Introduction
The 5th
Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) (AR5)
determined that global warming in the past 100 years is irrefutable (IPCC
2013). The average surface temperature of the earth increased by 0.85°C on
average during the 130 years from 1880 to 2012, and the rate of temperature
increase was 0.12°C·10 a-1 during 1951~2012, which is nearly twice
as fast as that since 1880. The earth has experienced three warmest 10 years
since 1983~2012, and the trends and characteristics of climate warming can be
observed at nearly all places worldwide. It is estimated that the average
surface temperature of the earth will continue to rise by 0.3~0.7°C during
2016~2035. Because of human activities, the concentration of CO2 in
the air has continually increased since 1750 and reached 391
μmol∙mol-1 in 2011. According to the low-emissions
scenario in the Representative Concentration Pathways 4.5 (RCP4.5) when the
radiation intensity is stabilized at 4.5 W∙m-2, the equivalent
concentration of CO2 will be stabilized at
approximately 650 μmol∙mol-1 after the year 2100
(IPCC 2013).
Stomatal
conductance, transpiration rates and soil evaporation rates are affected by
temperature changes; thus, crop water content circulation and
evapotranspiration are influenced (Rawson 1988; Zhou et al. 2011). A
warming experiment on potato that involved cultivation with mulching film to
influence the soil and plant ecological growing microenvironment directly by
improving the soil temperature and reducing moisture evaporation (Kar 2003;
Wang et al. 2005a) revealed improved crop yield and quality (Jenkins and
Gillison 1995; Lamont 2005; Luis et al. 2011;
Wang et al. 2011), and the water use efficiency (WUE) increased (Wang et
al. 2005a, b; Zhao et al. 2012). Similarly, an infrared radiator
farm warming experiment that simulated atmospheric warming showed that potato
physiology and ecology and the yield-formation processes were significantly
altered (Xiao et al. 2013 a, b). Under a scenario of future climatic
changes, warming during the tuber expansion stage will not adversely influence tuber
yields under irrigation (Carolina et al. 2017). Because of climate
warming, the potato inflorescence-forming stage in semi-arid regions has
advanced by 8~9 days, the blooming stage has advanced by 4~5 days, and the
potato growth period has increased in duration (Yao et al. 2010).
Warming in the spring and autumn is good for potato growth and yields, but
warming in the summer will aggravate the vulnerability of potato growth (Yao et
al. 2013; Zhao et al. 2015).
Many
studies have shown that elevated CO2 concentrations can promote the
total biomass and yield of potato (Sicher and Bunce
1999; Wheeler et al. 1999; Schapendonk et
al. 2000). For example, when CO2 concentrations were 370~740
μmol∙mol-1 higher than ambient concentrations, the potato
tuber yield increased by 27~49% (Wheeler et al. 1991). However, slightly
adverse effects have been observed during experiments (Finnan et al.
2005): increased CO2 supplies can accelerate leaf ageing and shorten
the blooming stage (Miglietta et al. 1998;
Lawson et al. 2001). Elevated CO2 concentrations can lower
potato leaf transpiration rates and increase photosynthesis and WUE (Ku et
al. 1977). When CO2 concentrations in an open-top chamber (OTC)
were elevated by 350~700 μmol∙mol-1, the canopy
photosynthesis improved by 80%, but the result varied with growth stages (Schapendonk et al. 2000). Sicher
and Bunce (1999) reported that the net photosynthesis rate in potato leaves
during the whole growth period increased and was higher than that in the
control leaves when the CO2 concentration was elevated. The results
of a controlled CO2 concentration experiment showed that the total
biomass, yield and WUE of potato improved when the CO2
concentrations were elevated (Fleisher et al. 2008).
Potato
(Solanum tuberosum L.) is planted in
157 countries worldwide, and the total yield reached 324 million tons in 2010.
In China, 5.33 million ha of potatoes were planted, and the annual yield led
the world by 80 million tons (Zhang et al. 2012). Potato is the fourth
major food crop after paddy rice, wheat and maize and is one of the most
promising high-yielding crop species. Potato is tolerant to drought and arid
conditions and is a particularly high-yielding crop species suitable in
semi-arid regions at mid-latitudes (Zhao et al. 2013). Potato growth and
yield formation are strongly influenced by climate warming, but studies
investigating the influence of atmospheric warming combined with CO2
concentrations on potato physiology, ecology and yield
formation in semi-arid regions at mid-latitudes are lacking. Therefore, it is
necessary to study the combined influence of elevated CO2
concentrations and atmospheric warming on potato physiology and ecology, build
an experimental base for potato physiology and ecology simulations, and provide
a scientific reference for industrial potato development in the background of
climatic change.
Materials and Methods
Climate and potato growth
outline in the study region
The study area belongs to the semi-arid
region of the Loess Plateau. The annual mean air temperature was 7.2°C, the mean
air temperature in July (the hottest month) was 19.2°C, and the mean air
temperature in January (the coldest month) was
-7.2°C. The annual mean precipitation was
377.1 mm. The precipitation from May to
October was 328.5 mm, which was 87.0% of that for
the whole year. The annual mean sunshine
duration was 243.7 h, and the annual mean
continuous frost-free duration was 145
days.
In the study region, potato was
usually planted during the first and middle ten
days of May. Furthermore, the seedling stage
occurred during the first and middle ten
days of June, the ramifying stage occurred
during the last ten days of June and during the
first ten days of July, the inflorescence-forming
stage occurred during the first and middle ten
days of July, the blooming stage occurred during
the middle and last ten days of July, and
harvest occurred during the first and middle ten days of October. The
whole growth period from sowing to harvest was 135~165 days. During
the whole growth period, the accumulated
temperature ≥ 0°C was approximately
2,729.7°C, the precipitation was approximately
346.2 mm, and the sunshine duration was
approximately 1,262.1 h.
The experiment was
performed at the Dingxi Drought and Ecological
Environment Experiment Station of the Lanzhou Institute of Arid Meteorology,
China Meteorological Administration from April to October 2016. The experiment
was performed in a new OTC to study the combined influence of atmospheric
warming and elevated CO2 concentrations on potato physiology and
ecology. The chamber was 18 m2 wide and 3 m high, and the top was
open.
RCP4.5
supposed that humans tried to reduce greenhouse gas emissions, the radiation
intensity stabilized at 4.5 W∙m-2, the equivalent CO2
concentration stabilized at 650 μmol∙mol-1
by the end of the 21st century, and the temperature increase at the
surface of the earth was within 2.0°C (IPCC, 2013). Therefore, two treatments
and one control were included in the experiment. In one treatment that included
warming (IT), a temperature monitor controller was used, and the air
temperature increase was controlled at 2.0±0.5°C (1.5 to 2.5°C). The other
treatment included both warming and an elevated CO2 concentration
(IT+IC). For this treatment, a CO2 concentration monitor controller
was used, and the CO2 concentration was controlled at 650±20 μmol∙mol-1,
CO2 exposure was carried out daily during daylight. For the contrast
check (CK), the ambient concentration of CO2 was approximately 370
μmol·mol-1. Each treatment was repeated three times.
A local potato cultivar “New Daping” served as experimental material, and all potato
seedlings were fertilized via free-air CO2
enrichment (FACE). This FACE fertilization period occurred from 07:00~18:00,
and the CO2 concentration in the chamber was kept stable during the
whole experimental period. The water content and fertilizer during the
experimental period were constant, and the soil relative humidity ranged from
60 to 65% of field capacity during the whole experimental period, there were no restricting
factors, such as plant diseases, insect pests or weeds.
Measured parameters and methods
The
leaf net photosynthesis rate (Pn), transpiration rate
(Tr), stomatal conductance (gs)
and intercellular CO2 (Ci) parameters were measured from 10:00~11:30
on sunny days via a Li-6400 photosynthesis measurement system developed by
LI-COR (US). The standard Li-6400 leaf chamber was used. Five representative
plants with healthy and identical growth were selected in each plot. The fourth
fully unfolded leaf was selected to measure the photosynthetic parameters. The
light intensity (PAR) was 1500 μmol·m-2·s-1, and the
air intake rate was 500 μmol·mol-1.
Chlorophyll
was measured by a SPAD-502 metre (Japan).
The SPAD values were measured at the widest part of the fourth fully
expanded leaf from top to bottom. Five representative plants were selected in
each plot, and measurements were repeated five times for each leaf. The average
value was used to represent the relative chlorophyll content of the leaf. Potato height and yield
components were measured in accordance with the agrometeorological observation
criterion of the China Meteorological Administration (1993).
Statistical analyses
The climatic element rate
of change tendency was calculated as follows: Xi=a+bti (i=1, 2, n) (Wei 2007). In the formula, Xi is the climatic element
variable, ti
is the time corresponding to Xi, a is the regression constant, b is the regression coefficient, and n is the number of samples. 10b is the climatic element tendency rate. Statistical analyses were
realized by analysis of variance, correlation analysis, linear regression
analysis and nonlinear regression analysis, which calculated the related
linearized coefficients and fitted the linearity and nonlinearity (Wei 2007).
Results
Characteristics of climate
change in the experimental region
Precipitation: The changes in precipitation in
the experimental region during 1958~2016 is shown in Fig. 1a. In the past 59
years, the annual precipitation decreased, and the decreasing precipitation
tendency was -12.171 mm (r=0.25, n=59, P=0.05) every 10 years. The annual
precipitation was 377.1 mm on average in the experimental region and fluctuated
from 245.7~720.1 mm; the precipitation ranged from -34.8~91.0% of the
percentage difference. The precipitation was high in the 1960s; the annual mean
precipitation was 447.6 mm. However, the precipitation was lowest in the 1990s;
the annual mean precipitation was 365.7 mm. The precipitation during the first
10 a of
the 21st century was also low; the annual mean precipitation was
375.6 mm.
Winter (December ~ February of the last year)
precipitation during previous years gradually increased by 0.420 mm (P>0.10)
every 10 years, but was not significant; the climatic tendencies of
precipitation in the spring (March ~ May) and summer (June ~ August) were -1.59
(P>0.10) and -5.431 mm (P>0.10) every 10 years, respectively; these
represented decreasing trends but were not significant. The climatic tendency
rate of precipitation decreased by -5.5967 mm
(P<0.10)
in autumn (September
~ November) and by -0.117 mm (P<0.10) every 10 years during the
potato growing stage (May ~ October), the latter of which was the longest
period of decreasing precipitation.
With
respect to the stability of changes in precipitation, the winter precipitation
difference among years was the largest and the most dynamic with coefficient of
variation was 54.4%. The coefficients of variation in other seasons among years
ranged from 25.9~44.8%. The annual difference in precipitation during the
potato growth period was the smallest; the coefficient of variation was 20.7%.
The stable period of annual changes in precipitation essentially matched the
potato growth period and was the period during which the precipitation rate
needed for planting potato was high.
Table 1: Changes in plant height at
different growth stages of potatoes treated with different factors
Treatment |
Ramifying |
Inflorescence-forming
forming |
Blooming |
Harvestable |
IT |
14.25±1.39aA |
18.83±1.21aA |
26.87±4.38aA |
38.30±3.73aA |
IT+IC |
12.55±0.90bB |
18.92±1.38aA |
25.46±2.49aA |
35.15±3.43bA |
CK |
11.65±1.49bB |
15.40±1.71bB |
17.26±13.0bB |
30.00±2.78cB |
Note: a, b and c mean α=0.05; A, B and C mean
α=0.01; the difference in letters between two rows means a significant
difference
Fig.
1: Changes in annual climatic
factors in the experimental region (1958~2016)
Fig. 2:
Changes in plant height (a) of and chlorophyll content (b) in potato in
response to different treatments
Air temperature: As shown in Fig. 1b, the air temperature significantly
increased during the past 59 years (during 1958~2016), and the linear
regression-based climatic tendency rate of the air temperature
was 0.417°C (r=0.808, n=59, P<0.001) every 10 years. There was an inverse
relation between the air temperature and the mean value during the 1960s~1980s;
the values were -0.5°C in the
1960s, -0.4°C in the 1970s,
-0.2°C in the 1980s, -0.5°C in the 1990s, 1.3°C in the first 10 years of the 21st
century, and 1.7°C during 2011~2016. The winter warming rate was rapid in the
past 59 years; the climatic tendency rate was 0.502°C (P<0.001) every 10
years. The climatic tendency rate of autumn warming was 0.423°C (P<0.001),
which was second only to the rate during winter. The climatic tendency rate of
summer warming was 0.373°C (P<0.001), and the spring warming rate was the
lowest at 0.354°C (P<0.001) every 10 years. The climatic tendency rate of
warming during the potato growth period was 0.373°C (P<0.001) every 10
years.
Influence of atmospheric
warming and elevated CO2 concentration on potato
Plant height: Fig. 2a and Table 1 show that the height of potato
plants treated with warming (IT treatment) were significantly higher than that
of plants treated with warming and elevated CO2 concentrations
(IT+IC treatment), and that of the CK
(P<0.01) at the ramifying stage. In addition, the height of potato
plants in the IT and IT+IC treatments was significantly higher than that of the
plants in the CK treatment, which implied that the fertilization effect of CO2
became increasingly distinct as the growth progressed. During the harvest
stage, the potato height followed the order of IT>IT+IC>CK and was
significant at 0.05 (Table 1). The difference among the three treatments was
significant, which proves that increased temperature can significantly increase
plant height.
Chlorophyll content: The chlorophyll content gradually increased as the
growth progression increased. This increase followed the order of
CK>IT+IC>IT during the early growth stage; it was the lowest in IT+IC on
July 1 but was not significant (P>0.05). Afterward, the chlorophyll content
gradually increased and peaked on July 31; the content within the different
treatments followed the order of IT+IC>CK>IT. The chlorophyll content
then started to decrease with as the potato leaves aged. From August 21 to the
end of harvest, the content decreased quickly as the leaves rapidly aged in the
CK treatment, but the content decreased gradually as the leaves slowly aged in
the IT treatment; the chlorophyll content accordingly followed the order of
IT>IT+IC>CK. These results prove that the chlorophyll content was higher
in the IT treatment than in the other treatments during later growth stages
(Fig. 2b).
Table
2: Results of the analysis of
variance of the net photosynthesis in different potato treatments during the
vegetative growth stage (blooming stage)
Variance source |
Degrees of freedom |
Sum of squares |
Mean square |
F value |
P value |
Between treatments |
2 |
448.827 |
224.414 |
20.931 |
0.0001 |
Within treatment |
20 |
214.430 |
10.722 |
|
|
Total variance |
22 |
663.257 |
|
|
|
Table 3: Results
of the analysis of variance of net photosynthesis in different potato
treatments during the reproductive growth stage (stem expansion stage)
Variance source |
Degrees of freedom |
Sum of squares |
Mean square |
F value |
P value |
Between treatments |
2 |
180.922 |
90.461 |
29.718 |
0.0001 |
Within treatment |
23 |
70.011 |
3.044 |
|
|
Total variance |
25 |
250.933 |
|
|
|
Fig. 3: Changes
in the net photosynthesis rate (a) and stomatal
conductance (b) of potato in response to different treatments
Influence of atmospheric warming
and elevated CO2 concentrations on leaf gas exchange
Net photosynthesis rate (Pn): The Pn in potato leaves during the vegetative growth stage and
the reproductive growth stage (the blooming stage and tuber expansion stage, respectively) followed the order: IT+IC>IT>CK.
The rate of IT+IC treatment was 18.6~23.8 μmol×m-2×s-1,
the rate in the IT treatment was 9.1~16.0 μmol×m-2×s-1,
and the rate in the CK treatment was 3.7~13.4 μmol×m-2×s-1
(Fig. 3a). The rate in the IT+IC treatment improved by 1~5 times and was, on
average, 2.1 times higher than that in the CK treatment. The rate in the IT
treatment improved by approximately 22 to 140% times and was, on average, 85% higher
than that in the CK treatment; both rates were significant at P<0.01 level
(Table 2). These results showed that, during the early growth stage of potato,
the atmospheric temperature was low and did not meet the most suitable
temperature for potato growth. Warming compensated for the low leaf
photosynthesis rate caused by low temperatures, and the rate improved in
response to warming. Furthermore, because CO2 concentrations
increased as the temperatures increased, the raw material required for leaf
photosynthesis increased under the IT+IC treatment, and the Pn
further improved. Therefore, the Pn in leaves treated
with warming and elevated CO2 concentrations was much higher than
that in the leaves of the other treatments.
Because
of warming and elevated CO2 concentrations during the early stage,
the potato plants grew quickly, but during the late growth stage, the plant
leaves started to age quickly; the change in net photosynthesis followed the
order of CK>IT>IT+IC, and the differences were significant or extremely
significant. The rate in the IT+IC treatment decreased by 210%~260% and was, on
average, 70% lower than that in the CK treatment, and the decreasing rate in
the IT treatment decreased by 110%~113% and was, on average, 55% lower than
that in the CK treatment. These results proved that the Pn
in potato leaves in the IT+IC treatment was higher than that in the other two
treatments during the vegetative growth stage. During the late growth stage,
the potato leaves started to age quickly because of warming and elevated CO2
concentrations during the early stage, and compared with that in the potato
leaves in the other treatments, the net photosynthesis in the potato leaves in
the IT+IC treatment decreased more quickly, and the differences were extremely
significant during the tuber expansion stage (Table 3).
Table 4: Results of the
analysis of variance of the potato leaf transpiration rate in different
treatments at the blooming stage
Variance source |
Degrees of freedom |
Sum of squares |
Mean square |
F value |
P value |
Between treatments |
2 |
1.874 |
0.937 |
0.644 |
0.536 |
Within treatment |
20 |
29.088 |
1.454 |
|
|
Total variance |
22 |
30.961 |
|
|
|
Table 5: Results of the
analysis of variance of the potato leaf transpiration rate in different
treatments at the tuber expansion stage
Variance source |
Degrees of freedom |
Sum of squares |
Mean square |
F value |
P value |
Between treatments |
2 |
31.210 |
15.605 |
4.315 |
0.025 |
Within treatment |
25 |
90.411 |
3.616 |
|
|
Total variance |
27 |
121.622 |
|
|
|
Fig. 4: Changes in the transpiration rate (a) and WUE (b) of
potato in response to different treatments
Stomatal conductance (gs): Potato
leaf gs increased but then decreased during the whole
growth period (Fig. 3b). Analysis of the gs of leaves revealed that, compared with that in the
CK treatment, the potato leaf gs under the IT+IC treatment decreased by 5~80% (44%
on average). The gs decreased slowly in the early growth stage and
quickly in the late growth stage. Compared with that in the CK treatment, the gs in
the IT+IC treatment increased by 17~46% in the early growth stage but then
decreased by 9~63% (32% on average) as the duration of warming increased.
Additional analyses revealed that the gs in the different treatments followed the order of
IT>CK>IT+IC in the early growth stage and CK>IT>IT+IC in the late
growth stage, and there was a significant or extremely significant difference
among the CK, IT and IT+IC treatments. These results prove that the gs of
leaves in the IT treatment is higher than that in the CK treatment during the
early growth stage and lower during the late stage. The gs
gradually decreased in response to elevated ambient CO2
concentrations; thus, the gs of leaves in the IT+IC treatment was lower than
that in the CK treatment.
Transpiration rate (Tr): The transpiration rate increased as warming
increased. The Tr in the IT treatment was highest,
followed by that in the CK treatment and that in the IT+IC treatment. Compared
with that in the CK treatment, the rate in the IT and IT+IC treatments
increased by 1~12% in the early growth stage, but it decreased by 4~27% in the
late growth stage (Fig. 4a). The difference in the Tr between different treatments
was not significant at the blooming stage (p>0.05; Table 4). With the
advancement of the fertilization process, the differences in transpiration
rates under the different treatments increased gradually. The transpiration
rate at the tuber expansion stage between the different treatments passed the
significance threshold of 0.05 (Tables 5–6).
Water use efficiency
(WUE): The
leaf WUE is the amount of CO2 assimilated per unit weight of water
content lost to transpiration and is normally represented by the ratio of net
photosynthesis to Tr, i.e., the potato leaf WUE can be calculated as follows:
WUE = Pn/Tr. Under high CO2
concentrations, the WUE of potato leaves was promoted by increasing the Pn and decreasing the transpiration rate caused by
decreasing the gs. During the blooming stage and tuber expansion stage, the potato leaf WUE
increased as the CO2 concentrations increased. The WUE among
treatments followed the order of IT+IC>CK>IT (Fig. 4b), compared with
that in the CK treatment, the average rate of WUE in the IT+IC treatment
increased by 31% at the blooming stage and by 1.4 times at the tuber expansion stage. There were differences in
the increase in ranges in different periods. Furthermore, compared with that in
the CK treatment, the WUE in the IT treatment decreased by 15~18%. During the
middle and late growth stages, the WUE followed the order of IT+IC>IT>CK.
At final harvest, the WUE followed the order of CK>IT>IT+IC.
Intercellular CO2
concentration (Ci): The
experimental results showed that the leaf Ci in the IT+IC treatment was
significantly (P<0.01) higher than that in the IT and CK treatments.
Compared with that of IT treatment, the Ci in the IT+IC treatment was 31~79%
higher and, on average, 54% higher, and the value was 40~83% higher and, on average,
61% higher than that in the CK treatment. Furthermore, compared with that in
the CK treatment, the Ci in the IT treatment was slightly higher (Fig. 5).
Influence of elevated CO2 concentration on potato
yield
Table 6: Multiple comparisons
(SSR method) of potato leaf transpiration rate in different treatments at the
tuber expansion stage
Treatment |
Mean |
a=0.05 |
a=0.01 |
IT |
6.4579 |
a |
A |
CK |
5.8000 |
bc |
A |
IT+IC |
4.0311 |
c |
A |
Table 7: Potato yield
structure change in response to different treatments
Treatment |
Potato weight per plant (g·plant-1) |
Scrap potato rate (%) |
Fresh stem weight (g·m-2) |
Ratio of potato to stem |
Theoretical yield (g·m-2) |
Actual yield (kg·hm-2) |
IT |
81.7 |
1 |
786.89 |
0.84 |
653.69 |
5529.07 |
IT+IC |
207.2 |
7 |
926.09 |
1.16 |
1168.03 |
8830.21 |
CK |
133.8 |
1 |
658.51 |
2.32 |
1780.96 |
7820.08 |
Fig. 5: Changes in intercellular CO2
concentrations in potato in response to different treatments
Analysis of the potato
yield in response to different treatments revealed that, compared with that in
the CK treatment, the tuber weight per plant in the IT treatment distinctly
decreased by 39% (Table 7). Furthermore, compared with that in the CK, the
tuber weight per plant in the IT+IC treatment improved by 55%. Compared with
that in the CK and IT treatments, the scrap potato rate in the IT+IC treatment was
distinctly improved (Table 7). The changing trend of
the fresh stem weight followed the order of IT+IC>IT>CK. Compared with
that in the CK treatment, the weight in the IT+IC and IT treatments increased
by 41 and 19%, respectively. The actual yield followed the order of
IT+IC>CK>IT, and the yield in the IT treatment was lowest. The ratio of
potato to stem followed the order of CK > IT+IC >IT, and the actual yield
followed the order of IT+IC>CK>IT. The ratio of potato to stem under the
CK increased by 1.16 and 1.14 compared with those under the IT+IC and IT
treatments, respectively. Additionally, the actual yield under the IT+IC
treatment increased by 60 and 13% compared with those under the IT and CK
treatments, respectively.
Discussion
Under the background of
global warming, the eco-environment in the semi-arid region of the Northwest
Loess Plateau has changed obviously, and the precipitation shows a decreasing
trend of fluctuation (Yao et al. 2016). In contrast, the trend of
temperature fluctuation is rising. In this study, the precipitation in the
region showed a decreasing tendency, with a decreasing rate of -12.171 mm every
10 years, and the air temperature increased at a rate of 0.417°C every 10
years, which is consistent with existing research conclusions (Yao et al.
2016). CO2 is the basic material for plant photosynthesis, and
changes in its concentration will inevitably have an important impact on plant
physiological and ecological characteristics. An elevated atmospheric CO2
concentration stimulates the productivity of a broad range of agricultural
crops (Kimball 1983, Lawlor and Mitchell 1991). The height of potatoes was
obviously affected by the change in CO2 concentration, and the
height of potatoes under the IT treatment was significantly higher those under
the IT+IC and CK treatments at the ramifying stage. In addition, during the
harvest stage, potato height followed the order of IT>IT+IC>CK. This
proves that warming can significantly increase potato height. The content of
chlorophyll in potato leaves affects the shape, yield and quality of potato,
and chlorophyll content is one of the physiological indexes of potato leaves
(Su et al. 2007). Overall, the chlorophyll content of leaves showed a
downward trend during the whole growth period and reached its highest level in
the mid-growth period. At the later stage of growth, the chlorophyll content
under IC treatment decreased compared with that under IT treatment but
increased compared with that under CK treatment. The response of different gas
exchange parameters to elevated CO2 was different. In the early
growing season, the Pn of potato leaves under the
IT+IC treatment were higher than those under the IT and CK treatments, and the
results were consistent with those of Olivo et al.
(2002). However, the increase in Pn was different;
this was mainly due to the differences in potato varieties and treatments. The Pn increased under the IC treatment, which may be due to
the competitiveness of the binding sites of the Rubisco enzyme, thus improving
the carboxylation efficiency (Zhang et al. 2014). Moreover, to some
extent, the inhibition of photorespiration by an elevated CO2
concentration is also one of the reasons for the increase in the Pn (Drake 1997). In addition, the increase in the Pn in different growth stages was different in the present
study, which is essentially consistent with existing research conclusions
(Kimball et al. 2002). The gs of plant leaves
generally decreased with increasing CO2 concentrations (Bunce 2000).
The change in the gs of potato leaves is sensitive to
high CO2 concentrations. To maintain the Ci partial pressure at a
level lower than the atmospheric partial pressure, stomatal opening and closing
is regulated. The gs of potato leaves decreased by an
average of 44% under the IC treatment in this study, which is consistent with
the results of Olivo et al. (2002) and Finnan
et al. (2005) under OTC test conditions. The Ci is related to the intensity
of plant photosynthesis and is an important parameter for characterizing the
physiological characteristics of plant photosynthesis (Wang et al.
2015). The amount of exogenous CO2 absorbed by the leaves increased
with an increase in the CO2 concentration, which correspondingly led
to an increase in CO2 entering mesophyll cells. The Ci under the
IT+IC treatment increased significantly, and the photosynthetic potential of
the plants increased accordingly. Moreover, a decrease in gs
led to an increase in resistance to water transpiration, which consequently led
to a decrease in transpiration and an increase in WUE, which is of great
significance for increased potato yields. Increasing the CO2
concentration concurrently with temperature provided sufficient basic material
for photosynthesis by potato leaves and further increased the Pn (Fleisher et al. 2008). WUE represents the
ability of plant leaves to fix CO2 under the conditions of equal
water consumption and is the basic physiological parameter of water use in
plant leaves (Ogutu et al. 2013). Under the
IT+IC treatment, the WUE increased correspondingly and was greater than those
under the IT and CK treatments, consistent with previous research conclusions
(Carolina et al. 2017). Previous studies (Schapendonk
et al. 2000) have shown that when the CO2 concentration
reached 350 μmol∙mol-1 to 700 μmol∙mol-1,
the average increase in tuber dry matter yield was 27%~49%; the actual potato
yield in this study increased by 60% and 13% compared with those in the IT and
CK treatments, respectively, which is similar to existing research conclusions
(Schapendonk et al. 2000, Olivo
et al. 2002). One difference in the results was that the range of increase
in the potato yields was different, mainly due to differences in potato
varieties and the control of the CO2 concentration. The potato yield
under the IT treatment decreased, mainly because potatoes are a cool-season
crop. The activity of photosynthetic enzymes was affected by elevated temperature
(Yao et al. 2013), and the photosynthesis rate decreased. Compared with
the potato yield under the CK treatment, the potato yield under the IT
treatment increased, mainly due to the relatively low temperature during the
early stage. The increase in temperature compensated for the
low-temperature-induced slow growth rate of potatoes to a certain extent, and
the vegetative growth period was more vigorous for those potatoes than for
potatoes under the CK treatment.
Conclusion
The annual precipitation
in the experimental region during the past 59 years tended to decrease. The air
temperature tended to significantly increase and has continued to increase
since the 1970s and was especially distinct in the 2000s.The plant height in
the IT treatment was higher than those in both the IT+IC and CK treatments.
Furthermore, the chlorophyll content of leaves was larger in the IT treatment was
higher than in the other treatments during the late growth stage. Under
combined treatment of warming and elevated CO2 concentration, the Pn of
leaves increased. But the stimulating effect of CO2 on
photosynthesis gradually weakens with the development of growth process.
When CO2 concentration increased, The gs of leaves decreased, on the contrary, transpiration rate
and water use efficiency increased. The Ci of leaves was distinctly higher than
that in the leaves in the CK and IT treatments. With the increase of CO2
concentration, the actual yield of potato distinctly increased. The yield of
potato was low under the treatment of warming because potato is suitable for
growing in cool weather, warming is not amenable to the growth of tubers,
excessive warming will stop the growth of tubers. Warming and elevated CO2
concentrations will stimulate the Pn and WUE of
leaves, which finally result in the increase of dry matter accumulation and
economic yields.
Acknowledgements
The National Natural
Science Fund (41575149), Gansu Province Higher School Industry Support and
Guidance Project (2020C-34), Special Project (Key Special Project) of Public
Welfare Industrial (Meteorology) Scientific Research (GYHY201506001-6), and
National Key Fundamental Research Development Plan (Plan 973) (2013CB430206).
Author Contributions
Yao yubi, Lei jun and Niu Haiyang planned the
experiments, Zhang xiuyun and Xiao guoju interpreted the results, Yao yubi
and Lei jun made the write up and statistically
analyzed the data and made illustrations.
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