INTRODUCTION The shortage of water resource is main environmental problem for the vegetation restoration and ecological management in arid and semi-arid areas. With the global climate change and the increase of greenhouse effect, the uneven distribution of precipitation in season was aggravated, and thereby the occurrence frequency, impact range and harm degree of drought are becoming more and more serious in some regions (Posch & Bennett 2009; Yan et al., 2017; Zheng et al., 2017). Drought stress is one of the main abiotic stress factors which affects plant physiology and ecology, and restricted the seedling survival, growth and development of afforestation species (Breshears et al., 2009; Allen et al., 2010; Xu et al., 2010; Hicke & Zeppel, 2013). These problems ultimately inhibited vegetation restoration and ecological management, especially during the establishment of forests (Monclus et al., 2006). Woody plants are important afforestation species in the north of China, which play an important role in vegetation restoration and ecological management (Wang et al., 2017). Overall, photosynthesis, growth, and development of the plants have been adversely affected due to small amount of rainfall and the uneven distribution of precipitation, which has received the extensive attention in recent years (Yan et al., 2017; Zheng et al., 2017; Liu et al., 2019). Photosynthesis is an important physiological metabolic process of plants, closely related to plant growth environment, and influenced by many external and internal factors. For example, soil water deficit can directly affect the photosynthetic pigments, photosynthetic activity of mesophyll cells, chlorophyll fluorescence, water use efficiency of plants, and ultimately triggers complex series of physiological and biochemical reactions of the plant (Condon et al., 2002; Dong et al., 2019), which has become one of the focus of physiological researches of plant drought resistance mechanism (Condon et al., 2002; Zhou et al., 2015; Liu et al., 2019). Under drought stress conditions, the photosynthetic center of plant leaves is often damaged, which usually leads to the decrease of net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs) and intercellular carbon dioxide concentration (Ci) (Marian et al., 2004). Hence, to adapt with drought environment, plants improve the water use efficiency (WUE) correspondingly. Previous studies have shown that photosynthesis of plant is more sensitive to drought stress (Sun et al., 2013), and stressed plants presented lower Pn and Gs (Cátia et al., 2018), which may be one of the main Pak. J. Agri. Sci., Vol. 57(4), 1237-1249; 2020 ISSN (Print) 0552-9034, ISSN (Online) 2076-0906 DOI: 10.21162/PAKJAS/20.504 http://www.pakjas.com.pk RESPONSES OF GAS EXCHANGE AND WATER-USE EFFICIENCY IN Platycladus orientalis AND Amorpha fruticosa TO DROUGHT EPISODE AND REWATERING Shulin Feng 1 , Ashim Sikdar 2,3 , Jinxin Wang 2,* , Boyuan Li 1 , Guoli Lv 1 and Xu Ma 1 1 Institute of Soil and Water Conservation, Northwest A&F University, Yangling, Shaanxi 712100; 2 College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi 712100; 3 Department of Agroforestry & Environmental Science, Sylhet Agricultural University, Sylhet, 3100, Bangladesh. *Corresponding author’s e-mail address: [email protected]To explore the changes in photosynthetic properties of two typical afforestation species (Platycladus orientalis and Amorpha fruticose) under drought stress and the subsequent recovery mechanism after rewatering, a pot experiment was carried out in greenhouse. The results showed that water deficit inhibited the photosynthesis of two afforestation plants. With the increase of drought stress, the seedlings of the two afforestation plants could enhance their adaptability and resistance to drought stress by decreasing the photosynthetic parameters and improving water use efficiency. The declined net photosynthetic rate (Pn) observed due to stomatal limitation in the three different drought stress treatments (87.87 % SWC, 70 % SWC and 52.16 % SWC). At 40 % SWC treatment, the reduction in Pn of two tree species was associated with the nonstomatal factors. The decrease in transpiration rate (Tr) was larger than that of Pn and the water-use efficiency improved to some extent. After rewatering, the photosynthetic parameters of three growth stages of two afforestation plants gradually recovered, indicating the stimulation in compensation effects on photosynthetic characteristics of two afforestation plants after rewatering, the photosynthetic indexes in two tree species could rapidly recovered close to control after 72h of rewatering. The results showed that photosynthesis of the two afforestation plants had a strong sensitivity and well adaptability to drought environment and suggested potential rapid recovery after rewatering. Keywords: Afforestation species; Drought stress; photosynthesis properties; recovery; rewatering.
Transcript
INTRODUCTION
The shortage of water resource is main environmental
problem for the vegetation restoration and ecological
management in arid and semi-arid areas. With the global
climate change and the increase of greenhouse effect, the
uneven distribution of precipitation in season was
aggravated, and thereby the occurrence frequency, impact
range and harm degree of drought are becoming more and
more serious in some regions (Posch & Bennett 2009; Yan
et al., 2017; Zheng et al., 2017). Drought stress is one of the
main abiotic stress factors which affects plant physiology
and ecology, and restricted the seedling survival, growth and
development of afforestation species (Breshears et al., 2009;
Allen et al., 2010; Xu et al., 2010; Hicke & Zeppel, 2013).
These problems ultimately inhibited vegetation restoration
and ecological management, especially during the
establishment of forests (Monclus et al., 2006).
Woody plants are important afforestation species in the
north of China, which play an important role in vegetation
restoration and ecological management (Wang et al., 2017).
Overall, photosynthesis, growth, and development of the
plants have been adversely affected due to small amount of
rainfall and the uneven distribution of precipitation, which
has received the extensive attention in recent years (Yan et
al., 2017; Zheng et al., 2017; Liu et al., 2019).
Photosynthesis is an important physiological metabolic
process of plants, closely related to plant growth
environment, and influenced by many external and internal
factors. For example, soil water deficit can directly affect the
photosynthetic pigments, photosynthetic activity of
mesophyll cells, chlorophyll fluorescence, water use
efficiency of plants, and ultimately triggers complex series
of physiological and biochemical reactions of the plant
(Condon et al., 2002; Dong et al., 2019), which has become
one of the focus of physiological researches of plant drought
resistance mechanism (Condon et al., 2002; Zhou et al.,
2015; Liu et al., 2019). Under drought stress conditions, the
photosynthetic center of plant leaves is often damaged,
which usually leads to the decrease of net photosynthetic
reasons for the decrease of plant biomass production under
lower water availability conditions. Generally, the reason for
the decrease of Pn can be attributed to the limitation of
stomatal factors or non-stomatal factors (Alfonso et al.,
2014).
The response of plants to drought stress and rewatering is
one of the complex metabolic processes. Therefore, the
adaptability and resistance of plants to the growing
environment are not only reflected in the process of drought
stress, but also reflected in the recovery process after post-
drought rewatering, which becomes another important part
of the overall plant physiological response to a drought
stress period (Alfonso et al., 2014). Whether plants can
quickly make up the damage caused by drought stress after
rewatering, is the key to their adaptability to drought
environment, and the recovery ability plays an important
role in drought adaptability (drought resistance and recovery
ability) (Chen et al., 2016). Previous studies have indicated
that physiological parameters of plants, such as Pn, Tr, Gs
and Ci, can be restored to different degrees after post-
drought rewatering (Miyashita et al., 2005). In recent years,
many afforestation plants have been extensively studied in
the north of China, but these studies mainly focused on a
single growth stage under drought stress and rewatering
conditions. Therefore, there is lack of in-depth reports on the
physiological response characteristics of typical afforestation
tree species in different growth stages to drought stress and
the recovery mechanism after rewatering. Moreover, the
physiological response characteristics and recovery
mechanism of photosynthetic capacity in typical
afforestation species have not been fully clarified.
Platycladus orientalis and Amorpha fruticosa are pioneer
afforestation species commonly used in northern China and
got more attention for resuming and rebuilding of vegetation
in drought-prone environment, due to their strong
adaptability to adverse environmental conditions and good
soil-water conservation characteristics (Zhou et al., 2015;
Yan et al., 2017). In this study, the changes of
photosynthetic properties and WUE of the seedlings of the
two species under drought stress, and the recovery
mechanism of photosynthetic physiology after rewatering
were investigated. The aim of this study was to reveal the
response characteristics of photosynthetic gas exchange
parameters of the leaves in different growth stages to
drought stress, and the dynamic restoration mechanism of
photosynthetic properties after rewatering, which could
provide theoretical and practical support for the application
of typical afforestation seedlings in vegetation restoration
and ecological construction in north China.
MATERIALS AND METHODS Plant material and treatments: The experiment was carried out in a greenhouse of Northwest A&F University,
Yangling, Shaanxi, Northwest China (34°16′N, 108°4′E). The precipitation mostly occurs between July and September at the study site with an annual mean of 650 mm. The average day and night temperatures are 27°C and 15°C,
respectively and relative humidity ranges from 35 % ～70
%. One-year-old seedlings of P. orientalis and A. fruticosa were used as test materials. Each pot (32 cm×27 cm×30 cm) was filled with 10 kg of air-dried loess soil having water holding capacity of 22.3 %. Two seedlings of P. orientalis and A. fruticosa were then transplanted into each pot in March. 1.2 kg of grits was used at the surface of each pot to prevent the evaporation of soil water. Three replicates were maintained for each treatment and pots were arranged following a randomized complete block design. According to the process of seedlings growth, the growth stages of P. orientalis and A. fruticosa were divided into three stages, i.e., the initial growth stage (IGS), the fast growth stage (FGS), and the late growth stage (FGS), respectively. Water supply was controlled artificially to simulate drought stress and the alteration in the photosynthetic indexes of two afforestation plants under different drought degrees (87.84 % SWC, 70 % SWC, 52.16 % SWC and 40 % SWC) and drought duration (15d, 30d, 45d and 60d in different growth stages) was evaluated. The soil relative water content (SWC) was maintained at 100 % as the control. Before starting the drought stress, all pots were watered with sufficient water for 1 month to facilitate seedling recovery from transplantation. For each treatment, P. orientalis and A. fruticosa seedlings rewatered for the rest of the growth period after the desired drought stress duration to maintain the soil water content at the control level. Plants were irrigated every day following the weighing method to maintain the desired moisture level during the entire experimental period. The plants were grown for 6 months (April to October) to investigate the impact of drought stress and rewatering in different growth stages. Photosynthetic parameters and water use efficiency measurements: Photosynthetic parameters were measured for mature leaves of P. orientalis and A. fruticosa seedlings from each test plant using a CIRAS-3 portable photosynthesis system (PP Systems, Amesbury, MA, USA). Red-blue, light-emitting diodes were maintained at a steady level of 1000 μmol∙m-2∙s-1. Measurements were made at the end of the drought stress period and after 2, 24, 48 and 72h of water application during rewatering. Leaf gas exchange parameters were measured between 8:00 AM and 12:00 AM. The photosynthesis system automatically recorded the net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci), and transpiration rate (Tr). Water-use efficiency (WUE) was calculated following the method of Liu et al. (2017). The LA of P. orientalis was calculated using the following equation (1) (Zhu et al., 2006). LA = 161×LW (1)
Where, LW is the leaf dry matter. The fresh leaves were put
into the oven and dried at 80°C until completely dried, and
the LA was obtained by substituting the formula.
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Data analysis: All the data are presented as mean±SD of
three replicates. Data were processed by EXCEL 2003 and
variance analysis was performed by SPSS17.0. To check
significant differences, multiple mean comparisons were
made using LSD at P < 0.05. All graphical works were
conducted with Origin 6.5.
RESULTS
Figure 1. Dynamics of net photosynthetic rate in Platycladus orientalis and Amorpha fruticosa seedlings in different
growth stages under different drought stress treatments after rewatering. Legend: (a), (e) and (i) represent the drought stress for 15 days; (b), (f) and (j) represent the drought stress for 30 days; (c), (g) and (k)
represent the drought stress for 45 days; (d), (h) and (l) represent the drought stress for 60 days. Data are mean ± SE (n =3). *
significant differences among different drought stress treatments (p ＜0.05).
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Effect of drought and rewatering on Pn of two tree species: Under soil water deficit treatments, response strategies of Pn of P. orientalis and A. fruticosa seedlings in different growth stages were different (Figure 1). With the decrease of soil moisture content, the change patterns of Pn in all the three different growth stages in two typical afforestation tree species were also different. In addition, the influential level of Pn was distinct to different drought stress, drought duration, tree species and growth stage. The Pn of A. fruticosa in FGS decreased at first, and then increased and lastly decreased under 15 days of drought stress, whereas the Pn in LGS, gradually decreased under four drought durations (15, 30, 45 and 60d). The Pn of two afforestation trees in other drought treatments first increased and then decreased. At 40 % SWC treatment, the Pn of the two afforestation species decreased to the lowest level, whereas the Pn of P. orientalis in all the three different growth stages, the Pn in the FGS and LGS of A. fruticose under four drought durations, and the Pn in the IGS of A. fruticosa under 15 days drought stress were significantly inhibited. The Pn in P. orientalis treated with 40 % SWC at 60d in IGS, in P. orientalis treated with 40 % SWC at 15d in FGS as well as treated with 40 % SWC at 45d in LGS decreased to the lowest value (68.15 %, 79.77 % and 38.28 % respectively), compared with the control group. Contrarily, the Pn in A. fruticosa treated with 40 % SWC at 15d in IGS, in A. fruticosa treated with 40 % SWC at 30d in FGS as well as A. fruticosa treated with 40 % SWC at 15d in LGS decreased to the lowest level (34.85 %, 44.58 % and 68.92 %, respectively). After rewatering, the recovery degree of Pn of P. orientalis and A. fruticosa seedlings was also different in all the three different growth stages, and the different compensation effects were also found due to different drought stress degree, stress duration, growth stage and tree species (Figure 1). In case of P. orientalis, the Pn of seedlings in different growth stages recovered to near control level after 72h of rewatering, and the Pn of P. orientalis in IGS treated with 87.84 % SWC at 30d was remarkably higher than the control. Among the drought stress treatments, the Pn of P. orientalis in IGS, FGS and LGS treated with 87.84 % SWC under 30 days of drought stress increased to the maximum value, which was 1.44, 1.18 and 1.22 times higher than those of the control, respectively. As for A. fruticosa, the compensation effects of the three growth stages of the seedlings stimulated by rewatering. After 72h of rewatering, the Pn of IGS and FGS returned to close to the control level, but in LGS, the inhibition of drought stress on Pn of A. fruticosa could not completely eliminate after 72h of rewatering, which may be related to the decrease of photosynthetic ability of plant leaf organs in LGS. Among the drought stress treatments, Pn in 70 % SWC treatment at 60d in IGS, in 70 % SWC treatment at 30d in FGS, and in 52.16 % treatment at 15d in LGS increased by 1.26, 1.76 and 1.31 times of the control, respectively after rewatering. Effect of drought and rewatering on Tr of two tree species: As shown in Figure 2, the response of transpiration rate (Tr)
of P. orientalis and A. fruticosa seedlings to water stress and rewatering treatments was different. In general, under drought stress condition, with the decrease of soil moisture content, the Tr of the two species in IGS, and LGS of A. fruticosa showed a gradual decline, while the changes in Tr at FGS of the two species were different due to the different degree of drought, stress duration and tree species. At 40 % SWC treatment, the Tr of two species were significantly inhibited, and the Tr of each growth stage of P. orientalis and A. fruticosa reached the lowest level. The Tr with 40 % SWC treatment at 60d in IGS of P. orientalis and 40 % SWC treatment at 30d in FGS and LGS of P. orientalis reached the lowest level by 86.39 %, 68.22 % and 50.35 % respectively, compared to the control. The Tr in A. fruticosa treated with 40 % SWC at 60d in IGS, with 40 % SWC at 30d in FGS, and with 40 % SWC at 15d in LGS reached the lowest level under four drought durations by 66.76 %, 54.08 % and 48.34 %, respectively with respect to the control. The change in Tr of P. orientalis and A. fruticosa after rewatering has been shown in Figure 2. For P. orientalis, after 48h of rewatering, the Tr of P. orientalis in IGS and LGS was restored to the control level. 72h after rewatering, there was no significant difference between the Tr of P. orientalis in all the three different growth stages in relation to the control. Among the drought stress treatments, the Tr with 87.84 % SWC treatment at 30d in IGS, with 87.84 % SWC treatment at 60d in FGS, and with 52.16 % SWC treatment at 45d in LGS increased by 1.15, 1.33 and 1.25 times of the control, respectively. As for A. fruticose, after 2h of rewatering, the Tr of IGS was almost restored to the control level. After 24h of rewatering, the Tr of FGS was almost the same as that of the control. After 72h of rewatering, the Tr of LGS returned to near the control level. In contrast, the Tr in 70 % SWC treatment at 60d in IGS, in 70 % SWC treatment at 30d in FGS, and in 52.16 % SWC treatment at 15d in LGS increased by 1.07, 1.29 and 1.86 times of the control, respectively. Effect of drought and rewatering on Gs of two tree species: The response of stomatal conductance (Gs) of P. orientalis and A. fruticosa seedlings at different growth stages was similar under drought stress conditions (Figure 3). With the decrease of soil moisture content, the change patterns of drought stress in all the three different growth stages of two typical afforestation tree species were different. The influential degree of Gs was distinct to different drought stress, drought duration, tree species and growth stage. Among the drought stress treatments, the Gs of P. orientalis in FGS under four drought durations (15, 30, 45 and 60d), and the Gs of A. fruticose in IGS under three drought durations (30, 15 and 60d) increased at first and then decreased. The Gs of two afforestation trees in other drought treatments had a continuous downward trend with the aggravation of soil drought. At two treatments with low soil moisture (52.16 % SWC and 40 % SWC), the stomatal movement of the seedlings of the two species inhibited significantly, and the Gs decreased to a lower level thereafter. The Gs of P. orientalis treated with 40 % SWC
Exchange and water-use efficiency
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treatment at 60d in IGS, with 40 % SWC treatment at 30d in FGS and LGS decreased by 77.04 %, 70.07 % and 71.46 % than those of the control, respectively. On the other hand, the
Gs of A. fruticosa treated with 40 % SWC treatment at 30d in IGS, with 40 % SWC treatment at 15d in FGS and LGS decreased by 49.61 %, 61.99 % and 44.71 %, respectively.
Figure 2. The changes in transpiration rate (Tr) in leaves of Platycladus orientalis and Amorpha fruticosa seedlings
subjected to different drought stress levels and recovery period. Legend: (a), (e) and (i) represent the drought stress for 15 days; (b), (f) and (j) represent the drought stress for 30 days; (c), (g) and (k)
represent the drought stress for 45 days; (d), (h) and (l) represent the drought stress for 60 days. Data are mean ± SE (n =3). * significant
differences among different drought stress treatments (p ＜0.05).
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1242
Figure 3. The changes of stomatal conductance (Gs) in leaves of Platycladus orientalis and Amorpha fruticosa seedlings
subjected to different drought stress levels and recovery period. Legend: (a), (e) and (i) represent the drought stress for 15 days; (b), (f) and (j) represent the drought stress for 30 days; (c), (g) and (k)
represent the drought stress for 45 days; (d), (h) and (l) represent the drought stress for 60 days. Data are mean ± SE (n =3). * significant
differences among different drought stress treatments (P＜0.05).
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1243
After rewatering, the response strategies of Gs between P.
orientalis and A. fruticose seedlings were different (Figure
3). As for P. orientalis, the Gs in IGS, increased firstly, then
decreased and finally increased slightly, the Gs in FGS first
decreased and then increased, and lastly remained relatively
stable. The Gs in LGS, first increased and then decreased,
and lastly remained relatively stable. After 48h of
rewatering, the Gs recovered to the control level in all the
three different growth stages. After 72h of rewatering, the
Gs of P. orientalis had no significant difference between
stressed plants and the control plants. Among the drought
stress treatments, the Gs with 87.84 % SWC treatment at
30d in IGS, with 52.16 % SWC treatment at 15d in FGS and
with 52.16 % SWC treatment at 45d in LGS increased by
8.23 %, 11.83 % and 29.96 % compared to the control
group, respectively. In case of A. fruticosa, there were
significant differences in Gs among the three growth stages
after rewatering. The Gs of IGS increased at first, then
decreased and lastly increased slightly under rewatering
conditions. In FGS, Gs decreased first and then increased. In
general, the change of Gs in LGS was opposite to that in
FGS, showing enhancement at first and then declination
(except for 87.84 % SWC at 45d). 72h after rewatering, the
Gs with 87.84 % SWC treatment at 60d in IGS, with 70 %
SWC treatment at 30d in FGS and with 52.16 % SWC
treatment at 15d in LGS increased by 2.7, 1.66 and 1.55
times higher than those of the control group, respectively.
Effect of drought and rewatering on the Ci of two tree
species: The changes of intercellular CO2 concentration (Ci)
in all the three growth stages of P. orientalis and A. fruticosa
seedlings were shown in Figure 4. In general, with the
aggravation of soil drought, the change strategy of drought
stress in all the three different growth stages in two typical
afforestation tree species were different. Among the drought
stress treatments, the Ci of P. orientalis in LGS under four
drought durations (15, 30, 45 and 60d), and the Ci of A.
fruticose in IGS under two drought durations (15 and 30d)
increased at first and then decreased. The Ci of two
afforestation trees under other drought treatments decreased
gradually with the aggravation of soil drought. Among the
drought stress treatments, in IGS of P. orientalis treated with
40 % SWC at 15d, in FGS of P. orientalis treated with 40 %
SWC at 15, 30 and 60d, and in LGS of P. orientalis treated
with 40 % SWC at 45d, the Ci was higher than that of 52.16
% SWC treatment. In the two treatments with low soil
moisture (52.16 % SWC and 40 % SWC), the Ci of the
seedlings of the two species decreased to a lower level. The
Ci of the two tree species decreased significantly at different
growth stages (except for LGS of P. orientalis treated with
52.16 % SWC at 15 and 30d, and with 52.16 % SWC and 40
% SWC at 45 and 60d). The Ci of P. orientalis with 40 %
SWC at 30d in IGS, with 52.16 % SWC at 15d in FGS, and
with 40 % SWC at 30d in LGS under four drought duration
treatments decreased by 32.52 %, 12.34 % and 15.52 %,
respectively compared with the control. For A. fruticose, the
Ci with 40 % SWC treatment at 60d in IGS, with 40 % SWC
treatment at 45d in FGS, and with 52.16 % SWC treatment
at 30d in LGS decreased by 42.38 %, 14.94 % and 24.70 %
respectively, compared with the control group. After
rewatering, the response of Ci in different growth stages of
P. orientalis seedlings was almost the same, the changes of
Ci in all the three different growth stages of A. fruticosa
seedlings were slightly different (Figure 4). In case of P.
orientalis seedlings, the Ci in all the three different growth
stages increased at first, then decreased and lastly increased
after rewatering. In general, the Ci of the seedlings returned
close to the normal level after 2h of rewatering in IGS，FGS and LGS. After 72h of rewatering, the Ci of P.
orientalis at different growth stages was not significantly
different from that of the control group. The Ci with 70 %
treatment at 30d in IGS, with 87.84 % treatment at 60d in
FGS, and with 87.84 % SWC treatment at 45d in LGS
increased to the maximum value among the drought stress
treatments, which was 1.06 times of the control in all the
three different growth stages. For A. fruticosa, there were
differences in the changes of Ci in different growth stages
after rewatering. In IGS, the Ci increased first, then
decreased and lastly increased. In FGS, the Ci increased first
and then decreased, and lastly increased slightly. However,
the Ci showed an upward trend in LGS. After 78h of
rewatering, the Ci of all the three different growth stages
returned close to the normal level. The Ci with 40 % SWC
treatment at 60d in IGS, with 52.16 % SWC treatment at 60d
in FGS, and with 70 % SWC treatment at 30d in LGS
increased by 1.26, 1.16 and 1.21 times of the control,
respectively. Effect of drought and rewatering on WUE of two trees: Under the condition of soil water deficit, the response of water use efficiency (WUE) was different in different growth stages of seedlings of the two tree species (Figure 5). In IGS, with the decrease of soil moisture content, the WUE of the two tree species was affected in varying degrees. Among the drought stress treatments, the WUE of P. orientalis treated with 40 % SWC at 45d was the highest, which was 2.66 times of the control. The WUE of A. fruticosa treated with 40 % SWC at 60d reached the maximum value, which was 2.50 times of the control. In FGS, with the increase of soil drought degree, the WUE in P. orientalis treated with 40 % SWC at 15 and 30d, and in A. fruticosa treated with 87.84 % SWC at 15d as well as with 70 % SWC at 45d was lower than that of the control respectively. The WUE of the two species trees under other drought treatments was higher than that of control. Among the drought stress treatments, the WUE in P. orientalis with 52.16 % SWC at 45d, in A. fruticosa with 40 % SWC treatment at 60d reached the maximum value, which was 1.27 and 1.51 times of the control, respectively. In LGS, the WUE in P. orientalis with 87.84 % SWC and 52.16 % SWC treatments at 30d, and with 87.84 % SWC treatment at 45d
Feng, Sikdar, Wang, Li, Lv & Ma
1244
were slightly lower than that of the control. The rest of the treatments improved the WUE of P. orientalis. Among the drought stress treatments, the WUE in 52.16 % SWC
treatment at 45d reached the maximum value, which was 1.74 times of the control. However, the WUE of A. fruticosa seedlings at different water-deficit treatments was lower than
Figure 4. The changes of intercellular CO2 concentration (Ci) in leaves of Platycladus orientalis and Amorpha
fruticosa seedlings subjected to different drought stress levels and recovery period. Legend: (a), (e) and (i) represent the drought stress for 15 days; (b), (f) and (j) represent the drought stress for 30 days; (c), (g) and (k)
represent the drought stress for 45 days; (d), (h) and (l) represent the drought stress for 60 days. Data are mean ± SE (n =3). *
significant differences among different drought stress treatments (P＜0.05).
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1245
that of the control.
After rewatering, there were differences in the dynamic
response characteristic of WUE between two species of
seedlings. Meanwhile, the response of WUE of P. orientalis
seedlings in all the three different growth stages was
different. In IGS, the WUE increased after 2h of rewatering
Figure 5. The changes in water use efficiency (WUE) in leaves of Platycladus orientalis and Amorpha fruticosa
seedlings subjected to different drought stress levels and recovery period. Legend: (a), (e) and (i) represent the drought stress for 15 days; (b), (f) and (j) represent the drought stress for 30 days; (c), (g) and (k)
represent the drought stress for 45 days; (d), (h) and (l) represent the drought stress for 60 days. Data are mean ± SE (n =3). *
significant differences among different drought stress treatments (P＜0.05).
Feng, Sikdar, Wang, Li, Lv & Ma
1246
(except for 40 % SWC treatment). After 24 and 48 h of
rewatering, the WUE took on ascend trend (except for 87.84
% SWC at 60d and 40 % SWC at 45d). The WUE 72h after
rewatering was lower than that of 48 h after rewatering. In
FGS, the WUE of different drought stress treatments
increased at first, then decreased and lastly maintained
relatively stable. 72h after rewatering, it returned close to the
normal level. In LGS, the WUE decreased firstly, increased
then, and lastly decreased slightly after rewatering. After
72h of rewatering, the WUE recovered almost to the control
level and then remained relatively stable. As for A. fruticosa,
the alteration in WUE at all the three different growth stages
was also different after rehydration. In IGS, with the
increase in time of rewatering, the overall WUE under 15
days drought stress decreased at first, then increased and
lfinally remained relatively stable. The WUE under 30, 45
and 60 days of drought stress treatments increased first and
then decreased. After 72h of rewatering, compared with 48h
of rewatering, the change in WUE under 15 and 30days of
drought stress attained constants thereafter. The WUE under
45 and 60 days of drought treatment continued the
downward tendency in general. Among the drought stresses,
the WUE under 70 % SWC treatment at 30 d reached the
maximum, which was 1.30 times higher than that of the
control. In FGS, 2～24h of rewatering, the WUE of 87.84 %
treatment increased under 15 and 30 days of drought stress,
and the change in WUE under other treatments was
relatively flat. 24～48h of rewatering, except the WUE at 70
% SWC treatment increased after 15 and 30 days of drought.
The WUE under other treatments decreased. After 72h of
rewatering, the WUE at each water stress treatment was
close to the normal level. The WUE under 70 % SWC
treatment at 30d reached the maximum value, which was
1.36 times of the control. In LGS, after 2 ～ 24h of
rewatering, the WUE of each water stress treatment tended
to be flat and the range of variation was small. After 24h of
rewatering, under four drought durations, each treatment
showed a rising trend in WUE. After 48h of rewatering, the
WUE in each treatment exhibited a increasing trend under
four drought durations (except 87.84 % SWC and 70 %
SWC treatments at 30d). After 72h of rewatering, the WUE
under 87.84 % SWC and 52.16 % SWC at 15d decreased,
and the WUE under the other treatments remained stable,
and the WUE under 87.84 % SWC treatment at 60d reached
the maximum value among the drought stresses, which was
1.10 times higher than that of the control.
DISCUSSION
Under drought stress and rewatering conditions, plants
usually adapt to changes in growth environment by adjusting
metabolic processes, for example, photosynthetic
metabolism (Santesteban et al., 2009; Alfonso et al., 2014),
antioxidative Metabolism (Marcinska et al., 2013; Khoyerdi
et al., 2016) and growth metabolism (Bagheri et al. 2011;
Zheng et al., 2017). In general, photosynthesis is the basis of
plant growth and development, and soil moisture fluctuation
directly affects the structure of photosynthetic apparatus and
photosynthetic enzyme activity of the plant. Study on the
response of plant photosynthetic physiological
characteristics is an effective way to reveal the adaptation
mechanism of plants to different drought environment. The
effect of soil water deficit on photosynthetic properties is
more prominent, and the decline in photosynthesis may be
caused by stomatal restriction or non-stomatal restriction
(Cátia et al., 2018). Previous reports have shown that
photosynthetic parameters of plants decreased with the
increase of drought stress (Lawlor et al. 2002; Naidoo et al.
2018). In the current research, soil water deficit reduced the
Pn, Tr, Gs and Ci. At 87.84 % SWC, 70 % SWC and 52.16
% SWC treatments, the Pn decreased with the decrease of
Gs and Ci, indicating the declined photosynthesis as a result
of stomatal restriction. This is similar to the research results
of Pompelli et al. (2010) and Arbona et al. (2005). However,
at 40 % SWC treatment, the Pn and GS decreased
significantly, but Ci increased to some extent under some
drought duration conditions. The results suggested that the
declination in photosynthetic capacity of P. orientalis and A.
fruticosa seedlings had a variational process from stomatal
limitation to the non-stomatal limitation, and non-stomatal
limitations on photosynthesis of two tree species were
observed, indicating that nonstomatal factors were involved
in Pn limitation. These findings are in line with the results of
Bacelar et al. (2009), which suggested that stomatal and
non-stomatal factors in different plant species existed
simultaneously under drought environment conditions (Cátia
et al., 2018). Due to the decrease in Gs, the Tr of P.
orientalis and A. fruticosa seedlings also decreased, which
was consistent with the results of Lefi et al. (2004).
The WUE is the amount of assimilation produced by plants
per unit of water consumption, which is an important
physiological index to evaluate the adaptation of plants to
the drought environment. Meanwhile, it is also one of the
important parameters to determine the water supply for plant
growth and development (Sun et al., 2002; Zhou et al.,
2015). The WUE is an important indicator of plant drought
resistance. Seedlings can adjust the water use efficiency and
maintain the balance of plant growth and water consumption
by coordinating the relationship between Pn and Tr. In the
current experiment, the results showed that the WUE of P.
orientalis and A. fruticosa seedlings improved under drought
stress conditions, this is because of the fact that soil water
deficit regulated the stomatal opening of the plant, improved
WUE to secure normal growth and development of plants
grown in drought environment.
In general, drought-resistant plants can recover quickly after
resuming normal water supply. Schimpl et al. (2019)
showed that water stress significantly reduced the Pn of
Exchange and water-use efficiency
1247
Brazil nuts (Bertholletia exelsa), but the Pn could be
returned to the control level after rewatering. Hamed et al.
(2016) showed that the photosynthetic parameters of
Pistacia atlantis and Pistacia vera were lower than the
control under water stress, and recovery ability of P. atlantis
regarding the photosynthetic parameters was higher than P.
vera after rewatering. However, the recovery of
physiological indexes indicated a potential recovery ability
of different plant species after rewatering. Post-drought
rewatering triggered photosynthetic compensation and
growth compensation behavior, playing an important role
during restoration after drought stress (Hao et al., 2010).
Previous researches showed the compensation effects are
different among various types of plant species under
rewatering after drought stress (Hamerlynck et al., 2016;
Wang et al., 2016). These results showed that rewatering
had different compensation effects on plant growth and
development, and compensatory growth from rewatering
seemed to be residual and delayed, since growth
compensation was usually observed during the later period
after water stress (Fang et al., 2006; Hao et al., 2010; Brossa
et al., 2015). At present study, the photosynthetic
characteristics of P. orientalis and A. fruticosa seedlings also
showed photosynthetic compensation or over-compensation
effects under rewatering conditions after drought stress, but
the compensation effect of rewatering after the drought was
slightly different between P. orientalis and A. fruticosa. This
may be due to the different degree of drought stress, drought
duration, plant species and growth stage (Ennahli & Earl,
2005; Galmés et al., 2007; Flexas et al., 2009).
Rewatering triggered different compensation effects to
compensate for the loss caused by drought stress on the
photosynthetic physiological process of P. orientalis and A.
fruticosa seedlings in different growth stages. Similar
compensation effects on photosynthetic physiology
characteristics have also been observed under drying and
wetting conditions (Dodd et al., 2015; Zhang et al., 2009).
Previous reports indicated that plants have some differences
in the recovery process under rehydration after drought
stress (Chen et al., 2010; Xu et al., 2010; Alfonso et al.,
2014). Based on the test results, photosynthetic parameters
of two tree species could recover rapidly after rewatering,
and the photosynthetic parameters of the seedlings of the
two species recovered close to normal level 72h after
rewatering. The compensation effects of photosynthesis of
the seedlings of the two species were stimulated by
rewatering after drought stress, which is similar to the results
of Schilmpl et al. (2019) and Hamed et al. (2016). Besides,
Urli et al. (2013) also indicated that the net photosynthetic
rate of Angiosperms could recover rapidly after rewatering.
Conclusions: The photosynthetic physiological metabolism
of P. orientalis and A. fruticosa seedlings showed sensitivity
to soil water deficit. Under drought stress conditions, the two
species adapted to and resist drought stress by reducing
photosynthetic parameters and improving WUE. In
particular, 40 % SWC treatment significantly inhibited the
photosynthetic ability of the two species, the decline of
photosynthetic capacity of two tree species has a variational
process of from stomatal limitation to the non-stomatal
limitation, and nonstomatal factors were involved in Pn
limitation, indicating that stomatal and non-stomatal factors
existed simultaneously in two tree species under drought
stress conditions. After rewatering, with the improvement of
soil moisture conditions, the photosynthetic parameters of
the two species recovered to different degrees. However, the
gas exchange parameters could recover close to the normal
level after 72h of rewatering, presenting physiological
compensation effects after rewatering. The results suggested
that the recovery capacity of the two species was slightly
different, but the recovery processes of two species were
highly effective and had better properties of potential
recovery, and the seedlings of P. orientalis and A. fruticosa
had strong adaptability to drought stress and rewatering.
Acknowledgments: This research was supported by the
National Natural Science Foundation of China [Grant
number 31670713], the research grants from the Nation Key
R&D Program of China [Grant number 2017YFC0504402]
and Shaanxi Province Science and Technology Co-
ordination Innovation Project [Grant number 2016KTCL03-
18]. We show our gratitude to Xiaoyang Liu, Lixia Yao and
Qiannan Dang for their assistance while experimenting.
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[Received 4 April 2020; Accepted 23 May; Published
