Introduction
Eggplant
(Solanum melongena L.) is one of the main vegetables
consumed in China (Lian et al. 2017).
The eggplant has tap root system and has low tolerance for drought. It is
highly sensitive to water supply; soil water deficit, excessive water content,
and variation in soil moisture content substantially affect the growth and
development and yield and quality of eggplant (Tong et al.
2013). Thus,
a method to reduce water loss and improve water use efficiency for eggplant is
required for quality production. At present, several water-saving irrigation
methods using sophisticated equipment are widely employed in agriculture such
as sprinkler irrigation, drip irrigation, and infiltration irrigation (Hu and Yu 2002; Li et al. 2004; Wu 2004). Although the aforementioned methods can
effectively improve water use efficiency, however, these are prone to surface
runoff resulting in wastage of water resources, loss of nutrients, and soil
compaction (Clinton et al. 2001).
The traditional flood irrigation method causes loss of water resources,
enhances evaporation and reduces soil temperature. Excessive evaporation causes
increased air humidity in greenhouses that reduces rate of transpiration of the
leaves, resulting in reduced root water uptake and increases probability of
groundwater pollution caused by diseases, pests, and percolation of nutrients (Wu et al.
2002). A
new type of automatic recharge water-saving irrigation technology was used in
this experiment in order to reduce wastage of water and improve water use
efficiency (Wang et al. 2015).
The stable negative pressure irrigation technology utilizes soil suction and
the ability of plant to absorb water actively to supplement soil moisture, and
regulates the soil moisture content in the root zone during the entire growth
period of the crop. This method promotes crop growth, improves crop yield and
helps control the disadvantages of traditionally followed irrigation methods (Liu et al.
2000a, 2000b). In
1908, Livingston first proposed the concept of water absorption using matrix
potential (Livingston 1908). Different scientists have
explored this concept theoretically, and verified the feasibility of negative
pressure irrigation with an automatic water supplying device (Richards and Loomis 1942; Kato 1982; Lei et al. 2005). Zou et al.
(2007)
found that the negative pressure irrigation system can automatically supply
water timely and appropriately without being affected by external factors and
reduces water loss from the soil caused by percolation and evaporation.
According to another report, if negative pressure irrigation is employed on a large scale it has a water
saving effect and requires less energy (Liu 2001). According to a report, Li et al. (2017) planted pepper using
negative pressure water supply device, and the results showed that negative
pressure irrigation at -5 kPa was beneficial for the growth and development of
pepper, promoted nutrient absorption and improved the quality of pepper.
Negative pressure irrigation technology has been applied in several other
crops. It can promote crop growth, improves water use efficiency, increases
yield, and improves fruit quality (Li et al. 2008a; Li et al. 2010; Xiao et al.
2015). During recent past, the application of negative pressure irrigation on
eggplant has been reported but those studies were only focused on the effect of
specific negative pressure irrigation on the growth and physiological
characteristics of eggplant (Li et al.
2016). However, the effects of different water supply pressures on the growth
and development of eggplant, water use efficiency, physiological
characteristics, and quality have not been reported. Therefore, in this study,
stable negative pressure irrigation device was used to control the soil water
potential in the range of -3 to -15 kPa for eggplant, to determine the effect
of different negative pressures on water consumption, dry matter accumulation,
physiological parameters, and quality of eggplant. The main objective of this
study was to find out the suitable negative pressure for irrigation of
eggplant.
Materials and Methods
Experiment material
The experiment was carried out from
May to October, 2017 in a rainproof plastic greenhouse with a steel frame
structure in the experimental base of Heilongjiang Bayi Agricultural
University, Daqing, Heilongjiang, China. The seedlings of eggplant cultivar
‘Black and Bright’ were obtained from the breeding base of Heilongjiang Bayi
Agricultural University, China for this study. ‘Black and Bright’ is a popular
cultivar of eggplant that is cultivated on a commercial scale in Heilongjiang province
of China. The upper 0–20 cm soil was used for pot filling and those pots were
used in this study. The soil used for this experiment was analyzed. Soil had a
pH of 8.4, organic matter 27.88 g·kg-1, available phosphorus 30.7
mg·kg-1, available potassium 168.5 mg·kg-1, and
alkali-hydrolyzed nitrogen 93.3 mg·kg-1. Each pot was filled with 33
kg of 1 cm sieved soil.
Stable negative pressure irrigation control device
The negative pressure irrigation device used in the
experiment was developed by Institute of Agricultural Resources and Regional
Planning of Chinese Academy of Agricultural Sciences (Long et al. 2014). It consists of three parts: the water outlet, water
storage barrel and negative pressure stabilizer. Each part is connected by an
organic transparent plastic hose. Among them, the water outlet is a "water
permeable and air impermeable" clay pipe (inner diameter 11 mm, outer
diameter 18 mm, length 250 mm), the water tank is 75 cm high, and the side wall
is equipped with 50 cm of high graduated tube; which is used to observe the
change of water level in the tank. The negative pressure stabilizer is mainly
composed of three parts: the negative pressure tank, digital display switch and
solenoid valve. The digital display switch sets the required negative pressure
value. The needed water for plant growth reduces the soil water potential
compared with pressure set by the negative pressure stabilizer; the water
within the water storage barrel permeates into the soil slowly under suction
pressure of the soil. When the water within the water storage barrel
enters the clay pipe through the plastic hose, the water level in the barrel is
dropped, the pressure within the barrel decreased until the internal pressure
of the negative pressure tank reached the set value of the digital display
switch and continued to decrease, and the solenoid valve is opened, causing a
certain amount of outside air to enter the negative pressure tank (Li et al.
2017). When the internal pressure of the negative pressure tank reaches the set
value of the digital display switch, the solenoid valve closes, to maintain a
stable negative water supply pressure. The size of the pot was 30 cm × 30 cm ×
45 cm, there were no holes in the bottom and the floor of pot was flat. The
clay pipe was inclined by 5 degrees and buried in the soil; it was positioned
10 cm from the front and back of the inner wall of the pot, 14.1 cm from the
left and the right inner wall, and 10 cm below the surface of the soil in the
pot. The pattern of the stable negative pressure irrigation device is
illustrated in Fig. 1. According to the principle of negative pressure
infiltration; negative pressure irrigation technology uses the difference
between soil water potential (soil suction) and water supply pressure of irrigation
system as the driving force for irrigation water to enter in the soil. This
fulfills an irrigation method that replenishes soil water in the crop root
zone, which is essentially a process in which irrigation water gradually wets
the soil in a certain area by means of capillary action through an irrigator
buried underground.
Experimental treatment
Four irrigation treatments were set
up for the pot experiment: C (normal irrigation, control), -3 (T1), -8 (T2) and
-15 kPa (T3). Basal dose of fertilizer was applied before transplanting and the
amount of fertilizer for application was calculated according to the local
recommendation (N = 150 mg·kg-1 soil; P2O5 =
100 mg·kg-1 soil; K2O = 150 mg·kg-1 soil). The
fertilizer and sieved soil were mixed in each pot. A completely randomized
experimental design was used for this study. Three pots containing four plants
in each pot were used for every treatment, and each treatment was replicated
three times. Pots were placed at 20 cm row spacing. Each set had an automatic
water supply device that controls a pot with a total of nine sets. The water
level of each automatic water supply device was recorded at 17:00 P.M. every day. The potted plants used
as control (C) were irrigated manually when 5 cm of the topsoil of pot become
dry, and every time 550 mL water was used for irrigation of a single pot.
Sample collection
The samples were harvested four times: at early flowering
stage (June 21, 2017), early fruit-bearing stage (July 12, 2017), full
fruit-bearing period (August 01, 2017) and late growth stage (September 01,
2017). Samples were harvested at 6:30 A.M,
however, sample for determining enzyme activities were harvested at 9:30
A.M. One representative plant was
harvested from each pot for each treatment. The samples of eggplant fruit were
taken four times: the first set of fruit (July 12, 2017), the second set of
fruit (July 26, 2017), the third set of fruit (July 31, 2017), and the fourth
set of fruit (August 31, 2017).
Water consumption determination
Water consumption was analyzed based on the following
formulas:
(1) Negative pressure irrigation:
Water consumption (kg strain–1) = Δh
(cm) × area (cm2) / total number of pots.
Where, Δh is the height difference between each
record; area is the internal floor area of storage barrel for the negative
pressure irrigation and total number is the number of eggplant pots.
(2) C: Water consumption (kg strain–1)
= Irrigation volume (L) used in a unit of time / total number of pots of
eggplant.
Where, irrigation volume (L) used in a unit of time was
550 mL for each pot and it was applied when upper 5 cm topsoil of pot become
dry.
Water use efficiency
Water use efficiency was calculated using the following
formula: Water use efficiency (g L–1) = Yield (g strain–1)
/ Water consumption (L strain–1).
Water consumption percentage
Water consumption percentage was calculated using the
following formula: Water consumption percentage = Water consumption at a
certain growth stage (mm) / total water consumption at the growth stages (mm) ×
100.
Nitrate reductase determination
Nitrate reductase activity was
determined using the aminobenzene sulfonic acid colorimetric method (Zhou
and Zhen 1985).
Determination of rhizosphere activity
Rhizosphere activity was determined
using the α-naphthylamine oxidation method (Chang et al. 2008).
Determination of chlorophyll
Chlorophyll a and
Chlorophyll b contents were determined using the alcohol extraction
method (Bai 1990).
Determination of vitamin C (VC) content
The VC content was determined using
2, 6-diohloroindophenol potentiometer titration (Zhao et al. 2006).
Determination of soluble sugars
The soluble sugar contents were
determined using anthrone colorimetric technique (Zou 2000).
Determination of soluble protein content
The soluble protein contents were
determined using Coomassie Brilliant Blue G250 staining (Li et al. 2002).
Determination of above ground dry weight
The above ground dry
biomass was measured by taking the above ground plant parts
such as leaves, stems, leaf stalks, and fruit of the eggplant at different
stages. Initially the samples were placed at 105°C for 30 min, and then dried
at 75℃ until the mass was constant, finally, the dry mass of each part was
weighed (Xu et al. 2014)
Data analysis
Microsoft Excel 2010 was used for
data coding. S.P.S.S. 19.0 (IBM 2010) was used for statistical analysis, and
multiple comparisons were done using least significant difference (LSD) test at
P ≤ 0.05.
Results
Effect of stable negative pressure irrigation on
water consumption, yield, and water use efficiency of eggplant
It was observed that under normal
irrigation (C) and negative pressure irrigation treatments, the water
consumption per plant and water consumption intensity increased gradually with
plant development during the Table 1: Effect of stable negative pressure irrigation on water consumption of
eggplant. C, normal irrigation; -3, -8 and -15 kPa
|
Growth period |
Treatments |
Water consumption (L plant-1) |
Water consumption intensity (L d-1) |
Total water consumption percentage (%) |
|
Early flowering stage |
Control |
3.71 a |
0.25 |
4.95 |
|
-3 kPa |
3.26 b |
0.22 |
5.48 |
|
|
-8 kPa |
2.51 c |
0.17 |
4.97 |
|
|
-15 kPa |
1.81 d |
0.12 |
8.06 |
|
|
Early fruit bearing stage |
Control |
15.40 a |
0.67 |
20.55 |
|
-3 kPa |
7.97 b |
0.35 |
13.39 |
|
|
-8 kPa |
7.25 b |
0.32 |
14.48 |
|
|
-15 kPa |
3.98 c |
0.17 |
17.64 |
|
|
Full fruit bearing period |
Control |
19.68 a |
1.09 |
26.26 |
|
-3 kPa |
15.81 b |
0.88 |
26.54 |
|
|
-8 kPa |
12.32 c |
0.68 |
24.43 |
|
|
-15 kPa |
5.06 d |
0.28 |
22.53 |
|
|
Late growth stage |
Control |
36.15 a |
0.95 |
48.24 |
|
-3 kPa |
32.52 b |
0.86 |
54.59 |
|
|
-8 kPa |
28.34 c |
0.75 |
56.21 |
|
|
-15 kPa |
11.63 d |
0.31 |
51.77 |
Different
letter along the mean values represent significant differences among the means
using least significant test at P ≤
0.05
Table 2: Effect of stable negative pressure irrigation on yield and water use
efficiency of eggplant. C, normal
irrigation; -3, -8 and -15 kPa
|
Indexes |
Treatments |
|||
|
Control |
-3 kPa |
-8 kPa |
-15 kPa |
|
|
Total water consumption(L) |
74.94 a |
59.57 b |
50.42 c |
22.48 d |
|
Yield (g strain-1) |
1316.20 b |
1479.86 a |
1084.32 c |
422.03 d |
|
Water use efficiency (g L-1) |
17.56 c |
24.85 a |
21.51 b |
18.87 c |
Different
letter along the mean values represent significant differences among the means
using least significant test at P ≤
0.05
Table 3: Effect of stable negative pressure irrigation on growth and development
of eggplant. C, normal
irrigation; -3, -8 and -15 kPa
|
Growth period |
Treatments |
(per
plant cm-1) |
(per plant g-1) |
|
|
Early flowering stage |
Control |
44.50 b |
6.81 b |
7.18 c |
|
-3 kPa |
55.90 a |
8.51 a |
11.53 a |
|
|
-8 kPa |
45.07 b |
7.24 ab |
8.74 b |
|
|
-15 kPa |
32.60 c |
6.02 b |
4.93 d |
|
|
Early fruit bearing stage |
Control |
60.80 b |
8.00 b |
20.09 a |
|
-3 kPa |
72.83 a |
9.28 a |
20.31 a |
|
|
-8 kPa |
62.23 b |
7.79 b |
16.29 b |
|
|
-15 kPa |
46.43 c |
6.42 c |
6.00 c |
|
|
Full fruit bearing period |
Control |
79.03 b |
11.75 a |
33.75 a |
|
-3 kPa |
91.03 a |
12.26 a |
34.66 a |
|
|
-8 kPa |
71.90 c |
10.20 b |
29.81 b |
|
|
-15 kPa |
57.63 d |
7.17 c |
13.92 c |
|
|
Late growth stage |
Control |
104.97 b |
12.10 b |
54.11 b |
|
-3 kPa |
126.17 a |
14.98 a |
60.99 a |
|
|
-8 kPa |
94.10 c |
11.72 b |
47.48 c |
|
|
-15 kPa |
74.83 d |
9.02 c |
34.96 d |
Different
letter along the mean values represent significant differences among the means
using least significant test at P ≤
0.05
growth period (Table 1). Water
consumption per plant and water consumption intensity was observed in the
following order: the early flowering stage < the early fruit-bearing stage
< the full fruit-bearing period < the late growth stage. The total water
consumption percentage was highest at the late growth stage, where it reached
to 48.24 to 51.77%. The results of this study showed that eggplant requires
more water in the middle and late growth stages.
With stable negative pressure irrigation, water consumption and water
consumption intensity per plant were lower compared with control (C).
Irrigation pressure was controlled between -3 and -15 kPa; it was observed that
the water consumption and water consumption intensity for each treatment was
decreased by reducing the irrigation pressure. The total water consumption per
plant for -3, -8 and -15 kPa treatments with stable negative pressure
irrigation were 20.51, 32.72 and 70.00% lower compared with control. And water
consumption in each growth period was apparently different from control (Table
2). The total water consumption intensities per plant in the -3 kPa, -8 and -15
kPa treatments were 22.18, 35.24 and 70.27% lower compared with control, and
the difference between the control and negative pressure irrigation was
significant. Considering the results of this study, we concluded that water
consumption of eggplant can be adjusted by controlling the pressure of
irrigation. By lowering the pressure, less water is consumed by the eggplant
that reduces water consumption intensity.
The results showed that negative pressure irrigation was positively
correlated with eggplant fruit yield and water use efficiency (Table 2). The
yield of eggplant for different irrigation treatment varies as: -3 kPa > CK
> -8 kPa > -15 kPa. The yield of eggplant for -3 kPa treatment was
significantly higher (12.43%) compared with control, and the yield of eggplant
for -8 and -15 kPa was 17.62 and 67.94% lower, respectively compared with
control. This indicated that controlling the irrigation pressure could increase
fruit yield; and when the irrigation pressure is too low that reduces fruit
yield.
The water use efficiency of -3, -8 and -15 kPa treatments with stable negative
pressure irrigation was 41.48, 22.45 and 7.45% higher compared with control.
The differences between -3 and -8 kPa, and control were significant while the
difference between the -15 kPa and control was non-significant. The results
show that water use efficiency of eggplant can be improved by controlling the
pressure the irrigation pressure between -3 to -8 kPa.
Effect of stable negative pressure irrigation on
the growth and development of eggplant
The results of the agronomic traits
and dry matter accumulation of eggplants at different growth stages (Table 3)
showed that plant height at -3 kPa (T1) was apparently higher compared with
control plants during the whole growth period. The plant height of plant at -3
kPa was increased by 25.62, 19.79, 15.18 and 20.20% at early flowering stage,
early fruit-bearing stage, full fruit-bearing period, and late growth stage,
respectively, compared with control. There was no significant difference
between the height of eggplants grown under control conditions compared with -8
kPa during the early growth period (from early flowering stage to early
fruit-bearing stage), however, the height of -8 kPa plants was lower compared
with control from full fruit-bearing period to the late growth stage. The
height of eggplant at -15 kPa was significantly lower compared with control
plants from the early fruit-bearing stage to the late growth stage.
%20427-435_files/image002.png)
Fig. 1: Sketch of stable negative pressure irrigation control device used in this
study
%20427-435_files/image004.png)
Fig. 2: Effect of stable negative pressure irrigation on nitrate reductase
activity of eggplant. FW, fresh weight;
C, normal irrigation; -3, -8 and -15 kPa. Error bars indicate SE.
Different letter above the bars represent significant differences among the
means using least significant test at P ≤
0.05
%20427-435_files/image006.png)
Fig. 3: Effect of stable negative pressure irrigation on the root activity of
eggplant. FW, fresh weight; C, normal irrigation; -3, -8 and -15 kPa. Error bars
indicate SE. Different letter above the
bars represent significant differences among the means using least significant
test at P ≤ 0.05
By analyzing the results, it was observed that stem diameter was improved
by up to 24.98% for the -3 kPa plant compared with control (C) at early
flowering stage, and there was no difference for other treatments compared with
control. From the early flowering stage to the late growth stage, the stem
diameter was increased for -3 kPa plants by 22.31% compared with control. The
stem diameter increment difference between -8 and -15 kPa treatment was
significantly lower compared with control.
The morphological indicators of the eggplant showed that plant height and
stem diameter was significantly improved (Table 3) compared with control during
the entire growth period of eggplant when the irrigation pressure was
maintained at -3 kPa. This indicates that negative pressure irrigation is
beneficial for the growth and development of eggplant.
From
transplanting to the early flowering stage, the dry matter accumulation of
eggplant varied in this order: -3 kPa > -8 kPa > CK > -15 kPa
(Table 3). From the early flowering stage to the full fruit-bearing period, the
dry matter accumulations for control plants was significantly higher compared
with -8 and -15 kPa plants. There was no significant difference
between the control and -3 kPa plants. At the late growth stage of eggplant, the
dry matter accumulation for the plants grown under negative pressure irrigation
was significantly increased compared with control. The plants grown at -3 kPa
had 12.71% higher dry matter accumulation compared with control, suggesting
that the negative pressure irrigation is beneficial for dry matter accumulation
of eggplant.
The number of eggplant fruits at different maturation stages increased
gradually with plant development (Table 4). There was no significant difference
for the number of eggplant fruits per plant between other treatments compared
with control, except -15 kPa. Under negative pressure irrigation (-3 and -15
kPa), the weight of eggplant fruits at different maturation stages increased
with the growth stages. The single eggplant fruit weight at different
maturation stages (the first, second and third set of fruit) for -3 kPa
treatment was 9.88, 27.56 and 30.83% higher compared with control,
respectively. The results showed that negative pressure irrigation at -3 kPa
was beneficial for eggplant fruit development.
Effect of stable negative pressure
irrigation on the nitrate reductase activity, root activity, and chlorophyll
content of eggplant
The results showed that the nitrate reductase activity of eggplant leaves
was increased with the development and it decreased after the full
fruit-bearing period (Fig. 2). The highest nitrate reductase activity of eggplant
leaves for all treatments was observed at the early fruit-bearing stage (-3 kPa
> CK > -8 kPa > -15 kPa). Nitrate reductase activity for -3 kPa
was 15.50, 13.06, 10.53 and 6.14%, higher at early flowering stage, early
fruit-bearing stage, full fruit-bearing period and late growth stage,
respectively, compared with control. The nitrate reductase activity of -8 kPa
Table 4: Effect of stable negative pressure irrigation on eggplant fruits. C,
normal irrigation; -3, -8 and -15 kPa
|
Fruiting section |
Indexes |
Treatments |
|||
|
Control |
-3 kPa |
-8 kPa |
-15 kPa |
||
|
First |
Number (per
plant) |
1 |
1 |
1 |
1 |
|
set of fruit |
Fresh weight (g) |
122.43 b |
134.53 a |
104.51 c |
63.50 d |
|
Second |
Number (per
plant) |
2 |
2 |
2 |
2 |
|
set of fruit |
Fresh weight (g) |
133.28 b |
170.02 a |
123.79 c |
79.49 d |
|
Third |
Number (per
plant) |
3 |
3 |
3 |
2 |
|
set of fruit |
Fresh weight (g) |
87.43 c |
114.38 a |
75.66 d |
99.78 b |
|
Fourth |
Number (per
plant) |
4 |
4 |
3 |
0 |
|
set of fruit |
Fresh weight (g) |
166.23 a |
165.54 a |
126.31 b |
0 |
Different
letter along the mean values represent significant differences among the means
using least significant test at P ≤
0.05
Table
5: Effect
of stable negative pressure irrigation on chlorophyll content of
eggplant. C, normal irrigation; -3, -8 and -15 kPa
|
Date |
Treatments |
Chlorophyll a (mg g-1) |
Chlorophyll b (mg g-1) |
Chlorophyll (a + b) (mg g-1) |
Chlorophyll a/b |
|
Early flowering stage |
Control |
1.37 c |
0.57 b |
1.94 ± 0.13 c |
2.41 |
|
-3 kPa |
2.25 a |
0.78 a |
2.95 ± 0.39 a |
2.76 |
|
|
-8 kPa |
1.89 b |
0.61 b |
2.5 ± 0.08 ab |
3.08 |
|
|
-15 kPa |
1.74 b |
0.62 b |
2.35 ± 0.12 bc |
2.84 |
|
|
Early fruit bearing stage |
Control |
1.36 b |
0.51 b |
1.87 ± 0.11 b |
2.68 |
|
-3 kPa |
1.66 a |
0.63 a |
2.29 ± 0.20 a |
2.62 |
|
|
-8 kPa |
1.11 bc |
0.46 b |
1.57 ± 0.19 bc |
2.39 |
|
|
-15 kPa |
0.93 c |
0.44 b |
1.37 ± 0.16 c |
2.14 |
|
|
Full fruit bearing period |
Control |
1.36 b |
0.54 ab |
1.90 ± 0.17 b |
2.54 |
|
-3 kPa |
1.67 a |
0.60 a |
2.28 ± 0.25 a |
2.78 |
|
|
-8 kPa |
1.26 b |
0.49 b |
1.76 ± 0.11 b |
2.58 |
|
|
-15 kPa |
0.95 c |
0.37 c |
1.31 ± 0.19 c |
2.58 |
|
|
Late growth stage |
Control |
1.27 b |
0.43 b |
1.71 ± 0.20 b |
2.93 |
|
-3 kPa |
1.64 a |
0.54 a |
2.18 ± 0.19 a |
3.05 |
|
|
-8 kPa |
0.97 c |
0.32 c |
1.29 ± 0.07 c |
3.04 |
|
|
-15 kPa |
0.76 d |
0.21 d |
0.97 ± 0.04 d |
3.58 |
Different
letter above the bars represent significant differences among the means using
least significant test at P ≤
0.05
plants at the early flowering stage
had no significant difference compared with control; however, it was
significantly lower compared with control at other growth stages. The nitrate
reductase activity of the -15 kPa plants was significantly lower compared with
control at all stages of plant growth and development.
Considering whole growth period of eggplant, the changing trends for root
activity was observed for different growth stages (Fig. 3). The highest root
activity was observed at early fruit-bearing stage, and then root activity was
decreased. For all growth stages of eggplant, the root activities of the -8 and
-15 kPa plants decreased to different degrees with decreasing irrigation
pressure compared with control. The root activity for -3 kPa plants was
significantly higher for all growth stages of the eggplant compared with
control (11.11–33.33%). From the results it was concluded that appropriate
negative irrigation pressure is conducive for the improvement of nitrate reductase activity and root activity that helps
improve eggplant plant growth and development. The nitrate reductase activity
and root activity were higher when the irrigation pressure is maintained at -3
kPa.
It was observed that negative pressure irrigation affected the chlorophyll
content of eggplant. The chlorophyll (a, b, and a + b)
contents at different growth stages of eggplant grown under stable negative
pressure irrigation and control conditions were highest at the early flowering
stage (Table 5). The changes in chlorophyll (a, b, and a +
b) contents under all treatments were relatively stable from the early
fruit-bearing stage to the full fruit-bearing period. The chlorophyll (a,
b, and a + b) content of eggplant increased gradually when
the irrigation pressure increased over the whole growth period. The
chlorophyll contents of -3 kPa plants were 51.58, 22.84, 20.04 and 27.66%
higher compared with control at early flowering stage, early fruit-bearing
stage, full fruit-bearing period, and late growth stage, respectively, compared
with control. The results showed that appropriate irrigation pressure (-3 kPa)
was conducive to increase the chlorophyll content, however, too low irrigation
pressure resulted in water deficiency, inhibiting the biosynthesis of
chlorophyll in eggplant leaves.
Effect of stable negative pressure
irrigation on the quality of eggplant
%20427-435_files/image008.jpg)
Fig. 4: Effect of stable negative pressure irrigation on the fruit quality of
eggplant. A: Effect of stable
negative pressure irrigation on the soluble sugar of eggplant, B: Effect of stable negative pressure
irrigation on the soluble protein of eggplant, and C: Effect of stable negative pressure irrigation on the VC of
eggplant. VC: Vitamin C; C, normal irrigation; -3, -8 and -15 kPa. Error bars
indicate SE. Different letter above the
bars represent significant differences among the means using least significant
test at P ≤ 0.05
Soluble sugar, soluble protein, and
vitamin C contents are important nutrients and considered as quality indicators
for the vegetables (Tavarini et al.
2008). Soluble sugar and protein contents of eggplant were different at
different maturation stages (the first, second, third, and fourth sets of
fruit). Soluble sugar and protein contents of eggplant were highest in the
second set of fruit, and vitamin C content was highest in the third set of
fruit (Fig. 4). The soluble sugar content of eggplant for -3 kPa was increased
by 47.22, 19.22, 19.43 and 14.47% at the first, second, third, and fourth sets
of fruit compared with control, respectively. The soluble sugar content in the
first set of -8 kPa fruits was 25.63% higher compared with control, however,
the soluble sugar content of eggplant fruit at other fruiting stages (second,
third, and fourth sets of fruit) were lower compared with control.
The soluble protein content of eggplant fruit grown under two irrigation
systems was analyzed. The soluble protein content of eggplant fruit
at different maturation stages for -3 kPa increased by up to 16.33–58.78%
compared with control. The first set of eggplant fruits of -8 kPa
has 31.20% increased soluble protein content compared with control, however for
other sets of fruits (second, third, and fourth set of fruit) the soluble protein
content were gradually decreased compared with control. There was an obvious
decrease in the soluble protein content of eggplant fruit at different
maturation stages, and soluble protein content of -15 kPa plants were reduced
from 26.25 to 34.43% compared with control.
It was observed that vitamin C content in eggplant fruit decreased
significantly when the irrigation pressure was decreased. The vitamin C
contents of eggplant grown at -3 kPa were 43.42, 25, 19.64 and 41.79% higher at
first, second, third and fourth set of fruit compared with control,
respectively. Except the fruits of first set of fruit of -8
kPa, vitamin C content in other treatments showed no significant difference
compared with control. The vitamin C content in other fruiting stages
(second, third, and fourth set of fruit) grown under -8 kPa were significantly
lower (0.42–37.31%) compared with control. It can be observed that
the quality of eggplant fruit can be improved when the irrigation pressure is
maintained at -3 kPa, however further reduction of irrigation pressure lead
towards the decline of eggplant fruit quality.
Discussion
The negative pressure irrigation
device is a closed water supply system that uses the water potential difference
between the system and the soil to achieve automatic crop water acquisition
(Geng et al. 2006). Considering the
energy analysis, it is proposed that the unit water potential inside the
irrigator is φin and the unit soil water potential outside the irrigator
is φout, when φin - φout is positive, the irrigation water will
flow into the soil from the irrigator; the condition for the irrigator to stop
the outflow is when φin - φout is zero (Cai et al. 2017). The results of this study showed that the eggplants
continuously absorbed water, and at this time φin > φout, the
irrigation water was automatically added to the root soil of the eggplant
during its growth. The water supply mode of the irrigation device was constant;
therefore, the soil water content could be maintained within a relatively
stable range. The water consumption during the growth period of the eggplant
could be effectively reduced, which is consistent with the findings of previous
reports (Li et al. 2008b; Liang et al. 2011). The results showed that
the water consumption of eggplant grown under control conditions and negative
pressure irrigation was increased with the development of the eggplant,
which was different from the results of Du et
al. (2018). This may be due to the differences in greenhouse environment,
and the longer growth period from the full fruit-bearing period to the late
growth period of eggplant. Higher indoor temperature during the daytime
increases the consumption of unproductive water such as water used for
evapotranspiration, which leads to an increase in water consumption at the late
growth stage of the eggplant. The results also showed that the water
consumption of eggplant with negative pressure irrigation was apparently
decreased by decreasing irrigation pressure during the whole growth period. The
water consumption of eggplant was significantly decreased compared with control.
This may be because of increase in the photosynthetic rate during the negative
pressure irrigation treatment, which significantly decreases the leaf water
potential and stomatal conductance, thus affects plant transpiration (Liu et al. 2010).
Water use efficiency (WUE) is one of the most important indicators of crop
water use (Li et al. 2008b). With the
help of the difference between the soil water potential (soil suction) and the
pressure of the irrigation system, the negative pressure irrigation treatment
was used as the driving force for the irrigation water to enter the rhizosphere
of the soil (Jiang et al. 2006). The
realization that the purpose of demand-based water supply provided an
appropriate water environment for the growth of eggplant also laid a foundation
for the efficient use of water. The results showed that negative pressure
irrigation could significantly improve the WUE of eggplant compared with
control. The water use efficiency was increased by enhancing the negative
pressure used for irrigation; the water
use efficiency was highest at -3 kPa and lowest at -15 kPa for eggplant. The
highest yield per plant was obtained from the plants grown at -3 kPa and the
lowest yield per plant was obtained from
the plants grown at -15 kPa. Our results were different from the findings of Bian et al.
(2018); this may be due to the fact that water requirement of eggplant is increased when plants enter into the full
fruit-bearing period. During that time, the water supply rates at a negative
pressure irrigation of -8 and -15 kPa did not meet the normal growth
requirements of the eggplant, thus yield is reduced. Meanwhile, it was observed
that the water consumption from the full fruit-bearing period to the late
growth stage accounts for a large proportion of water consumption during the whole growth period. Therefore, low
irrigation pressure was not conducive to improve the water use efficiency of
eggplant at -8 and -15 kPa.
Conclusion
In this study eggplants were grown
under a stable negative pressure of -3 to -15 kPa. Negative pressure irrigation
reduced the water consumption of eggplant by up to 20.51–70.00%, decreased the
total water consumption intensity of plants by up to 22.18–70.27%, and
increased the water use efficiency by 7.45 to 41.48%. Negative pressure
irrigation also promoted the accumulation of dry matter of eggplant. When the
irrigation pressure was maintained at -3 kPa, plant height, stem diameter, and
dry weight of eggplant at the late growth stage was increased by up to 20.20,
23.81 and 12.71%, respectively compared with control. Similarly, the yield of
eggplant was improved by 12.43% compared with control. Negative pressure
irrigation (-3 kPa) also improved the nitrate reductase activity (6.14–15.50%),
root activity (11.11–33.33%), and chlorophyll content (20.04–51.58%) of
eggplant at different growth stages compared with control. The soluble sugar,
soluble protein, and vitamin C contents of eggplant fruit were increased by
14.47–47.22%, 16.33–58.78%, and 19.64–43.42%, respectively, compared with
control. The stable negative pressure irrigation (-3 kPa) reduced the water
consumption and improved the water use efficiency. The stable negative pressure
irrigation improves eggplant growth and development, improves the physiological
activity, increases the yield, and improves the quality of eggplant.
Acknowledgements
This research was financially supported by the Mechanisms of
temporal variation of soil moisture affecting crop water use efficiency and
nutrient uptake (2018YFE0112300).
Author
Contributions
All the authors declared that everyone contributed adequately
to all the procedures of the experiment and manuscript writing. J.Z. conceived
and designed the research, performed experiments and analyzed the data. J.Z.
also participated in drafting the manuscript. P.W. designed the research and
revised the manuscript critically for the main content. H.L. revised the
manuscript critically for the main content. X.H. and J.S. analyzed the data.
All authors approved the final manuscript for publication and agreed to be
accountable for all aspects.
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