Introduction

Drought is considered to be one of the most devastating natural hazards and is a pervasive climate phenomenon across the world. It has been predicted that drought will become increasingly frequent and severe due to climate change in the interior of numerous continents1,2. Additionally, global warming may offset any modest increases in precipitation by increasing evapotranspiration, which will eventually result in further decreases in soil moisture3. Decreased soil moisture will inhibit plant growth (even causing mortality) and negatively affect ecosystems4,5.

Water is a key resource for plant growth and survival and can shape the nature of plant–plant interactions in a wide range of terrestrial ecosystems. Plants perceive and rapidly respond to alterations (even small ones) in water status via a series of parallel physiological, cellular, and molecular events6. Plant responses to drought are complex, involving adaptive changes and/or deleterious effects7, and the various responses are modulated by the plant species as well as the intensity, duration, and rate of progression of the imposed stress8. Drought stress has profound effects on general plant physiology, i.e., both the gas exchange and water status depend on the rapidity, severity and duration of the drought event. The earliest response to a leaf water deficit is stomatal closure to prevent desiccation9,10, which occurs before any change in leaf water potential (LWP) and/or relative water content (RWC)11,12. It is now well established that there is drought-induced root-to-leaf signaling, such as that involving abscisic acid (ABA), which reaches the leaves through the transpiration stream and induces the closure of stomata11. The photosynthesis rate (A) is subsequently affected by the internal water deficiency, so A is unavoidably reduced due to decreased CO2 availability at the level of the chloroplast13. In recent years, stomatal closure has generally been accepted as the main determinant for decreased A under drought conditions11,14, and the primary role of the stomata might be to avoid damage from plant water deficits15. However, another possibility is that control of the transpiration rate (Tr) by the stomata plays a role in maintaining leaf temperature under drought conditions10. The drought-stress-induced limitation on plant growth is mainly caused by reductions in carbon assimilation, which depends on the balance between A and respiration16. The response of A to drought stress has received considerable attention in the past, and efforts have been made to generalize the responses of photosynthetic parameters to drought in higher plants17,18,19. It has been emphasized that a high degree of co-regulation of stomatal conductance (gs) and A is usually observed11,20. Determining the effect of a given change in gs on A and Tr can be fairly straightforward, but analyzing the role of stomata in the control of these changes is complicated because the changes in A and Tr that result from changes in gs can themselves affect gs. Decreased A and Tr can affect the LWP, which then changes gs through a feedback10. The gs and A of leaves also decrease as water status declines21, but the precise relationship is dependent on several factors, including the study species, drought history and environmental conditions during drought11.

Plant responses to drought and the relationship between gas exchange and water status, which depend on drought intensity, metabolic CO2 assimilation and biological form, are clearly complex22. In past years, numerous works have focused on plant gas exchange and water status under drought, and all reports showed the decrease of gas exchange and water status trails in response to drought. However, the magnitudes of decrease differed greatly among the various studies. These differences were mainly the result of complex interactions between drought stress and the high variability of various plant types. Thus, it is difficult to compare the results of individual studies, and the responses of different plant types are unlikely to be effectively resolved by studies performed at individual sites. Therefore, to determine the central tendency and identify different patterns of plant response to drought, it is necessary to integrate results across studies. To this end, we conducted a meta-analysis based on global plant gas exchange and water status under drought and aimed to determine the changes in gas exchange (A, gs and Tr) and water status (leaf water potential and relative water content) of different types of plants and different drought intensities. The following issues were addressed in this study: (1) how gas exchange and water status respond to different drought intensities, photosynthetic pathways (C3 and C4) and growth forms (herbs, shrubs, trees and lianas) and the differences between C3 and C4 plants under the same drought intensities; (2) whether the relationships of both A and Tr with gs shift in C3 and C4 plants, herbs, shrubs, trees and lianas during drought; and (3) the relationships between gas exchange and water status in various photosynthetic pathways and growth forms. Our results could improve the understanding of the response of plant gas exchange and water status to drought.

Results

Effects of drought on leaf gas exchange

Our results showed that the response ratios of A, gs and Tr were −0.759 ± 0.020, −0.975 ± 0.026 and −0.713 ± 0.026 (P < 0.0001) (Fig. 1a–c), respectively, across all studies. When taking drought intensity into consideration, we found that the response ratio of A significantly decreased under drought compared to the control with values of −0.328 ± 0.025, −0.627 ± 0.038 and −1.024 ± 0.040 (P < 0.0001) under mild, moderate and severe drought, respectively. Furthermore, the response ratio of gs also decreased significantly under drought compared to the control with values of −0.4479 ± 0.058, −0.8140 ± 0.097 and −1.1649 ± 0.093 (P < 0.0001) under mild, moderate and severe drought, respectively. When we divided the plant species into four growth forms (herbs, shrubs, trees and lianas), we found that herbs and shrubs showed the lowest and highest A, gs and Tr response ratios (Fig. 1a–c), respectively. When the two photosynthetic pathways (the C3 and C4 cycles) were considered, we found that the response ratios of A, gs and Tr in C4 plants (−0.619 ± 0.065, −0.716 ± 0.067 and −0.514 ± 0.069, respectively; P < 0.0001) were lower than those in C3 plants (−0.777 ± 0.021, −1.012 ± 0.028 and −0.742 ± 0.028, respectively; P < 0.0001) (Fig. 1a–c), indicating that C4 plants performed better in the context of drought.

Figure 1: Weighted response ratios (lnRR++) of the plant photosynthesis rate (A, panel a), stomatal conductance (gs, panel b) and transpiration rate (Tr, panel c) under different drought intensities (mild, moderate and severe) for different CO2-assimilation metabolic pathways (C3 and C4 plants) and in different growth forms (herbs, shrubs, trees and lianas).
figure 1

The numbers above the symbols specify the number of data points. The error bars indicate 95% CI. Symbols (***) after the number indicate statistical significance (P < 0.001).

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Effects of drought on leaf gas exchange in C3 and C4 plants under different intensities

Our results showed that the response ratios of A, gs and Tr in C3 plants were higher than in C4 plants under the same drought intensities (Fig. 2a–c). The response ratios of A, gs and Tr under severe drought stress were −1.086 ± 0.042, −1.228 ± 0.050 and −0.937 ± 0.052, respectively, in C3 plants and −0.578 ± 0.103, −0.750 ± 0.120 and −0.613 ± 0.120 in C4 plants. The response ratios of A and gs did not differ under mild and moderate drought stress in C4 plants.

Figure 2: Weighted response ratios (lnRR++) of the plant photosynthesis rate (A, panel a), stomatal conductance (gs, panel b) and transpiration rate (Tr, panel c) under different drought intensities (mild, moderate and severe) for different CO2-assimilation metabolic pathway (C3 and C4 plants).
figure 2

The numbers above the symbols specify the number of data points, and the error bars indicate 95% CI. The symbols (*, **, ***) after the number indicate statistical significance (P < 0.05, P < 0.01, P < 0.001, respectively).

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Effects of drought on leaf water status

Leaf water potential (LWP) and relative water content (RWC) were the main indices used to reflect the water status of plants suffering from a drought, and the RWC and LWP response ratios across all studies were −0.211 ± 0.012 and 0.739 ± 0.042 (P < 0.0001) (Fig. 3a,b), respectively. The RWC and LWP response ratios differed according to drought intensity, resulting in values of −0.055 ± 0.006 and 0.367 ± 0.033, −0.138 ± 0.009 and 0.539 ± 0.040, and −0.379 ± 0.028 and 0.890 ± 0.065 (P < 0.0001) under mild, moderate and severe drought, respectively. The response ratios of RWC and LWP differed among growth forms with herbs exhibiting the highest RWC but a lower LWP and shrubs showing the highest LWP; lianas showed the lowest RWC and LWP values among the four plant types. C4 plants exhibited a higher RWC but a lower LWP.

Figure 3: Weighted response ratios (lnRR++) of the relative water content (RWC) and leaf water potential (LWP) under different drought intensities (mild, moderate and severe) for different CO2-assimilation metabolic pathways (C3 and C4 plants) and in different growth forms (herbs, shrubs, trees and lianas).
figure 3

The numbers above the symbols specify the number of data points. The error bars indicate 95% CI. Symbols (*, **, ***) after the number indicate statistical significance (P < 0.05, P < 0.01, P < 0.001, respectively).

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Relationships between stomatal conductance and both photosynthetic and transpiration rate

The stomata are the gates through which CO2 and water pass, and the results showed that the response ratio of stomatal conductance (lnRR(stomatal conductance)) correlated significantly with the response ratio of photosynthetic rate (lnRR(photosynthetic rate) = 0.63 lnRR(stomatal conductance) −0.16, P < 0.0001, R2 = 0.55) in all of the studies (Fig. 4a). When divided into the two photosynthetic pathways, the stomatal conductance response ratio also correlated significantly with the photosynthetic rate response ratio in C3 plants (lnRR(photosynthetic rate) = 0.60 lnRR(stomatal conductance) −0.19, P < 0.0001) (Fig. 4b) and C4 plants (lnRR(photosynthetic rate) = 0.92 lnRR(stomatal conductance) + 0.06, P < 0.0001) (Fig. 4c), which could explain 52% and 81% of the photosynthetic rate, respectively. The lnRR(stomatal conductance) also exhibited a significant correlation with lnRR(photosynthetic rate) in all four growth forms (P < 0.0001) (Fig. 4d–g), explaining 55%, 47%, 55% and 71% in herbs, shrubs, trees and lianas, respectively. The lnRR(stomatal conductance) correlated significantly with the transpiration rate response ratio (lnRR(transpiration rate)) (P < 0.0001) (Fig. 4h) and explained 74% of the lnRR(transpiration rate) in all of the studies; 73% and 81% of the lnRR(transpiration rate) in C3 and C4 plants, respectively (Fig. 4i,j); and 67%, 73%, 89% and 81% in herbs, shrubs, trees and lianas (Fig. 4k–n). The results also indicated that lnRR(stomatal conductance) contributes more to water exchange than to CO2 exchange (Fig. 4).

Figure 4
figure 4

The relationships between the response ratios (lnRR) of the stomatal conductance (gs) with the photosynthesis rate (A) and transpiration rate (Tr) in all plants (panels a,h), C3 plants (panel b,i), C4 plants (panel c,j), herbs (panels d,k), shrubs (panels e,l), trees (panels f,m) and lianas (panels g,n).

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Relationship between gas exchange and water status

The results showed that the response ratios of A, gs and Tr (P < 0.001) were significantly correlated with water status, including RWC and LWP across all studies (Figs 5 and 6). C3 plants showed a significant correlation between the response ratios of RWC and LWP and A, gs and Tr (Figs 5a–c and 6a–c), but there were no correlations between the RWC and LWP response ratios and Tr in C4 plants. The RWC response ratio contributed more substantially to the A and gs response ratios in C4 than in C3 plants. The response ratio of A showed a significant correlation with the response ratios of RWC and LWP in all four growth forms (Figs 5d and 6d) and a significant correlation between the gs response ratio and LWP in all four growth forms (Fig. 6e) but no significant correlation between the response ratios of RWC and gs in shrubs (Fig. 5e). Finally, no correlation was found between the Tr response ratio and RWC or LWP in trees (Figs 5f and 6f).

Figure 5: The relationship between the response ratios (lnRR) of the leaf relative water content (RWC) and gas exchange in all plants.
figure 5

The regression equation represents the correlation across all studies, and the black and blue lines represent the correlations of the C3 and C4 plants (panel a–c), respectively; the black, blue, yellow and red lines represent the correlations of the herbs, shrubs, trees and lianas (panels d–f), respectively.

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Figure 6: The relationship between the response ratios (lnRR) of the leaf water potential (LWP) and gas exchange in all plants.
figure 6

The regression equation represents the correlation across all studies, and the black and blue lines represent the correlation of the C3 and C4 plants (panels a–c), respectively; the black, blue, yellow and red lines represent the correlations of the herbs, shrubs, trees and lianas (panels d–f), respectively.

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Discussion

In this study, we conducted a meta-analysis of global gas exchange and water status data, and we analyzed the response ratios of these parameters under different drought intensities (mild, moderate and severe drought) and different plant growth forms (herbs, shrubs, trees and lianas) and photosynthetic pathways (C3 and C4). We also analyzed the response ratios of C3 and C4 plants under different drought intensities (Figs 1, 2, 3) as well as the relationship between gas exchange and water status in the different plant types and photosynthetic pathways (Figs 4, 5, 6). The stomata provide a means for controlling water loss from plants while allowing photosynthesis, so they play a fundamental role in determining plant transpiration and assimilation. We found that the gs response ratio was greater than those of A, Tr, RWC and LWP under different drought intensities, plant growth forms and photosynthetic pathways, and the decrease in lngs could explain 55% of the change in lnA and 74% in lnTr across all studies, respectively, which indicates that gs plays an important role in controlling photosynthesis and transpiration rates. The gs response was greater than the decrease in gas exchange, RWC and/or LWP, which is consistent with the results of Gollan et al.18, Chaves et al.7 and Jensen et al.19. Stomata are the gates through which carbon and water exchange occurs, and stomatal closure helps reduce water loss from transpiration, which could be regulated by signals from the roots in drying soil23,24. Although a high degree of gs and A co-regulation was usually observed11,20, the decrease in A was smaller than that in gs because of metabolic adjustments by the plants11,21.

The stress tolerance of different growth forms is an important factor that may affect the way plants respond to drought. In this study, there were no tolerant or sensitive plant genotypes, and all of them survived the imposed stress. Among the four growth forms, we observed that the gas exchange response ratio was smallest in herbs, indicating that their ability to adapt to drought conditions through regulation is relatively low and that a higher Tr in herbs could accelerate plant death in the context of drought. Shrubs and trees exhibited more rapid responses to drought and might have enhanced drought resistance due to their lower Tr and large roots. Gas exchange in C4 plants was less influenced by drought, and the C4 plants showed a smaller response than C3 plants under the same drought conditions (Fig. 3). This is consistent with reports that C4 plants have some advantages under drought conditions compared with C3 plants25.

It is generally accepted that the accurate measurement of plant water status is critical in experiments investigating the effects of drought and that such measurements must be considered when defining the experimental conditions in terms of both the treatments applied and the effects on the plants. Leaf water status depends on the soil water deficit, which can be regarded as a stressor26. In the study, the water status of plants showed a slight decrease under mild drought, followed by a large decrease under severe stress in this study, which is consistent with the results of Galmés et al.17. RWC, as the metabolically available water, could reflect the metabolic activity in plant tissues, and it declines with continuing drought. LWP, which could reflect the water transport, also declines with drought; thus, both RWC and LWP could as indicators for plants under drought. In this study, we found that the RWC response ratio was smaller than the LWP response ratio, indicating that LWP was more sensitive than RWC. This finding establishes LWP as an earlier indicator of drought than RWC, which differs from the conclusion of Sinclair and Ludlow27, who proposed that RWC was a better indicator. The lianas used in this meta-analysis come from four studies, and all the plant material was Vitis vinifera L. We found that the response ratio of water status in the lianas was the smallest among the growth forms, which may be due to the near-isohydric behavior of Vitis vinifera28,29,30. There were no significant differences in the response ratios of RWC in C3 (based on 152 papers and 139 species) and C4 (based on 21 papers and 13 species) plants, but the LWP response ratio in C3 plants was higher than that in C4 plants. This may be due to a difference in drought resistance between C3 and C4 plants25, which requires further investigation.

The stomata occupy a central position in the pathways for both water loss from plants and CO2 exchange. The debate regarding the main determinant of decreased A under drought has been ongoing since the publication of studies of the effects of drought on A31,32,33, which generally conclude that stomatal closure is the main determinant11,14. Very few studies have directly examined how the relationship between A and gs is affected by drought in different plant growth forms. Stomata often close in response to drought before any change occurs in LWP and/or RWC12. The regulation of gs is related to species and genotype, making it difficult to define a pattern of photosynthetic responses to drought. Furthermore, a high degree of A and gs co-regulation is usually observed11,20; the decrease in gs could explain 55% of the decline in A in all of the studies, indicating that, consistent with Cornic and Massacci14 and Medrano et al.11, the decrease in gs is primarily responsible for the decline in A under drought conditions. Besides, the non-stomatal limitation was also responsible for the decline in A11,31, such as the decrease of mesophyll conductance, which was an important limiting factor in photosynthesis. However, due to the lack of mesophyll conductance data in our dataset, or the literature did not meet the other criteria for inclusion, the contribution of CO2 diffusion within the leaf could not be accurately obtained due to the limited data on mesophyll conductance, so this parameter was not included in this manuscript despite its importance. In C4 plants, the decrease in gs explained 81% of the decline in A, suggesting that the decrease in gs played a more important role in the decline in A under drought in the C4 plants; this finding is consistent with the findings of Da Silva and Arrabaca34, Ripley et al.35, Ghannoum et al.36 and Ghannoum37, who reported that the A in C4 plants under drought was mainly limited by the decline in gs caused by stomatal closure. Moreover, we found that the growing temperature also influenced the decline in A (Supplementary information, Figure S1), indicating that warming temperatures may strengthen the severity of the effects of drought on plants.

In general, we know that Tr depends on gs, the air saturation deficit, temperature, wind speed, and other factors38. Rising temperatures could increase the Tr of plants (Figure S1), which could accelerate the loss of soil water. Although the role of stomata in the control of transpiration has been the subject of debate for many years, the role of stomata in controlling transpiration can be analogously defined as the relative change in Tr for a given relative change in gs10. Across all studies, we found that a decrease in gs could explain 74% of the variance in Tr, which is higher than that of A, indicating that maintaining plant water status may the most important function under drought stress. This finding was consistent with Cowan39,40, Parkhurst and Loucks41 and Jones10, who suggested that stomata operate in a manner that minimizes water loss relative to the ratio of CO2 uptake to soil moisture decrease. We also found that the decrease of gs could explain the decline of Tr in C4 plants more than in C3 plants and that it was higher in trees than in herbs, shrubs and lianas, indicating that decrease of gs caused by drought is likely primarily responsible for the decline of Tr in C4 plants and trees.

Gas exchange is known to be closely related to the status of leaf water, which could be considered to be an indicator of stress under drought conditions26. In this study, we found that gas exchange had a close relationship to leaf water status, as previous works reported that the A in plants decreased as the RWC and LWP decreased14,21,42. We also found that the decrease of RWC in C4 plants caused by drought was a major reason for the decrease of A. Additionally, A showed a faster decrease with the decline of leaf water status in C4 than C3 plants. Moreover, we also found that a decrease in leaf water status caused by drought could explain the decreased A to greatest extent in the lianas among the four plant types. In either case, plant water status had a significant relationship with gs10, and the results supported the notion that leaf water status influences the stomatal response under drought. In all of the studies, leaf water status showed a significant relationship with the gs, and gs showed a faster decrease with the decline in leaf water status in C4 plants and lianas. The change of leaf water status also showed a significant relationship with Tr. The absence of a strong relationship between water status and gas exchange indicated that other factors are involved in regulating gas exchange, such as the air saturation deficit, temperature, and wind speed38.

Our meta-analysis was based on the global scale and focused on the response of the gas exchange and water status to drought and the relationship between these two factors. Based on our data set, we concluded that drought should decrease the gas exchange and water status slightly under mild drought and substantially under severe stress. The gs showed a larger decrease than other physiological traits, and the gas exchange declined the most in shrubs, compared with herbs, trees and lianas. Gas exchange also showed a more substantial decrease in C3 plants than in C4 plants. Moreover, the results showed that the decrease in gs under drought conditions primarily reduced Tr. Additionally, the decrease in gs could explain 55% of the decrease in A and 74% of the decrease in Tr under drought, and it plays a relatively important role in the decrease in A in C4 plants and lianas. The gas exchange also showed a close relationship with the leaf water status, as RWC was less sensitive than LWP, and gas exchange showed a faster decrease in C4 plants and lianas as the water status decreased.

Methods

Data preparation

Peer-reviewed journal articles were searched using the Web of Science and the online databases of the Chinese Academy of Sciences with the following search term combinations: drought/water stress and photosynthesis/gas exchange. To avoid bias in the selection of publications, the studies were chosen based on the following criteria: (1) the experiments were conducted using at least two datasets (control and treatment) and included drought intensity, photosynthetic pathway (C3 and C4) and growth forms (herbs, shrubs, trees and lianas); (2) only experiments conducted under controlled conditions were included, and studies were excluded when the study plant was described as having both tolerant or sensitive genotypes and was not subject to drought-related mortality; and (3) the means, standard deviations/errors and sample sizes of the variables in the control and treatment groups could be directly extracted from the context, tables or digitized graphs. In addition, plant species, photosynthetic pathways (C3 or C4 plants), growth forms (herbs, shrubs, trees or lianas), drought intensity (mild, moderate or severe) and relative soil water content (RSWC) were recorded directly from the papers, and when the drought intensity was not provided by the study, it was grouped into one of three categories according to the RSWC: mild drought (55%<RSWC<70%), moderate drought (40%<RSWC<55%) and severe drought (RSWC<40%).

In total, 167 published papers involving 152 plant species (not include tolerant or sensitive genotypes) and reporting drought and/or water stress studies that satisfied our selection criteria for the meta-analysis were selected from more than 5,000 published papers (Supplementary information and Dataset), including 1,058 observations of A, 908 observations of gs, 594 observations of Tr, 342 observations of RWC and 245 observations of LWP. All original data were extracted from the text, tables, figures and appendices of the publications.

When data were presented graphically, numerical data were obtained using Get–Data Graph Digitizer (ver. 2.20, Russian Federation). To test differences in the responses of plant gas exchange and water status to drought, three drought intensities, mild stress (55 plant species, 47 papers), moderate stress (69 plant species, 69 papers) and severe stress (85 plant species, 78 papers); two photosynthetic pathways, C3 (139 plant species, 152 papers) and C4 (13 plant species, 21 papers); and four growth forms, herbs (62 plant species, 87 papers), shrubs (33 plant species, 30 papers), trees (53 plant species, 51 papers) and lianas (4 plant species, 4 papers) were included.

Analysis

We followed the methods of Hedges et al.43 to evaluate the responses of gas exchange and water status to drought. A response ratio (lnRR, the natural log of the ratio of the mean value of a variable of interest in the drought treatment to that in the control) was used to represent the magnitude of the effects of drought as follows:

where Xe and Xc are the response values of each individual observation in the treatment and control, respectively. Because the LWP is a negative value, when calculating the lnRR of LWP, we used the absolute value of LWP. The corresponding sampling variance for each lnRR was calculated according to Eq. 2:

where ne, nc, Se, Sc, Xe and Xc are the sample sizes, standard deviations and mean response values in the experimental and control groups, respectively. The reciprocal of its variance (W = 1/vi) was considered as the weight of each lnRR. The mean weighted response ratio (RR++) was calculated from lnRR of individual pairwise comparisons between the treatment and control, lnRRij (i = 1, 2,…, m; j = 1, 2,…, k), as below:

here, m is the number of groups (e.g., plant types), and k is the number of comparisons in the ith group.

The meta-analyses were performed using the R software package (version 3.1.1)44. The natural logs of the RRs for the individual and combined treatments were determined by specifying the studies as a random factor in the model in the “metafor” package. The effects of drought on gas exchange and water status were considered significant if the 95% confidence intervals (CIs) of lnRR did not overlap with zero. To compare the responses of gas exchange and water status to drought of different photosynthetic pathways (C3 and C4) and different growth forms (herbs, shrubs, trees and lianas) with the control, we tested whether the interactions between multiple treatments were significant by using the “rma.uni models” in the “metafor” package with treatments as the categorical variables. Regression analysis was conducted to detect relationships between the lnRR of gas exchange and water status under drought in the two photosynthetic pathways (C3 and C4) and four growth forms.

Additional Information

How to cite this article: Yan, W. et al. A meta-analysis of leaf gas exchange and water status responses to drought. Sci. Rep. 6, 20917; doi: 10.1038/srep20917 (2016).