Growth, photosynthesis and productivity of greenhouse tomato cultivated in open or closed rockwool systems

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1 Growth, photosynthesis and productivity of greenhouse tomato cultivated in open or closed rockwool systems X. Hao and A. P. Papadopoulos Agriculture and Agri-Food Canada, Greenhouse and Processing Crops Research Centre, Harrow, Ontario, Canada N0R 1G0. Received 21 June 2001, accepted 6 May Hao, X. and Papadopoulos, A. P Growth, photosynthesis and productivity of greenhouse tomato cultivated in open or closed rockwool systems. Can. J. Plant Sci. 82: Two full spring season tomato crops (Lycopersicon esculentum Mill. Trust ) were grown in an open rockwool system with standard rockwool feeding formulae (O-R; conventional method), and in closed rockwool systems with standard rockwool (C-R) or Nutrient Film Technique (C-NFT) feeding formulae (modified in 1997) in 1996 and 1997 to examine the feasibility of a fully closed rockwool production system with appropriate feeding formulae. The closed rockwool system with optimized feeding formulae achieved high marketable yield, similar to that of the open rockwool system. There were no differences in early plant growth, plant biomass or biomass partitioning, and in total fruit yield, size and grades except for the closed rockwool system with the standard rockwool feeding formulae (C-R), which had lower yield than C-NFT in the last month of harvest in The photosynthesis of old foliage was higher and the root systems at the end of the experiments were rated healthier in plants grown in the closed (C-R and C-NFT) systems than in plants grown in the open (O-R) system. Over 30% of water and fertilizer was saved with the closed systems in comparison to the conventional open system. These results demonstrated that closed rockwool systems with optimized nutrient feedings are economically and environmentally sound alternative methods for greenhouse tomato production in Ontario. Key words: Lycopersicon esculentum, tomato, yield, recycling, rockwool, greenhouse Hao, X. et Papadopoulos, A. P Croissance, photosynthèse et productivité des tomates de serre cultivées sur laine de roche en circuit ouvert ou fermé. Can. J. Plant Sci. 82: Les chercheurs ont produit deux cultures de tomates (Lycopersicon esculentum Mill. Trust ) de printemps dans de la laine de roche, en circuit ouvert avec la formule nutritive habituelle (O-LR; méthode classique) ou en circuit fermé avec la formule nutritive normale (F-LR) ou sur film nutritif (F-NFT) (version modifiée de 1997), en 1996 et L objectif était de voir si on peut adopter un système de production sur laine de roche à circuit totalement fermé avec la bonne formule nutritive. Le système à circuit fermé et à formule nutritive optimisée permet aux tomates d atteindre un rendement commercialisable semblable à celui du système à circuit ouvert. Les chercheurs n ont noté aucune variation chez les plants au début de la croissance, pas plus que dans la biomasse végétale et la répartition de cette dernière, le rendement total en fruits, le calibre des fruits et leur classement, sauf pour le système en circuit fermé avec formule nutritive normale (F-LR), où le rendement était inférieur à celui du système F-NFT lors du dernier mois de la récolte, en Les vieilles feuilles photosynthétisaient mieux et les racines étaient plus saines à la fin de l expérience chez les plants cultivés en circuit fermé (F-LR et F-NFT) que chez ceux cultivés en circuit ouvert (O-LR). On a récupéré plus de 30 % de l eau et des engrais avec le système à circuit fermé, comparativement au système classique à circuit ouvert. Les résultats indiquent que les systèmes de culture sur laine de roche à circuit fermé reposant sur une solution nutritive optimisée constituent une méthode de culture plus économique et moins dommageable pour l environnement pour la tomate de serre en Ontario. Mots clés: Lycopersicon esculentum, tomate, rendement, recyclage, laine de roche, serriculture 771 Rockwool is the most popular growth medium for soilless greenhouse vegetable production (Jensen and Malter 1995). In Ontario, more than 95% of the greenhouse vegetables are grown in rockwool (Khosla, personal communication, 2001). Rockwool can provide good aeration; it is inert and has low buffering capacity, which makes almost 100% of the supplied nutrients available for plant use. However, the low buffering capacity of this substrate makes plants more vulnerable to toxic levels of salts and nutrient deficiencies if the plants are not appropriately fertigated. An extra 20 30% water and fertilizers are usually applied to greenhouse vegetables grown on rockwool to prevent water shortage or nutrient imbalance (Papadopoulos 1991). The excessive application of water and fertilizers is no longer acceptable because consumers have become increasingly aware of the environmental pollution from run-off fertilizers. Legislation in The Netherlands required that growers produce greenhouse vegetables in closed systems when the systems became feasible (Baars 1992; van Os 1999). Improving fertigation management by adding appropriate amounts of water and fertilizers to the recycled drainage solution may contribute to reductions in water and fertilizer Abbreviations: BER, blossom-end rot; CF, catfaced; C-NFT, closed rockwool system with Nutrient Film Technique feeding formulae; C-R, closed rockwool system with standard rockwool feeding formula; EC, electrical conductivity; O-R, open rockwool system with standard rockwool feeding formulae

2 772 CANADIAN JOURNAL OF PLANT SCIENCE use (Mankin and Fynn 1992), and thus increase fertigation efficiency. However, with the closed system, there is a risk of low yield due to potential nutrient imbalance (Jensen and Collins 1985), accumulation of phytotoxic compounds (Yu and Matsui 1993) and of deleterious microorganisms, as well as an increased risk of widespread contamination with root pathogens (Wohanka 1991; Hardgrave 1993; van Os et al. 1998). Poor quality of water alone can necessitate partial dumping of nutrient solutions. The fertigation efficiency of partially closed systems may not be higher than that of open systems if the fraction of nutrient solution dumped is high (Voogt and Sonneveld 1997). Thus, to be feasible and more efficient, the closed systems need not only support similar yield and quality as the open systems, but also have low or zero nutrient solution discharge. Although closed rockwool systems have been used for tomato production in Western Europe, only a short tomato crop grown with a fully closed system has been reported to have achieved the same fruit yield and quality as similar crops grown in open rockwool systems (Zekki et al. 1996). Many closed systems still need to periodically dump nutrient solution to avoid harmful mineral imbalance and buildup because of poor water quality (Voogt and Sonneveld 1997). A full-season tomato crop grown in a fully closed system (100% reuse) has not been reported to have achieved the same yield and quality as a crop grown in an open rockwool system. There are still concerns about sustained growth and productivity of a full-season tomato crop grown in a fully closed system. In a closed soilless cultivation system, the input of water and nutrients needed to be adjusted frequently to achieve optimum nutrient concentrations in the root environment to obtain better nutrient uptake and optimum yield and product quality (Voogt and Sonneveld 1997). For a specific crop or cultivar, the nutrient supply is usually adjusted based on plant growth stage and climate because these factors have major influence on nutrient uptake, plant growth, yield and quality. In open rockwool or recycling production systems, the feeding formulae also need to be tuned to the demand of plants as much as possible to increase fertigation efficiency. The existing feeding formulae developed for open rockwool and recycling soilless production systems have already incorporated the available knowledge on plant nutrient demand and uptake (such as variations with growth stages and local water quality and climate). By investigating the effects of these feeding formulae on plant growth, photosynthesis, fruit yield and quality, and the dynamics of nutrient concentrations in closed systems, we can identify the deficiencies in these formulae when used with fully closed systems, improve them and develop better nutrient feeding formulae for a fully closed rockwool production system. Greenhouse tomato has been successfully grown with rockwool and nutrient film technique (NFT) (Papadopoulos 1991). The feeding formulae developed for NFT tomato cropping may be more suitable for tomato grown in a fully closed rockwool system than the feeding formulae developed for the open rockwool growing system because NFT is practically a closed system. The objectives of our study were: (1) to investigate the effects of fully closed rockwool systems (100% reuse) on growth, photosynthesis and productivity of a full-season tomato crop in comparison to an open rockwool system, and (2) to examine the effects of different feeding formulae (including the NFT feeding formulae) on the productivity and the dynamics of nutrient concentration in open and closed rockwool systems for developing appropriate feeding formulae for tomato production in a fully closed rockwool system. MATERIALS AND METHODS Experiments were conducted in a glass greenhouse at the Greenhouse and Processing Crops Research Centre, in Harrow, Ontario, Canada from January to August 1996 and from January to July Tomato cv. Trust seed was sown in small rockwool cubes ( cm 3, Pargro Inc., Orillia, ON) in seedling packs on 30 November 1995 and 2 December After seedlings emerged and cotyledons fully unfolded, the seedlings were transplanted into large rockwool blocks ( cm 3 ; Pargro Inc., Orillia, ON). The transplants were raised on benches and fertigated using a Harrow Fertigation Manager (Papadopoulos and Liburdi 1989) according to standard recommendations (Papadopoulos 1991). On 17 January 1996 and 23 January 1997, at the fourto five-leaf stage, transplants were transferred onto sleeved rockwool slabs ( cm 3, Pargro Inc., Orillia, ON) at a density of 3.3 plants m 2. The plants were cultured with three methods: C-R, closed rockwool with rockwool nutrient feeding formulae/schedules (Tables 1 and 2); C-NFT, closed rockwool with NFT nutrient feeding formulae/schedules (Tables 1 and 2); and O-R, open rockwool with rockwool nutrient feeding formulae/ schedules (Tables 1 and 2). Three slabs were wrapped with polyethylene film to form a row (plot) and to facilitate collection of run off nutrient solution. Nine plants were planted in each row. The leaching solution from each row was collected in a pail. For the closed systems (C-R and C-NFT), the leaching solution was pumped back to a holding tank where it was mixed with fresh nutrient solution for further feeding of the plants; 100% of leaching solution was collected and reused. The nutrient formulae of fresh solution for C-R was the same as those of O-R in The target feeding electrical conductivity (EC) (Tables 1 and 2) was maintained by adding tap water into the holding (feeding) tanks or by diluting the input fresh nutrient solution when the EC in the systems was too high. The tap water contained 1 ppm N, 0 ppm P, 1.2 ppm K, 32.8 ppm Ca, 7.8 ppm Mg, 6.7 ppm S, 9.2 ppm Na, and 22.4 ppm Cl. No fresh nutrient solution, but only water was added to the holding (feeding) tanks in the closed systems in the last 2 wk of the experiment in For the open system, the leaching solution was collected only for measurement of its volume and nutrient contents, and was not used again for feeding the plants. The ph of the feeding solution was maintained at 5.5 with nitric acid. In the 1997 experiment, the nutrient inputs for the closed systems (C-R and C-NFT) were modified (Table 2). Sulfur, magnesium and calcium inputs were reduced and potassium and phosphorus inputs were

3 HAO AND PAPADOPOULOS TOMATO IN OPEN OR CLOSED ROCKWOOL SYSTEM 773 Table 1. Nutrient feeding formulae and seasonal fertigation schedules in 1996 z EC Nutrient concentration (ppm) Days y (ms cm 1 ) NO 3 -N NH 4 -N P K Ca Mg S Rockwool end NFT end z The ph of the nutrient solution was adjusted to 5.5. The NO 3 -N nutrient concentrations listed include the NO 3 -N from any nitric acid added for ph control. Number of days after planting. Table 2. Nutrient feeding formulae and seasonal fertigation schedules in 1997 z EC Nutrient concentration (ppm) Days y (ms cm 1 ) NO 3 -N NH 4 -N P K Ca Mg S Rockwool end NFT end z The ph of the nutrient solution was adjusted to 5.5. The NO 3 -N nutrient concentrations listed include the NO 3 -N from any nitric acid added for ph control. Number of days after planting. increased starting just before the first fruit harvest (8 wk from planting in the greenhouse). The treatments were replicated four times. Day/night heating temperature was set at 21/17 C in 1996 and at 19/18 C in 1997 while ventilation temperature was set at 23 C. Day/night relative humidity was set at 70/65%.

4 774 CANADIAN JOURNAL OF PLANT SCIENCE The climate in the greenhouse was controlled and recorded with a Priva-Integro Control System. The plants were trained to a single stem according to commercial practice. An electrical vibrator was used to assist plant pollination. The feeding and leaching nutrient solutions were sampled for analyzing macro- and micronutrients twice a week in 1996 and once a week in The volume of the nutrient solution in the systems was recorded at the time of nutrient solution sampling. Based on the results of nutrient analysis and volume measurement, the water and nutrient uptake by plants were calculated. The data from the first 2 wk of the experiments were not used in the calculations because of the anticipated large experimental error due to the small size of the plants and the potential adsorption of nutrients at the initial stages of cultivation by rockwool. Plant height and leaf number of three plants in each row were measured just before plants reached the overhead wires (2-m high). In 1997, all leaves removed from the plants were collected and their leaf area and dry weight were determined; stem dry weight was also determined at the end of the experiment. Leaf area was measured with a LI-COR 3100 leaf area meter (LI-COR Inc., Lincoln, NE 68504), and leaf and stem dry weights were determined after being dried in brown paper bags at 65 C for 2 3 wk. Plant root growth was visually assessed and Pythium infection was examined in a laboratory. Gas exchange parameters were measured with a LI-COR 6400 portable photosynthesis system (LI-COR Inc., Lincoln, NE 68504, USA), with the LI-COR LED light source on sunny days in To reduce influence of diurnal effects on leaf photosynthesis, the measurements were done between 1000 and Fruit reaching breaker stage was harvested and graded as grade #1: extra large, large, and small; commercial; grade#2; and unmarketable: blossom-end rot (BER) and catfaced (CF) and other unmarketable grades, according to commercial grading standards (Ontario Ministry of Agriculture, Food and Rural Affairs, Regulation 378/90, 1990). Fruit was harvested from 4 April to 6 August in 1996 and from 24 April to 12 July 12 in1997. Fruit quality parameters (total dry matter content, soluble solids, ph, EC) were measured three times (early, middle and late harvests) in For quality analysis, four to five large or extra-large fruit of grade no. 1 in breaker stage were harvested from each plot, then these fruits were stored at 20 C and 70% relative humidity until the table-red stage. Total dry matter content was measured by drying sliced fruit at 65 C for 3 wk. Soluble solids were measured with a portable digital refractometer (model PR-101, Atago Co., Tokyo, Japan), and EC and ph were measured with portable EC (Dist WP 4, HANNA Instruments, Woonsocket, RI 02895) and ph meters (phep 3, HANNA Instruments, Woonsocket, RI 02895) on homogenized fruit samples. Total fruit dry matter was calculated from data on yield and fruit dry matter content. Data were subjected to analysis of variance using the SAS GLM procedure. If the treatment effects were significant, their means were separated with the Duncan s multiplerange test. RESULTS AND DISCUSSION Early Plant Growth and Fruit Production The early growth and development of tomato grown in open and closed rockwool systems were similar. Plant height, leaf number per plant, cluster number per plant and leaf chlorophyll of 5th and 10th fully expanded leaves measured on 24 February and 7 April 1997 were similar (P > 0.05) in all three treatments (data not shown). There was also no difference in total fruit weight, marketable fruit yield, fruit size, percentage of grade no. 1 fruit in marketable fruit or fruit grades in early harvests (Table 3). These findings are similar to those of Zekki et al. (1996) in a short-term experiment; it would appear that plants grown in fully closed rockwool cultivation systems can achieve similar growth and productivity as plants grown in open rockwool systems early in the season. In the 1996 experiment, the incidence of BER in the open system was higher than in the closed systems. However, there was no difference on incidence of BER in the 1997 experiment. The exact cause is not clear. There was no difference among treatments on fruit dry matter content, soluble solids, EC and ph during early fruit production (Table 4). Late Growth, Photosynthesis and Fruit Production The growth and productivity of plants grown in the open and closed systems were different in the late growing stage. The root systems of plants in the closed systems at the end of the experiments were rated healthier than those of plants in the open system in both 1996 and The root system of plants grown in the C-NFT was the best, followed by the C-R, while the root systems in O-R treatment were in poor condition. There was no difference in gas exchange for the mid- or top foliage of plants grown in the open or closed systems (Fig. 3). However, the leaf photosynthesis and stomatal conductance of bottom leaves of plants in the closed systems were significantly higher than those in the open system. Therefore, the plants in the closed systems might senesce slower than plants grown in the open system. In the closed systems, there were higher concentrations of Ca, Mg and S due to their accumulation (Figs. 1 and 2). The magnesium uptake capacity of tomato declined with plant age, and magnesium supply to tomato needs to be increased late in the growth season (Sonneveld 1987). Therefore, the accumulation of magnesium in closed rockwool systems might have naturally met the plant needs, delaying the senescence of the plants, and promoting a better root system, especially late in the growth season. A higher concentration of magnesium has been shown to promote root growth (Schwartz and Bar-Yosef 1983). This finding also indicated the need of a study to refine the Mg nutrition of the open rockwool production system. One of other possible reasons for better root systems in the closed systems might be related to microorganism interaction in rhizosphere. A parallel study (Tu et al. 1999) on the closed rockwool systems indicated that there was an accumulation of beneficial bacteria in the closed systems. The population of beneficial bacteria in the closed systems

5 HAO AND PAPADOPOULOS TOMATO IN OPEN OR CLOSED ROCKWOOL SYSTEM 775 Table 3. Early (first month), late (last month) and seasonal total accumulated yield and fruit grades of tomato grown in open or closed rockwool systems with NFT or rockwool nutrient feedings in 1996 (and 1997 z ) Total fruit Marketable fruit weight Weight Size Grade 1 Grade 2 Comm. Unmarketable fruit (%) x Treatment y (kg m 2 ) (kg m 2 ) (g fruit 1 ) (%) w (%) w (%) w BER CF Other (a) First month (4 April to 4 May) in 1996 C-R 10.33a 9.97a 165.3a 95.5a 4.5a 4.7a 0.15b 3.53a 0.00a C-NFT 8.88a 8.58a 168.9a 88.8a 6.5a 0.0a 0.00b 3.37a 0.00a O-R 10.20a 9.44a 173.6a 93.5a 4.2a 2.3a 3.76a 3.53a 0.00a (b) Last month (6 July to 6 Aug.) in 1996 C-R 7.72b 7.03b 130.0a 97.3a 2.0a 0.7a 0.00a 0.00a 8.63a C-NFT 9.24a 8.98a 135.3a 95.8a 4.2a 0.0a 0.00a 0.00a 2.65a O-R 8.22ab 7.95ab 137.7a 96.7a 3.0a 0.3a 0.54a 0.00a 2.98a (c) Total: 4 April to 6 Aug. in 1996 C-R 34.06a 33.40a 145.3a 96.1a 3.7a 0.1a 0.08b 1.79a 2.17a C-NFT 36.51a 35.97a 147.0a 95.1a 3.7a 1.1a 0.05b 1.20a 1.63a O-R 36.20a 33.33a 146.5a 94.4a 4.8a 0.7a 1.75a 1.23a 1.27a z In 1997, first month marketable yields for C-R, C-NFT and O-R were 11.35, 11.29, and kg m 2, respectively; total marketable yields for 3 mo of harvest in C-R, C-NFT and O-R were 30.16, and kg m 2, respectively; percentage of blossom-end rot ranged from 0.16 to 0.79%. There was no significant difference among C-R, C-NFT and O-R on total fruit weight, marketable fruit weight, fruit size, percentage of grade no.1, commercial, BER, CF and other unmarketable (P > 0.05). y C-R, closed rockwool with rockwool nutrient feeding formulae/schedule; C-NFT, closed rockwool with NFT nutrient feeding formulae/schedule; and O-R, open rockwool with rockwool nutrient feeding formulae/schedule. x Fresh weight as a percentage of total fruit weight. w Fresh weight as a percentage of marketable fruit weight. a, b Different letters in the same column indicate a significant difference (P < 0.05) according to Duncan s multiple-range test. Table 4. Fruit quality of tomato grown in open or closed rockwool systems with NFT or rockwool nutrient feedings in 1997 z Total dry Soluble matter solids EC Date Treatment y (%) (Brix ) (ms cm 1 ) ph 2 May C- R 4.23a 3.70a 4.60a 4.65a C-NFT 4.22a 3.70a 4.53a 4.50a O-R 4.55a 3.75a 4.45a 4.50a 2 June C- R 4.95ab 3.93ab 4.00a 4.57ab C-NFT 5.01a 4.02a 3.97a 4.53b O-R 4.68b 3.72b 3.83a 4.63a 16 July C- R 5.17ab 4.35b 4.07ab 4.60a C-NFT 5.52a 4.81a 4.23a 4.60a O-R 5.07b 4.32c 3.87b 4.60a z C-R, closed rockwool with rockwool nutrient feeding formulae/schedule; C-NFT, closed rockwool with NFT nutrient feeding formulae/schedule; and O-R, open rockwool with rockwool nutrient feeding formulae/schedule. a c Different letters in the same column indicate a significant difference (P < 0.05) according to Duncan s multiple range test. 140 d after planting was higher than in the open system. The Pythium population in the rhizosphere was lower and roots were less infected by Pythium in the closed systems than in the open system. Therefore, if good sanitation is practiced in closed systems, the disease pressure will not be any higher than in open systems, since 85 90% of rhizospheric pathogens tend to stay in the rhizosphere and only a very small portion of them are leached out. On the other hand, a higher proportion of beneficial bacteria may leach out in open systems. The buildup of beneficial bacteria in the closed rockwool systems can depress root-rot pathogen populations. The last-month marketable yield of the C-NFT in 1996 was higher than that in the C-R, whereas the marketable yield in the O-R was between those of the two closed systems (C-NFT and C-R, Table 3). The lower marketable yield in the C-R might be related to high S in the system (Table 1). The input of N, P, K, Ca and Mg was similar in C-NFT and C-R in However, the S input was much higher in C-R than C-NFT. Sulfur can easily accumulate in closed systems (Voogt and Sonneveld 1997). In 1997, even though the input of S had been reduced, the S concentration late in the season still accumulated to two to three times ( ppm) the input level ( ppm) (Table 2 and Fig. 2). A high level of S is detrimental to plant growth (Cerda et al. 1984). However, Nukaya et al. (1991) found that the yield of tomato was not affected in a long-term tomato crop with S ranging from 160 ppm to 320 ppm. Lopez et al. (1996) reported that tomato can tolerate S up to 650 ppm in a short experiment when the ratios between the major elements and S are appropriate. The S concentration in our 1996 experiment was not likely to have reached this high level, even in the late stage of the experiment. Therefore, the reduction in yield in 1996 might be more related to nutrient imbalance resulting from high S input than to the toxic effects of high concentration of S. The supply of fresh nutrient solution to closed systems was based on total solution EC in this study (i.e., by keeping the total EC on target) just as in commercial production (Voogt and Sonneveld 1997). Starting in early April (70 80 d after planting), we needed to frequently dilute the solutions using tap water in the closed systems, especially in the C-R, to bring down the EC, which might have caused nutrient imbalance. The P and K concentration in the C-R system

6 776 CANADIAN JOURNAL OF PLANT SCIENCE Fig. 1. Macronutrient concentrations in open or in closed rockwool tomato systems with rockwool or NFT feeding formulae/schedule in d after planting, decreased to below 15 mg L 1 and 100 mg L 1, respectively, and stayed at this low levels until the end of the experiment. Less than 100 mg L 1 K in the nutrient solution causes yield loss (Adams 1986). The P and K concentrations in C-NFT also declined; however, they soon recovered, so did not affect the yield in C-NFT. In 1997, after we adjusted the feeding formulae by reducing the input of S and increasing P and K inputs, there was no difference in late fruit harvest (data not shown). The fruit EC, dry matter and soluble solids contents in the closed systems were similar to or higher than those in the open system (Table 4). The higher EC in the closed systems due to Ca, Mg and S accumulation might have contributed to the higher fruit dry matter content and soluble solids because high EC is known to increase fruit dry matter content and soluble solids (Dorais et al. 2001). Total Plant Growth, Biomass Distribution and Fruit Production The total leaf area, total biomass and biomass distribution were similar in all three treatments. In 1997, g biomass per plant (not including roots) was produced; 70 to 71.1% allocated to fruit, 9.7 to 10.9% allocated to stem and 18.8 to 19.2% allocated to leaves. There was no significant

7 HAO AND PAPADOPOULOS TOMATO IN OPEN OR CLOSED ROCKWOOL SYSTEM 777 Fig. 2. Macronutrient concentrations in open or in closed rockwool tomato systems with rockwool or NFT feeding formulae/schedule in difference in total marketable yield or fruit size (Table 3). In 1997, there was less grade#2 fruit in the closed systems (4.6 and 5.1 for C-R and C-NFT, respectively) than in the open system (6.6 for O-R; P < 0.05). There was no difference in percentage of grade no. 1, no. 2 and marketable fruit in 1996, and no difference in percentage of grade no. 1 and unmarketable fruit in The marketable yields (33 36 kg m 2 for a 4-mo harvest in 1996 and kg m 2 for a 3-mo harvest in 1997) were similar to or higher than the yields achieved in commercial greenhouses in south-

8 778 CANADIAN JOURNAL OF PLANT SCIENCE western Ontario. As shown in Table 3 and Fig. 3, the C-NFT tended to perform better late in the season. Fig. 3. Leaf gas exchange of tomato plants grown in open or closed rockwool systems, with rockwool or NFT feeding formulae/schedule. Gas exchange was measured on 5 and 6 June 1997 at 1000 µmol m 2 s 1 PPFD, 29 C, 65% RH and ambient concentration of CO 2. The leaf position was counted from the top leaf longer than 10 cm. Water and Fertilizer Consumption in Open and Closed Systems Water and fertilizer usage in the closed systems was much lower than that in the open system (Table 5). A major reduction in water and fertilizer usage was due to savings in leaching solution, as expected. In 1996, the uptake of water and nutrients in C-R was about 30% lower than in C-NFT and O-R. Due to the high S input of C-R in 1996, it was very difficult to bring down its solution EC. Therefore, the actual EC of the nutrient solution in C-R could be higher than the EC of the nutrient solution in C-NFT and O-R. High EC reduces water and nutrient uptake (Dorais et al. 2001). At a similar rate of water uptake between C-NFT and O-R in 1996, there was a higher uptake of Ca and Mg and a lower uptake in P and K in C-NFT than in O-R, due to Ca and Mg accumulation and lower P and K concentrations in C-NFT (Table 5 and Fig. 1). In 1997, a compromise on EC target was made to facilitate nutrient balance, which led to the actual EC in the closed systems sometimes being 0.5 ms cm 1 higher than in O-R from day 80 after planting to the end of the experiment. Therefore, the uptake of water in the

9 HAO AND PAPADOPOULOS TOMATO IN OPEN OR CLOSED ROCKWOOL SYSTEM 779 Table 5. Nutrient and water usage of tomato grown in open or closed rockwool culture systems with NFT or rockwool nutrient feedings z water uptake Nutrient uptake (g plant 1 ) Treatment z (L plant 1 ) N P K Ca Mg S February 13 to 16 July 1996 C-R C-NFT O-R O-R leaching January 31 to 25 June 1997 C-R C-NFT O-R O-R leaching z C-R, closed rockwool with rockwool nutrient feeding formulae/schedule; C-NFT, closed rockwool with NFT nutrient feeding formulae/schedule; and O-R, open rockwool with rockwool nutrient feeding formulae/schedule. closed systems was lower than in the open system in The nutrient uptake in the closed systems also declined, at least partially due to a reduction in water uptake. The reduction in water and nutrient uptake in the closed systems had no adverse effect on plant growth, photosynthesis, fruit yield and quality. Nutrient Dynamics and Fertigation Management in Closed Systems The N concentration in the closed systems was around or above 200 mg L 1 during most of the experiment in both 1996 and 1997 (Figs. 1 and 2), which was sufficient to meet plant needs (Adams 1986). The P and K concentration in the closed systems, 8 wk after planting, just before starting fruit harvest, declined to quite low levels in The P and K concentration in the C-R system, 80 d after planting, decreased to below 15 mg L 1 and 100 mg L 1, respectively, and stayed at this low levels until the end of experiment. The K concentration in the C-NFT system declined to below 100 mg L 1 for a shorter period of time (a month) and then recovered; this did not affect fruit production in the late production period. The dipping of K corresponded to a period of high K demand and uptake as reported by Voogt (1993) in a long tomato crop. From week 7 (56 d after planting) to week 14 (100 d after planting), the uptake of K linearly increased with fruit loads (Voogt 1993). In our experiments, the plants reached their highest fruit load around the time of first harvest, which was about d from planting. In the 1997 experiment, despite the increased P and K inputs (Table 2), the P and K concentration still dipped to 15 and 100 mg L 1 at 80 d after planting. However, because the levels of P and K soon recovered, it did not affect the fruit yield and quality. There was some accumulation of S, Ca and Mg in the closed systems (Fig. 2). However, after the adjustment in the fertilizer inputs in 1997, their concentrations were in acceptable ranges. The highest S concentration in 1997 was 200 mg L 1 ; this S concentration was not harmful to plant growth (Nukaya et al. 1991; Lopez et al. 1996). Several studies on Ca/Mg nutrition of greenhouse tomato (Sonneveld 1987; Hao and Papadopoulos, unpublished data) have demonstrated that plant growth and fruit yield in the latter growth stage benefited from high concentration of Ca and Mg. Therefore, the Ca and Mg inputs in the closed systems in 1997 did not need to be reduced further unless their accumulation caused too high an EC. The period around the beginning of fruit harvest is the crucial one for nutrient management in closed systems. Several researches (Gassim and Hurd 1986, van Noordwijk 1990) have shown that the roots of tomato stop growing about 56 d after planting because of heavy fruit loads competing for carbohydrates. This halt in root growth in combination with climatic factors may reduce nutrient uptake, but not water uptake, thus leading to low nutrient uptake concentration. Large decline in Ca, S, P and N uptake concentration in a year-round tomato crop at d after planting has been reported by Voogt (1993). In southwestern Ontario, this period usually corresponds to a major climate change, which is characterized with strong solar radiation, and consequently active ventilation. The strong solar radiation and ventilation stimulate plant transpiration and water uptake, and thus lead to low nutrient uptake concentration and high EC in the nutrient solution of closed systems. Considerable quantities of low EC nutrient solution or tap water have to be added to the closed systems to appropriately dilute nutrient solutions, which then prevents the replenishment of nutrients to the closed systems. In our tap water, there was considerable Ca (32.8 mg L 1 ), Mg (8 mg L 1 ) and S (7 ppm), but no P (0 ppm) and little K (1.2 ppm). Therefore, Ca, Mg and S accumulated in the closed systems while P and K declined to very low levels. In recent studies (Dorais et al. 2000; Hao et al. 2000), it was found that the target EC for tomato can be increased up to 40% above the recommended EC (Papadopoulos 1991) without compromising fruit yield, which makes it possible to supply more P and K (higher than those used in 1997) around the beginning of fruit harvest. In our experiments, the concentration of Na and Cl in C-R was ppm (seasonal average: 75 ppm) and ppm (seasonal average: 53.6 ppm), respectively. In C-NFT, the concentration of Na and Cl was ppm (seasonal average: 68.1 ppm) and ppm (seasonal average: 68.8 ppm), respectively. The maximum acceptable Na and Cl concentrations in the root environment for tomato are 276 ppm and 426 ppm, respectively (Voogt

10 780 CANADIAN JOURNAL OF PLANT SCIENCE and Sonneveld 1997). Therefore, the Na and Cl concentrations in our closed systems were well below the maximum allowed levels and there is no need for partial dumping of the solution from the system. Thus, the quality of our tap water is sufficiently good for producing greenhouse tomato in a fully closed rockwool system. The quality of water used in commercial greenhouse tomato production in Leamington, Ontario, the largest and most concentrated greenhouse tomato production area in North America, is similar to the quality of water used in our experiments. Therefore, water quality should not be a problem for a fully closed rockwool system for most commercial tomato production units in Ontario. In summary, the fully closed rockwool systems, as used in our experiments, with optimized nutrient feedings, support similar plant growth, fruit yield and quality as the open system while substantial savings in water and fertilizer can be achieved. 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