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Effects of Plant Growth Regulators on Water Deficit-Induced Yield Loss in Soybean

Mingcai Zhang, Liusheng Duan, Zhixi Zhai, Jianming Li, Xiaoli Tian, Baomin Wang, Zhongpei He and Zhaohu Li

Centre of Crop Chemical Control, Department of Agronomy, State Key Lab of Plant Physiology and Biochemistry, China Agricultural University, Beijing, 100094, P. R. China www.cau.edu Email: lizhaohu@cau.edu.cn

Abstract

The effects of PGRs, including benzyladenine (6-BA), uniconazole (S3307), brassinolide (Br), and abscisic acid (ABA), on leaf water potential (Ψleaf), chlorophyll (Chl), photosynthetic rate (Pn), PSⅡ photochemical efficiency (FV/FM) and seed yield of soybean (Glycine max Merr.), cv. Keng 5, were studied under water deficit. PGRs were foliar applied at R1 of 50, 100, 0.1 and 50 mg L-1 for 6-BA, S3307, Br and ABA, respectively. Two levels of soil moisture (–0.02 and –0.06 MPa for well-watered and water-deficit, respectively) were applied at R3. The results indicated that water deficit decreased biomass of stems and leaves, and induced yield loss significantly. PGRs treatments increased soybean yields both under well-watered and water deficit conditions, expect 6-BA under water deficit. However, the yields of PGR treatments under water deficit were still lower than that of the well-watered control. PGRs increased roots and nodule biomasses under both water conditions, except nodule biomass of 6-BA under water deficit condition. ABA and Br treatments increased stems and leaves biomass under well-watered condition. Water deficit significantly decreased Ψleaf, Chl, Pn, and FV/FM. Under water stress condition, PGRs treatments significantly increased water potential and FV/FM, and improved Chl (expect 6-BA) and Pn (except S3307 and Br) compared to the control. Under well-watered condition, PGRs treatments did not affectΨleaf, Ch1 and FV/FM in our experiments, but increased Pn. It was concluded that PGRs treatments minimized the yield loss coursed by water deficit.

Media summary

The plant growth regulators benzyladenine, uniconazole, brassinolide, and abscisic acid minimized the soybean yield loss caused by water deficit.

Key Words

Soybean; Plant Growth Regulator; Water deficit; Yield

Introduction

Drought is a major abiotic stress for crop production. Various agronomic and physiological practices are applied to minimize the adverse effect of moisture stress on plant growth. Plant growth regulators (PGRs) have been used in field crops for many years. Various experiments had shown that some PGRs improved crop resistance to different stresses, including water deficit.Benzyladenine(6-BA) is an active plant growth substance. It can increase the drought resistance of different plants (Shang et al., 2000). Dong et al. (1997) found that applied 10-5M 6-BA significantly increased the RuBPCase activity of wheat and corn in seedling under water deficit. Triazole plant growth regulators such as paclobutrazol and uniconzaole (S3307) are potent gibberellin biosynthesis inhibitors and shoot growth retardants. In addition, triazoles induced a variety of other responses in plants (Davis et al. 1988; Gao et al. 1988). Fletcher and Hofstra (1988) reviewed the biological activities of triazole compounds in relation to plant stress tolerance. Plants treated with triazole compounds are better able to withstand several adverse environmental conditions including drought, atmospheric pollutants and extreme temperatures. Brassinolides (Br) have been evaluated for use in increasing crop yield and stress tolerance. They were reported to increase ear weight and grain weight in wheat and grain yield in maize (Takematsu et al., 1983; Yokota and Takahashi, 1986). Applied Br helped crops to overcome environmental stresses (Yokota and Takahashi, 1986). Abscisic acid (ABA) is considered to be a stress hormone, and it regulates responses common to many environment stresses (Boussiba et al., 1975). ABA accumulates in leaves of many species during dehydration (Bunce , 1990).Soybean is a very important field crop. Drought stress is the primary constraint for increasing soybean yield (Cooper et al., 1991). The objective of this research is to study weather PGR 6-BA, S3307, Br and ABA could be used to improve or maintain soybean productivity under water deficit.

Materials and Methods

The experiment was conducted at China Agricultural University. Soybean cultivar “Keng 5” was grown in 30-cm height by 25-cm diameter pots. Each pot was filled with 15 kg sandy loam soil. The pots were placed in a field and sheltered from rain by a removable polyethylene shelter. Eight seeds were sown in each pot. At the three-leaf stage, plants were thinned to four plants per pot (equivalent to a density of 40 plants m-2). The plants were watered daily by hand to maintain the soil water content.

The experiment was a 2 5 (two levels of soil moisture, five plant growth regulators) factorial design with 10 treatment combinations. Each treatment was replicated 10 times in individual pots. Four plant growth regulators, benzyladenine(6-BA), uniconazole(S3307), brassinolide(Br), and abscisic acid(ABA) were foliar applied at R1 growth stage (Fehr et al.,1971). Tap water was used as control. The concentrations of 6-BA, S3307, Br and ABA were at 50, 100, 0.1 and 50 mg L-1, respectively. The concentrations had been found to be most effective in preliminary experiment. Two levels of soil moisture were applied by controlled watering at R3 stage (7 days after PGR application) up to harvest at R6 stage. The well-watered and water-deficit treatments were maintained at –0.02 and –0.06 MPa, respectively (Gonzalez et al., 1995).

Effects of treatments on growth and yield were determined by measurement of accumulated biomass of various organs. At harvest the plants were removed carefully from the pots, then dried for 48 h at 70o C for determination of dry matter production.

The physiological measurements were made on day 7 after the different water treatments were applied, which was 14 days after PGR application. The middle leaflets of fully expanded leaves (developed under the treatments) were used for the measurements. Leaf water potential (Ψleaf) was measured at mid-day (9.30 h) with a pressure chamber (Model 3000, Soil Moisture Equipment Corp, Santa Barbara, CA, USA). Chlorophyll contents were recorded by a chlorophyll meter (Minolta SPAD-502). The photosynthetic rates (Pn) were made during 09.00-11.00h when photosynthetically active radiation above the canopy was 1000-1100 mol m-2s-1 with a gas analyzer (CID-PS CO2 analyzer System, CID, Vancouver, WA, USA). The chlorophyll fluorescence dynamic parameters were measured after 30 min dark adaptation by FMS2 fluorescence meter (Hanstch, UK). All the measurements were made with 12 leaves for each treatment.

Four pots of each treatment were harvested individually as a replicate for biomass determination. All the physiological measurements were made with 12 leaves for each treatment. All data were subject to ANOVA test and means were compared using the appropriate Fisher’s protected LSD (P < 0.05).

Results

Table 1. Effects of water deficit and plant growth regulators on roots, nodules and total biomasses and seed yield of soybean Keng 5.

Water treatment

PGR treatment

Roots biomass (g/plant)

Nodules biomass (g/plant)

Stems + leaves biomass (g/plant)

Seed yield (g/plant)

Water-deficit (soil water potential at –0.06 MPa)

CK

1.15 c

0.02 c

6.17 a

2.27 b

6-BA

1.39 ab

0.02 c

6.54 a

2.48 ab

S3307

1.28 b

0.01 d

7.01 a

2.79 a

Br

1.46 a

0.05 a

7.18 a

2.83 a

ABA

1.47 a

0.03 b

6.61 a

2.78 a

Mean

1.35 A

0.03 A

6.70 B

2.63 B

Well-watered (soil water potential at –0.02 MPa)

CK

1.15 d

0.01 d

8.94 c

3.14 d

6-BA

1.55 c

0.04 c

9.34 c

3.35 c

S3307

1.42 c

0.02 d

9.32 c

3.71 b

Br

1.90 b

0.06 b

10.2 b

3.51 c

ABA

2.10 a

0.24 a

11.8 a

3.96 a

Mean

1.62 A

0.07 A

9.91 A

3.53 A

Values in each column followed by the lower-case letters for comparison between PGRs within same water treatment and upper-case letters for comparison between water deficit and well-watered at P<0.05.

Biomass and yield

Water deficit decreased biomass of stems and leaves, and induced significant yield loss (Table 1). This was agreeing with the others (Chandel et al., 1995). The biomasses of roots and nodules were not affected in our experiments (Table 1). The results indicated that PGR treatments increased soybean yields both under well-watered and water deficit conditions compare to the control with same water treatments, expect 6-BA under water deficit. However, the yields of PGRs treatments under water deficit were still lower than that of the well-watered control. PGR treatments increased roots and nodule biomasses under both water conditions, except nodule biomass of 6-BA under water deficit condition. ABA and Br treatments increased stem and leaf biomass under well-watered condition.

Ψleaf , Chl, Pn and FV/FM

The results indicated that water deficit significantly decreasedΨleaf, Chl, Pn, and FV/FM (Table 2). Under water stress condition, PGRs treatments significantly increased water potential and FV/FM, and improved Chl (except 6-BA) and Pn (except S3307 and Br) compared to the control. Under well-watered condition, PGR treatments did not affectΨleaf, Ch1 and FV/FM in our experiments, but increased Pn.

Table 2. Effects of water deficit and plant growth regulators on leaf water potential (Ψleaf), chlorophyll (Chl), photosynthetic rate (Pn) and PSⅡ photochemical efficiency (FV/FM) of soybean Keng 5.

Water deficit treatment

PGRs

Ψleaf leaf(-MPa)

Chl(SPAD value)

Pn(mg.CO2.dm-2.hr-1)

FV/FM (%)

Water-deficit (soil water potential at –0.06 MPa)

CK

2.24 b

42.85 c

19.07 c

69.80 c

6-BA

1.93 a

43.88 abc

20.38 b

73.37 b

S3307

1.80 a

44.95 ab

19.78 bc

76.77 a

BR

1.80 a

43.25 ab

19.80 bc

72.13 b

ABA

1.94 a

45.50 a

24.03 a

77.33 a

Mean

1.94 A

44.09 B

20.61 B

73.88 B

Well-watered (soil water potential at –0.02 MPa)

CK

0.50 ab

47.90 a

23.27 c

85.03 a

6-BA

0.53 b

48.13 a

25.29 b

85.50 a

S3307

0.43 a

48.40 a

25.05 b

85.17 a

BR

0.45 ab

48.20 a

25.78 b

84.97 a

ABA

0.50 ab

48.13 a

28.21 a

84.97 a

Mean

0.48 B

48.15 A

25.52 A

85.13 A

Values in each column followed by the lower-case letters for comparison between PGRs within same water treatment and upper-case letters for comparison between water deficit and well-watered at P<0.05.

Conclusions

PGR treatments increased soybean yields, roots and nodules biomasses both under well-watered and water deficit conditions compared with the control at the same water levels, expect 6-BA under water deficit. ABA and Br treatments increased stems and leaves biomass under well-watered conditions. Water deficit significantly decreased Ψleaf, Chl, Pn, FV/FM, resulted in significant yield loss. Under water stress condition, PGRs treatments significantly increased water potential and FV/FM, and improved Chl (expect 6-BA) and Pn (except S3307 and Br) compared to the control. Under well-watered conditions, PGR treatments did not affectΨleaf, Ch1 and FV/FM, but increased Pn. It was concluded that PGR treatments minimized the yield loss caused by water deficit.

Acknowledgement

The authors thank for support of the National High Technology Research and Development Program of China.

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