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Peer-reviewed

Research Article

Atmospheric CO_two concentration furnishings on rice water utilize and biomass production

• Uttam Kumar,
• William Paul Quick,
• Marilou Barrios,
• Pompe C. Sta Cruz,
• Michael Dingkuhn

x

• Published: Feb 3, 2017
• https://doi.org/10.1371/periodical.pone.0169706

Figures

Abstruse

Numerous studies have addressed effects of ascension atmospheric CO_two concentration on rice biomass production and yield but furnishings on ingather h2o use are less well understood. Irrigated rice evapotranspiration (ET) is equanimous of floodwater evaporation and awning transpiration. Crop coefficient Kc (ET over potential ET, or ET_o) is ingather specific according to FAO, but may decrease equally CO₂ concentration rises. A sunlit growth chamber experiment was conducted in the Philippines, exposing 1.44-m² canopies of IR72 rice to 4 constant CO_two levels (195, 390, 780 and 1560 ppmv). Crop geometry and direction emulated field conditions. In two wet (WS) and 2 dry out (DS) seasons, final aboveground dry out weight (agdw) was measured. At 390 ppmv [CO₂] (current ambient level), agdw averaged 1744 g yard^-two, similar to field although solar radiations was simply 61% of ambient. Reduction to 195 ppmv [CO₂] reduced agdw to 56±v% (SE), increase to 780 ppmv increased agdw to 128±8%, and 1560 ppmv increased agdw to 142±v%. In 2013WS, crop ET was measured by weighing the water extracted daily from the chambers by the air conditioners controlling air humidity. Sleeping accommodation ET_o was calculated according to FAO and empirically corrected via observed pan evaporation in sleeping accommodation vs. field. For 390 ppmv [CO_ii], Kc was near 1 during crop establishment simply increased to about iii at flowering. 195 ppmv CO₂ reduced Kc, 780 ppmv increased information technology, but at 1560 ppmv it declined. Whole-season ingather water use was 564 mm (195 ppmv), 719 mm (390 ppmv), 928 mm (780 ppmv) and 803 mm (1560 ppmv). With increasing [CO_ii], crop water use efficiency (WUE) gradually increased from one.59 g kg^-1 (195 ppmv) to ii.88 g kg^-1 (1560 ppmv). Transpiration efficiency (TE) measured on flag leaves responded more strongly to [CO₂] than WUE. Responses of some morphological traits are also reported. In conclusion, increased CO₂ promotes biomass more than water use of irrigated rice, causing increased WUE, but it does not help saving water. Comparability with field weather is discussed. The results will exist used to railroad train crop models.

Citation: Kumar U, Quick WP, Barrios Thou, Sta Cruz PC, Dingkuhn M (2017) Atmospheric CO₂ concentration furnishings on rice water use and biomass product. PLoS 1 12(2): e0169706. https://doi.org/10.1371/journal.pone.0169706

Editor: Fábio M. DaMatta, Universidade Federal de Viçosa, BRAZIL

Received: September 8, 2016; Accepted: December 20, 2016; Published: Feb 3, 2017

Copyright: © 2017 Kumar et al. This is an open access commodity distributed under the terms of the Artistic Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript and its Supporting Information files.

Funding: This research was supported by the Climate Change, Agriculture and Food Security programme of the CGIAR (CCAFS) and the Bill and Melinda Gates Foundation through the C4-Rice project.

Competing interests: The authors have alleged that no competing interests exist.

Abbreviations: agdw, Aboveground dry out weight; DS, Dry season, corresponding to December-May at the site; ET, Evapotranspiration [mm d^-1]; ET_o, Potential evapotranspiration according to Penman-Monteith equation [mm d^-1]; Kc, Crop coefficient for evapotranspiration, Kc = ET / ET_o; LAI, Leaf area alphabetize (unitless); ppmv, Parts per 1000000 by volume for gases. Since this is a ratio, it is unitless.; TE, Transpiration efficiency at foliage level [mmol (CO_ii) mol^-1 (H₂O)]; WS, Wet flavor, respective to June-November at the site; WUE, Water use efficiency at crop level [mg (agdw) g^-1 (water used)]

Introduction

The current and anticipated impact of climate change and the associated increase of atmospheric CO₂ concentration on rice product are of corking economical and social importance. This is particularly true for the tropics where rice is the dominant staple crop, and for the intensified irrigated (flooded) rice ecosystems which contribute 75% to global rice production [1].

The atmospheric concentration of CO₂ volition double by the finish of this century [2], and the current level of nearly 400 ppmv already represents a 43–63% increment over pre-industrial levels [2,3]. Carbon dioxide is a growth limiting resource, especially for C3 crops like rice. [4] reported yield increment between 3 and 18%, depending on rice cultivar, in Gratuitous-Air Carbon Dioxide Enrichment (FACE) experiments in Nippon, where CO_two concentration was increased by 200 ppmv over current levels. [5] reported an increase in rice biomass production nether similar weather. [6] observed a 12.eight% grain yield increase caused past the same CO_ii handling in a Face experiment in China. Then far, no Face up experiments for rice have been conducted in the tropics, but there is little doubt that rising [CO₂] volition increase yield potential if h2o is non limiting and heat stress does not critically reduce spikelet fertility. In fact, water is crucial for rice to avert heat impairment through transpiration cooling [7], and increased [CO₂] tends to cause warmer canopies through partial stomatal closure [eight]. Rice water requirements in a irresolute climate are thus a major concern, both because of the need to ensure effective transpirational cooling of the awning and because of the globally increasing scarcity of irrigation water resources.

Irrigated rice systems mainly consume water through evapotranspiration (ET) whereas percolation losses are unremarkably pocket-sized in puddled fields [9]. Evapotranspiration may decrease nether college [CO₂] levels considering information technology causes fractional stomatal closure and thereby increases leaf transpiration efficiency (TE), which translates into improved field level h2o use efficiency (WUE) [x]. Although leaf TE tin can increment dramatically under college ambient [CO₂] [5,11,12,13,fourteen], WUE is a more circuitous parameter that depends on leaf area dynamics and ground cover, respiration losses and crop-generated microclimate [8,10] that are not a straight function of [CO₂]. [five] reported an increase of WUE by 19% under +200 ppmv [CO₂], whereas water use decreased past just ix%. This effect can be expected to be variable because stomatal sensitivity to [CO₂] in the field is highly environment dependent [10] and [CO₂] may thus impact on biomass or water employ in a variable way. Uncertainty is specially big for tropical climates because of scarce data. Crop-level water balance data for irrigated rice under tropical, CO₂-enriched atmospheric condition are non-real to our knowledge—probably because water balance studies are considered virtually relevant for drought-prone, non-flooded systems; and besides because FACE experiments for rice and so far do not be in the tropics.

Evapotranspiration is extremely variable considering it is driven by the evaporative demand of the atmosphere. A normally used judge of this demand is potential ET, or ET_o, as formulated past [fifteen] to describe the ET of a curt moist grass awning at whatsoever given weather situation, and further refined for FAO as a global standard past [sixteen]. The crop coefficient Kc, defined equally a crop's ET divided past ET_o, is a useful parameter to approximate ET for unlike crops and environments. [16] proposed Kc estimates for many crops, including rice, for early on, mid and late flavour Kc values (e.grand., 1.2 for rice in midseason in the absence of h2o deficit). From an analytical perspective, the concept of Kc permits to normalize observed ET values confronting fluctuating weather situations and thus, to distinguish betwixt meteorological and crop-related causes of variation in ET. Potential effects of variable atmospheric [CO₂] on ET via ingather canopy transpiration, caused by stomatal sensitivity to [CO_ii], are leap to touch Kc unless they are compensated by changes in LAI.

The present study attempted to evaluate the issue of sub- and supra-ambient [CO_ii] on the dry mater product, water utilise and WUE of rice canopies in sunlit but closed chambers. The concepts of ET_o and Kc, which were originally designed for field crops and atmospheric condition data obtained from weather condition stations not located inside the field, were adapted for the purpose. Specifically, the study tested the hypothesis that the increased biomass production and TE of rice under super-ambience atmospheric [CO_two] would be accompanied by reduced water requirements not merely at the leaf level merely also at constitute population level under field-similar cultivation. This information is needed to parameterize the h2o employ algorithms of ingather models for the prediction of climate modify impacts on tropical irrigated rice.

Materials and methods

Experiments

The principal report on water use and biomass production was conducted in naturally sunlit, CO₂ controlled, temperature and humidity adjusted growth chambers during moisture flavour of 2013 (2013 WS) at the International Rice Research Institute (IRRI) in Los Baños, Philippines. The same experiment was also conducted in the 2011 DS, 2011 WS and 2012 WS but only phenology and final crop aboveground dry weight (agdw) are reported here. The seasons DS and WS refer to calendar periods and had no consequence on h2o resources or humidity in the chambers, but were associated with dissimilar solar radiation (Table i). Each of four chambers corresponded to i [CO₂] treatment (195 ppmv, 390 ppmv [electric current ambient], 780 ppmv and 1560 ppmv) and had a i.44-1000ⁱⁱ planted expanse. The semidwarf (100–105 cm maximal plant height), high-tillering, high-yielding, brusque to medium duration (ca. 110–115 d seed to seed), indica rice diversity IR72 was grown as a transplanted (14 d after sowing) as a continuously flooded ingather. The crop was exposed to the CO₂ treatment from sowing to physiological maturity, except 1 h at pre-dawn and 1 h at post-dusk each twenty-four hour period to affluent out trace gases.

Technical setup and environment control

Dimensions of walk-in growth chambers were two.01 m (W) x ii.41 one thousand (L) x 1.96 m (H), with a i.ii m (W) 10 1.2 ten (L) 0.56 one thousand (H) metal basin placed at its lesser to receive soil and plants (S1 Picture). The chambers were covered with Mylar (polyethylene) transparent plastic sheeting on all sides and all environment control equipment was installed inside the chamber to ane side of the bowl, thus limiting shading to ane side just. The chambers were located in a large greenhouse hangar having glass walls on all except 1 side, which was on the same side at that where the equipment was installed within the chambers. At least iii.v one thousand gratuitous space was provided around each sleeping accommodation in all directions to permit maximal lateral illumination. Daily average solar radiation levels were 61% of that outside the greenhouse. The greenhouse structure thereby intercepted 27% of ambient solar radiation, and the bedroom construction 17% of the remaining solar radiations in the greenhouse. Air within chambers was mixed with ii fan systems located above canopy tops to the side of the planted plot, aspiring air from blow and bravado it confronting the chamber ceiling on pinnacle of the planted area. This acquired a turbulent apportionment from lesser to top in the not-planted sector and from top to lesser through the constitute canopy. A second air circuit was used to pass air through an activated charcoal filter to blot air contaminants.

Carbon dioxide was scrubbed/injected as controlled with an infrared gas analyzer. Temperature was set to 27°C day and 25°C dark (cooling but) and relative humidity (RH) to almost 75%. The system was not always able to maintain these values at midday, resulting in the mean Tmax and RHmin values presented in Table 2. Global radiation within the chamber was recorded with a Davis^™ weather station. Daily maximum temperature (Tmax), minimum temperature (Tmin), photosynthetically agile radiations (PAR) and relative humidity (RH) were recorded with PT100 (T), LiCor Line Quantum Sensor (PAR) and EE16 (RH) sensors. Wind speed could not be measured reliably because of the turbulent air movement. Condensation water from the cooling and humidity command system was nerveless and quantified with a tipping bucket gauge.

Crop management and sampling

The crop civilization basins were filled with puddled topsoil from IRRI paddy rice fields to a depth of 0.5 m and were irrigated to maintain 3–v cm continuing h2o throughout the crop cycle, causing anaerobic atmospheric condition. IR72 pre-germinated seed was sown onto flat seedling nursery trays and grown for xiv d inside the chambers, then transplanted as single seedlings at twenty cm x 20 cm spacing. Sowing for 2013 WS was on 09 September.

Weeds were managed by mitt picking and insects were managed past spraying recommended pesticides equally needed. The plants were fertilized with 135.7–121.8–345.1–ten.fifteen kg ha^-ane N, P, One thousand and Zn respectively. P, Thou, and Zn fertilizers were applied as basal dose earlier transplanting and N fertilizer (every bit urea) was split (16% applied at 7–eight days after transplanting (DAT), 52% at 35–56 DAT, 25% at 63–84 DAT and 7% at 91–99 DAT.

Measurements.

Canopy growth: Leaf area measurements were done by 2 methods, at 81 DAS (about flowering) a non-subversive measurement around midday using Accupar LP-80 (Decagon Inc., Pullman, WA, United states) calorie-free interceptor system with the extinction coefficient set to 0.6; and at physiological maturity (PM) destructive measurement of physical foliage surface area using LiCor-3100 (LiCor, Nebraska, USA).

Leaf gas substitution: a LI-6400XT portable photosynthesis arrangement (LiCor, Nebraska, USA) was used between 29 Oct and 02 November 2013 (51 DAS to 55 DAS) while setting the instrument to the chamber'southward CO₂ concentration and uniform RH and T settings at saturating PAR (observed air temperature in cuvette: 27.3°C at 195 ppmv, 27.7°C at 390 ppmv, 27.seven°C at 780 ppmv, 28.0°C at 1560 ppmv); block temperature at xxx°C; RH at seventy%; photosynthetically active radiations at 1500 μmol photons one thousand^-two due south^-one). Only transpiration efficiency (TE) is presented in this paper.

Evapotranspiration (ET): The condensing h2o from the air conditioners was trapped and measured past tipping bucket rain gauges (Model; TR-525M, 25 mm collector, Metric. Texas Electronics, Inc. 5529 Redfield Street, Dallas, TX 75235, United states of america) on a subsample of days (Table 3). Based on the daily amount of water nerveless and the cropped surface expanse, ET (mm d^-i) was calculated.

Tabular array iii. Daily measured evapotranspiration (ET), calculated potential evapotranspiration (ET_o), and derived crop coefficient (M_C = ET ETo-1) for 27 days in the growth chambers having different CO₂ concentration.

https://doi.org/10.1371/journal.pone.0169706.t003

Calculation of potential evapotranspiration (ET_o)

Potential evapotranspiration (ET_o) was calculated from the climate variables in the chambers Penman-Monteith equation recommended by FAO [16], as follows: Where:

ET_o reference evapotranspiration [mm day^-1],

R_{due north} net radiation at the crop surface [MJ grand^-two twenty-four hour period^-one],

One thousand soil rut flux density [MJ m^-2 day^-i],

T mean daily air temperature at two thou top [°C],

u₂ current of air speed at ii 1000 acme [g s^-ane],

e_south saturation vapor pressure [kPa],

eastward_a bodily vapor pressure level [kPa],

e_south−due east_a saturation vapor pressure arrears [kPa],

Δ slope vapor force per unit area curve [kPa °C^-one],

γ psychrometric abiding [kPa °C^-1].

The term (R_{due north}-1000) [MJ m^-2 d^-1] is non commonly bachelor only was derived for short found canopies co-ordinate to [xvi] from the average shortwave radiations measured with a pyranometer. Since wind speed was not measurable in the chambers due to turbulent conditions, a value was estimated empirically. The linear correlation between pan evaporation and ET_o data in the field as provided by the local weather station was compared was used to conform right chamber ET_o information past varying current of air speed input to the Penman-Monteith equation, based on pan evaporation measured in the chambers in the presence of flooded soil but no crop. The advisable wind speed value for the chamber to obtain the field-based ET_o vs. pan evaporation relationship was about 1 chiliad south^-one.

Calculation of crop coefficient Kc, total cumulative ET and WUE

Daily ET_o values throughout the ingather wheel were needed considering ET was measured only on 27 days (Tabular array three) and had to be interpolated for the other days, in order to calculate WUE from final agdw and cumulative ET. This was washed by the post-obit steps, past using the observed dynamics of ingather coefficient for evapotranspiration Kc:

1. Estimation of daily ET_o for all days of the ingather cycle
2. Calculation of Kc = ET ET_o ^-i [xvi] for the days where ET was measured
3. Calculation of mean Kc for iii ingather development periods (22–29 DAS, early vegetative stage; 68–86 DAS, heading and flowering stages; 95–117 DAS, tardily maturation stage) (S1 Fig, Panel C)
4. Establishing an approximately sigmoidal, empirical growth office for Kc (S1 Fig, Panel A)
5. Establishing an approximately bell shaped, empirical, overall response function of Kc vs. [CO_two] (S1 Fig, Panel B); whereby we considered Kc = i before crop establishment, corresponding to an open up h2o surface
6. Establishing a combined model predicting Kc from growth stage and [CO₂] by multiplying function (4) with part (5) (S1 Fig, Panel C), and testing its accurateness with the measured information (S1 Fig, Panel D; R² = 0.96)
7. Calculating ET for each 24-hour interval of the growth wheel with this model for all [CO₂] treatments

Finally, WUE was calculated by dividing terminal agdw past the cumulative ET, with WUE = agdw (∑ET)^-1. For daily atmospheric condition information refer to S1 Table.

Statistical analysis

Analysis of variance (ANOVA, Blazon III sum of squares assay) was conducted with XLSTAT (V2016, Addinsoft, Inc.) in conjunction with Excel V14.0 (Microsoft, Inc.). Regression analyses and curve fitting were conducted with SigmaPlot V13 (Systat Software, Inc.).

Results

Atmospheric CO₂ and season furnishings on biomass

Dry out and wet seasons differed in solar radiation (Table 1) and thus gave different levels of biomass (Table 4). Final agdw at maturity responded strongly to atmospheric [CO₂] (Fig 1A). Although the response approached saturation beyond 780 ppmv (corresponding to doubling of current levels), the current level of 390 ppmv was distinctly sub-optimal for rice biomass production. In absolute terms, final agdw observed in the chambers was similar to or slightly in a higher place the values that were independently observed in the field for the same cultivar IR72 (field information reported by [17]). Daily solar radiations (Rs) levels in the chambers averaged at 11.9, 9.iii, 13.0 and 9.0 MJ m^-two d^-ane for 2011 DS, 2011 WS, 2012 DS and 2013 WS, respectively, representing 61% of those in the field (details in Tabular array 1). Consequently, the bedroom crops produced similar biomass as field crops in comparable seasons [17], just with 39% less solar radiations.

Table 4. Observations and ANOVA for measured days from sowing to flowering, days from sowing to grain maturity and aboveground dry out weight (agdw) for four CO_two concentration treatments and 4 seasons.

https://doi.org/10.1371/journal.pone.0169706.t004

Fig ane.

A: Response of concluding to a higher place ground dry weight (agdw) to atmospheric CO₂ concentration for two dry seasons (DS) and ii wet seasons (WS), differing in solar radiation levels. B: Dynamics of potential ET (ET_o, Tabular array 3) and crop ET in 2013 WS. C: Dynamics of ingather coefficient Kc (ET/ET_o) in 2013 WS. D: Response of Kc to atmospheric CO₂ concentration during 3 periods of crop development. Fault confined represent SEM for multiple measurements on different plants inside a sleeping room.

https://doi.org/ten.1371/journal.pone.0169706.g001

Between the two wet seasons and between the two dry out seasons, accented agdw and its response pattern to [CO₂] were like, indicating that the results were highly reproducible. Dry out season crops produced greater agdw than WS crops due to greater Rs. Mean agdw across all seasons was 1744 g thousand^-2 for 390 ppmv (current ambient level), and information technology decreased by 4456% at 195 ppmv (0.5 x ambient), increased past 29% at 780 ppmv (2 x ambient) and increased by 42% at 1560 ppmv (4 x ambience) (Table iv). The flavour consequence on agdw was significant (P < 0.05) and the [CO_ii] effect was highly meaning (P < 0.001).

Atmospheric CO_ii effects evapotranspiration

Potential evapotranspiration (ET_o) was betwixt 2 and 3 mm d^-1, which is close to the values reported in before studies [18,19] for the wet season in the Philippines (Fig 1B). Ingather ET increased with crop development and attained a maximum at virtually flowering stage, with nearly 6 mm d^-1 for the 390 ppmv treatment. This translated into a crop coefficient Kc (Kc = ET ET_o ^-one) of nearly 1 during seedling stage and about 3 effectually flowering for the 390 ppmv treatment (Fig 1C). The latter value is much higher than the standard value estimated for irrigated rice by FAO [16] and this observation will exist discussed in the succeeding section.

The response of Kc to atmospheric CO₂ concentration is shown in Fig 1D for three stages of crop evolution. The Kc was highest for the 780 ppmv handling and tended to decrease at higher concentrations. Consequently, increased [CO₂] compared to electric current levels (390 ppmv) increased crop water use, whereas reduced CO₂ reduced ingather water apply.

Atmospheric CO₂ effects on phenology and some morphological parameters

Effects of [CO₂] and season on phenology were pocket-sized affecting days to flowering and days to maturity by 5% or less (Table 4). The [CO₂] effects were non-meaning (P > 0.05) but the season effect on days from sowing to maturity was highly significant despite its small magnitude (P < 0.001). Consequently, phenology did not contribute to the strong [CO₂] effects on agdw.

Number of leaves appeared on the main stem at flowering (without counting the prophyll) was xi at 390 ppmv [CO_ii] (Fig 2A). Increased [CO₂] had no significant outcome but lower levels (195 ppmv) increased foliage number significantly (P<0.05), whereby leaves were considerably smaller (leafage size data not presented). Opposite effects were observed for tiller number at flowering (Fig 2B), which was strongly decreased at 195 ppmv [CO₂]. It showed a bell-shaped response to CO₂ concentration, the maximum occurring at the super-ambient concentration of 780 ppmv.

Fig 2. Response to atmospheric CO₂ concentration of number of leaves appeared on main culm (A), number of tillers produced per hill (B) and foliage area index (C) at flowering stage.

Mistake bars indicate SEM of means of biological replications within a chamber and non replications of treatment.

https://doi.org/x.1371/periodical.pone.0169706.g002

Leaf expanse alphabetize at flowering was in the typical of range of values found for IR72 in the field, with half-dozen.6 at for 390 ppmv [CO₂] and similar values at greater concentrations (Fig 2C). The sub-ambient concentration, however, strongly decreased LAI. Measurements of LAI were indirect and non-subversive, and therefore just gave trend information.

Crop water apply

In order to judge total crop water use in the absence of ET measurements for some periods of the crop cycle (Tabular array 3), we established an empirical relationship between Kc, DAS and atmospheric CO₂ concentration and predicted ET with it for all days of the crop bicycle (S1 Fig). The model was assembled from the mean responses of Kc-1 to DAS (S1 Fig, Panel A; three^rd gild power part forced through origin) and to CO_two concentration (S1 Fig, Panel B; two^nd guild). Multiplication of both models and addition of 1 (for ET_o) gave a 3D surface of Kc response to both variables (S1 Fig, Panel C) and a good fit of calculated vs. observed Kc (S1 Fig, Panel D; R^two = 0.96).

Straight measured water use (in terms of ET) during the 27 days of observation scattered over the crop cycle (Table 3) was 112 (195 ppmv), 131 (390 ppmv), 171 (780 ppmv) and 149 (1560 ppmv) mm, indicating and increase past 31% when the current, ambient [CO_ii] was doubled to 780 ppmv (Table five). Calculated total water use from sowing to maturity was 565, 719, 928 and 803 mm d^-1 (= kg thousand^-2) for 195, 390, 780 and 1560 ppmv CO₂, respectively (Fig 3A). Doubling of current, ambient [CO₂] increased water apply by 29% (Table 5). The extrapolation of measured ET to the whole crop cycle thus conserved the proportions among handling effects. However, the extrapolated values are more meaningful than the raw data in Table three because they (ane) encompass the complete crop cycle and (2) take into account the slight differences in the atmospheric conditions among the chambers.

Fig 3.

A: Response to atmospheric CO_ii concentration of final total agdw (TDW) and cumulative crop water use. Water utilize was calculated from daily calculations of Kc as shown in S1 Fig. B: Response of leaf-level transpiration efficiency (TE) and crop-level water utilize efficiency (WUE).

https://doi.org/10.1371/periodical.pone.0169706.g003

The results indicated that sub-ambient CO₂ concentration reduced water use and super-ambience CO₂ concentration increased information technology, just with a declining trend at 1560 ppmv. This declining tendency was not observed for biomass, and consequently h2o use efficiency (WUE) increased at high CO₂ concentration (Fig 3B).

Since only iv points were bachelor for the response of WUE to CO_ii concentration (although based on 27 ET observations per treatment), and no error term could be calculated (because whole-bicycle ET extrapolation gave a unmarried value), the shape of the response remained uncertain. Even so, a linear tendency with a positive slope is a plausible interpretation of the information. WUE was 1.six g kg^-1 at 195 ppmv [CO_ii], two.9 g kg^-one at 1560 ppmv [CO₂], and intermediate at intermediate [CO₂]. Transpiration efficiency (TE) of the flag leaf at flowering responded much more strongly to [CO_ii] than did crop level WUE, and approached a plateau towards the highest CO₂ concentration (Fig 3B).

Word

Biomass production

Final agdw was 1540 grand m^-2 (SE = 6; Northward = 2) in the WS and 1948 one thousand one thousand^-2 (SE = 36; Due north = two) in the DS at ambient [CO_two] levels (390 ppmv) (Fig 1A). These values are like to field observations for the same variety at the site, and growth duration was also near-identical [17]. Although these results may betoken that chamber weather were representative of the field, some circumspection is warranted because the sleeping accommodation wall and the greenhouse roof together intercepted 39% of the natural solar radiation. A lower biomass production was thus expected in the chambers, merely the dimming effect was probably compensated by (1) some lateral light interception due to small-scale plot size (1.44 thousand²) despite a 1-row planted border; (2) the higher proportion of lengthened radiation due to scattering by chamber and greenhouse wall/roof material; and (iii) the highly protected conditions in the chambers.

In terms of agdw response to CO₂, a typical asymptotic response was observed with diminishing slope as it approached saturation. The maximal agdw at saturating CO_ii concentration (1560 ppmv) was 2183 1000 m^-ii (SE = 130) in the WS and 2768 g grand^-two (SE = 180) in the DS, or +42% in both seasons equally compared to the ambience treatment. This confirmed the highly limiting nature of the carbon resource for irrigated rice, and is in line with numerous previous studies on rice [20] (review: [21]) and wheat [22,23,24]. An important validity examination is the comparing with the +200 ppmv (590 ppmv) scenarios investigated in rice FACE studies in Japan and China (for comparison of sites: [25]). By interpolation, our results signal a 22% increase in agdw for the 590-ppmv scenario, equally compared to a 29% increase observed by [25] in Cathay on average for 3 cultivars and a xiii% increment observed by [26] in Nihon for 8 cultivars. Genotypic differences were large, with indica and high-tillering cultivars responding more than strongly to enhanced CO₂ concentration. In our written report the high-tillering, indica cv. IR72 was used. We conclude that the chamber-based observations on agdw response to CO₂ are fully supported by the two Face studies conducted on rice.

Causes of loftier Kc values in chambers

The observed Kc value at 390 ppmv [CO_two] at crop flowering (3.0) was far college than the standard value proposed by FAO for the field (1.2; [16]). [27] observed a Kc of about 1.4 for fully developed, flooded rice canopies at the landscape scale. [28] observed a similar Kc of 1.42 at the field scale (indica materials). [10] reported a Kc of 1.24 for japonica rice in a FACE report under ambient [CO_two], and [19]. reported even lower values. The high Kc values observed here in the chambers were probably not due to underestimations of ET_o because Kc values were realistic (near 1) at the beginning of the ingather bicycle (open water surface with no crop canopy). According to [sixteen,29], the ET of open water surfaces is similar to that of a wet, curt grass canopy and thus ET_o, corresponding to Kc = 1.

The likely crusade of the high Kc observed in mid and late flavor resides in the fact that Kc under field conditions is referenced past weather condition observed at 2m peak on terrain non located in the cropped area, whereas in the chambers the temperature and humidity were measured nearly canopy tops. A large boundary exists between field rice canopies and weather stations. The crop develops its distinct microclimate (haven effect) that is different from the conditions measured at a weather station, whereas in the chambers the turbulent air mixing and brusk concrete distances provided for no such boundary. The aerodynamic coupling of the canopy to the atmosphere contributes to the magnitude of ET [30]. The absenteeism of an oasis effect in the chambers may therefore explain the high apparent Kc but this should not cause a bias in the relative effects of the [CO_two] treatments. FACE experiments are besides affected by this problem but to a smaller extent [31,32].

The application of the ET_o and Kc concepts to chamber studies is obviously problematic if the objective is to derive water remainder information for field extrapolation. In this report, notwithstanding, the Kc concept was employed for the purpose of extrapolation of ET from the 27 observed days (Table iii) to all days of the crop cycle, in order to calculate cumulative ET and WUE. By this modeling procedure, the originally observed [CO₂] furnishings on ET were conserved (Tabular array 5), but crop ET totals were obtained, and effects of the slight differences in conditions amid chambers were compensated for.

CO₂ effects on evapotranspiration

We observed an increment of Kc from 390 to 780 ppmv [CO₂] (Fig 1C; S1 Fig, Panel B), followed past a decrease from 780 to 1560 ppmv. This stands in contrast with several studies reporting a decline in evapotranspiration in CO_two-enriched crops [11,12,xiii,20,]. To our knowledge, [x] published the just FACE field report so far on [CO_ii] effect on rice crop ET. They found ET throughout the crop cycle to be identical between ambient and +200 ppmv [CO₂] during early and late season, but slightly reduced at midseason when temperatures were elevated (estrus sensitive japonica rices were planted). This issue reduced overall Kc from 1.24 to 1.17, as indicated by the slopes reported between ET and ET_o. [ten] conclude that although leaf gas exchange measurements consistently indicate reductions in water use under enhanced [CO_ii], effects on h2o use at the crop scale are much smaller and quite unlike.

CO_ii effects on TE and WUE

The stimulation of TE by increased atmospheric CO_ii concentration has two components, a decrease in transpiration (due to partial stomatal closure) and an increment in photosynthesis. Both are linked past a physiological tradeoff, whereby the fractional stomatal closure usually has the smaller contribution to TE, east.g. 20% in the case of soybean [33]. [In these studies TE, expressed as canopy CO_two assimilation rate over ET, is termed WUE and must not be dislocated with crop-level WUE which is equal to dry weight over either cumulative ET or h2o apply.] Atmospheric CO₂ effects on TE are much greater than those on WUE because all processes constituting TE are direct CO₂ dependent, whereas soil/floodwater surface evaporation and found respiration are non, but contribute to WUE. This was too the case in our study (Fig 3B).

Co-ordinate to the measurements on irrigated rice by [34], which have since been supported past like reports, WUE on a grain weight basis varied seasonally between about 0.87 and 1.32 mg g^-1, and WUE on agdw basis would be most twice every bit high (ca. ane.vii–2.6). [Inclusion of variable percolation rates and other water losses can substantially reduce that value.] At 390 ppmv [CO₂], we observed a similar value for WUE of well-nigh ii.0 mg g^-1, and experimental variation of [CO₂] made it range from 1.6 to 2.9 mg g^-1. We did non find reports on [CO₂] effects WUE in the agronomic sense (final biomass over either cumulative water use or cumulative ET), and even the otherwise complete mega analysis by [21] (a review of 125 studies) only reports [CO₂] effects on TE (thereby termed "foliage-level WUE"). According to [21], TE increases by 37% for the scenario of doubled [CO₂]. In the present report TE increased was by 58% (780 vs. 390 ppmv [CO₂]), while the respective increase of WUE was simply nearly ca. 17% (based on linear trend in Fig 3B).

CO₂ furnishings on morphology

Most CO_ii enrichment studies reported the absence of significant effects of elevated [CO₂] on LAI in rice [11,35,36], and also for wheat and winter barley [37]. [10] reported that LAI of rice was increased during early stages of growth but was decreased at later development stages. The meta-analysis of [21] concluded that although [CO₂] doubling stimulates agdw past 28% and belowground dw by 42%, LAI remains constant and is associated with a small increase in tiller number (+14%). Consequently, tillers become both more than numerous and heavier, merely accept reduced leafage area per tiller.

The trends observed in this written report support this assessment. The LAI was strongly reduced at sub-ambient [CO_two] but supra-ambient [CO_two] did not increment it. In that location were inverse effects of [CO_two] on tiller number vs. total leafage number appeared per main culm, indicating that within the historical and anticipated ranges (represented by 195, 390 and 780 ppmv treatments), [CO₂] stimulates tillering but reduces the number of leaves developed per tiller.

Conclusion

This study had the objective to test the following hypothesis: Increasing atmospheric [CO_ii] reduces water requirements of irrigated rice. Water requirements of a crop in the field are usually expressed by Kc, an approach that normalizes crop ET past the atmospheric evaporative demand ET_o. Although the Kc measured in the confined experimental system was different from that in the field due to different purlieus weather condition, it was withal a valid arroyo to normalize ET across variable atmospheric weather and thus permitted evaluating effects of crop development stage and [CO₂] treatments on h2o utilise. On this ground, nosotros did confirm that increasing [CO_two] increased leaf level TE and crop-level cumulative WUE, but accented water utilise and Kc tended to increment too, and clearly did not decrease as hypothesized. This result has implications for crop water residual modeling for future climate scenarios, simply needs validation at the field calibration for tropical indica rice because no such data have been reported to date.

Supporting information

S1 Fig. Modeling of Kc.

A: Dynamics of mean [Kc-1] across CO₂ treatments described by iii^rd-social club power regression, assuming Kc = 1 in the absence of crop. B: Response of mean [Kc-1] across developmental stages described past 2^nd-guild power regression. C: Three-dimensional surface of response of calculated Kc (Kc = ane + Eq ane * Eq 2) vs. the predictor variables as in A and B. D: Relationship between simulated (as in C) and corresponding observed Kc.

https://doi.org/10.1371/journal.pone.0169706.s002

(TIF)

Acknowledgments

This enquiry was supported past the Climate change, Agriculture and Food Security program of the CGIAR (CCAFS) and the Beak and Melinda Gates Foundation through the C4-Rice project.

Author Contributions

1. Conceptualization: MD UK.
2. Data curation: MD UK.
3. Formal analysis: Medico Britain MB.
4. Funding acquisition: MD WPQ.
5. Investigation: Britain.
6. Methodology: Doctor WPQ UK MB.
7. Project administration: MD.
8. Resources: WPQ MD.
9. Supervision: Doctor.
10. Validation: PCSC.
11. Visualization: UK Md.
12. Writing – original draft: UK.
13. Writing – review & editing: Physician PCSC.

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