Responses of water use in Moso bamboo (Phyllostachys heterocycla) culms of different developmental stages to manipulative drought

Abstract

Background

Understanding the mechanisms underlying plant water use in response to drought is critical to predicting the trend of ecosystem development under a changing environment, while the responses of water use in clonal plants with interconnected ramets to drought are far less clear than trees.

Method

In this study, we conducted manipulated drought by excluding 50% annual throughfall at a stand level in a Moso bamboo (Phyllostachys heterocycla) forest, to determine how water use of the Moso bamboo responds to drought stress, and whether bamboo culms of different developmental stages show synchronized responses given the existence of physiological integration. Three developmental stages of bamboo culms (i.e., young, mature and old) were investigated and measurements were made on sap flow density (J_s), leaf water potential at predawn (ψ_pre) and midday (ψ_mid), leaf water potential at turgor loss (ψ_TLP), leaf stomatal density and size, and the maximum stomatal conductance (g_w(max)) was calculated.

Results

Under the manipulative drought conditions, the values of J_s significantly decreased in the mature culms but not in the young and old culms. g_w(max) remained unchanged in mature culms, and increased in the young and old culms. Drought significantly reduced ψ_mid of the young culms, resulting in more negative ψ_TLP in the young culms compared with the old ones.

Conclusions

The results imply that the young culms adopted the drought tolerance strategy, while the old culms tended to adopt the drought avoidance strategy. Both the young and old culms are capable of maintaining relatively stable J_s under drought by structural and physiological adjustments. Our findings demonstrate the variable responses of water use in bamboo culms of different developmental stages to drought, suggesting an unsynchronized responses of water use to water stress among culms in clonal plants.

Background

Frequent and severe drought events triggered by climate change have been monitored in subtropical regions of China during the past half century (Zhou et al. 2011). Climate change model projections also indicate that the risk of exacerbated drought events is very likely to double in Southwest China than the other regions of China (Wang and Chen 2014). These drought events are expected to endanger plant water use (Barbeta et al. 2015), water-carbon relations (Sevanto et al. 2014) and consequently affect forest ecosystem functions and services (Anderegg et al. 2013).

As one of the most important physiological characteristics, plant water use under drought stress was extensively studied, by evaluating water loss via transpiration estimated from sap flow monitoring and plant water status reflected by leaf water potential (ψ_leaf) (Du et al. 2011; Meinzer et al. 2013; Grossiord et al. 2014; Meinzer et al. 2016; Yan et al. 2016). Investigations indicated that drought decreased sap flow and ψ_leaf with varying degrees across both tree species growing in the same habitat (Forner et al. 2014; Bretfeld et al. 2018) and intraspecies individuals acclimating to different habitats (Attia et al. 2015; Hochberg et al. 2018). The different drought sensitivities in sap flow and ψ_leaf among species could be attributed to the inherent difference in structural (e.g. stomatal density and size) (Pinheiro et al. 2005; Skelton et al. 2015) and physiological traits (e.g., ψ_TLP) related to water use. For instance, stomatal density and size predominantly determine the maximum stomatal conductance, and therefore, plants with lower stomatal density or smaller stomatal size would have a slight decrease in transpiration in response to drought (Liu et al. 2012; Lawson and Blatt 2014). On the other hand, plants with less negative ψ_TLP could maintain relatively stable water status by earlier stomata closure under drought (Maréchaux et al. 2015). Similarly, there are plastic responses of stomatal density and size as well as ψ_TLP to varying soil water conditions, leading to different sensitivities in sap flow and ψ_leaf among intraspecies individuals (Fraser et al. 2009; Xu et al. 2009; Sun et al. 2014). Higher stomatal density, smaller stomatal size (Bosabalidis and Kofidis 2002) and more negative ψ_TLP (Bartlett et al. 2014) would occur under inadequate soil water condition.

Despite considerable investigations on water use across different tree species and habitats in response to drought (Schultz 2003; Forner et al. 2014; Mata et al. 2014; Attia et al. 2015), it is still barely investigated how the clonal plants with interconnected ramets, respond to drought, and unclear whether intraspecies individuals growing in the same habitat show different responses in sap flow and ψ_leaf to drought. Generally, the interconnected ramets of clonal plant are at different developmental stages which differ in resource acquisition due to their varying structural and physiological characteristics (Dong et al. 2015; Zhang et al. 2016; Zhang et al. 2017). Despite some studies have reported that there is water use integration among ramets of different developmental stages in response to soil water heterogeneity (van Kleunen and Stuefer 1999; You et al. 2013; Roiloa et al. 2014), little is known about the drought response of ramets of different developmental stages in a homogeneous habitat. As a typical clonal plant of Poaceae, bamboo normally form pure forest under intensive management regime, which is consist of culms of different developmental stages growing in an association from an interconnected network of rhizomes. Moso bamboo (Phyllostachys heterocycla), as one of the monopodial bamboo species for the production of shoots and culms, is widely planted in southwest China. Recent studies indicated that Moso bamboo culms at different developmental stages experienced diverse water use and carbon acquisition patterns (Song et al. 2016; Zhao et al. 2016; Zhao et al. 2017). The increasing risk of drought due to climate change in southwest China is likely to affect the growth and productivity of Moso bamboo plantation, and consequently bamboo-based industry development. Therefore, there is a need to study the physiological responses of Moso bamboo to drought, focusing on water use and foliar stomatal changes of the bamboo culms at different developmental stages. In this study, we set up a manipulated throughfall reduction experiment in a Moso bamboo forest. We aim to elucidate (1) how foliar stomatal structural and physiological characteristics (e.g., stomatal density, size and ψ_TLP) of Moso bamboo respond to manipulated drought; and (2) the effects of manipulated drought on sap flow and ψ_leaf of culms at different developmental stages; in order to answer (3) the water use differences among developmental stages of Moso bamboo in response to drought.

Methods

Study site

This study was conducted in Changning Bamboo Forest Ecosystem Research Station (28°27′–28°27′N, 105°00′–105°01′E) located in Changning county, Sichuan Province China. The study area is a humid mid-subtropical region characterized with continental monsoon climate. The mean annual air temperature and average annual precipitation are 18.3 °C and 1141.7 mm, respectively. The predominant soil in the study area is Cambisols (according to the Food and Agriculture Organization of United Nations and United Nations Educational, Scientific, and Cultural Organization soil classification system). Pure Moso bamboo forest is the dominant forest type in this county with variety of management intensities for different purposes (e.g. tourism, food production, or industrial bamboo material).

Experimental design

Four replications of permanent blocks with throughfall reduction (TRT) and control treatments of each (20 m × 20 m) were established in 2016. The TRT is imposed using plastic roofs made of polyolefin film attached to rails about 2 m above the ground, which drained the collected rain out of the plot. These roofs direct about 50% of throughfall away from the 20 m × 20 m TRT area. The ambient plot served as a control without plastic roofs. To prevent from lateral water movement and get rid of the clonal integration from bamboos outside the plot, we trenched the plots and buried a 0.3 cm thick plus 80 cm depth PVC board along the trench leaving 10 cm height above the ground. The TRT was initiated in April 2017.

In experimental stands, newly emerging bamboos were tagged in spring of each year from 2014 to 2018. Three developmental stages of bamboo culms were identified. Those new culms reaching full height in current year were chosen as young culms. We defined 2–3 years old culms and older than four years as the mature and old, respectively. In total 72 culms were selected (3 culms for each developmental stage per plot) in 2017, and another 12 young culms (6 culms for each treatment) emerging in 2018 were also monitored subsequently.

Meteorological observation

Local climate data were continuously collected from an automatic weather station (Onset Computer Corporation, Pocasset, MA, USA), about 500 m away from the study plots, including photosynthetically active radiation (PAR, μmol∙m^− 2∙s^− 1), air temperature (T, °C), relative humidity (RH, %), wind speed (WS, mph), and precipitation (Precip., mm). All the meteorological data were measured at 30 s intervals, and 30 min averages were recorded on the U30-NRC-000-10-S100–000 data logger (Onset Computer Corporation, Pocasset, MA, USA). Vapor pressure deficit (VPD, kPa) was calculated from T and RH (Allen et al. 1998). Additionally, SM-150 T sensors (Delta-T Devices, Cambridge, UK) mounted at three measurement depths (10, 30, 60 cm) in each plot to measure the soil water content (SWC, m³∙m^− 3) and soil temperature (°C) at 10 s intervals and a DL2e data logger (Delta-T Devices, Cambridge, UK) recorded data with every 30 min averages.

Sap flow measurements

Three bamboo samples at each age were selected in each plot to measure sap flow density. The measurements were conducted during growing seasons of 2017 and 2018. The sap flow was monitored using self-made thermal dissipation probes following Granier’s prototype (Granier 1987). A probe set consists of two 10-mm long needles installing in vertical at 1.3 m height of each sample culm on the south side sheltering with a plastic box and a film to protect from rainfall, solar radiation and thermal gradient. The upper needle was heated with a 120-mA direct current, and the lower one remained unheated as a reference. The sap flow density was measured every 30 s and stored every 10 min using a data logger (DL2e, Delta-T Devices, Cambridge, UK). Due to the remarkable difference of the hydraulic traits between bamboo and the dicot species from which Granier’s standard equation (Granier 1987) was derived, we used the calibrated equation described by Zhao et al. (2016) basing on monitoring sap flow in the P. pubesens with an accurate modified 10-mm probe. Thus, the sap flow density (J_s, g H₂O∙m^− 2∙s^− 1) of P. pubesens equation (Zhao et al. 2016) is given as follows:

$$ {J}_{\mathrm{s}}=360.44\times {\left(\frac{\varDelta {T}_{\mathrm{m}}-\varDelta T}{\varDelta T}\right)}^{1.746} $$

(1)

where ΔT_m is the maximum temperature difference obtained under zero sap flow conditions and ΔT is the temperature difference between two needles; 360.44 and 1.746 are the suggested values for corrected empirical constants of original equation.

Leaf water potential and turgor loss point measurement

The ψ_leaf was measured using a pressure chamber (SKPM1400, Skye, Llandrindod Wells, UK, n > 4 for each developmental stages per treatment) at predawn (5:00–7:00) and midday (11:00–13:00) in sunny days during June to August. The ψ_leaf at the turgor loss or wilting point (ψ_TLP) was derived from pressure-volume curves described by Koide et al. (1989). Four to five healthy, non-necrotic and sunlit branches were excised in bamboo canopy of each developmental stage per treatment in the predawn after raining in 16th July, 2018. These branches were immediately re-cut underwater and rehydrated until ψ_leaf > − 0.05 MPa. After that, three shoots per branch were collected to be weighed and measured for ψ_leaf repeatedly at regular time intervals until ψ_leaf > − 4 MPa. Then these shoots were weighted after being desiccated at 65 °C for 48 h. The pressure-volume curves were built with inverse ψ_leaf and leaf relative water content. The distance from ψ_TLP was calculated as the difference between ψ_TLP and ψ_mid.

Stomatal density, size and g _w(max) calculation

Stomatal attributes were measured in leaves fully expanded and exposed to direct sunlight from seven culms of each developmental stage per treatment (n = 35 leaves for each developmental stage classification per treatment). Nail polish imprint was torn down from the abaxial leaf surface by a tweezer and then placed it under light microscopy (Olympus BX51, Tokyo, Japan) to calculate stomatal density (D), guard cell pair width (W) and length (p). Stomatal size was calculated as the guard cell pair width multiply guard cell length (Franks et al. 2015). g_w(max) was calculated based on the formula (Franks et al. 2009):

$$ {g}_{\mathrm{w}\left(\max \right)}=\frac{d}{v}\bullet D\bullet {a}_{\mathrm{max}}/\left(l+\frac{\pi }{2}\sqrt{a_{\mathrm{max}}/\uppi}\right) $$

(2)

where d is the diffusion coefficient of H₂O in air (cm²∙s^− 1), v is the molar volume of air (m³∙mol^− 1), a_max is the average value of maximum stomatal pore area, which, in Moso bamboo, is close to a circle with the stomatal pore length equal to a diameter, that is, a_max = πp²/4, l is the stomatal pore depth, which is approximated as half of the stomatal width for fully hydrous guard cells (Franks and Farquhar 2007). All the leaves were collected in June 20th, 2018, when the renewed leaves had been mature.

Statistical analysis

One-way analysis of variance (ANOVA) was used to test the difference in SWC and ST between ambient and TRT. Two-way analysis of variance was used to identify the difference in stomatal density and size, g_w(max) as well as ψ_TLP between the treatment and the developmental stages of culms. Repeated measures ANOVA was used to identify the difference in ψ_leaf and its distance from ψ_TLP between treatments in each developmental stages of culms. Linear mixed models were used to assess differences in J_s among the three developmental stages of culms. In the model, developmental stage, treatment and their interaction were fixed effects, and culms within each group were counted as a random effect. R-package “LME4” (Bates et al. 2015) was applied to perform Linear mixed model analyses and R-package “MULTCOMP” (Hothorn et al. 2008) was used to implement pairwise comparisons. To find out the environmental driver of J_s, relative importance metrics of environmental factors were calculated applying LMG method (Lindeman et al. 1980) which divided model contribution among all predictors. Confidence intervals for relative importance metrics were obtained by bootstrapping (1000 runs). Linear regression models were built with meteorological factors (predictors) and average J_s (response) in the daytime of different developmental stages of culms per treatment, and analysis of covariance (ANCOVA) was performed to compare the slopes of the regressions lines to validate whether J_s sensitivity to average VPD in the daytime and total daily PAR was different between treatments. Further, to test whether SWC impacts the variation of J_s which VPD and PAR couldn’t explain, we fitted linear models using the residuals J_s and average daily SWC. The residuals J_s generated by the linear regression of J_s and VPD as well as J_s and PAR. To clarify the sensitivity of J_s to ψ_leaf, J_s at midday (11:00–13:00) and ψ_mid were selected to build linear regression model for each developmental stage. The slopes of regression lines were compared by ANCOVA. All data analyses and graphics were performed in the statistical program R 3.4.3 (R Core Team 2017).

Results

Soil temperature and moisture

The TRT caused a significant decrease in SWC by 31.4% in 2017 and 35.6% in 2018 comparing with ambient treatment (all p < 0.01; Fig. 1a and b). There was no inter-annually difference in average SWC between 2017 and 2018 (p > 0.05). Moreover, soil temperature was not affected by TRT (p > 0.05, Fig. 1c and d).

Fig. 1

Soil volumetric water content (a, b; %) andsoil temperature (c, d; °C) in the top 10 cm of the soil in ambient and throughfall reduction treatment (TRT) in 2017 (a, c) and 2018 (b, d). The band inside the boxes represent the median, the whiskers upside and underside of box represent the first and third quartiles (25th and 75th percentiles, respectively), and dots correspond to values beyond 1.5× inner quartile range (IQR) from the first and third quartiles. Asterisks (*) denote significantly different in SWC between ambient and TRT (α < 0.05, ANOVA)

Sap flow

The TRT reduced J_s (p < 0.05) of the mature rather than that of the old and young culms (Fig. 2b). The variation of J_s was best described by VPD and PAR (Table 1), and positive linear relationships between J_s and VPD (Fig. 3a, c, d) as well as J_s and PAR (Fig. 4a, c, d) were detected in three developmental stages of culms. In contrast, SWC could not explain the fluctuation of J_s (Table 1). Furthermore, when the effects of VPD and PAR were controlled, residual J_s could not explained by SWC as well (Fig. 3b, d, f, and 4b, d, f), suggesting SWC had no direct effect on J_s. Interestingly, TRT significantly lowered the slopes of the relationships between J_s and VPD (Fig. 3a, c, d), and the similar results occurred between J_s and PAR (Fig. 4a, c, d), suggesting TRT caused significant decreases in sensitivities of J_s to VPD and PAR.

Fig. 2

Linear mixed model results of diel sap flow density in consecutive sunny days. The band inside the boxes represent the median, the whiskers upside and underside of box represent the first and third quartiles (25th and 75th percentiles, respectively), and dots correspond to values beyond 1.5× inner quartile range (IQR) from the first and third quartiles. Asterisks (*) denote significantly different between ambient and throughfall reduction treatment (TRT) within a given developmental stage (α < 0.001; Tukey method)

Table 1 Relative importance metrics and confidence intervals

Fig. 3

Linear regression (a, c, d) of average vapor pressure deficit (VPD, kPa) and sap flow density (J_s, g∙m^− 2∙s^− 1) in the daytime, and linear regression (b, d, f) of residuals plotted against soil water content (SWC, %) in old (a, b), mature (c, d), young (e, f) in ambient (A) and throughfall reduction treatment (TRT). The significance level (p < 0.001) is marked by Asterisks (*) for these significant fitting model. Different letters behind r-square value denote the significant difference in the slopes of the regression lines between ambient and drought (ANCOVA, (a) F (1, 22) = 58.096, p < 0.001; (c) F (1, 22) = 5.677, p = 0.018; (e) F (1, 34) = 65.727, p < 0.001). Data only in sunny days during growing seasons in 2017 and 2018 were included (n = 211)

Fig. 4

Linear regression (a, c, d) of total daily photosynthetically active radiation (PAR, μmol∙m^− 2∙d^− 1) and average sap flow density (J_s, g∙m^− 2∙s^− 1) in the daytime, and linear regression (b, d, f) of residuals plotted against soil water content (SWC, %) in old (a, b), mature (c, d), young (e, f) in ambient (A) and throughfall reduction treatment (TRT). The significance level (p < 0.001) is marked by Asterisks (*) for these significant fitting model. Different letters behind r-square value denote the significant difference in the slopes of the regression lines between ambient and drought (ANCOVA, (a) F (1, 22) = 59.339, p < 0.001; (c) F (1, 22) = 10.343, p = 0.001; (e) F (1, 34) = 73.831, p < 0.001). Data only in sunny days during growing seasons in 2017 and 2018 were included (n = 211)

Leaf water potential and ψ _TLP

The TRT significantly decreased ψ_mid of the mature and young culms (all p < 0.01, Fig. 5b, d, f) rather than that of the old. Moreover, negative linear relationships between J_s and ψ_mid were found for the three developmental stages of culms regardless of treatments (Fig. 6), and the slope for the old culms was higher than that of the young culms both in ambient and TRT (A: p < 0.01, TRT: p = 0.04, ANCOVA). Additionally, the TRT decreased ψ_TLP irrespective of developmental stages of culms (all p < 0.01, Fig. 7) and the ψ_TLP in the old culms was the least negative in the three developmental stages of culms (all p < 0.01), suggesting that the stomata of the old culms close earliest under drought. The distance from ψ_TLP, indicating that the stomatal aperture, decreased in all developmental stages of culms from June to July under TRT (all p < 0.01, Fig. 8).

Fig. 5

Leaf water potential at predawn (ψ_pre, a) and midday (ψ_mid, b) for the old, mature and young culms during June to August in 2018. Error bars represent ± SEM

Fig. 6

Relationship between sap flow density (J_s) and leaf water potential at midday (11:00–13:00) for old (a), mature (b) and young culms (c) in ambient (green) and throughfall reduction (red) during June to August in 2018

Fig. 7

Mean turgor loss point (ψ_TLP) for the leaves of three developmental stages of bamboo culms in ambient (black) and throughfall reduction (grey). Different capital letters below the columns indicate significant difference between treatments and the lowercase letters indicate significant difference between three developmental stages of culms (α < 0.05; Tukey method)

Fig. 8

The difference between leaf water potential (ψ_mid) and turgor loss point (ψ_TLP) for the old (a), mature (b) and young (c) culms in ambient (green) and throughfall reduction (red). Error bars represent ± SEM

Stomatal density, size and g _w(max)

The TRT increased the stomatal density by 45.4% (p = 0.03) and 59.9% (p = 0.03) in the old and young culms, respectively (Fig. 9a), while decreased the stomatal size of the old culms by 43.5% (p < 0.01, Fig. 9b), which became the smallest among the three developmental stages of culms. Due to the variation of stomatal density and size, g_w(max) was increased by 13.0% (p = 0.03) and 49.1% (p < 0.01) in the old and young culms under TRT, respectively (Fig. 9c).

Fig. 9

Stomatal density (a), stomatal size (b) and maximum stomatal conductance (g_w(max)) (c) in three developmental stages of bamboo culms. The band inside the boxes represent the median, the whiskers upside and underside of box represent the first and third quartiles (25th and 75th percentiles, respectively), and dots correspond to values beyond 1.5× inner quartile range (IQR) from the first and third quartiles. Different capital letters below the columns indicate significant difference between treatments and the lowercase letters indicate significant difference between three developmental stages of culms (α < 0.05; Tukey method)

Discussion

The response of sap flow to throughfall reduction

The responses of J_s to TRT were not identical in the three developmental stages of culms, J_s decreased in the mature culms rather than the old and young under TRT. Generally, the mature culms often had the highest specific leaf area and leaf area index (LAI) in all three developmental stages, suggesting highest transpiration demands (Banik 2000; Banik and Islam 2005), and thus they probably were most susceptible to soil water stress. Another reason for the declined J_s in the mature would be the declining stomatal aperture (Hsiao 1973; Cornic 2000; Kuromori et al. 2018). As stomatal aperture directly control the sensitivity of J_s to VPD and PAR (Lawson and Blatt 2014), in turn, the sensitivity of J_s to VPD and PAR reflected the stomatal aperture. In detail, the decreasing sensitivity of J_s to VPD and PAR in the mature culms elucidated their stomatal aperture down regulating. Moreover, the decreasing distance from ψ_TLP supported that the stomatal aperture decreased under TRT. However, reduced stomatal aperture occurred in the old and young culms, which didn’t elicit the decrease of J_s. The consistent J_s in the young under TRT probably was helped by the water compensated from the mature culms. This was reported that more than 20% of compensative water was transported to the young culms from culms at other developmental stages (Zhao et al. 2016), and the compensative water helped the young culms maintaining water use, gas exchange activity and net photosynthesis under drought (Adonsou et al. 2016). Moreover, plants could plastically adjust their stomatal traits to respond to drought (Liu et al. 2016). Therefore, the young and the old probably changed their stomatal characteristics to sustain J_s under drought.

Leaf water potential response to throughfall reduction

The three developmental stages of culms exhibited distinct patterns of ψ_mid variation after TRT, which were attributed to the different responses of J_s to TRT. In other words, plant water status was the result of their water use under drought (Nadezhdina 1999). In fact, negative relationships between J_s and ψ_mid showed in all three developmental stages of culms. Thereby, the declining J_s in the mature culms caused a raise of ψ_mid, and similarly, the unchanged J_s in the old culms coupled with consistent ψ_mid under TRT. However, unlike the old, the unchanged J_s accompanied with the decreasing ψ_mid in the young culms. This was explained by the more negative ψ_TLP in the young culms than the old, suggesting the young closed stomatal latter under drought and thus lost more water than the old. The developmental stage-specific responses of ψ_leaf to TRT probably indicated different water use strategies under drought, where the old culm tended to take drought avoidance strategy to conserve water, while the young and mature culm preferred to take drought tolerant strategy.

Stomatal response to throughfall reduction

Consistent with the different response of J_s among developmental stages, the stomatal density and size also responded differently to drought among developmental stages. The g_w(max) was increased only for the young and old culms by changing stomatal density and size, but not for the mature culms. We speculate that the young and old culms maintain consistent J_s through the increased g_w(max) in the context that the stomatal aperture was reduced under TRT, while the decreased J_s of the mature probably was due to the unchanged g_w(max). Interestingly, different from the young which solely increased stomatal density without changing size, the old culms increased stomatal density but decreased stomatal size. Studies suggested that smaller stomata responded faster to environmental and internal cues (Drake et al. 2013; Kardiman and Ræbild 2017), which means stomata with smaller size closing faster under water stress. Therefore, the risk of hydraulic failure following increased stomatal density could be avoided by lowering stomatal size (Franks et al. 2009). On account of lacking secondary growth, the quantity and quality of functional stem vessels declines with developmental stages, and thus the ability for water uptake and transportation of old bamboos are the worst in three developmental stages of culms (Gielis 2015). Therefore, the old culms encountered the risk of potential hydraulic failure while maintaining water use under drought, but the reduced stomatal size coincidently was beneficial to avoid hydraulic failure. Actually, the different patterns of stomatal density and size in response to drought generated different drought sensitivity of water use of the culms at different developmental stages. Water use of the young and old culms were less sensitive to drought than the mature due to their stomatal adjustments. Furthermore, different developmental stages of culms probably triggered various water use strategies under drought.

Different water use strategies to drought

Disparate trends in J_s and ψ_leaf under TRT indicated different water use strategies in the three developmental stages of culms. The consistent J_s and decreased ψ_leaf in the young culms under TRT were characteristic of a drought tolerant strategy, where J_s was sustained at highly negative ψ_leaf. In contrast, a drought avoidance strategy was found in the old culms under TRT, where the ψ_mid did not change and J_s was more sensitive to decreasing ψ_leaf than the young culms, suggesting that the old was more conservative to drought than the young culms. However, a recent study showed that all of three developmental stages of a sympodial bamboo species (Bambusa chungii) adopted the tolerant strategy under drought (Zhang et al. 2017). As the rhizome systems of the sympodial and monopodial bamboo are pronouncedly different, and the sympodial bamboo with tight clumping habit are native to tropical regions with a long dry season (Kleinhenz and Midmore 2001). In the relatively homogeneous habitat, different developmental stages of culms of the sympodial bamboo would be better to resist water deficit by taking the same water use strategy.

Conclusion

We found different responses of J_s and ψ_leaf in three developmental stages of culms to manipulative drought, which were attributable to the developmental stage-specific structural and physiological variations of stomata. Importantly, the water use of the mature culms was the most sensitive under drought among three developmental stages. Further research is needed to quantify the role of clonal integration in the regulation of structural and physiological traits of bamboo culms at different developmental stages, and how Moso bamboo adjusts the composition of different developmental stages of culms, as well as feedbacks in this adjustment.