Effects of Potassium and Calcium on Root Exudates and Grain Quality During Grain Filling

ACTA AGRONOMICA SINICA Volume 37, Issue 4, April 2011 Online English edition of the Chinese language journal Cite this article as: Acta Agron Sin, 2011, 37(4): 661–669.

RESEARCH PAPER

Effects of Potassium and Calcium on Root Exudates and Grain Quality During Grain Filling LIU Li-Jun, CHANG Er-Hua, FAN Miao-Miao, WANG Zhi-Qin, YANG Jian-Chang* Key Laboratory of Crop Genetics and Physiology of Jiangsu Province, Yangzhou University, Yangzhou, 225009, China

Abstract: The purposes of this study were to understand the root activity and root exudates of rice (Oryza sativa L.) in response to different levels of potassium (K) and calcium (Ca) nutrients and the effect of K and Ca on rice grain quality. Cultivars Yangdao 6 (indica) and Yangjing 9538 (japonica) were cultured in hydroponical solutions and treated with different Espino nutrition solutions from heading to maturity, which were supplemented with complete K and Ca (standard Espino nutrition solution, control), half K (1/2K), zero K (0K), half Ca (1/2Ca), and zero Ca (0Ca) nutrition. Compared to the control, the 0K treatment significantly decreased root activity, ATPase activity, and concentrations of citric acid, Ca2+, K+, and NH4+ in the root exudates, but accelerated root senescence; and the 0Ca treatment significantly decreased concentrations of oxalic acid, Ca2+, K+, and NH4+ in the root exudates. The oxalic acid concentration in root exudates was positively correlated (P < 0.01) with chalky grain percentage, chalkiness, and gel consistency of rice grain; whereas the citric acid concentration in exudates was negatively correlated (P < 0.01) with chalky grain percentage, chalkiness, and breakdown value, but positively correlated (P < 0.05) with setback value. At early (10 d after heading) and mid grain-filling (20 d after heading) stages, the concentrations of K+ and Ca2+ in root exudates were negatively correlated (P < 0.05) with chalky kernel percentage, chalkiness, and amylose contents, and the NH4+ concentration was also negatively correlated (P < 0.05) with amylose content of grain. This result indicates that root exudates are closely associated with grain quality. K and Ca nutrition regulate root exudates and consequently affect grain quality of rice. Keywords: rice; potassium; calcium; root exudates; grain quality

Potassium (K) is one of the massive essential elements in plant growth. As the activator of many enzymes in plant and the osmotic regulator of cell solute potential, K plays an important role in plant growth and metabolism [1–3]. In rice (Oryza sativa L.), increased application of K fertilizer can significantly improve grain quality and milled quality, such as increasing percentages of brown rice, milled rice, and head milled rice, reducing chalkiness, and enhancing grain protein content [4]. Calcium (Ca) is a macroelement in rice plant, which is also pivotal to regulate plant growth, metabolism, and physiological and biochemical processes [5–8]. The Ca concentration in rice plant has a close relationship to amylose content and starch viscosity characteristics of rice grains [9]. Root activity reflects oxidation ability of root, which is regarded as an indicator of senescence. Previous studies indicated that root activity was closely associated with plant

growth as its H+-ATPase activity influenced the nutrient uptake in rice plants [10–13]. The main function of plasma membrane H+-ATPase is to hydrolyze ATP and transport protons, resulting in H+ transported from cytoplasm to outside plasma membrane. Root ATPase activity is closely related to the nutrient absorption capacity [10, 11]. Root exudates are also influenced by plant nutrition status. However, the effects of K+ and Ca2+ levels on root exudates and their relationship with rice grain quality are not clear. In this study, we used an indica and a joponica cultivar to investigate the variations of root activity and major components of root exudates under different K and Ca treatments, and the correlations between grain quality indices and root exudates were also studied during grain-filling period of rice. The result could not only help to understand the regulatory mechanism of K and Ca on rice growth and development but also provide a guide for rice

Received: 26 September 2010; Accepted: 6 January 2011. * Corresponding author. E-mail: [email protected] Copyright © 2011, Crop Science Society of China and Institute of Crop Sciences, Chinese Academy of Agricultural Sciences. Published by Elsevier BV. All rights reserved. Chinese edition available online at http://www.chinacrops.org/zwxb/ DOI: 10.1016/S1875-2780(11)60018-7

LIU Li-Jun et al. / Acta Agronomica Sinica, 2011, 37(4): 661–669

fertilization aiming at high yield and elite quality.

1

Materials and methods

1.1

Cultivars and treatments

Rice cultivars Yangdao 6 (indica) and Yangjing 9538 (japonica) were grown in hydroponic pools of the Key Laboratory of Crop Genetics and Physiology of Yangzhou University, Yangzhou, Jiangsu Province, China in 2006 and 2007. Seeds were sown on seedbed on 10 May. On 4 June, the seedlings 25-day-old, which were fixed with sponge panels, were transplanted to the hydroponic pools at the hill density of 15 cm × 18 cm. Cultivars were cultured in separated pools, and each cultivar had 148 hills in 4 m2 with double plants per hill. The prescription of hydroponic solution followed the Espino formulation by the International Rice Research Institute (Los Baños, Philippines). Seedlings at initial heading were divided into 5 groups to be treated with different solutions: standard Espino nutrition solution with complete K and Ca dosages (control), half dosage of K in the standard Espino nutrition solution (1/2K), no K in the standard Espino nutrition solution (0K), half dosage of Ca in the standard Espino nutrition solution (1/2Ca), and no Ca in the standard Espino nutrition solution (0Ca). The hydroponic solution was maintained at pH 5.0 modulated every day with H2SO4 (1 mol L1) or NaOH (1 mol L1). The hydroponic solutions were refreshed once a week. All treatments had 2 replicates. 1.2 Collection of root exudates and concentrations of exudates components Five hills of plants were sampled 10, 20, and 30 d after heading and placed into, separately, sealed beakers filled with deionized water after surface cleaning by tap water and distilled water. The plants were then incubated for 4 h at light intensity of 700–800 ȝmol mí2 sí1 and canopy temperature of 28–30 °C. Solutions in the beakers were collected to analyze the root exudates of rice. Roots of samples were oven-dried and weighed. Concentrations of organic acids, NH4+, NO3, and PO43 were determined using the DIONEX DX-500 Ion Chromatography System (Dionex Corporation, San Francisco, CA, USA). The concentrations of K+, Ca2+, and Na+ were measured using the Varica SpectrAA-20 Atomic Absorption Spectrometer (South East Chemicals & Instruments Ltd., Hongkong, China). The concentrations of citric acids or ions were designed as the amounts of organic acids or ions produced in 4 h per capita dry root weight. The standard organic acids were bought from Sigma (St Louis, MO, USA). 1.3 Determination of root activity and plasma membrane ATPase activity Plants from 5 hills were sampled 10, 20, and 30 d after

heading and cleaned with tap water. Part of the roots from each sample was used to determine the root activity with the alpha-naphthylamine (Į-NA) oxidation method [14]. The remaining roots were used for measuring enzyme activities. The root samples were ground in 3 mL of extraction buffer, which contained 0.1 mol L1 Tris-Maleate buffer (pH 6.5) and 1% PVP (W/V). The mortar was rinsed twice with 2 mL of extraction buffer. After centrifuge for 10 min under 50,000× g, the supernatant was used for determination of ATPase activity as described in Laboratory Guide for Modern Plant Physiology [15]. The enzyme activity was expressed in ȝmol Pi mg1 protein h1. The experiments were repeated thrice for each treatment. 1.4

Determination of Q enzyme activity in grain

From the fifth day after flowering, 25 spikelets were sampled from the primary branch in the central part of a panicle. The activity of Q enzyme in developing grains was determined according to the method described by Zhao et al. [16]. The enzyme activity unit was expressed as absorbed spectrum increased per grain per minute. Each treatment had 2 replicates and the average was adopted as an observation. 1.5

Evaluation of grain quality

After harvest, rice grains were air-dried and deposited for 3 month at room temperature. The amylose content of milled rice was measured using Infratec 1241 Grain Analyzer (FOSS Tecator, Denmark). Other quality parameters were determined according to the National Standard for Good Quality of Rice Grains (GB/T17891-1999) [17]. The Rapid Viscosity Analyzer (RVA) profiles of starch were measured previously [18]. 1.6

Statistical analysis

Analyses of variance and correlation were performed in Statistical Analysis System (SAS 6.12) package, and means of treatments or cultivars were compared at P < 0.05 using least significant difference (LSD) method. The figures were drawn using Sigmaplot 10.0 (http://www.sigmaplot.com/). We obtained similar results in both years, and this article was based on the data from 2007.

2 2.1

Results Effects of K and Ca on grain quality

2.1.1 Appearance quality The effects of K and Ca on grain appearance quality were significant in both cultivars (Table 1). The chalky grain percentage and chalkiness degree were significantly increased in 1/2K, 1/2Ca, 0K, and 0Ca treatments compared to those of the control. This indicated that the levels of K and Ca during grain filling could regulate appearance quality of rice grain.

LIU Li-Jun et al. / Acta Agronomica Sinica, 2011, 37(4): 661–669

Table 1 Appearance quality of rice grains in different K and Ca treatments (%) Treatment

Chalky grain

Chalk size

Chalkiness

Yangdao 6 Control

13.5 c

23.8 b

3.2 c

1/2K

21.5 b

21.4 c

4.6 ab

0K

27.0 a

20.1 c

5.4 a

1/2Ca

17.5 bc

24.9 ab

4.4 b

0Ca

20.0 b

26.9 a

5.4 a

Yangjing 9538 Control

64.5 c

20.9 ab

13.5 b

1/2K

71.0 b

21.0 ab

14.9 ab 15.5 a

0K

77.0 a

20.1 b

1/2Ca

73.0 ab

21.7 ab

15.9 a

0Ca

71.5 b

22.9 a

16.4 a

Control: Rice plants were cultured in standard Espino nutrient solution at grain-filling stage; 1/2K and 0K: Rice plants were cultured in Espino nutrient solutions with half and zero K nutrient, respectively; 1/2Ca and 0Ca: Rice plants were cultured in Espino nutrient solutions with half and zero Ca nutrient, respectively. In each cultivar, means followed by different letters are significantly different at P < 0.05 according to LSD test.

2.1.2 Cooking quality and nutritional quality There were similar effects of K and Ca in both cultivars (Table 2). In 0K and 0Ca treatments, the amylose content and breakdown were increased, whereas the gel consistency and setback were significantly decreased compared to the controls. The K and Ca levels had no significant effects on protein content of grain.

2.2 Effects of K and Ca on root activity and root exudates components 2.2.1 Root oxidation ability and ATPase activity In both cultivars, the root oxidation abilities were decreased significantly with the decline of K nutrient. In contrast, Ca had slight effect on root oxidation during grain filling except for the 0Ca treatment (Fig. 1). Root ATPase activities of both cultivars showed consistent variations with the root oxidation abilities in response to K and Ca deficiencies (Fig. 2). These results indicated that K deficiency could accelerate root senescence and decrease root ATPase activities rather than Ca deficiency. 2.2.2 Concentrations of organic acids and ions in root exudates The concentrations of organic acids in rice root exudates decreased with plant growth, and K and Ca had significant effects on the concentrations of oxalic acid and citric acid (Table 3). Compared to the control, the oxalic acid concentration of the root exudates significantly reduced in 1/2Ca and 0Ca treatments; and the citric acid concentration was significantly decreased in 1/2K and 0K treatments. For other organic acids in root exudates, K and Ca had no significant effects on their concentrations during grain filling. Under different levels of K and Ca, the concentrations of metal ions in root exudates varied with the process of grain filling. Compared to the controls, the concentration of K+ or Ca2+ decreased significantly in 0K and 0Ca treatments, whereas the concentration of Na+ was less influenced. The 2 cultivars had consistent trends in ion concentration variations (Fig. 3).

Table 2 Cooking and nutrient qualities of rice under different K and Ca levels during grain-filling stage Treatment

Protein content (%)

Amylose content (%)

Gel consistency (mm)

Breakdown (cP)

Setback (cP)

Yangdao 6 Control

12.4 a

13.1 b

68.8 a

621.5 d

251.8 a

1/2K

12.4 a

14.4 a

66.0 ab

757.3 ab

179.3 ab 136.5 b

0K

12.3 a

14.9 a

64.8 b

762.5 a

1/2Ca

12.9 a

14.0 ab

63.2 b

699.3 c

199.3 ab

0Ca

13.2 a

14.4 a

63.6 b

717.8 bc

139.8 b

174.0 a

Yangjing 9538 Control

10.6 a

13.5 c

84.5 a

716.3 b

1/2K

11.0 a

14.8 ab

76.5 c

737.5 ab

0K

10.3 a

15.1 a

75.5 c

806.0 a

65.5 c

1/2Ca

10.3 a

14.3 b

81.0 b

737.0 ab

139.5 ab

0Ca

10.5 a

15.2 a

79.5 b

785.3 ab

102.0 bc

96.0 bc

Control: Rice plants were cultured in standard Espino nutrient solution at grain-filling stage; 1/2K and 0K: Rice plants were cultured in Espino nutrient solutions with half and zero K nutrient, respectively; 1/2Ca and 0Ca: Rice plants were cultured in Espino nutrient solutions with half and zero Ca nutrient, respectively. In each cultivar, means followed by different letters are significantly different at P < 0.05 according to LSD test.

LIU Li-Jun et al. / Acta Agronomica Sinica, 2011, 37(4): 661–669

Control

Fig. 1

Root oxidation activity of rice in different K and Ca treatments during grain filling

Control

Fig. 2 Activity of plasma membrane ATPase of rice root in different K and Ca treatments during grain filling

Table 3 Concentrations of organic acid in root exudates of rice under different K and Ca levels during grain filling (ȝg gí1 DW) Process of grain filling

Oxalic acid

Citric acid

Tartaric acid

Malic acid

Acetic acid

Maleic acid

Succinic acid

Control 1/2K 0K 1/2Ca 0Ca

101.4 a 100.6 a 103.1 a 77.3 b 70.9 c

261.6 a 180.1 b 150.5 c 243.5 a 243.5 a

834.5 829.9 818.6 837.0 815.8

505.4 528.1 503.5 510.4 518.5

922.1 935.5 908.6 942.1 938.9

11.9 11.0 11.7 12.8 12.6

128.5 130.4 129.0 132.5 124.6

20 d after heading

Control 1/2K 0K 1/2Ca 0Ca

85.3 a 91.5 a 87.1 a 63.4 b 49.1 c

197.5 a 148.4 b 129.4 c 193.4 a 195.9 a

716.5 702.7 706.1 718.1 712.3

400.9 383.8 400.0 389.0 394.9

823.9 806.0 807.8 793.9 809.9

7.6 7.3 8.6 7.4 8.1

94.5 93.5 93.6 90.2 96.5

30 d after heading

Control 1/2K 0K 1/2Ca 0Ca

34.6 a 34.9 a 33.2 ab 30.6 b 31.5 b

134.4 a 123.4 b 125.1 ab 134.5 a 129.1 ab

657.5 601.1 586.8 606.4 647.4

281.5 224.3 214.3 265.0 281.5

776.0 752.7 744.2 771.5 765.2

5.8 6.3 5.5 5.3 6.1

54.7 54.7 52.9 52.9 50.6

Yangdao 6 10 d after heading

Treatment

Yangjing 9538 10 d after heading

Control

555.1 a

132.5 a

813.6

501.8

232.2

18.7

82.5

1/2K

539.7 a

101.0 b

833.4

508.9

216.0

19.7

86.6 83.8

0K

529.8 ab

91.5 b

829.5

512.0

218.4

17.7

1/2Ca

456.9 b

127.5 a

829.9

516.2

217.8

18.5

86.7

0Ca

372.4 c

122.5 a

811.4

528.8

225.6

17.7

84.5

(To be continued)

LIU Li-Jun et al. / Acta Agronomica Sinica, 2011, 37(4): 661–669

Table 3 (Continued) Process of grain filling 20 d after heading

30 d after heading

Treatment

Tartaric acid

Malic acid

Acetic acid

Maleic acid

Control

Oxalic acid 482.5 a

Citric acid 93.5 a

715.9

382.5

181.3

16.7

Succinic acid 62.9

1/2K

489.0 a

69.3 b

711.9

384.5

177.1

16.0

64.4

0K

479.6 a

51.7 c

722.3

397.5

173.4

15.0

60.7

1/2Ca

396.1 b

89.4 a

720.2

379.7

174.0

15.7

60.8

0Ca

333.0 c

91.9 a

714.0

394.6

166.8

17.0

64.7

Control

252.0 a

45.8 a

525.8

309.5

147.4

12.1

50.5

1/2K

232.5 a

47.4 a

494.8

305.5

137.0

13.0

48.5

0K

230.5 a

49.6 a

479.9

301.5

143.0

12.7

52.8

1/2Ca

262.5 a

45.5 a

505.4

295.6

131.0

10.6

50.6

0Ca

250.9 a

44.3 a

491.6

288.0

141.5

11.8

50.2

Control: Rice plants were cultured in standard Espino nutrient solution at grain-filling stage; 1/2K and 0K: Rice plants were cultured in Espino nutrient solutions with half and zero K nutrient, respectively; 1/2Ca and 0Ca: Rice plants were cultured in Espino nutrient solutions with half and zero Ca nutrient, respectively. Data are the means of 2 biological replicates. Concentrations of oxalic and citric acids have significant differences among treatments, and the variations in other acid concentrations are not significant at P <0.05.

Concentration of K+ (ȝmol gí1 DW)

Yangdao 6

Yangjing 9538

60

60

50

50

40

40

30

30

20

20

10

10

0

0

Concentration of Na+ (ȝmol gí1 DW)

10

20

30

10

160

160

120

120

80

80

40

40

0

Concentration of Ca2+ (ȝmol gí1 DW)

30

30

20

30

20

30

0 10

20

30

30

20

20

10

10

0

10

0 10

20

30

10

Days after heading (d) Control

Fig. 3

20

Days after heading (d) 1/2K

0K

1/2Ca

0Ca

Concentrations of metal ions in root exudates in different K and Ca treatments during grain filling of rice

LIU Li-Jun et al. / Acta Agronomica Sinica, 2011, 37(4): 661–669

The concentrations of NH4+ and PO43í in root exudates decreased significantly with the process of grain filling, but NO3 concentration had small changes in the observed period. At early and mid grain-filling stages (10 d and 20 d after heading), K and Ca had significant effects on NH4+ concentration in root exudates. Compared to the control, NH4+ concentration was significantly decreased in 1/2K, 1/2Ca, 0K, or 0Ca treatment. In addition, NO3 concentration was also reduced significantly in the 0Ca treatment. The concentrations of NH4+ and NO3 at late grain-filling stage (30 d after heading) and PO43 concentrations during the whole filling period had no significant changes in different K and Ca treatments (Fig. 4). Clearly, K and Ca nutrition levels at grain-filling stage had temporal effects on the concentrations of ions in root exudates of rice. 2.2.3 Q enzyme activity in rice grains Compared to the controls, Q enzyme activities in grains of both cultivars were declined significantly in 0Ca or 0K treatment at early-, mid-,

and late-filling stages. The levels of K and Ca had great influences on Q enzyme activity during grain filling of rice (Fig. 5). 2.3

Correlation between root exudates and grain quality

The concentrations of organic acids and ions in root exudates were highly correlated with grain quality parameters during grain filling. For instance, oxalic acid concentration was positively correlated with chalky grain percentage, chalkiness, and gel consistency of rice grain, and the correlation coefficients were as high as 0.93–0.98; citric acid concentration was negatively correlated with chalky grain percentage, chalkiness, and breakdown value and positively correlated with setback value. The concentrations of K+ and Ca2+ in root exudates at early or mid filling stage were also negatively correlated with chalky grain percentage, chalkiness, and amylose content. The concentration of NH4+ was negatively correlated with amylose content (Table 4).

(ȝmol gí1 DW)

Concentration of NH4+

Yangdao 6

Yangjing 9538

5

5

4

4

3

3

2

2

1

1

0

0

(ȝmol gí1 DW)

Concentration of NO3í

10

20

30

10

40

40

30

30

20

20

10

10

0

(ȝmol g DW)

20

30

25

20

20

15

15

10

10

5

5

0

10

20

30

20

30

0 10

20

30

10

Days after heading (d) Control

Fig. 4

30

0 10

í1

Concentration of PO4í

25

20

Days after heading (d) 1/2K

0K

1/2Ca

0Ca

Concentrations of NH4+, NO3, and PO43 in root exudates of rice in different K and Ca treatments during grain filling

LIU Li-Jun et al. / Acta Agronomica Sinica, 2011, 37(4): 661–669

Control

Fig. 5

Q-enzyme activity in grains of rice in different K and Ca treatments during grain filling

  Table 4 Correlations of root exudates concentration with grain quality parameters Root exudates

Days after heading (d)

Oxalic acid

Citric acid

K+

Ca2+

NH4+

Chalky grain percentage

Chalkiness

Amylose content

Breakdown

Setback

Gel consistency

10

0.95**

0.93**

0.25

0.41

–0.59

0.91**

20

0.95**

0.93**

0.27

0.42

–0.60

0.90**

30

0.98**

0.97**

0.31

0.44

–0.53

0.93**

10

–0.90**

–0.87**

–0.54

–0.76**

0.80**

–0.12

20

–0.94**

–0.91**

–0.57

–0.70*

0.78**

–0.18

30

–0.99**

–0.98**

–0.46

–0.60

0.74**

–0.48

10

–0.65*

–0.78**

–0.83**

–0.59

0.34

0.43

20

–0.78**

–0.88**

–0.87**

–0.18

0.29

0.12

30

–0.45

–0.18

–0.56

–0.38

0.48

0.38

10

–0.92**

–0.89**

–0.88**

0.54

–0.29

0.20

20

–0.52

–0.35

–0.38

0.54

–0.40

0.41

30

–0.38

–0.21

–0.28

0.33

–0.24

0.40

10

–0.44

–0.42

–0.92**

0.43

–0.25

0.46

20

–0.56

–0.37

–0.87**

0.32

–0.46

0.51

30

–0.39

–0.24

–0.64*

0.13

–0.14

0.27

*P < 0.05; **P < 0.01. 

3

Discussion

3.1 Effects of K and Ca on root activity and root exudates during grain filling The H+-ATPase activity in root affects nutrient uptake of rice plants [10–13]. We found that root activity and H+-ATPase activity in root were significantly decreased under K-deficient conditions during grain filling, particularly in the 0K treatment. Therefore, absence of K nutrition may accelerate plant senescence and consequently affect nutrient absorption

of rice root. Unlike K stress, absence of Ca nutrition has less impact on root activity and H+-ATPase activity, which is probable because K+ and ATPase participate in transmembrane transport of ion pump. The levels of K and Ca nutrition during grain filling were closely related to root exudates compositions. The concentrations of citric acid and oxalic acid in root exudates were significantly decreased under K-deficient and Ca-deficient conditions, respectively. In addition, the concentrations of K+, Ca2+, and NH4+ in root exudates were significantly decreased under no K or Ca condition. The

LIU Li-Jun et al. / Acta Agronomica Sinica, 2011, 37(4): 661–669

changes of root exudates compositions indicate the status of plant metabolism and development. When exposed to nutritional stresses, plants could acclimate to the environment through adjusting the components of root exudates in rhizosphere [19–21]. Therefore, we inferred that the responses of root exudates to K and Ca levels might be the expression of environmental adaptation in rice plant. However, this requires validation, and the mechanism of root exudates variation should be further studied.

4

Conclusions

The concentrations of organic acids (oxalic acid and citric acid) and ions (K+, Ca2+, and NH4+) in root exudates were closely related to grain quality in rice. K+ and Ca2+ regulate the concentrations of organic acids and ions in root exudates, and further impact grain quality. Under K deficient stress at grain filling stage, K fertilizer supplement can improve rice appearance quality and cooking quality.

3.2 Effects of K and Ca levels at grain filling stage on grain quality

Acknowledgments

Some researches pointed out that the K content in rice grain, especially the content ratio of K to Mg, might be positively correlated with chalkiness and amylose content [22–24]. However, others believed that K nutrition could improve appearance quality and processing quality of rice [25–27], and the responses of appearance quality and processing quality to K nutrition varied in different rice cultivars [28]. These controversial conclusions might be related to soil K supply ability and K fertilizer application during rice growing periods. In this study, we observed the effects of K and Ca on grain quality during grain filling. Under the conditions of insufficient K and Ca supplies, chalky grain percentage, chalkiness, amylose content, and breakdown value were increased significantly compared to the control, whereas gel consistency and setback value were decreased significantly, leading to worsening appearance quality and cooking quality. Therefore, we conclude that rice grain quality can be improved by increasing K supply under K deficiency stress at grain filling stage.

This study was supported by the National Natural Science Foundation of China (31061140457 and 30800670), the Natural Science Foundation of Jiangsu Province, China (BK2009005), and the Research Funds for Doctoral Program of Higher Education from the Ministry of Education of China (200811170002).

3.3 Correlations between grain quality and root exudates of rice

[4]

The grain quality parameters were significantly correlated with the concentrations of root exudates components, such as oxalic acid, citric acid, and several ions. This result indicates that grain quality might be regulated by the management of K and Ca supplies, which affects the compositions and concentrations of root exudates. In our previous study, we found that the organic acid concentrations and components in root exudates affected the synthesis of hormones in rice root; the formation of grain quality was under the synergic impact of both organic acids in root exudates and endogenous hormones in root [18]. Q enzyme plays a key role in the regulation of grain filling [29, 30]. In this study, we observed the significant decreases of Q enzyme activity under K and Ca deficient conditions. This result is a supplement to our earlier findings and confirms the regulation chain from K and Ca supplies to root exudates, to hormones in root and the key enzymes involved in starch synthesis, and ultimate formation of grain quality.

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