 2a, b, g, and h).This result was also demonstrated by Ainsworth and Mc-Grath (2010).Comparing P n and T r in the two species, a lower WUE ge in Q. variabilis was obtained due to its different physiological and morphological traits, such as larger leaf area, rapid growth, and higher stomatal conductance than that in P. orientalis (Adiredjo et al., 2014).Medlyn et al. (2001) reported that stomatal conductance of broadleaved species is more sensitive to elevated [CO 2 ] than conifer species.There is no agreement on the patterns of iWUE at the leaf level, related to SWC (Yang et al., 2010).The WUE ge values in P. orientalis and Q. variabilis were enhanced with soil drying, as presented by Parker and Pallardy (1991), DeLucia and Heckathorn (1989), Reich et al. (1989), andLeakey (2009).Bögelein et al. (2012) confirmed that WUE cp was more consistent with daily mean WUE ge than with WUE phloem (calculated with the δ 13 C of phloem).The WUE cp of the two species demonstrated similar variations to those in δ 13 C WSC , which differed from those of WUE ge .Pons et al. (2009) noted that of leaf soluble sugar is coupled with environmental dynamics over a period ranging from a few hours to 1-2 days.The WUE cp of our materials responded to [CO 2 ] × SWC treatments over a number of cultivation days, whereas WUE ge was characterized as the instantaneous physiological change in plants as a response to new conditions.Consequently, WUE cp and WUE ge had different degrees of variation in response to different treatments.

Influence of mesophyll conductance on the fractionation after carboxylation
Mesophyll conductance, g m , has been identified to coordinate with environmental factors more rapidly than stomatal conductance (Galmés et al., 2007;Tazoe et al., 2011;Flexas et al., 2007).During our 7-day cultivations, g m increased and WUE ge decreased with increasing SWC.It has been documented that g m can improve WUE under drought pretreatment (Han et al., 2016).However, the mechanism by which g m responds to the fluctuation of [CO 2 ] is unclear.Terashima et al. (2006) demonstrated that CO 2 permeable aquaporin, located in the plasma membrane and inner envelope of chloroplasts, could regulate the change in g m .In our study, g m is species-specific to the [CO 2 ] gradient.The g m in P. orientalis significantly decreased by 9.1-44.4% from C 600 to C 800 at 60-80 % of FC; this is similar to the results of Flexas et al. (2007).A larger g m value in Q. variabilis under C 800 was observed compared to P. orientalis.Furthermore, g m contributed to the total 13 C fractionation that followed carboxylation, while photosynthate was not transported to the sapling twigs.The 13 C fractionation of CO 2 from the air surrounding the leaf to substomatal cavities may be simply explained by stomatal resistance, which also contains the fractionation derived from mesophyll conductance between substomatal cavities and the site of carboxylation in the chloroplast that cannot be neglected and should be elucidated (Pons et al., 2009;Cano et al., 2014).In estimating the post-carboxylation fractionation, g m -sourced fractionation must be subtracted from the total 13 C fractionation (the difference between δ 13 C WSC and δ 13 C model ), which is closely associated with g m (Fig. 8, p = 0.01).Variations ing m -sourced fractionation are coordinated with those in g m with changing environmental conditions (Table 2).

Post-carboxylation fractionation generated before photosynthate moves out of leaves
Photosynthesis, a biochemical and physiological process (Badeck et al., 2005), is characterized by discrimination in 13 C, which leaves an isotopic signature in the photosynthetic apparatus.Farquhar et al. (1989) reviewed the carbon fractionation in leaves and covered the significant as-pects of photosynthetic carbon isotope discrimination.The post-carboxylation or post-photosynthetic fractionation associated with the metabolic pathways of non-structural carbohydrates (NSCs; defined here as soluble sugars + starch) within leaves, and fractionation during translocation, storage, and remobilization prior to tree ring formation, is unclear (Epron et al., 2012;Gessler et al., 2014;Rinne et al., 2016).The synthesis of sucrose and starch before transportation to twigs falls within the domain of post-carboxylation fractionation generated in leaves.Hence, we hypothesized that 13 C fractionation may exist.When we completed the leaf gasexchange measurements, leaf samples were collected immediately to determine the δ 13 C WSC .Presumably, 13 C fractionation generated in the synthetic processes of sucrose and starch was contained within the 13 C fractionation from the site of carboxylation to cytoplasm before sugar transportation.Comparing δ 13 C WSC with δ 13 C obs , the total 13 C fractionation in P. orientalis ranged from 0.0328 to 0.0472 ‰, which was somewhat less than that in Q. variabilis (from 0.0384 to 0.0466 ‰).Post-carboxylation fractionation contributed 75.3-98.9% to total 13 C fractionation, determined by subtracting the fractionation in g m from total 13 C fractionation.Gessler et al. (2004) reviewed the environmental components of variation in photosynthetic carbon isotope discrimination in terrestrial plants.Total 13 C fractionation in P. orientalis was enhanced by the increase in SWC, consistent with that in Q. variabilis, except at 100 % of FC.The 13 C isotope signature in P. orientalis was depleted with elevated [CO 2 ].However, 13 C depletion was weakened in Q. variabilis for C 600 and C 800 .Linear regressions between g s and total 13 C fractionation indicated that the post-carboxylation fractionation in leaves depends on the variation in g s and that stomata aperture was correlated with environmental change.

Conclusions
Through orthogonal treatments of four [CO 2 ] × five SWC, WUE cp values calculated by δ 13 C WSC and WUE ge derived from simultaneous leaf gas-exchange were estimated to differentiate the δ 13 C signal variation before leaf-level translocation of primary assimilates.The influence of g m on 13 C fractionation between the sites of carboxylation and ambient air is important.It requires consideration when testing the hypothesis that the post-carboxylation contributes to the 13 C fractionation from the site of carboxylation to cytoplasm before sugar transport.In response to the interactive effects of [CO 2 ] and SWC, WUE ge decreased with increasing SWC in both tree species and increased with elevated [CO 2 ] at 35-80 % of FC.We concluded that relative soil drying, coupled with elevated [CO 2 ], can improve WUE ge by strengthening photosynthetic capacity and reducing transpiration.WUE ge in P. orientalis was significantly greater than that in Q. variabilis, while the opposite was the case for WUE cp .The g m and post-carboxylation both contributed to the total 13 C fractionation.Rising [CO 2 ] and/or moistening soil generated increasing disparities between δ 13 C WSC and δ 13 C model in P. orientalis; nevertheless, the differences between δ 13 C WSC and δ 13 C model in Q. variabilis increased when [CO 2 ] was less than 600 ppm and/or water stress was alleviated.Total 13 C fractionation in the leaf was linearly dependent on g s .With respect to carbon isotope fractionation in post-carboxylation and transportation processes, we note that 13 C fractionation derived from the synthesis of sucrose and starch is likely influenced by environmental changes.A clear description of the magnitude and environmental dependence of post-carboxylation fractionation is worth considering.
Data availability.The relevant data are presented in the Supplement.The Supplement related to this article is available online.
Author contributions.NZ and YH collected field samples and performed the experiments.NZ analyzed the data and wrote the paper.PM commented on the theory and study design.XY revised and edited the paper.
Competing interests.The authors declare that they have no conflict of interest.
Special issue statement.This article is part of the special issue "Ecosystem processes and functioning across current and future dryness gradients in arid and semiarid lands".It does not belong to a conference.

Figure 1 .
Figure 1.Diagram of the automatic drip irrigation device used in this study; numbers indicate the individual parts of the irrigation device (nos.1-12).The lower-left corner of this figure presents the detailed schematic for the drip irrigation component (nos.8-12).

Figure 3 .
Figure 3. Carbon isotope composition of water-soluble compounds (δ 13 C WSC ) extracted from leaves of P. orientalis (a) and Q. variabilis (b) for four CO 2 concentration × five soil volumetric water content treatments.Means ± SDs, n = 32.

Figure 4 .
Figure 4. Instantaneous water use efficiency through gas-exchange measurements (WUE ge ) for leaves from P. orientalis (a) and Q. variabilis (b) for four CO 2 concentration × five soil volumetric water content treatments.Means ± SDs, n = 32.

Figure 5 .
Figure 5. Instantaneous water use efficiency estimated by δ 13 C of water-soluble compounds (WUE cp ) from leaves of P. orientalis (a) and Q. variabilis (b) for four CO 2 concentration × five soil volumetric water content treatments.Means ± SDs, n = 32.
Zhao et al.:Interaction of CO 2 concentrations and water stress 3.4 13 C fractionation from the site of carboxylation to cytoplasm before sugar transportationWe evaluated the total 13 C fractionation from the site of carboxylation to the cytoplasm by gas-exchange measurements and WSCs in leaves (Table 2), which can help track the path of 13 C fractionation in leaves.Comparing δ 13 C WSC with δ 13 C model from Eqs. (4, 7-9), the total 13 C fractionation in P. orientalis ranged from 0.0328 to 0.0472 ‰, which was less than that in Q. variabilis (0.0384 to 0.0466 ‰).The total fractionation in P. orientalis was magnified with increasing SWC, especially when SWC reached 35-80 % of FC from C 400 to C 800 (increasing by 21.3-42.0%).The total fractionation for C 400 and C 500 was amplified as SWC increased until 50-60 % of FC in Q. variabilis.Conversely, they increased at 50-80 % of FC and decreased at 100 % of FC for C 600 and C 800 .Elevated [CO 2 ] enhanced the mean total fractionation in P. orientalis, while fractionation in Q. variabilis declined sharply from C 600 to C 800 .Total 13 C fractionation in P. orientalis, with increased SWC, increased more rapidly than it did in Q. variabilis.800 at all SWCs for Q. variabilis.The g m in Q. variabilis was clearly greater than that in P. orientalis under the same treatment conditions.

Table 2 .
Carbon-13 isotope fractionation in P. orientalis and Q. variabilis under four CO 2 concentration × five soil volumetric water content treatments.