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Increase of greenhouse tomato fruit yields by plant growth-promoting rhizobacteria (PGPR) inoculated into the peat-based growing media
Soil Biol. Biochem. Vol. 25, No. 2, pp. 269-212, 1993 Printed in Great Britain. All rights reserved
0038-0717/93$6.00+ 0.00 Copyright 0 1992Pqamon Press Ltd
INCREASE OF GREENHOUSE TOMATO FRUIT YIELDS BY PLANT GROWTH-PROMOTING RHIZOBACTERIA (PGPR) INOCULATED INTO THE PEAT-BASED GROWING MEDIA SERGE GAGNB, LEILA DEHBI, DOMINIQUELE QuI%, FRANCE CAYER, JEAN-LUC MORIN, RICHARD LEMAYand NIC~LE FOURNIER Premier Research Center, Premier Peat Moss, 454 Tkniscouata, P.O. Box 2600, Riviere-du-Loup, Quebec, Canada G5R 4C9 (Accepted 5 August 1992) Summary-The
effect of some plant growth-promoting
rhizobacteria (PGPR) on greenhouse tomato fruit
yields was determined during a spring and a fall production. The bacteria were inoculated into commercial peat-based substrates. In the spring crop, the bacterial strains tested increased the marketable and Grade No. 1 fruit yields between 5.6 and 9.6% but these results were not statistically significant. None of the bacteria affected significantly the fruit size in this experiment. On the other hand, in the fall experiment where the plants were grown under suboptimal environmental conditions, Pseudomonasfluorescensstrain 63-28 increased significantly the marketable fruit yield by 13.3% (P Q 0.1) and Grade No. 1 fruit weight by 18.2% (P < 0.05). The average size of harvested fruits was also increased by 11.1% (P C 0.05) in this assay and the plants treated with strain 63-28 produced 12% of unmarketable fruits as compared to 23% for the control. These results show the potential to use PGPR in order to improve yields in greenhouse tomato crops.
INTRODUCIION The
use of plant
growth-promoting
rhizobacteria
(PGPR) to increase plant yields has been extensively studied for field crops such as potato (Burr et al., 1978; Kloepper et al., 1980), sugarbeet (Suslow and Schroth, 1982b), canola (Kloepper et al., 1988), peanut (Turner and Backman, 1991), wheat (Weller and Cook, 1986), soybean (Kloepper et al., 1991) barley (Iswandi et al., 1987b), and maize (Lalande et al., 1989). However, their effect on greenhouse vegetable crops has received little attention, especially concerning their potential to increase fruit yields. Moreover, Digat (1988) showed that peat-based horticultural substrates seemed particularly well adapted to bacterization with PGPR but this approach has not been investigated to any extent as compared to seed or field inoculation. We have assessed the effect of PGPR strains on greenhouse tomato fruit yields by introducing the bacteria into commercial peat-based substrates for tomato.
MATERIALS AND METHODS
The experiments were carried out at Riviire-duLoup, Quebec, Canada, in a greenhouse covered with a double polyethylene film. Two trial assays were done, the first one during the spring-summer period and the second during the autumn-winter period when natural lighting conditions are suboptimal for
tomato growth. No artificial light was used during both experiments. The bacterial strains tested were obtained from Esso Ag Biologicals, Saskatoon, Saskatchewan, Canada. Strains Rl7-FP2, QPS and Rl5-A4 are fluorescent pseudomonads isolated from peat bog plant roots. Strain 63-28 was isolated from a canola field and identified as Pseudomonas fluorescens (Kloepper et al., 1988). These bacteria had been shown to promote plant growth or to inhibit plant pathogens in screening tests. The inoculum was prepared by growing the bacteria on Pseudomonas agar F (Difco) 72 h at 28°C. Then, the cells were harvested in phosphate buffer (PB) (10 mM, pH 7.0), centrifuged 30 min at 4500 rev min-i and resuspended in PB. The bacterial suspensions were diluted to lo* cells ml-’ by spectrophotometer measurement (D.O. = 0.05 at 780 nm) and added to the peat-based substrates (75 ml inoculum 1-l substrate). The controls received the same amount of PB only. The bacteria were inoculated in the sowing and in the pricking out mixes 1 week before use and in the transplantation mix 2 days before the plants were transplanted. Tomato seeds (Lycopersicon esculentum Mill. cv. Vision) were sown in multicell flats (162 inverted pyramid cells of 2.5 x 3.8 cm, side length x depth) filled with Pro-Mix Germinating Mixfl”) (Premier Peat Moss Ltd, Riviere-du-Loup, Quebec). The plantlets were pricked out in 12cm dia bottomless pots containing blond sphagnum peat and perlite (70: 30, vol:vol). After the appearance of the first 269
SERGE GAGNOet al.
270
flower cluster, the plants were transplanted in AllegrooM) growbags (Premier Peat Moss Ltd). Each growbag contains 24 litres of a peat-based mix and supports three tomato plants. The plant density for both experiments was 3.5 plants mW2.The plants were grown following the common practices of Quebec greenhouse tomato growers and according to the recommendations of the Conseil des Productions Vegetales du Quebec (1984, 1990). The plants were drip-ir~gated with a nutrient solution containing (fig 1-I): N03, 180; Pop, 45; K, 285; Ca, 155; Mg, 50; Fe, 2.0; Mn, 1.0; Zn, 0.4; B, 0.3; Cu, 0.1; MO, 0.06. During harvesting periods, the K concentration was increased to 34Opg 1-t. In the first experiment, sowing was done on 5 March and harvesting from 14 June to 31 July. The fall crop was sown on 30 July and tomatoes were harvested from 9 November to 11 January. The fruits were harvested once or twice a week according to the needs and they were classified and weighed each time, Marketable fruits are those with no defect and weighing 270 g. Grade No. 1 fruits have no imperfection and weigh 2 90 g. In the second experiment, the plant height, the stem diameter, and the number of leaves were determined at transplantation in order to know if the bacteria affected plant growth at this stage. The experimental design was a randomized complete block with five replications and three plants per ex~rimental unit. Significant treatment differences were tested by analysis of variance and protected least significant difference (LSD) statistical test (Steel and Torrie, 1980). RESULTS
The bacteria produced no effect on the vegetative growth of tomato transplants as far as plant height, stem diameter and number of leaves are concerned (data not shown). During the first experiment (spring crop), environmental conditions were much more favorable for
plant growth and fruit ripening than during the second experiment (fall crop) when natural light was suboptimal and the greenhouse temperature was cooler. The fruit yields were much lower in the fall than in the spring crop. In the first experiment, none of the bacterial strains produced statistically significant effect on total as well as on marketable or Grade No. 1 fruit yields (Table 1). However, yields were higher for the plants grown with PGPR than for the controls. For example, strains Rl7-FP2, 63-28, and QPS increased marketable fruit yields by 9.4, 8.7 and 5.6% respectively. The fruit size was not significantly affected by bacteria in this experiment. In the second experiment (Table 1), P. fruorescens strain 63-28 was the most beneficial, increasing significantly the marketable and Grade No. 1 fruit yields by 13.3% (P ~0.10) and 18.2% (P
Table i. Effect of some PGPR strains on greenhouse tomato fruit yields Total harvested fruits Fruit weight/plant Strain
g
%’
Marketable fruits’
Fruit size g
%’
Fruit weight/plant g E~~~jrnenf
RI 7.FP2 63-28
6.4 6.8 3.1
Control
4790 4806 4640 450 1
162 158 156 163
RI 7-FP2 63-28 QP5 RIS-A4 Control
1908 1946 1779 1940 1862
2.5 4.5 - -4.5 4.2
101 110 94 103 99
QPS
%’
Grade No.
Fruit size g
%I
1 fruits2
Fruit weight/plant
Fruit size
g
%oJ
g
%’
-0.1 -2.8 -3.0
4458 4414 4302 4068
9.6 8.5 5.1
166 163 163 168
-0.9 -2.9 -2.8
-0.9 4.4 -6.2 2.7
1371 1508 1149 1358 1276
7.4 18.2*’ -10.0 6.4
1 (Spring crop)
-0.5 -2.5 -3.9
4627 9.4 160 4597 8.7 155 4466 5.6 155 4230 160 Experiment 2 (Autumn crop) 2.0 1723 6.2 112 11.1** 1837 13.3* 118 -5.1 I520 -6.3 106 4.0 1665 2.7 116 1622 113
Values are the mean of five replicates and each experimental unit contains three plants. *Statistically significant at P 4 0.10 using protected LSD. **Statistically significant at P d 0.05 using protected LSD. ‘Marketable: fruits 2 70 g without imperfection. *Grade No. 1: fruits 390 g without imperfection. 3Percentage change from control.
125 -0.8 131 4.0 118 -6.3 12.8 1.6 126
Increase of tomato fruit yields by PGPR
271
1
q lwFP2 n 63.23 QP5
q RI544 Cl
CONTROL
EXPERIMENT 1
EXPERIMENT 2
Fig. 1. Percentage of unmarketable fruit number harvested from tomato plants (cv. Vision) grown in peat-based substrates inoculated with PGPR strains. Experiment 1 was a spring crop while experiment 2 was a autumn crop.
plants treated with strains 63-28 and R17-FP2, which produced respectively 12 and 18% unmarketable fruits as compared to 23% for the control. Strains QP5 and R15-A4 had no effect. DISCUSSION
The introduction of PGPR in peat-based mixes for greenhouse tomato production can significantly increase the marketable fruit yield. Some PGPR strains can stimulate tomato seed emergence (Digat, 1988) as well as shoot and root growth of young tomato plants (Elad et al., 1987; van Peer and Schippers, 1989), but to our knowledge, our work is the first to show that PGPR increase marketable tomato fruit yield. Although bacterial populations were not monitored during these experiments, we have found in another assay using rifampicin-resistant mutants of strains R17-FP2 and 63-28 that the bacteria were still present on the tomato roots at the end of harvesting period at log 3 cells g-’ fresh root (unpubl. results). This indicates that the bacteria remain viable and successfully colonize the roots under normal production conditions. The mechanism by which the bacteria increased fruit production is unknown. The fact that they had no effect on shoot development at the transplantation stage suggests that the plants are likely to benefit from PGPR mainly at the moment they need more energy to ripen fruits. This benefit is possibly attributable to a better root system, whether caused by competition with other organisms that reduce plant growth or by some direct interaction with the plant.
Several studies have shown that the effect of PGPR on plant growth can be associated with the inhibition of deleterious microorganisms (Iswandi et al., 1987a; Kloepper and Schroth, 1981; Suslow and Schroth, 1982a; van Peer and Schippers, 1989; Yuen and Schroth, 1986). Deleterious pseudomonads have been shown to be present on the root surface and in the endorhizosphere of tomato plants grown on artificial substrates (van Peer et al., 1990). Inhibition of Pythium spp is also a possible explanation of the beneficial effect of bacteria since the strains tested, especially P. jluorescens 63-28 have been shown to inhibit efficiently P. ultimum in peat-based substrates artificially infested with the pathogen (unpubl. data). Although Pythium is not generally considered as a major pathogen in greenhouse tomato production, it is recognized as a problem causing yield reduction especially when the plants are subjected to stress conditions. This is in accordance with the fact that the effect of bacteria on fruit yield was more significant in the autumn crop when environmental conditions were more favorable to P. ultimum than during the spring crop. Growth promotion by PGPR may also be attributed to the production of plant growth regulating 1991; substances (Arshad and Frankenberger, Brown, 1974; Frankenberger and Poth, 1988; Lifshitz et al., 1987; Nieto and Frankenberger, 1989; Sobieszczanski et al., 1989). Frankenberger and Arshad (1991) reported that the application of L-tryptophan (a precursor of the plant hormone indole-3-acetic acid) on melon seedlings resulted in fruit yield increases but no vegetative growth re-
272
SERGEGAG&
sponse was noted before transplanting into the field. They suggested that auxin production by microorganisms on plant roots may involve the synthesis of auxin conjugates that are stored in the plants and activated upon the reproductive stage. Although many technical and biological considerations remain to be verified before commercialization, our results show the potential to use PGPR in order to improve yields in greenhouse tomato crops. Acknowledgements-This research was supported by the National Research Council of Canada and the Centre Quebecois de Valorisation de la Biomasse. REFERENCES
Arshad M. and Frankenberger W. T. Jr (1991) Microbial production of plant hormones. In The Rhizosphere and Plant Growth (D. L. Keister and P. B. Cregan, Eds), pp. 327-334. Kluwer, Dordrecht. Brown M. E. (1974) Seed and root bacterization. Annual Review of Phytopathology 12, 181-197.
Burr T. J., Schroth M. N. and Suslow T. (1978) Increased potato yields by treatment of seedpieces with specific strains of Pseudomonas Jluorescens and P. putida. Phytopathology 68, 1377-1383.
Conseil des Productions Vegetales du Quebec (1984) ragUmes de Serre, Culture. Minis&e de l’Agriculture, des Pkcheries et de 1’Alimentation du Quebec. Conseil des Productions Vtgetales du Quebec (1990) L&umes de Serre, Culture sur Film Nutritif (NFT), sur Laine de Roche et en Milieux Tourbeux. Minis&e de
I’Agriculture, des Picheries et de I’Alimentation du Quebec. Digat B. (1988) The bacterization of horticultural substrates and its effects on plant growth. Acta Horticulturae 221, 279-288.
Elad Y., Chet I. and Baker R. (1987) Increased growth response of plants induced by rhizobacteria antagonistic to soilborne pathogenic fungi. Plant and Soil 98,325-330. Frankenberger W. T. Jr and Arshad M. (1991) Yield response of watermelon and muskmelon to L-tryptophan applied to soil. HortScience 26, 35-37. Frankenberger W. T. Jr and Poth M. (1988) L-tryptophan transaminase of a bacterium isolated from the rhizosphere of Festuca octofora (graminae). Soil Biology & Biochemistry 20, 299-304.
Iswandi A., Bossier P., Vandenabeele J. and Verstraete W. (1987a) Relation between soil microbial activity and the effect of seed inoculation with the rhizopseudomonad strain 7NSK2 on plant growth. Biology and Fertility of Soils 3, 147-151.
Iswandi A., Bossier P., Vandenabeele J. and Verstraete W. (1987b) Effect of the seed inoculation with the rhizopseudomonad strain 7NSK2 on the root microbiota of maize (Zea mays) and barley (Hordeum vulgare). Biology and Fertility of Soils 3, 153-158. Kloepper J. W. and Schroth M. N. (1981) Relationship of in vitro antibiosis of plant growth-promoting rhizobacte-
et al.
ria to plant growth and the displacement of root microflora. Phytopathologv 71, 102Ck1024. Kloepper J. W., Sch&h M. N. and Miller T. D. (1980) Effects of rhizosphere colonization by plant growthpromoting rhizobacteria on potato plant development and yield. Phytopathology 70, 1078-1082. Kloepper J. W., Zablotowicz R. M., Tipping E. M. and Lifshitz R. (1991) Plant growth promotion mediated by bacterial rhizosphere colonizers. In The Rhizosphere and Plant Growth (D. L. Keister and P. B. Cregan, Eds), pp. 3 15-326. Kluwer, Dordrecht. Kloepper J. W., Hume D. J., Scher F. M., Singleton C., Tipping B., Lalibertt M., Frauley K., Kutchaw T., Simonson C., Lifshitz R., Zaleska I. and Lee L. (1988) Plant growth-promoting rhizobacteria on canola (rapeseed). Plant Disease 72, 4246. Lalande R., Bissonnette N., Coutlee D. and Antoun H. (1989) Identification of rhizobacteria from maize and determination of their plant-growth promoting potential. Plant and Soil 115, 7-l 1. Lifshitz R., Kloepper J. W., Kozlowski M., Simonson C., Carlson J., Tipping E. M. and Zaleska I. (1987) Growth promotion of canola (rapeseed) seedlings by a strain of Pseudomonas putida under gnotobiotic conditions. Canadian Journal of Microbiology 33, 390-395.
Nieto K. F. and Frankenberger W. T. Jr (1989) Biosynthesis of cytokinins by Azotobacter chroococcum. Soil Biology & Biochemistry 21, 967-972.
van Peer R. and Schippers B. (1989) Plant growth responses to bacterization with selected Pseudomonas spp strains
and rhizosphere microbial development in hydroponic cultures. Canadian Journal of Microbiology 35, 456463. van Peer R., Punte H. L. M., de Weger L. A. and Schippers B. (1990) Characterization of root surface and endorhizosphere pseudomonads in relation to their colonization of roots. Applied and Environmental Microbiology 56, 246222470.
Sobieszczanski J., Stempniewicz R. and Krzysko T. (1989) Pseudomonas sp. as producer of plant growth regulators. In Interrelationships Between Microorganisms and Plants in Soil (V. Vancura and F. Kunc, Eds), pp. 201-205. Elsevier, Amsterdam. Steel R. G. D. and Torrie J. H. (1980) Principles and Procedures of Statistics, a Biometrical Approach, 2nd Edn. McGraw-Hill, New York. Suslow T. V. and Schroth M. N. (1982a) Role of deleterious rhizobacteria as minor pathogens in reducing crop growth. Phytopathology 72, 11l-l 15. Suslow T. V. and Schroth M. N. (1982b) Rhizobacteria of sugarbeet: effects of seed application and root colonization on yield. Phytopathology 72, 199-206. Turner J. T. and Backman P. A. (1991) factors relating to peanut yield increases after seed treatment with Bacillus subtilis. Plant Disease 75, 347-353.
Weller D. M. and Cook R. J. (1986) Increased growth of wheat by seed treatments with fluorescent pseudomonads, and implications of Pythium control. Canadian Journal of Plant Pathology 8, 328-334.
Yuen G. Y. and Schroth M. N. (1986) Interactions of Pseudomonasfluorescens strain E6 with ornamental plants and its effect on the composition of root-colonizing microflora. Phytopathology 76, 176180.
0038-0717/93$6.00+ 0.00 Copyright 0 1992Pqamon Press Ltd
INCREASE OF GREENHOUSE TOMATO FRUIT YIELDS BY PLANT GROWTH-PROMOTING RHIZOBACTERIA (PGPR) INOCULATED INTO THE PEAT-BASED GROWING MEDIA SERGE GAGNB, LEILA DEHBI, DOMINIQUELE QuI%, FRANCE CAYER, JEAN-LUC MORIN, RICHARD LEMAYand NIC~LE FOURNIER Premier Research Center, Premier Peat Moss, 454 Tkniscouata, P.O. Box 2600, Riviere-du-Loup, Quebec, Canada G5R 4C9 (Accepted 5 August 1992) Summary-The
effect of some plant growth-promoting
rhizobacteria (PGPR) on greenhouse tomato fruit
yields was determined during a spring and a fall production. The bacteria were inoculated into commercial peat-based substrates. In the spring crop, the bacterial strains tested increased the marketable and Grade No. 1 fruit yields between 5.6 and 9.6% but these results were not statistically significant. None of the bacteria affected significantly the fruit size in this experiment. On the other hand, in the fall experiment where the plants were grown under suboptimal environmental conditions, Pseudomonasfluorescensstrain 63-28 increased significantly the marketable fruit yield by 13.3% (P Q 0.1) and Grade No. 1 fruit weight by 18.2% (P < 0.05). The average size of harvested fruits was also increased by 11.1% (P C 0.05) in this assay and the plants treated with strain 63-28 produced 12% of unmarketable fruits as compared to 23% for the control. These results show the potential to use PGPR in order to improve yields in greenhouse tomato crops.
INTRODUCIION The
use of plant
growth-promoting
rhizobacteria
(PGPR) to increase plant yields has been extensively studied for field crops such as potato (Burr et al., 1978; Kloepper et al., 1980), sugarbeet (Suslow and Schroth, 1982b), canola (Kloepper et al., 1988), peanut (Turner and Backman, 1991), wheat (Weller and Cook, 1986), soybean (Kloepper et al., 1991) barley (Iswandi et al., 1987b), and maize (Lalande et al., 1989). However, their effect on greenhouse vegetable crops has received little attention, especially concerning their potential to increase fruit yields. Moreover, Digat (1988) showed that peat-based horticultural substrates seemed particularly well adapted to bacterization with PGPR but this approach has not been investigated to any extent as compared to seed or field inoculation. We have assessed the effect of PGPR strains on greenhouse tomato fruit yields by introducing the bacteria into commercial peat-based substrates for tomato.
MATERIALS AND METHODS
The experiments were carried out at Riviire-duLoup, Quebec, Canada, in a greenhouse covered with a double polyethylene film. Two trial assays were done, the first one during the spring-summer period and the second during the autumn-winter period when natural lighting conditions are suboptimal for
tomato growth. No artificial light was used during both experiments. The bacterial strains tested were obtained from Esso Ag Biologicals, Saskatoon, Saskatchewan, Canada. Strains Rl7-FP2, QPS and Rl5-A4 are fluorescent pseudomonads isolated from peat bog plant roots. Strain 63-28 was isolated from a canola field and identified as Pseudomonas fluorescens (Kloepper et al., 1988). These bacteria had been shown to promote plant growth or to inhibit plant pathogens in screening tests. The inoculum was prepared by growing the bacteria on Pseudomonas agar F (Difco) 72 h at 28°C. Then, the cells were harvested in phosphate buffer (PB) (10 mM, pH 7.0), centrifuged 30 min at 4500 rev min-i and resuspended in PB. The bacterial suspensions were diluted to lo* cells ml-’ by spectrophotometer measurement (D.O. = 0.05 at 780 nm) and added to the peat-based substrates (75 ml inoculum 1-l substrate). The controls received the same amount of PB only. The bacteria were inoculated in the sowing and in the pricking out mixes 1 week before use and in the transplantation mix 2 days before the plants were transplanted. Tomato seeds (Lycopersicon esculentum Mill. cv. Vision) were sown in multicell flats (162 inverted pyramid cells of 2.5 x 3.8 cm, side length x depth) filled with Pro-Mix Germinating Mixfl”) (Premier Peat Moss Ltd, Riviere-du-Loup, Quebec). The plantlets were pricked out in 12cm dia bottomless pots containing blond sphagnum peat and perlite (70: 30, vol:vol). After the appearance of the first 269
SERGE GAGNOet al.
270
flower cluster, the plants were transplanted in AllegrooM) growbags (Premier Peat Moss Ltd). Each growbag contains 24 litres of a peat-based mix and supports three tomato plants. The plant density for both experiments was 3.5 plants mW2.The plants were grown following the common practices of Quebec greenhouse tomato growers and according to the recommendations of the Conseil des Productions Vegetales du Quebec (1984, 1990). The plants were drip-ir~gated with a nutrient solution containing (fig 1-I): N03, 180; Pop, 45; K, 285; Ca, 155; Mg, 50; Fe, 2.0; Mn, 1.0; Zn, 0.4; B, 0.3; Cu, 0.1; MO, 0.06. During harvesting periods, the K concentration was increased to 34Opg 1-t. In the first experiment, sowing was done on 5 March and harvesting from 14 June to 31 July. The fall crop was sown on 30 July and tomatoes were harvested from 9 November to 11 January. The fruits were harvested once or twice a week according to the needs and they were classified and weighed each time, Marketable fruits are those with no defect and weighing 270 g. Grade No. 1 fruits have no imperfection and weigh 2 90 g. In the second experiment, the plant height, the stem diameter, and the number of leaves were determined at transplantation in order to know if the bacteria affected plant growth at this stage. The experimental design was a randomized complete block with five replications and three plants per ex~rimental unit. Significant treatment differences were tested by analysis of variance and protected least significant difference (LSD) statistical test (Steel and Torrie, 1980). RESULTS
The bacteria produced no effect on the vegetative growth of tomato transplants as far as plant height, stem diameter and number of leaves are concerned (data not shown). During the first experiment (spring crop), environmental conditions were much more favorable for
plant growth and fruit ripening than during the second experiment (fall crop) when natural light was suboptimal and the greenhouse temperature was cooler. The fruit yields were much lower in the fall than in the spring crop. In the first experiment, none of the bacterial strains produced statistically significant effect on total as well as on marketable or Grade No. 1 fruit yields (Table 1). However, yields were higher for the plants grown with PGPR than for the controls. For example, strains Rl7-FP2, 63-28, and QPS increased marketable fruit yields by 9.4, 8.7 and 5.6% respectively. The fruit size was not significantly affected by bacteria in this experiment. In the second experiment (Table 1), P. fruorescens strain 63-28 was the most beneficial, increasing significantly the marketable and Grade No. 1 fruit yields by 13.3% (P ~0.10) and 18.2% (P
Table i. Effect of some PGPR strains on greenhouse tomato fruit yields Total harvested fruits Fruit weight/plant Strain
g
%’
Marketable fruits’
Fruit size g
%’
Fruit weight/plant g E~~~jrnenf
RI 7.FP2 63-28
6.4 6.8 3.1
Control
4790 4806 4640 450 1
162 158 156 163
RI 7-FP2 63-28 QP5 RIS-A4 Control
1908 1946 1779 1940 1862
2.5 4.5 - -4.5 4.2
101 110 94 103 99
QPS
%’
Grade No.
Fruit size g
%I
1 fruits2
Fruit weight/plant
Fruit size
g
%oJ
g
%’
-0.1 -2.8 -3.0
4458 4414 4302 4068
9.6 8.5 5.1
166 163 163 168
-0.9 -2.9 -2.8
-0.9 4.4 -6.2 2.7
1371 1508 1149 1358 1276
7.4 18.2*’ -10.0 6.4
1 (Spring crop)
-0.5 -2.5 -3.9
4627 9.4 160 4597 8.7 155 4466 5.6 155 4230 160 Experiment 2 (Autumn crop) 2.0 1723 6.2 112 11.1** 1837 13.3* 118 -5.1 I520 -6.3 106 4.0 1665 2.7 116 1622 113
Values are the mean of five replicates and each experimental unit contains three plants. *Statistically significant at P 4 0.10 using protected LSD. **Statistically significant at P d 0.05 using protected LSD. ‘Marketable: fruits 2 70 g without imperfection. *Grade No. 1: fruits 390 g without imperfection. 3Percentage change from control.
125 -0.8 131 4.0 118 -6.3 12.8 1.6 126
Increase of tomato fruit yields by PGPR
271
1
q lwFP2 n 63.23 QP5
q RI544 Cl
CONTROL
EXPERIMENT 1
EXPERIMENT 2
Fig. 1. Percentage of unmarketable fruit number harvested from tomato plants (cv. Vision) grown in peat-based substrates inoculated with PGPR strains. Experiment 1 was a spring crop while experiment 2 was a autumn crop.
plants treated with strains 63-28 and R17-FP2, which produced respectively 12 and 18% unmarketable fruits as compared to 23% for the control. Strains QP5 and R15-A4 had no effect. DISCUSSION
The introduction of PGPR in peat-based mixes for greenhouse tomato production can significantly increase the marketable fruit yield. Some PGPR strains can stimulate tomato seed emergence (Digat, 1988) as well as shoot and root growth of young tomato plants (Elad et al., 1987; van Peer and Schippers, 1989), but to our knowledge, our work is the first to show that PGPR increase marketable tomato fruit yield. Although bacterial populations were not monitored during these experiments, we have found in another assay using rifampicin-resistant mutants of strains R17-FP2 and 63-28 that the bacteria were still present on the tomato roots at the end of harvesting period at log 3 cells g-’ fresh root (unpubl. results). This indicates that the bacteria remain viable and successfully colonize the roots under normal production conditions. The mechanism by which the bacteria increased fruit production is unknown. The fact that they had no effect on shoot development at the transplantation stage suggests that the plants are likely to benefit from PGPR mainly at the moment they need more energy to ripen fruits. This benefit is possibly attributable to a better root system, whether caused by competition with other organisms that reduce plant growth or by some direct interaction with the plant.
Several studies have shown that the effect of PGPR on plant growth can be associated with the inhibition of deleterious microorganisms (Iswandi et al., 1987a; Kloepper and Schroth, 1981; Suslow and Schroth, 1982a; van Peer and Schippers, 1989; Yuen and Schroth, 1986). Deleterious pseudomonads have been shown to be present on the root surface and in the endorhizosphere of tomato plants grown on artificial substrates (van Peer et al., 1990). Inhibition of Pythium spp is also a possible explanation of the beneficial effect of bacteria since the strains tested, especially P. jluorescens 63-28 have been shown to inhibit efficiently P. ultimum in peat-based substrates artificially infested with the pathogen (unpubl. data). Although Pythium is not generally considered as a major pathogen in greenhouse tomato production, it is recognized as a problem causing yield reduction especially when the plants are subjected to stress conditions. This is in accordance with the fact that the effect of bacteria on fruit yield was more significant in the autumn crop when environmental conditions were more favorable to P. ultimum than during the spring crop. Growth promotion by PGPR may also be attributed to the production of plant growth regulating 1991; substances (Arshad and Frankenberger, Brown, 1974; Frankenberger and Poth, 1988; Lifshitz et al., 1987; Nieto and Frankenberger, 1989; Sobieszczanski et al., 1989). Frankenberger and Arshad (1991) reported that the application of L-tryptophan (a precursor of the plant hormone indole-3-acetic acid) on melon seedlings resulted in fruit yield increases but no vegetative growth re-
272
SERGEGAG&
sponse was noted before transplanting into the field. They suggested that auxin production by microorganisms on plant roots may involve the synthesis of auxin conjugates that are stored in the plants and activated upon the reproductive stage. Although many technical and biological considerations remain to be verified before commercialization, our results show the potential to use PGPR in order to improve yields in greenhouse tomato crops. Acknowledgements-This research was supported by the National Research Council of Canada and the Centre Quebecois de Valorisation de la Biomasse. REFERENCES
Arshad M. and Frankenberger W. T. Jr (1991) Microbial production of plant hormones. In The Rhizosphere and Plant Growth (D. L. Keister and P. B. Cregan, Eds), pp. 327-334. Kluwer, Dordrecht. Brown M. E. (1974) Seed and root bacterization. Annual Review of Phytopathology 12, 181-197.
Burr T. J., Schroth M. N. and Suslow T. (1978) Increased potato yields by treatment of seedpieces with specific strains of Pseudomonas Jluorescens and P. putida. Phytopathology 68, 1377-1383.
Conseil des Productions Vegetales du Quebec (1984) ragUmes de Serre, Culture. Minis&e de l’Agriculture, des Pkcheries et de 1’Alimentation du Quebec. Conseil des Productions Vtgetales du Quebec (1990) L&umes de Serre, Culture sur Film Nutritif (NFT), sur Laine de Roche et en Milieux Tourbeux. Minis&e de
I’Agriculture, des Picheries et de I’Alimentation du Quebec. Digat B. (1988) The bacterization of horticultural substrates and its effects on plant growth. Acta Horticulturae 221, 279-288.
Elad Y., Chet I. and Baker R. (1987) Increased growth response of plants induced by rhizobacteria antagonistic to soilborne pathogenic fungi. Plant and Soil 98,325-330. Frankenberger W. T. Jr and Arshad M. (1991) Yield response of watermelon and muskmelon to L-tryptophan applied to soil. HortScience 26, 35-37. Frankenberger W. T. Jr and Poth M. (1988) L-tryptophan transaminase of a bacterium isolated from the rhizosphere of Festuca octofora (graminae). Soil Biology & Biochemistry 20, 299-304.
Iswandi A., Bossier P., Vandenabeele J. and Verstraete W. (1987a) Relation between soil microbial activity and the effect of seed inoculation with the rhizopseudomonad strain 7NSK2 on plant growth. Biology and Fertility of Soils 3, 147-151.
Iswandi A., Bossier P., Vandenabeele J. and Verstraete W. (1987b) Effect of the seed inoculation with the rhizopseudomonad strain 7NSK2 on the root microbiota of maize (Zea mays) and barley (Hordeum vulgare). Biology and Fertility of Soils 3, 153-158. Kloepper J. W. and Schroth M. N. (1981) Relationship of in vitro antibiosis of plant growth-promoting rhizobacte-
et al.
ria to plant growth and the displacement of root microflora. Phytopathologv 71, 102Ck1024. Kloepper J. W., Sch&h M. N. and Miller T. D. (1980) Effects of rhizosphere colonization by plant growthpromoting rhizobacteria on potato plant development and yield. Phytopathology 70, 1078-1082. Kloepper J. W., Zablotowicz R. M., Tipping E. M. and Lifshitz R. (1991) Plant growth promotion mediated by bacterial rhizosphere colonizers. In The Rhizosphere and Plant Growth (D. L. Keister and P. B. Cregan, Eds), pp. 3 15-326. Kluwer, Dordrecht. Kloepper J. W., Hume D. J., Scher F. M., Singleton C., Tipping B., Lalibertt M., Frauley K., Kutchaw T., Simonson C., Lifshitz R., Zaleska I. and Lee L. (1988) Plant growth-promoting rhizobacteria on canola (rapeseed). Plant Disease 72, 4246. Lalande R., Bissonnette N., Coutlee D. and Antoun H. (1989) Identification of rhizobacteria from maize and determination of their plant-growth promoting potential. Plant and Soil 115, 7-l 1. Lifshitz R., Kloepper J. W., Kozlowski M., Simonson C., Carlson J., Tipping E. M. and Zaleska I. (1987) Growth promotion of canola (rapeseed) seedlings by a strain of Pseudomonas putida under gnotobiotic conditions. Canadian Journal of Microbiology 33, 390-395.
Nieto K. F. and Frankenberger W. T. Jr (1989) Biosynthesis of cytokinins by Azotobacter chroococcum. Soil Biology & Biochemistry 21, 967-972.
van Peer R. and Schippers B. (1989) Plant growth responses to bacterization with selected Pseudomonas spp strains
and rhizosphere microbial development in hydroponic cultures. Canadian Journal of Microbiology 35, 456463. van Peer R., Punte H. L. M., de Weger L. A. and Schippers B. (1990) Characterization of root surface and endorhizosphere pseudomonads in relation to their colonization of roots. Applied and Environmental Microbiology 56, 246222470.
Sobieszczanski J., Stempniewicz R. and Krzysko T. (1989) Pseudomonas sp. as producer of plant growth regulators. In Interrelationships Between Microorganisms and Plants in Soil (V. Vancura and F. Kunc, Eds), pp. 201-205. Elsevier, Amsterdam. Steel R. G. D. and Torrie J. H. (1980) Principles and Procedures of Statistics, a Biometrical Approach, 2nd Edn. McGraw-Hill, New York. Suslow T. V. and Schroth M. N. (1982a) Role of deleterious rhizobacteria as minor pathogens in reducing crop growth. Phytopathology 72, 11l-l 15. Suslow T. V. and Schroth M. N. (1982b) Rhizobacteria of sugarbeet: effects of seed application and root colonization on yield. Phytopathology 72, 199-206. Turner J. T. and Backman P. A. (1991) factors relating to peanut yield increases after seed treatment with Bacillus subtilis. Plant Disease 75, 347-353.
Weller D. M. and Cook R. J. (1986) Increased growth of wheat by seed treatments with fluorescent pseudomonads, and implications of Pythium control. Canadian Journal of Plant Pathology 8, 328-334.
Yuen G. Y. and Schroth M. N. (1986) Interactions of Pseudomonasfluorescens strain E6 with ornamental plants and its effect on the composition of root-colonizing microflora. Phytopathology 76, 176180.