Introduction

 

Water is considered as one of the most critical resources for human beings. Over the past 60 years, global demand for water has increased because of many reasons, mainly including rapid population and economic growth (Kaur et al. 2010). As population and economic growth will continue, more food will be needed to be produced in the future. Therefore, water demand will have been increased by more than 40% by 2050 (Nazari et al. 2018). Water scarcity will be a remarkable issue in the near future (Doungmanee 2016). Agriculture is the sector responsible for the highest amount of water use, consuming 70% of the total water use worldwide (Yavuz et al. 2012; Kang et al. 2017). As such, improving agricultural water productivity is an important measure for ensuring global food security. In this regard, researchers must develop ways to improve the efficiency of water consumption in agriculture and to implement agricultural water saving technologies (Farooq et al. 2009; Chai et al. 2016; Xu et al. 2017).

Plant CGPDR theory states that reduced plant growth due to drought stress is offset or exceeded when water supply becomes available. Based on the theory, agricultural water-saving technologies, including deficit irrigation, regulated deficit irrigation and supplemental irrigation in dryland farming, have been widely used in crop production (Rowland et al. 2018; Farooq et al. 2019; Ramlow et al. 2019). Therefore, crop CGPDR is important for water-saving agriculture.

Zhang et al. (2018) found that applying fertilisers containing nitrogen, phosphorus pentoxide and potassium can increase the water use efficiencies of corn and cotton. Wang et al. (2011) reported that improving soil fertility can enhance the compensation growth of rewatered drought-stressed winter wheat. Wang et al. (2016) indicated that root-induced leaf cytokinin caused by root nitrate (NO3−) absorption increases corn CGPDR. Thus, soil fertiliser, especially NO3−, plays a vital role in crop CGPDR. In general, soil releases NO3− via nitrification. Soil nitrification likely participates in crop CGPDR. However, the connection between soil nitrification and the CGPDR has not been reported. Thus, the effect of soil nitrification on the CGPDR mechanism should be further investigated to provide insights into the underlying mechanism in plants and soils.

Corn is a high-water-consuming crop and the third-largest produced crop worldwide. It is also the most produced crop in China. In northern China, corn production is usually hampered by water scarcity, indicating that the efficient use of water in corn production should be increased. Corn seedlings are sensitive to drought, and their growth variation can be easily detected as they grow rapidly. Therefore, in the present study, corn seedlings were used to reveal the regulatory mechanism of soil nitrification in crop CGPDR. The nitrification inhibitor 3, 4-dimethylpyrazole phosphate (DMPP) was added to soils to retain soil nitrification. Plant hormones in leaves and xylem saps, photosynthesis indices and soil nitrification rate were estimated to ascertain the regulatory mechanism.

 

Materials and Methods

 

Experimental design

 

The study took place at the experimental farm in Henan University of Science and Technology (34°32′ N, 112°16′ E, altitude 138m) in Luoyang City, Henan Province. This area receives an annual rainfall of 601 mm with a temperature of 14.2°C. ‘Zhengdan-958’ was selected as the type of corn (Zea mays L.) cultivar because it has the advantages of drought resistance, wide adaptability and wide cultivation in China. The study was carried out under a rain shelter with potting.

The experiment was conducted for 41 days from 13 May 2018 to 23 June 2018. On 13 May 2018, 12 corn seeds were planted in 150 pots for the selection of pots with uniformly grown seedlings. The diameter of the pot mouth was 21.5 cm, and the height of each pot was 20.0 cm. Each pot contained 5.8 kg of soil with organic carbon and total nitrogen contents of 24.3 and 2.16 g/kg, respectively. Only five seedlings showing strong growth were selected 6 days after emergence, and they grew for 10 days. Other seedlings were pulled out. Then, 36 pots with uniformly grown seedlings were selected from 150 pots. The schematic in Fig. 1 clearly displays the trial time course, treatment setting and related indicators measured in the 36 pots.

Afterwards, 36 pots were split into four groups, with 9 pots in each group. They were then subjected to two growth periods for 10 days: drought stress and rewatering. During the drought stress period, the first two groups were given sufficient water, while the remaining groups were subjected to drought stress. Then, 13 mL of DMPP solution (1 g/L) was added to the second and fourth groups every day during

the drought stress period to restrain soil nitrification during the rewatering period. In the rewatering period, all the groups received sufficient water. Thus, the four treatments comprising nine pots each were as follows: (1) wetness (WT), (2) wetness with DMPP added (WR), (3) rewatering (DT) and (4) rewatering with DMPP added (DR).

 

Fig. 1: Schematic diagram of the experimental design

The 3th, 5th, 6th, 9th and 10th days refer to the 3th, 5th, 6th, 9th and 10th days of the 10-day drought stress or rewatering periods. WT, WR, DT and DR indicate treatments of wetness, wetness with DMPP added, rewatering and rewatering with DMPP added, respectively. “Wet” and “Dry” refer to the exposure of corns to wetness and drought stress, respectively. “DMPP” indicates the DMPP added to soil. “3 pots each” corresponds to the three pots in each treatment. “10 days” denotes the 10-day duration. “Damaged” implies that the corns were taken to lab to be completely harmed for measurement. “No damage” means that corns were not harmed during measurement. All indices refer to indices that were measured in the present study. Pn, Sc, and Tr represent the net photosynthetic rate, stomatal conductance and transpiration rate, respectively

 

 At the beginning of the drought stress, the corns in this trial were in the growth period of 4–5 leaves. At the end of the rewatering period, the corns were in the growth period of 7–8 leaves. Our preliminary experiment found that after adding DMPP to the soil, its inhibitory effect on soil nitrification appeared after several days. This phenomenon was the main reason for choosing this time to add DMPP to soil. Wetness and drought stress were induced by the addition of the weighed amount of water that allowed the soil water content to be maintained at 75–80% and 50–55% of the field water capacity, respectively. During the time period 6:00 a.m. to 9:00 p.m., when the soil water content reached lower than 75% of the field water capacity, the water content was maintained at 75–80% of the field water capacity by adding water. Water was not added to the soil on the first 2–3 days to allow soil water dissipation. Afterwards, the pots were weighed, and the soil water content was maintained at 50–55% of the field water capacity by adding water.

The soil water content at each treatment was calculated using Formula (1) in accordance with previously described methods (Xiong et al. 2007):

 

SWC =Χ 100%                     (1)

 

where SWC is the soil water content, Wt is the temporary whole pot weight, Wd is the net weight of dried soil in the pot, We is the weight of the empty pot, Wp is the estimated fresh weight of all plants in the pot, and FWC is the field water capacity. The estimated fresh weight of all plants in one pot was determined in advance by measuring the fresh weight of the plants in extra pots in each test period.

 

Measurements and data analysis

 

At the end of the drought stress period and 3, 6 and 9 days post rewatering, net photosynthetic rate (Pn), transpiration rate (Tr) and stomatal conductance (Gs) were measured using LI-6400 photosynthesis equipment at 11:00 a.m. in each treatment.

At the end of the drought stress period and 5 and 10 days post rewatering, three pots from each group were taken to lab. After the stem bases were clipped, the stem wounds were immediately covered with 1.0 g of absorbent cotton. After 12 h, the cotton’s weight was determined. The quantity of xylem was equal to the increased weight in the absorbent cotton. The volume of sap was determined by dividing the increased weight by 1 g/cm3. Then, the cotton was compactly placed at the end of a 10 mL syringe by using a piston. The sap was pooled from each sample to 5 mL centrifuge tubes to measure the concentrations of zeatin riboside (ZR) and abscisic acid (ABA) in the xylem sap. Immediately after the stem bases were clipped, the weights of the clipped stems and leaves were determined, and leaf samples were collected for the measurement of indole-3-acetic acid (IAA), gibberellic acid (GA3), ZR and ABA concentrations in leaves.

After the xylem sap was collected, soil samples were obtained to measure soil ammonium (NH4+) and NO3− contents and soil nitrification rate. The roots of the corn seedlings were separated from the soil by washing, and the roots and stem bases were weighed. Root samples were also collected to measure the root activity and solute carbohydrate content in roots. The fresh samples of roots, stems and leaves were dried at 65°C for 72 h to determine their water and dry matter contents. The product of the fresh weight and dry matter content of the sample was adopted to determine the biomasses. Aboveground biomass was calculated as the sum of the stem and leaf biomasses. The sum of the stem, leaf, and root biomasses was then used to calculate the total biomass.

IAA, GA3, ZR and ABA concentrations in the leaves or xylem saps were estimated via an enzyme-linked immunosorbent assay in accordance with previously described methods (Qin and Wang 2019). The test kits for IAA, GA3, ABA and ZR were produced at the Phytohormone Research Institute of China Agricultural University. Soil NH4+ and NO3− contents were measured through indophenol blue and phenol disulfonic acid colorimetry, respectively (Lu 1999).

The soil net nitrification rate was determined via the culture method (Lu 1999), which was slightly modified. All the soil samples except the soil samples of DT and DR at the end of the drought stress period were cultured at 25°C for 7 days under the SWC of 75–80% of the field water capacity to simulate the wetness condition. The soil samples of DT and DR at the end of the drought stress period were cultured at 25°C for 7 days under the SWC of 50–55% of the field water capacity to simulate the drought stress condition. The NO3− contents of the soil before and after being cultured were measured, and their differences were divided by the culturing time of 7 days to obtain the soil net nitrification rates. Anthrone method was used to measure the soluble carbohydrate contents in roots (Li et al. 2000). Root activity was determined using the triphenyltetrazolium chloride method (Li et al. 2000). The abbreviations used in the text are listed in Table 1.

 

Statistical analysis

 

Statistical analyses were conducted with Microsoft Excel 2007. The values in the tables or graphs corresponded to average values. The effects of water, including drought stress and rewatering, and DMPP addition to soil on corn growth were analysed using two-way variance analysis and Duncan’s multiple range tests (P= 0.05).

 

Results

 

Table 1: Symbol definition

 

Symbol

Definition

Symbol

Definition

CGPDR

compensatory growth of post-drought rewatering

WT

wetness

DMPP

Nitrification inhibitor 3,4-dimethylpyrazole phosphate

WR

wetness with DMPP added

ZR

zeatin riboside

DT

rewatering

NO3−

Soil nitrate

DR

rewatering with DMPP added

NH4+

soil ammonium

SWC

soil water content

GA3

gibberellic acid

FWC

field water capacity

ABA

abscisic acid

Wt

temporary whole pot weight

IAA

indole-3-acetic acid

Wd

net weight of dried soil in the pot

Pn

net photosynthetic rate

We

weight of empty pot

Tr

transpiration rate

Wp

fresh weightof all plants in the pot

Sc

stomatal conductance

 

 

 

Table 2: Aboveground and total biomasses, Pn, Sc, and Tr in different treatments

 

 

Days after rewatering (d)

Treatments

F-Value

WT

WR

DT

DR

DMPP effect

Water effect

WaterΧDMPP

Abovegro-und biomass (g/pot)

0

3.44a±0.13

3.02b±0.34

2.44c±0.22

1.79d±0.28

13.07*

57.46*

0.63

5

3.68a±0.12

3.11b±0.33

3.24b±0.25

1.93c±0.27

41.80*

30.84*

6.54*

10

4.00a±0.18

3.22b±0.32

3.81a±0.23

2.31c±0.28

59.14*

13.65*

5.76*

Total biomass (g/pot)

0

5.66a±0.24

5.00b±0.48

4.20c±0.08

3.11d±0.27

25.53*

92.91*

1.51

5

6.07a±0.21

5.12b±0.49

5.42b±0.25

3.21c±0.29

68.94*

45.45*

11.17*

10

6.63a±0.18

6.33b±0.39

5.31a±0.47

3.83c±0.26

93.47*

20.41*

9.14*

Pn (μmol/m2•s)

0

14.67a±0.84

5.88c±0.49

9.11b±0.83

3.83d±0.24

351.94*

102.77*

21.82*

3

11.57b±0.81

3.55c±0.30

15.50a±1.31

4.35c±0.29

435.12*

26.54*

11.63*

6

10.60b±0.95

2.27c±1.69

16.87a±0.19

1.77c±0.14

431.26*

26.09*

35.93*

9

10.19b±1.10

3.95c±1.42

13.27a±0.25

3.32c±0.14

237.19*

5.42*

12.44*

Sc (μmol/m2•s)

0

0.12a±0.005

0.06b±0.005

0.06b±0.006

0.03c±0.004

222.07*

191.68*

27.65*

3

0.08bc±0.003

0.08c±0.007

0.09a±0.006

0.04d±0.004

89.82*

18.51*

72.54*

6

0.07b±0.007

0.02c±0.010

0.10a±0.001

0.02c±0.002

289.62*

12.85*

28.86*

9

0.06b±0.006

0.03c±0.009

0.08a±0.001

0.02d±0.002

216.78*

3.84

28.85*

Tr (mmol/m2•s)

0

1.66a±0.10

1.01b±0.12

0.99b±0.05

0.64c±0.07

92.44*

98.35*

8.17*

3

2.70b±0.11

2.35b±0.26

3.36a±0.37

1.55c±0.13

60.61*

0.26

27.86*

6

0.81b±0.07

0.28c±0.10

1.22a±0.03

0.20c±0.02

411.23*

18.17*

41.83*

9

1.27b±0.11

0.65c±0.06

1.68a±0.06

0.40d±0.06

460.211*

3.20

56.22*

The values are the mean ± 1 standard deviation (n = 3). Different letters in each row indicate significant differences (P <0.05). “*” means “P<0.05”. Pn, Sc and Tr represent the net photosynthetic rate, stomatal conductance, and transpiration rate, respectively. WT, WR, DT and DR indicate treatments of wetness, wetness with DMPP added, rewatering and re-watering with DMPP added, respectively

 

Biomass and photosynthetic characteristics

 

Table 2 shows that the aboveground and total biomasses in WT significantly increased compared with that in WR, DT and DR during drought stress and 5 days after rewatering. These biomasses still significantly increased in WT compared with those in WR and DR, but no significant differences in these biomasses were found between WT and DT 10 days after rewatering. Furthermore these biomasses in WT, DT and WR also significantly increased compared with those than in DR during drought stress and rewatering period. Pn, Tr and Sc in WT significantly increased compared with those in WR, DT and DR prior to rewatering. These parameters also significantly increased in DT compared with those in WT, WR and DR and in WT compared with those in WR and DR 6 and 9 days after rewatering. No significant differences in Pn were found between WR and DR during the drought rewatering period. The interaction between water and DMPP significantly affected corn growth and its photosynthetic characteristics during drought stress and rewatering periods. Therefore, drought stress and DMPP addition to soil inhibited corn growth and photosynthesis, but rewatering without adding DMPP to soil increased them obviously.

 

Plant hormones and xylem sap quantity

 

Table 3 shows that the leaf ZR contents and its concentration in the xylem saps significantly increased in WT compared with that in DT prior to rewatering and in DT compared with that in WT 5 days post rewatering. The leaf ZR content significantly increased in WT compared with that in WR and in DT compared with that in DR during the rewatering period. No significant differences in leaf ZR and its concentration in the xylem saps were detected between WR and DR during the rewatering period and between DT and WT 10 days post rewatering. The interaction between drought stress and DMPP significantly affected the leaf ZR contents and its concentrations in the xylem saps. The primary type of cytokinin was ZR. These findings indicated that drought stress and DMPP addition to soils decreased the leaf cytokinin content. Rewatering without DMPP addition increased the cytokinin contents in the leaves and the xylem saps during the rewatering period, but this effect gradually diminished during rewatering.

The leaf IAA contents in WR were significantly higher than those in WT. Similarly, the leaf IAA contents in DR were significantly higher than those in DT before rewatering and 5 and 10 days post rewatering. The leaf ABA content and its concentration in the xylem saps were significantly higher in DT and DR than in WR and WT prior to rewatering, respectively. The xylem sap quantity per unite increased in WT compared with that in DT, DR and WR. This parameter also significantly increased in DT compared with that in DR prior to rewatering. Conversely, the xylem sap quantity per unit time significantly decreased in WR compared with that in WT and in DR compared with that in DT 5 and 10 days post rewatering. The interaction between water and DMPP slightly affected the leaf IAA content, but significantly influenced the leaf ABA content and its concentration in the xylem sap during rewatering. These results indicated that adding DMPP to soil increased the leaf IAA contents, and drought stress increased the ABA content in the leaves and xylem saps. Drought stress and DMPP addition to soil decreased the xylem sap quantity per unit time. Adding DMPP to soil and rewatering slightly affected leaf GA3.

 

Soil nitrification rate, soil ammonium and nitrate nitrogen, root soluble carbohydrate and root activity

 

In Table 4, the nitrification rate significantly increased in WT compared with that in WR, DT and DR prior to rewatering. This parameter also significantly increased in WT and DT compared with those in WR and DR 5 and 10 days after rewatering, respectively. No significant differences were observed in soil nitrification between WT and DT and between WR and DR 5 and 10 days post rewatering. The soil NH4+ content significantly increased in WR compared with that in WT and in DR compared with that in DT before rewatering and 5 and 10 days after rewatering. The soil NO3− content significantly increased in WT compared with that in WR, DT and DR. It also significantly increased in DR compared with that in DT before rewatering. Similarly, the soil NO3− content significantly increased in WT compared with that in WR 5 and 10 days post rewatering. No significant differences in the soil NO3− content were detected between WT and DT 5 and 10 days after rewatering. The interaction of water and DMPP significantly affected soil NH4+ and NO3− contents. Their interaction also significantly influenced the soil nitrification rate before rewatering. Thus, DMPP addition to soil and drought stress without the added DMPP restrained soil nitrification and decreased the soil NO3− content. Without DMPP, rewatering resumed soil nitrification, and the soil NO3− content was similar to wetness.

The root soluble carbohydrate contents and root activity were significantly higher in DT than in WT, WR and DR before rewatering. These parameters were also significantly higher in DT than in WT and in DR than in WR 5 days after rewatering. The interaction of water and DMPP slightly influenced the root soluble carbohydrate contents and root activity. These results showed that drought stress caused an increase in the root soluble carbohydrate content and root activity, and this effect decreased with rewatering days.

 

Discussion

 

When the reduction of the biomass of rewatered corns induced by drought stress was offset in the rewatering period, the CGPDR occurred. Drought stress decreased corn growth in DT compared with that in WT. After 10 days of rewatering, the similar aboveground and total biomass between WT and DT indicated that corns experienced the CGPDR without DMPP addition to soil. By contrast, 10 days of rewatering growth did not cause the CGPDR in DR when DMPP was added. Therefore, the absence of DMPP addition to soil played a key role in the CGPDR.

Without DMPP, the leaf cytokinin concentration in DT was higher than that in WT during the rewatering period. A previous study showed that cytokinin concentration in leaves has a vital role in corn’s CGPDR (Wang et al. 2016, 2018). Increased cytokinin levels in leaves help enhance their net photosynthetic rate (Tamaki and Mercier 2007; Kobayashi et al. 2010, 2012). By nature, the CGPDR is a fast production and accumulation process of organic matter via photosynthesis. Pn in DT was higher than that in WT during the rewatering period, thereby causing the occurrence of CGPDR without DMPP added. However, no high leaf cytokinin content was observed in DR compared with that in WR during the rewatering period, resulting in similar Pn in them. As a result, no CGPDR occurred when DMPP was added. Thus, the absence of DMPP in soil was closely related to the CGPDR on the basis of leaf cytokinin.

 

Table 3: ZR, ABA, IAA, and GA3 concentrations in the leaves; ZR and ABA concentrations in the xylem saps; and xylem sap quantity in different treatments

 

 

Days after rewatering (d)

Treatments

F-Value

WT

WR

DT

DR

DMPP effect

Water effect

Water Χ DMPP

ZR content in leaves (΅g/g)

0

119.41a ± 6.41

95.24b ± 9.72

68.47c ± 9.22

78.67c ± 8.90

1.95

45.62*

11.83*

5

111.53b ± 12.50

93.08c ± 8.14

137.51a ±10.16

98.32c ± 8.28

25.27*

7.41*

3.27

10

122.10a ± 9.39

100.12b ± 4.57

128.80a ± 11.02

104.58b ± 10.84

18.40*

1.08

0.04

ZR content in xylem sap (΅g/ml)

0

78.19c ± 7.62

57.94 c ± 9.42

308.40a ±32.47

218.18b ± 21.45

22.04*

275.31*

8.84*

5

63.72a ± 5.30

68.45ab ±9.26

78.79 a ± 4.09

77.79 a ± 9.96

0.18

7.78*

0.43

10

57.85b ± 9.69

65.12 a ± 12.68

63.47 a ± 6.34

71.96 a ± 2.31

2.48

1.55

0.02

IAA content in leaves (μg/g)

0

294.36bc±30.78

360.10a ±33.30

266.82c ±35.75

342.60ab±33.00

13.58*

1.38

0.07

5

295.93b ±31.99

413.59a ±37.10

237.09b ±32.02

395.49a ± 17.24

60.33*

4.69

1.31

10

241.62b ±32.02

314.03a ±32.85

244.03b ±23.51

323.08a ± 21.51

22.06*

0.13

0.04

ABA content in xylem sap (΅g/mL)

 

0

192.33d±26.54

356.53c ±54.96

798.18b ±32.98

917.60a ± 45.17

35.21*

596.12*

0.88

5

146.68b ±22.09

163.63b ± 6.89

64.50c ± 13.00

200.24a ± 34.08

37.48*

3.34

22.69*

10

75.45c ± 1.66

148.49a ±14.50

121.33b ±14.14

88.82c ± 6.01

10.98*

1.27

74.46*

ABA content in leaves (΅g/g)

0

192.33d±26.54

356.53c ±54.96

798.18b ±32.98

917.60a ± 45.17

35.21*

596.12*

0.88

5

146.68b ±22.09

163.63b ±6.89

64.50c ± 13.00

200.24a ± 34.08

37.48*

3.34

22.69*

10

75.45c ± 1.66

148.49a ±14.50

121.33b ±14.14

88.82c ± 6.01

10.98*

1.27

74.46*

GA3 content in leaves (΅g/g)

0

8.88b ± 0.94

7.77b ± 0.33

11.14a ± 0.94

10.39a ± 1.49

2.52

17.42*

0.10

5

14.83a ± 2.14

13.22a ± 0.77

10.59b ± 0.91

5.75c ± 0.57

19.72*

64.77*

4.93

10

9.18ab ± 1.03

6.56c ± 0.77

8.17b ± 0.68

9.99a ± 1.22

0.54

4.89

16.44*

Root xylem sap quantity (mg/plant•h)

0

42.50 a ± 4.38

27.19b ± 2.61

8.71c ± 0.73

7.68c ± 0.66

29.70*

316.00*

22.69*

5

48.57 a ± 4.61

39.54b ± 2.72

49.20a ± 4.52

32.89c ± 2.40

35.19*

1.98

2.90

10

52.38a ± 5.22

37.34b ± 2.34

56.98a ± 5.38

42.59b ± 4.54

31.40*

0.02

3.51

The values are the mean ± 1 standard deviation (n = 3). Different letters in each row indicate significant differences (P < 0.05). “*” means “P < 0.05”. WT, WR, DT and DR indicate treatments of wetness, wetness with DMPP added, rewatering and rewatering with DMPP added, respectively

 

Table 4: Soil nitrification rate, soil ammonium and nitrate nitrogen, solute carbohydrate content in roots, and root activity in different treatments

 

 

Days after rewatering (d)

Treatments

F-Value

WT

WR

DT

DR

DMPP effect

Water effect

WaterΧDMPP

Soil nitrification rate (mg/kg•d)

0

0.31 a ± 0.03

0.08c ± 0.01

0.18b ± 0.03

0.09c ± 0.03

88.65*4

14.427*

16.625*

5

0.33a ± 0.04

0.08b ± 0.01

0.27a ± 0.03

0.09b ± 0.05

101.1*39

1.600

2.209

10

0. 23a ± 0.01

0.11b ± 0.01

0.28a ± 0.05

0.14b ± 0.01

50.58*6

4.804

0.505

NH4+-N content in soil (mg/kg)

0

14.85c ± 1.49

18.21b ± 1.15

13.51c ± 1.23

20.94a ± 1.00

57.73*

0.95

8.26*

5

11.11b ± 0.88

16.43a ± 1.10

12.17b ± 0.67

17.99a ± 1.53

78.25*

4.31

0.17

10

8.08c ± 0.60

12.41b ± 0.88

10.12c ± 0.89

17.38a ± 1.00

121.95*

40.64*

13.77*

NO3−-N content in soil (mg/kg)

0

9.97a ± 0.93

2.03c ± 0.14

7.70b ± 0.84

8.51b ± 0.41

87.76*

30.46*

131.54*

5

7.08a ± 0.62

3.87b ± 0.30

7.94a ± 0.38

7.34a ± 0.71

38.74*

50.00*

18.28*

10

6.06a ± 0.29

4.42c ± 0.35

5.48ab ± 0.54

5.09b ± 0.34

21.97*

0.55

9.75*

Solute carbohydrate content in roots (mg/kg)

0

121.54b ±12.61

135.63b ±14.84

178.89a±16.94

169.39a ±11.62

0.079

31.082*

2.084

5

128.95b ±9.84

119.75b ±8.59

157.67a±20.16

149.85ab ±12.62

1.181

14.102*

0.008

10

137.57a ±14.37

129.86a ±8.46

143.58a±11.20

139.55a ±10.74

0.798

1.424

0.078

Root activity (mg/pot•h)

0

219.17b ±20.65

200.91b ±21.09

274.96a±24.58

267.54a ±20.91

1.03

23.50*

0.18

5

257.49b ±17.71

233.52b ±14.43

302.98a±30.71

285.26a ±21.37

2.71

14.76*

0.06

10

235.8ab ± 22.82

208.44b ±12.08

268.58a±18.33

242.49ab ±20.92

5.95*

9.30*

0.01

The values are the mean ± 1 standard deviation (n = 3). Different letters in each row indicate significant differences (P < 0.05). “*” means “P < 0.05”.WT, WR, DT and DR indicate treatments of wetness, wetness with DMPP added, rewatering and rewatering with DMPP added, respectively

 

In general, as the main sites of cytokinin synthesis, roots synthesise cytokinin and deliver them to leaves via the xylem sap (Zaicovski et al. 2008; Lu et al. 2009). The product of xylem sap quantity and its cytokinin concentration showed the quantity of cytokinin delivered from roots to leaves. Transpiration is an indispensable factor because it promotes the migration of sap from the roots to the leaves. During the rewatering period, the cytokinin concentration in the xylem sap and Tr increased in DT compared with that in WT, although the same xylem sap quantity was found in WT and DT treatments. This result showed that the roots of DT could deliver more cytokinins to the leaves, leading to an increased leaf cytokinin concentration. However, in the presence of DMPP, no high cytokinin concentration in the xylem sap, xylem sap quantity and Tr were found in DR compared with that in WR during rewatering period. As a result, no high leaf cytokinin content was observed in DR compared with that in WR. Therefore, the absence of DMPP addition to soil increased the leaf cytokinin content mainly by enhancing its delivery rate from roots to leaves.

Without DMPP, NO3− is released from soil via nitrification. Soil NO3− directly induces roots to synthesise cytokinins in various plants (Criado et al. 2009; Lu et al. 2009). In our study, similar soil nitrification rates and soil NO3− contents between WT and DT during the rewatering period showed the close amount of NO3− released from soil and the close amount of NO3− being in soil. Therefore, their roots had similar chances to approach NO3− in this case. However, the leaf cytokinin content in DT was higher than that in WT during the rewatering period. A previous study found that high root absorption in rewatering corns causes them to have more chances to approach NO3− when NO3− is added to the roots of corn planted in sand; as a result, large amounts of cytokinin are synthesised in the roots and delivered to the leaves to promote the CGPDR (Wang et al. 2016, 2018). In the present study, during the rewatering period, the high root absorption in DT promoted its roots to deliver more cytokinins to the leaves than it did in WT, so the CGPDR was promoted.

Although the soil NO3− content in DR was high, the similar cytokinin concentration of xylem sap, Tr and xylem sap quantity in DR and WR showed that the same amounts of cytokinins were synthesised and delivered to the leaves by the roots during the rewatering period. This phenomenon might occur because adding DMPP to soil decreased the soil nitrification rate, resulting in less NO3− released from the soil. This caused less stimulations of NO3− on roots; as a result, the roots synthesising cytokinins were inhibited, and the delivery of these substances to the leaves was retained in WR and DR. Under this condition, the high root absorption in DR did not increase its root-induced leaf cytokinin compared with that in WR. CGPDR was also retained. Thus, in comparison with soil NO3−, soil nitrification was a relatively reliable soil environmental factor that influenced root-induced leaf cytokinin.

The soil NO3− concentration was similar between DT and DR during the rewatering period. Although the cytokinin concentration of the xylem sap in DR and DT was similar, high Tr and xylem sap quantity in DT showed that more cytokinins were delivered to the leaves compared with those in DR. This finding could be attributed to the high soil nitrification rate in DT. This further indicated that in the present study soil NO3− weakly affected root-induced leaf cytokinin, and soil nitrification was the key soil environmental factor regulating it. Under drought stress, the soil nitrification rate in DT was lower than that in WT, but rewatering changed this result. The soil nitrification rates were similar in the two treatments during the rewatering period. The increased soil nitrification rate was beneficial to root-induced leaf cytokinin. Thus, rewatering was a key factor influencing soil nitrification.

The soil NO3− concentration was lower in WR than in WT during the rewatering period because DMPP retained soil nitrification. However, the soil NO3− concentration was similar between DT and DR during the rewatering period. This finding could be mainly due to rapid growth after rewatering using a large amount of soil NO3− in DT, and in the same time slow growth caused by drought stress and DMPP added reduced the use of NO3− in soil in DR. The high root carbohydrate concentration during the rewatering period played a role in enhancing the root absorption of mineral nutrition and NO3− in DT. The root absorption of mineral nutrients required energy. With sufficient organic substances and adequate water, roots could increase their rate of mineral nutrient and NO3− absorption.

 

Conclusion

 

Corns without DMPP added to soil encountered CGPDR. In the absence of DMPP in soil, rewatering increased the leaf cytokinin content and its delivery rate from roots to leaves, and leaf cytokinin improved the growth of corn after being rewatered. Without DMPP in soil, soil nitrification increased the ability of rewatered corn to synthesise cytokinin and deliver it from roots to leaves. Therefore, soil nitrification is a key soil environmental factor that influences the CGPDR of corn.

 

Acknowledgements

 

This work was supported by the National Natural Science Foundation of China (U1304326), the Excellent Youth Foundation of He’nan Scientific Committee (174100510004), and the Scientific Research Program of Henan Provincial Education Department in China (2011A180012).

 

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