Scientia Horticulturae 112 (2007) 191–199 www.elsevier.com/locate/scihorti Vermicompost as a substitute for peat in potting media: Effects on germination, biomass allocation, yields and fruit quality of three tomato varieties Johann G. Zaller * Institute of Organic Agriculture, University of Bonn, Germany Received 8 March 2006; received in revised form 10 November 2006; accepted 6 December 2006 Abstract Commercial potting media often contain substantial amounts of peat that was mined from endangered bog and fen ecosystems. The main objectives of this study were to assess (1) whether the amendment of 0, 20, 40, 60, 80 and 100% (v/v) of vermicompost (VC) to a fertilized commercial peat potting substrate has effects on the emergence, growth and biomass allocation of tomato seedlings (Lycopersicon esculentum Mill.) under greenhouse conditions, (2) whether possible impacts on seedlings can affect tomato yields and fruit quality even when transplanted into equally fertilized field soil, and (3) whether effects are consistent among different tomato varieties. Amended VC was produced in a windrow system of food and cotton waste mainly by earthworms Eisenia fetida Sav. Vermicompost amendments significantly influenced, specifically for each tomato variety, emergence and elongation of seedlings. Biomass allocation (root:shoot ratio) was affected by VC amendments for two varieties in seedling stage and one field-grown tomato variety. Marketable and total yields of field tomatoes were not affected by VC amendments used for seedling husbandry. However, morphological (circumference, dry matter content, peel firmness) and chemical fruit parameters (contents of C, N, P, K, Ca, Mg, L-ascorbic acid, glucose, fructose) were significantly affected by VC amendments in seedling substrates; these effects again were specific for each tomato variety. Overall, vermicompost could be an environmentally friendly substitute for peat in potting media with similar or beneficial effects on seedling performance and fruit quality. However, at least for tomatoes, variety-specific responses should be considered when giving recommendations on the optimum proportion of vermicompost amendment to horticultural potting substrate. # 2006 Elsevier B.V. All rights reserved. Keywords: Soilless substrate; Peat moss replacement; Seedling husbandry; Earthworms; Solid organic wastes; Vermicompost 1. Introduction Sphagnum peat moss is used extensively as a soilless potting substrate in horticulture because of its desirable physical characteristics and high nutrient exchange capacity (Raviv et al., 1986). However, in recent years there has been increasing environmental and ecological concerns against the use of peat because its harvest is destroying endangered wetland ecosystems worldwide (Barkham, 1993; Buckland, 1993; Robertson, 1993). Several studies revealed that peat can be substituted by various compost types without any negative effects on a variety of crops raised in these substrates (e.g., Inbar et al., 1986; * Present address: Institute of Zoology, Department of Integrative Biology and Biodiversity Research, University of Natural Resources and Applied Life Sciences Vienna, Gregor Mendel Strasse 33, A-1180 Vienna, Germany. Tel.: +43 1 47654 3205; fax: +43 1 47654 3203. E-mail address: johann.zaller@boku.ac.at. 0304-4238/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2006.12.023 Bugbee and Frink, 1989; Beeson, 1996; Eklind et al., 2001; Hashemimajd et al., 2004). Vermicompost, in contrast to conventional compost is the product of an accelerated biooxydation of organic matter by the use of high densities of earthworm populations without passing a thermophilic stage (Domı́nguez et al., 1997; Subler et al., 1998). Research has shown that different earthworm species are able to consume a wide range of organic residues such as sewage sludge (Mitchell et al., 1980; Domı́nguez et al., 2000), animal wastes (Edwards et al., 1985; Chan and Griffiths, 1988; Wilson and Carlile, 1989; Atiyeh et al., 2000b), crop residues (Mba, 1996; Shanthi et al., 1993; Orozco et al., 1996) and industrial wastes (Albanell et al., 1988; Kaushik and Garg, 2003; Maboeta and van Rensburg, 2003). These earthworm-processed organic wastes are finely divided peat-like materials with high porosity, aeration, drainage, and water-holding capacity (Edwards and Burrows, 1988). Compared to conventional compost which passes a thermophilic stage, vermicompost usually has a much finer 192 J.G. Zaller / Scientia Horticulturae 112 (2007) 191–199 structure and larger surface area providing strong absorbability and retention of nutrients (Shi-wei and Fu-zhen, 1991). Nutrients in vermicompost are present in readily available forms for plant uptake such as nitrates, exchangeable phosphorus, potassium, calcium, and magnesium (Edwards and Burrows, 1988; Orozco et al., 1996). Vermicompost additionally also contains substances that stimulate and regulate plant growth (Krishnamoorthy and Vajranabhaiah, 1986; Tomati et al., 1988). Accordingly, vermicomposts have early been suggested to have a great potential as plant growth media (Edwards and Burrows, 1988). Several studies assessed the effect of vermicompost amendments in potting substrates on seedling emergence and growth of a wide range of marketable fruits cultivated in greenhouses (Arancon et al., 2003, 2004a; Atiyeh et al., 2000c,d), as well as on growth, yields (Mba, 1996; Karmegam et al., 1999; Atiyeh et al., 2000a; Arancon et al., 2004b, 2005). Effects of vermicompost applications on fruit quality of fieldgrown tomatoes have rarely been investigated (Premuzic et al., 1998; Zaller, 2006). Providing that all nutrients are supplied by mineral fertilization, studies show greatest plant growth responses when vermicomposts constituted a relatively small proportion (10–20%) of the total volume of the substrate mixture, with higher proportions of vermicomposts in the mixture not always improving plant growth (Subler et al., 1998; Atiyeh et al., 2000d). The main objectives of the current study were to assess whether (1) the amendment of different proportions of vermicompost to a fertilized commercial peat potting substrate can affect the emergence, growth and biomass allocation of seedlings of tomato plants under greenhouse conditions, (2) whether possible effects on seedling performance can translate into effects on yields and fruit quality even when these seedlings were transplanted into equally fertilized field soil, and (3) whether effects are consistent among different tomato varieties. It was hypothesized that if vermicompost amendments are affecting seedlings this should also be manifested in their yield and fruit quality. Results should help to answer the question whether peat in potting media could be replaced by VC and should additionally stress the importance of substrate quality in seedling husbandry for fruit quality. 2. Materials and methods 2.1. Experimental setup The experiment was conducted in 2003 using the greenhouse and field facilities of the certified organic research farm of the University of Bonn, Germany (65 m a.s.l.; 78170 E, 508480 N). Long-term mean annual air temperature at this location is 9.5 8C, mean annual precipitation is about 770 mm. The year 2003 was exceptional warm (mean annual air temperature: 10.2 8C) and dry (annual precipitation: 708 mm). Three, classical globe-shaped, medium-sized (mean fruit fresh mass 85–90 g) tomato varieties (Lycopersicon esculentum Mill.) with red, three to four chambered fruits and medium shelf life were used: cv. Diplom F1 (cv. D) is a very early maturing hybrid with medium yields, cv. Matina (cv. M) is very early maturing and high yielding, cv. Rheinlands Ruhm (cv. RR) is characterised by mid-season maturation with high yields. 2.2. Substrate mixtures Six substrate mixtures were used by substituting a commercial peat medium with vermicompost (VC) in the proportions of 0, 20, 40, 60, 80 and 100% (v/v). Commercial peat substrate consisted of about 70% peat moss, 20% green waste compost and additional organic fertilizer in its formulation (Klasmann BioPotgrond, Groß Hesepe, Germany; average nutrient concentrations pH 5.8, N = 100 mg L 1, P2O5 = 300 mg L 1, K2O = 400 mg L 1, Mg = 150 mg L 1). Vermicompost was produced from organic food and cotton waste using Eisenia fetida in windrows (Tacke Regenwurmfarm, Borken, Germany; average nutrient concentrations pH 6.5, N = 640 mg L 1, P2O5 = 1600 mg L 1, K2O = 6000 mg L 1, Mg = 710 mg L 1). 2.3. Seeding, seedling emergence and growth In a greenhouse, for each substrate mixture twenty seeds of each tomato variety were sown into cell plug trays filled with the particular substrates and arranged in a randomized design. Seedling emergence was monitored on average every second day after seeding and was expressed as number of seedlings emerged relative to number of seeds sown per tomato variety. Seedling elongation was measured on average every 3 days from soil surface until maximum height of the plant. After growing in plug trays for 32 days, dependent on the emergence rates at least five seedlings per variety and treatment were transferred into 11 cm diameter plastic pots containing VCpeat mixtures corresponding to those in plug trays again in a randomized design. Of the potted plants, growth, number of leaves, number of flower buds and flowers were measured weekly. Seedlings grew in pots for 24 days before they were transplanted into field soil in a randomized design (soil type: fluvisol, row distance: 0.8 m, within row distance of plants: 0.4 m). Tomato plants in plug cells, pots and field were watered when needed using a drip irrigation system. No additional fertilizer was applied to seedlings in plug trays and pots, whereas all field plants were fertilized weekly with 10 g (fresh mass) of VC applied on the soil surface near each plant followed by a subsequent watering (amounting to an application rate of 600 g VC m 2 for the field period). 2.4. Biomass allocation and yields Biomass allocation was determined on five subsamples per substrate mixture and tomato variety before seedlings were transplanted from plug cells into pots and before plants were transplanted from pots into the field. For field plants biomass allocation was determined at the end of the experiment by cutting aboveground parts at soil surface and by determining root biomass on a defined 20 cm  20 cm 20 cm soil volume excavated around each plant. Data of aboveground biomass production also include shoots regularly clipped during the J.G. Zaller / Scientia Horticulturae 112 (2007) 191–199 growing season necessary to produce high yields and fruit quality. Harvested plant biomass was dried at 80 8C for at least 24 h and weighed. Marketable yield was calculated per tomato plant as the sum of orange and red fruits successively harvested until the end of the experiment. Total yields additionally include green and lowquality fruits present at the end of the experiment or harvested at an earlier date. 2.5. Morphological and chemical parameters of fruit quality Fruit quality was assessed on fully orange and red fruits harvested from similar heights of insertion on the tomato plant on three harvesting dates during the experiment. After harvesting, the following morphological properties were measured on at least five fruits per plant: circumference at the fruit equator, peel firmness on three randomly chosen places along the fruit equator using a mechanical hardness tester with a Shore A hardness scale ranging from 0 to 100 units (Type HP; Bareiss, Oberdischingen, Germany), fruit volume was calculated by water displacement. After the morphological measurements, fruits were chopped thoroughly using a household mixer. About half of these fruits were then used to determine ascorbic acid, the remaining half was freeze dried for further analyses. Ascorbic acid was determined on fruit sap of filtered and homogenized fruit material based on the formation of formazan (Deneke et al., 1978) using continuous flow analysis (Photometer 6000, Skalar, Breda, The Netherlands; Brunsch et al., 2000). All other chemical fruit parameters were determined on the freeze-dried material. C and N content was measured using a CHN-analyzer (type NA 1500N; Carlo Erba Instruments, Rodano, Italy). 193 Nitrate-N, ammonium-N and P concentrations were analyzed using a continuous flow method on a photometer (type 6010; Skalar, Breda, The Netherlands). K, Ca, Mg concentrations were determined on an atomic-absorption-spectrometer (type 2380; Perkin Elmer, Wellesley, MA, USA) after microwave extraction (type MLS 1200; Milestone S.r.l., Sorisole, Italy). D-Glucose and D-fructose was determined on enzymatically produced NADPH (Schmidt, 1961) using an UV spectrophotometer (Lamda 2, Perkin Elmer, Wellesley, MA, USA). Because there was little variation of fruit quality data between sampling dates, averages across dates were used for statistical analyses. 2.6. Statistical analyses Data were analyzed with a two-way ANOVA with tomato variety and VC proportion as the two factors by using the general linear model approach in SAS (Version 8.02, SAS Institute, Cary, NC, USA). Time course data on seedling emergence and elongation were analyzed with repeated measures ANOVAs using the GLM approach. In addition to the overall analysis, ttests on the effects of vermicompost proportion were carried out separately for each variety to determine the response patterns in more detail. All ANOVA analyses were performed using Type III sums of squares and were followed by Tukey’s least squares means test for multiple comparisons. 3. Results 3.1. Seedling emergence and growth Seedling emergence was significantly different between varieties and VC amendment (repeated measures ANOVA Fig. 1. Seedling emergence (relative to number of seeds sown) and elongation of three tomato varieties (cv. D, cv. M, cv. RR) grown in plug cells with substrate mixture containing different proportions of vermicompost. P-values derived from repeated measures ANOVAs for individual varieties. Means (n = 15–20). 194 J.G. Zaller / Scientia Horticulturae 112 (2007) 191–199 results; variety effect: P < 0.001, VC effect: P < 0.001; VC  variety interaction: P = 0.052). While the 100% VC substrate led to earlier emergence than other VC proportions in two varieties (cv. D, cv. M), emergence of the third variety (cv. RR) was highest in substrate containing 20% VC and lowest in the 100% VC substrate (Fig. 1). Seedling elongation of the three varieties was significantly different (repeated measures ANOVA results; variety effect: P = 0.043) and affected by VC proportion (VC effect: P = 0.021; Fig. 1). For two varieties (cv. D, cv. M) pure commercial peat substrate showed highest elongation, one variety (cv. RR) showed highest elongation at 100% VC amendment (Fig. 1). All varieties showed lowest elongation with VC amendments between 2 and 60% (Fig. 1). 3.2. Biomass allocation Shoot mass was significantly different between varieties for tomato plants in plug cells, pots and the field (variety effect: P < 0.001, P = 0.010 and P = 0.038 for plug cells, pots and field, respectively). Across varieties VC amendment did not affect shoot mass in plug cells, pots or in field grown plants. Seedlings in plug cells also showed a significant interaction between variety and VC proportion (P = 0.004; Fig. 2). When the varieties were tested individually, only shoot mass of seedlings in plug cells of cv. D and cv. M was significantly affected by VC proportion (Fig. 2) while shoot mass of plants in pots or field remained unaffected. Fig. 2. Above- and below-ground biomass production of three tomato varieties (cv. D, cv. M, cv. RR) growing in plug cells and pots containing substrate mixture with different proportions of vermicompost and of plants which have been transplanted into equally fertilized field soil. P-values derived from ANOVAs for individual variety, R/S indicates tests of root–shoot ratios. Different letters above and below bars indicate significant differences at P = 0.05 (Tukey LSD test). Means  S.E. (n = 3–5). Small error bars are not depicted. 195 J.G. Zaller / Scientia Horticulturae 112 (2007) 191–199 Table 1 Marketable yield and mass per fruit of orange and red fruits and total yield (marketable plus green fruits) of three field-grown tomato varieties (cv. D, cv. M, cv. RR) raised in commercial peat potting substrate with different portions of vermicompost (VC). Means (n = 2–3). Different letters after means indicate significant differences at P < 0.05 (Tukey LSD test) VC (%) 0 20 40 60 80 100 Marketable yield (g plant 1) Total yield (g plant 1) Mass per fruit (g fresh mass) cv. D cv. M cv. RR cv. D cv. M cv. RR cv. D cv. M cv. RR 3184  1391 3512  803 3121  475 2286  949 2321  233 2040  227 3329  1000 1513  645 662  317 2856  721 1979  761 1449  567 2923  1772 612  234 1651  229 3150  1095 3326  890 762  245 95  7 ab 112  8 a 97  10 ab 86  15 ab 86  13 b 93  7 ab 69  10 51  11 67  11 75  3 61  7 55  19 77  27 63  2 76  14 95  9 96  16 59  11 4734  1846 4912  957 4835  686 3964  1604 3621  51 3605  468 4484  1515 1744  484 2283  413 4111  1004 2986  949 2164  902 4693  3040 2433  312 2932  258 5338  2073 5390  1613 1183  378 Root mass varied significantly between varieties in plug cells, pots and the field (variety effect: P < 0.001, P = 0.001 and P = 0.008 for plug cells, pots and the field, respectively). Across varieties, VC proportion affected root mass in plug cells and pots but not in the field (VC effect: P = 0.047, P = 0.017 and P = n.s. for plug cells, pots and field, respectively). Only root mass of seedlings in plug cells showed a significant interaction between variety and VC mixture (P = 0.015; Fig. 2). When the variety responses were tested individually, only root mass of potted plants of cv. M were significantly affected by vermicompost proportion (P = 0.029; Fig. 2). Root masses of cv. M grown in plug cells and cv. RR in the field were nonsignificantly affected by vermicompost proportion. Root–shoot ratio was significantly different between varieties in plug cells (P = 0.010) and the field (P = 0.002; Fig. 2) but not for potted plants. Across varieties, VC proportion in substrate mixture affected root–shoot ratios of tomato plants in all three developmental stages (VC effect: P = 0.029, P = n.s. and P = 0.018 for seedlings, potted plants and field plants, respectively; Fig. 2). In the overall analysis, plants in pots and the field also showed a significant variety  VC proportion interaction (P = 0.014, P = 0.017 for potted and field plants, respectively). Individual analyses showed that root–shoot ratio was significantly affected for seedlings in plug cells of cv. M (P < 0.001) and cv. RR (P = 0.038) and field plants of cv. M (P = 0.011, Fig. 2). 3.3. Yields Marketable yield (no variety effect, VC effect: P = 0.040), fresh mass per fruit (variety effect: P < 0.001, no VC effect) and total yield (variety effect: P = 0.047, no VC effect) were different between tomato varieties and affected by VC proportion of substrate mixture (Table 1). Analysis for individual variety showed that only fruit mass of one variety (cv. D) was affected by VC proportion in the substrate mixture (P = 0.038, Table 1). All other parameters were unaffected by VC proportions of substrate mixture (Table 1). 3.4. Fruit quality Fruit circumference was significantly different between varieties (P < 0.001) however was across varieties unaffected by VC proportion in substrate mixture (Table 2). Individual analysis of each variety showed that VC proportions in substrate significantly affected circumference of cv. D (P = 0.003) and cv. RR (P = 0.003) but did not affect cv. M (Table 2). Fruit dry matter content was significantly different between varieties (P < 0.001) and across varieties not affected by VC proportions in substrate (variety  VC interaction: P < 0.001, Table 2). Individual analysis showed that peat substitution significantly affected dry matter content of cv. D (P = 0.029) and cv. M (P = 0.002) and not significantly affected cv. RR (Table 2). Peel firmness was significantly different between varieties (P < 0.001) and varied significantly between different levels of peat substitution (P < 0.001; variety  VC interaction: P < 0.001; Table 2). Individual analysis showed that peel firmness was significantly affected by peat substitution for cv. D (P = 0.004) and cv. RR (P = 0.003) but not for cv. M (Table 2). Table 2 Circumference, dry matter content and peel firmness (Shore A scale: 0 . . . soft, 100 . . . hard) of three field-grown tomato varieties (cv. D, cv. M, cv. RR) raised in commercial peat potting substrate with different portions of vermicompost (VC). Means (n = 5–8). Different letters after means indicate significant differences at P < 0.05 (Tukey LSD test) VC (%) Circumference (cm) cv. D 0 20 40 60 80 100 18.1  0.2 18.7  0.3 18.0  0.3 17.7  0.3 17.7  0.3 18.1  0.3 ab a ab ab b b Dry matter content (%) cv. M cv. RR 15.9  0.3 14.3  0.2 14.9  0.2 16.0  0.2 15.3  0.2 15.1  0.3 16.5  0.3 16.8  0.1 17.2  0.4 17.7  0.3 16.3  0.3 16.2  0.3 cv. D ab ab ab a b ab 6.6  0.2 6.0  0.3 6.4  0.2 7.3  0.4 6.0  0.2 5.9  0.2 Peel firmness (Shore A scale) cv. M ab ab ab a b ab 6.7  0.4 7.9  0.2 8.5  0.6 7.1  0.3 8.1  0.4 6.1  0.2 b ab a ab a b cv. RR cv. D 7.0  0.2 6.4  0.6 6.2  0.1 6.4  0.2 5.9  0.6 8.3  0.1 31.8  1.9 44.3  1.3 36.1  2.0 36.7  3.9 42.1  1.4 42.5  2.9 b a ab b ab ab cv. M cv. RR 37.8  2.2 32.4  0.8 31.9  1.0 36.8  1.9 35.9  1.5 37.9  2.7 27.1  3.1 38.5  3.8 52.9  5.4 54.0  3.3 46.5  3.8 36.8  2.2 b b ab a ab ab 196 J.G. Zaller / Scientia Horticulturae 112 (2007) 191–199 Table 3 Nitrogen, phosphorus and potassium concentration in marketable fruits of three field-grown tomato varieties (cv. D, cv. M, cv. RR) raised in commercial peat potting substrate with different portions of vermicompost (VC). Means (n = 2–4). Different letters after means indicate significant differences at P < 0.05 (Tukey LSD test). VC (%) Nitrogen content (g kg 1) cv. D 0 20 40 60 80 100 27  2 28  1 26  1 26  1 21  1 23  1 cv. M ab a b b b b 27  1 21  1 23  1 26  1 24  1 25  1 Phosphorus (mg kg 1) cv. RR a b b ab ab ab 25  2 22  1 22  0 27  1 24  0 18  2 a ab ab a ab b cv. D cv. M 52  2 51  2 51  3 50  2 51  3 50  3 48  2 44  0 43  1 48  1 46  1 49  2 Chemical composition of marketable fruits varied significantly between varieties and across varieties substrate mixtures significantly affected fruit nitrogen (P < 0.001, Table 3), Lascorbic acid (P < 0.001, Fig. 3), glucose and fructose (P < 0.001, Fig. 3), and did not affect phosphorus (Table 3), Potassium (mg kg 1) cv. RR a a b a a a 48  0 48  1 47  1 46  2 45  1 40  2 a a a a a b cv. D cv. M cv. RR 501  25 a 441  13 b 505  6 a 446  17 b 453  12 b 446  17 b 475  12 444  37 447  23 463  17 474  34 488  8 433  20 a 459  17 a 432  9 a 444  25 a 400  7 b 419  6 a potassium (Table 3), calcium (Fig. 3), magnesium (Fig. 3) and carbon concentrations (Table 3). When the response of each variety to peat substitution was analyzed individually most parameters on fruit quality tested showed highly significant differences in all three varieties (Table 3, Fig. 3). Fig. 3. Calcium, magnesium, L-ascorbic acid and sugar (glucose + fructose) concentrations of marketable, field-grown fruits of three tomato varieties (cv. D, cv. M, cv. RR). Plants were raised in substrate mixture with different proportions of vermicompost until the flowering stage and have been transplanted into equally fertilized field soil. P-values derived from ANOVAs for individual variety. Different letters above bars indicate significant differences at P = 0.05 (Tukey LSD test). Means  S.E. (n = 2–3). Small error bars are not depicted. J.G. Zaller / Scientia Horticulturae 112 (2007) 191–199 4. Discussion 4.1. Seedling emergence and growth Generally the results of this study show that a substitution of fertilized commercial peat potting substrate with vermicompost is possible without detrimentally affecting emergence and growth of seedlings of the three tomato varieties tested. However, the tested varieties differed greatly in their response and no general relationship between proportion of vermicompost amendment and seedling emergence and growth could be detected. Studies on the effect of VC amendment to growth media for tomatoes in greenhouses either showed a maximum growth at VC proportions of around 20% in the growth mixture (Atiyeh et al., 1999, 2000a) or a steady increase in growth with increasing VC amendment (Arnold et al., unpublished data). This discrepancy between findings in the literature could be explained by the use of different tomato varieties and by the use of VC of different origin which has been shown to cause great differences in vermicompost quality (e.g., Atiyeh et al., 2000d; Domı́nguez, 2004; Edwards, 1988). An additional explanation for contrasting results in the current study might also be that no additional mineral fertilization was supplied during the course of the experiment. This however also indicates that vermicompost contains a well-balanced composition of nutrients and at least for tomato seedling husbandry no additional supply of mineral nutrients seems to be required. Recent work confirmed this by showing that the quality of tomato transplants cultivated in vermicompost was only slightly reduced (tested only up to 20% amendment) compared to media containing no VC with no negative effects in the performance of field tomatoes (Paul and Metzger, 2005). 4.2. Biomass allocation of seedlings, potted plants and field plants It is well established that changes in allocation patterns largely determine the ability of plants to capture resources (Poorter et al., 1990) and that plants may change their allocation patterns in response to the environment and especially to the availability of soil nutrients (Brouwer, 1962). In the current experiment shoot and root biomass production of seedlings and potted plants was significantly altered by peat substitution through vermicompost and different between tomato varieties. Compared to the commercial peat mixture, shoot biomass of seedlings was only lower in mixtures containing 80% (cv. D) and 100% vermicompost (cv. M), while all other VC amendments led to similar shoot biomass production than in fertilized peat medium. Root biomass production seemed to be mainly unresponsive to peat substitution with the exception of one variety (cv. M) where seedlings in 100% vermicompost showed 30% less root biomass than those in all other VC-peat substrate mixtures. Shifts in biomass allocation to roots (expressed as altered root–shoot ratio) occurred in seedlings of two varieties (cv. M and cv. RR) when grown at VC proportions higher than 40%. The absence of a clear relationship between VC proportion in the growth media and 197 tomato biomass production also suggests that not only purely physical and chemical properties of VC are stimulating plant growth but there is also the possibility that indirect effects via the inhibition of plant pathogen infection (Szczech, 1999; Zaller, 2006), effects on the rhizosphere microflora (de Brito Alvarez et al., 1995), nitrate uptake kinetics (Dell’Agnola and Nardi, 1987; Muscolo et al., 1999), effects on beneficial microorganisms (Atiyeh et al., 2000d), plant growth regulators (Tomati et al., 1988) or mycorrhizal colonisation of roots (Cavender et al., 2003) might override pure nutrient effects. Clearly, more experimental research aiming to specifically addressing interactions between VC applications, microorganisms and their consequences for crop plants seems necessary. 4.3. Yields and fruit quality One of the central aims of this experiment was to test whether tomato seedlings that were raised in different substrate types have acquired some predisposition that affected yields or fruit quality. In terms of marketable and total yields this was clearly not the case; i.e. different VC proportions in seedling substrates did not influence yields in the field. Fresh mass per fruit, however, was increased by 18% for the hybrid tomato variety (cv. D) when seedling substrates contained 20% vermicompost compared to plants raised in 0, 40, 60 and 80% vermicompost; the other two varieties tested remained unaffected. Perhaps the most remarkable result of the current study is that across tomato varieties nearly all determined parameters of morphological and chemical fruit quality (circumference, dry matter content, firmness of peel, contents of C, N, P, K, Ca, Mg, Vitamin C, glucose, fructose) were significantly affected by the substrate mixture used to raise the seedlings. Among the substrates containing vermicompost also the proportion of vermicompost amended often correlated with certain fruit quality parameters. Impact on fruit quality of vermicompost could also be shown in comparison to hydroponic substrates (Premuzic et al., 1998) or when applied as foliar spray (Zaller, 2006). Generally, ascorbic acid and sugar concentrations of tomato fruits have been shown to be affected by plant nutrition, water supply and light intensity (Neubert, 1959; Mozafar, 1993; Veit-Köhler et al., 1999). Especially contents of ascorbic acid and sugars are also directly linked to tomato flavour attributes (Auerswald et al., 1999). Since effects on fruit quality do not correlated with differences in growth or allocation patterns of plants it can be assumed that vermicompost in the substrate alters seedling performance and/or the above-mentioned association with rhizosphere organisms that translate into altered fruit quality. Although VC-induced alterations of morphological and chemical fruit quality parameters were not different for each variety it is noticeable that the only hybrid tomato variety tested (cv. D) was the most responsive to substrate mixtures. In conclusion, results of the current experiment show that firstly, vermicompost in potting media has no detrimental but rather stimulatory effects on emergence, growth and biomass allocation of tomato seedlings and has thus considerable 198 J.G. Zaller / Scientia Horticulturae 112 (2007) 191–199 potential for substituting peat in horticultural potting substrates. Secondly, fruit quality of tomatoes can be altered by the substrate mixture used to raise seedlings even when seedlings were transplanted into equally fertilized field soil. This influence of substrate quality may also have implications for tomato pest and disease resistance (Hoffland et al., 2000, 1999; Edwards et al., 2004) or its susceptibility to abiotic stress (e.g. chilling; Starck et al., 2000). Finally, the current results also highlight differences of vermicompost effects between crop varieties, an aspect that has been ignored in the literature so far. Especially the latter finding should be considered when giving recommendations on the optimum proportion of vermicompost amendment to horticultural potting substrates. 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