S. Bonachela, J.A. Vargas and R.A. Acuña Departamento de Producción Vegetal, Universidad de Almería , Almería Spain

Transcription

1 Effect of Increasing the Dissolved Oxygen in the Nutrient Solution to Above-Saturation Levels in a Greenhouse Watermelon Crop Grown in Perlite Bags in a Mediterranean Area S. Bonachela, J.A. Vargas and R.A. Acuña Departamento de Producción Vegetal, Universidad de Almería. 412, Almería Spain Keywords: Citrullus lanatus, fruit quality, fruit yield, oxygen deficiency, perlite substrate Abstract Oxygen deficiency can occur even in well-aerated substrates as the result of various factors. In crops grown in perlite bags, where perlite is wrapped in a polyethylene film with little surface open to the atmosphere, the dissolved oxygen concentration ([O 2 ]) may be depleted quite rapidly at high growth rates and high substrate temperatures. This situation may occur in spring cycles of greenhouse vegetable crops on the Spanish Mediterranean coast. This work studied the behaviour of a watermelon crop (Citrullus lanatus L.) grown in perlite bags in a commercial greenhouse in El Ejido (Almería). A crop irrigated with a nutrient solution super-saturated with dissolved oxygen (O 2 +) was compared with a standard irrigated crop for which the [O 2 ] in the nutrient solution was below or about saturation values (O 2 -). [O 2 ] values measured at the dripper outflow were higher for the O 2 + treatment throughout the whole crop cycle (average value of 13.5 mg L -1 ) than for the O 2 - treatment (average value of 5.9 mg L -1 ). Moreover, [O 2 ] values in the nutrient solution extracted from perlite bags at the end of the watermelon cycle were slightly higher for the O 2 + treatment during the whole diurnal period measured, and the average values were 4.5 and 3.7 mg L -1 for O 2 + and the O 2 - treatments, respectively. In both treatments, [O 2 ] values were above 3 mg L -1, except for a short time interval in the O 2 - treatment. No significant differences were found between oxygen treatments for total and marketable production of watermelon, although it was slightly higher for the O 2 + treatment. Mean values of final marketable production were 7.2 and 6.9 kg m -2 for the O 2 + and O 2 - treatments respectively. Additionally, none of the quality parameters of seedless fruits were significantly affected by the nutrient solution oxygen content. INTRODUCTION The Mediterranean coast of South-east Spain, mostly the Almería coast, constitutes one of the largest greenhouse areas in the world, with approximately 35, ha of greenhouses dedicated to intensive vegetable production. In this area, substrate cultures, mostly open systems, have been used increasingly in the last fifteen years and the perlite culture, mostly in bags, is the main commercial soilless system (Pérez-Parra and Céspedes, 21). Numerous reports demonstrate that oxygen deficiency in the rhizosphere can induce lower root growth and ion and water uptake (Armstrong and Drew, 22). The main advantage of most substrates, such as rockwool and perlite, over soil cultivation is their ability to provide sufficient levels of both water and oxygen to the roots. Oxygen exists in porous media in both gaseous and dissolved forms, but plant roots consume only dissolved oxygen in water. Oxygen availability to roots usually depends on the oxygen diffusion rate (ODR) in the media, which is much lower in dissolved forms than in air. Oxygen deficiency can occur even in well-aerated substrates as the result of various factors (Raviv et al., 24). Most substrate-grown plants, cultivated in greenhouses under supposedly near-optimal production conditions, have high growth rates associated with high root respiration rates and water demands. This situation requires the supply of ample water, which makes it harder to allow enough air into the medium. Moreover, the higher greenhouse temperatures, compared to crops grown outdoors, increase root respiration Proc. IS on Soilless Cult. and Hydroponics Ed: M. Urrestarazu Gavilán Acta Hort. 697 ISHS 25 25

2 rates and could decrease dissolved oxygen concentrations in the substrate solution. Lower productivity or quality of substrate-grown horticultural crops induced by oxygen deficiency has been described for ornamentals (Bass et al., 1991) and vegetable crops (Bowen, 1983; Guri and Marfà, 1999). In crops grown in perlite bags, where the substrate is wrapped in a polyethylene film with little surface open to the atmosphere, the oxygen content may be depleted quite rapidly at high root respiration rates and substrate temperatures. This situation may occur in spring cycles of greenhouse vegetable crops on the Almería coast. Several methods of nutrient solution oxygen-enrichment (bubbling, agitation, forced aeration, etc) have been used to improve the oxygen supply to the roots in hydroponics systems. The supply of pure, pressurized, oxygen gas to the nutrient solution is another oxygen-enrichment method mainly used for research purposes (Chun and Takakura, 1994; Goto et al., 1996). The latter has been improved and adapted in order to be used in commercial horticultural greenhouses on the Spanish Mediterranean coast and has been called oxifertigation (Marfà and Guri, 1999). This work studied the behaviour of a watermelon crop (Citrullus lanatus L.) grown in perlite bags and irrigated with a nutrient solution whose dissolved oxygen concentration was maintained at super-saturated levels (oxifertigation facility). Watermelon is one of the main greenhouse spring crops on the Almería coast. MATERIALS AND METHODS The experiment was carried out from February to May 21 in a commercial Parral-type greenhouse in El Ejido (Almería). Iris, a seedless watermelon cultivar, was interplanted with Sweet Marvel, a standard seeded pollinator cultivar, at.77 plants m -2. Plants were grown in two-year-old reused perlite bags of 4 L and the most common perlite type (< 5 mm particle size) was used. A crop irrigated with a nutrient solution super-saturated with dissolved oxygen (O 2 +) was compared with a standard irrigated crop for which the dissolved oxygen concentration was below or about saturation values (O 2 -). The experiment was arranged in a randomised complete-block design. During each irrigation event, pure, pressurized, oxygen gas was dissolved in the nutrient solution of the O 2 + treatment with a gas injector within the irrigation pipe. Irrigation water of 1.5 ds m -1 electrical conductivity was supplied. Dissolved oxygen concentration [O 2 ] was measured with an oxygen probe (Handylab OX 1/SET, Schott-Geräte/Glas, Germany) of ±.1 mg L -1 resolution. [O 2 ] values in the nutrient solution supplied were measured every 2-3 days at the dripper outflow. Moreover, the diurnal [O 2 ] dynamic in the substrate solution was measured on a sunny day (28 April 21) at the end of the crop cycle. Solution was extracted from the lower and central part of the perlite bags with a soil moisture sampler (Rhyzon SMS, Eijkelkamp, Giesbeek, The Netherlands) after irrigation. Total and marketable watermelon yield was measured separating seedless from seeded fruits. Four seedless fruits were randomly selected from each replicate plot for quality analysis. Morphology parameters (transversal and longitudinal perimeter) of each fruit were measured. The fruit was then cut in half, and the rind thickness (cm) and core firmness (kg cm -2 ) were determined. Watermelon juice was extracted from a piece of core and total soluble solids (%) and ph were measured. Finally, a representative fruit sample was dried at 7 ºC for a week to determine percentage of dry-matter (%). Aerial or shoot biomass and its partitioning into fruits and vegetative parts were also measured at the end the crop cycle. RESULTS AND DISCUSSION Nutrient and Substrate Solution The watermelon crop was managed following common local practices. In both oxygen treatments, daily values of percentage of drainage water were higher at the beginning of the cycle (Fig. 1), maintained between 3 and 45% during flowering, fruit setting and fruit growth, and were decreased progressively by 5-1% at the end of the 26

3 cycle. This final reduction is a common local practice aimed at increasing fruit quality. Moreover, values were slightly higher for the oxygen-enriched treatment (O 2 +) during most of the growth cycle (Fig. 1). In general, values of percentage of drainage water were slightly higher than those recommended for the irrigation water used (Requena, 1999), but this is a common practice of most local growers. The electrical conductivity (EC) of the nutrient solution supplied was maintained at about 2.5 ds m-1 during most of the watermelon cycle in both oxygen treatments (Fig. 2), except at the end of cycle when it was progressively increased to values about 3.5 ds m-1 in order to enhance fruit quality. On the other hand, EC values of the drainage water were maintained at a level between.5 and 1.5 ds m -1 higher than those of the nutrient solution supplied during most of the watermelon cycle, except at the end when they were much higher (Fig. 2). EC values of the substrate solution were not measured, but they were between the values of the solution supplied and that of the drainage water, i.e. they should be between 3 and 4 ds m -1. Moreover, EC values of drainage water were slightly lower for O 2 + during most of the cycle, possibly due to the slightly higher percentage of drainage water in this treatment (Fig. 1). Values of dissolved oxygen content in the nutrient solution [O 2 ], measured at the dripper outflow, were higher and more variable for the O 2 + than for the O 2 - treatment throughout the whole watermelon cycle (Fig. 3). The average seasonal value of [O 2 ] for O 2 + was 13.7 mg L-1, although there were differences between the two oxygen-enriched irrigation sectors (Fig. 2). [O 2 ] was higher in the first irrigation sector [O 2 +(1)] than in the second one [O 2 +(2)] during the whole measured period, with average seasonal values of 16.2 and 11.2 mg L-1, respectively. [O 2 ] values for O 2 +(1) were within the interval recommended by Marfà and Guri (1999) for oxygen enrichment of nutrient solution by oxifertigation. [O 2 ] values for the O 2 - treatment ranged from 3 to 8 mg L -1 (below saturation values). Diurnal [O 2 ] dynamic in the nutrient solution extracted from perlite bags was measured at the end of the watermelon cycle, 73 days after planting (Fig. 4). Oxygen availability in the rhizosphere during this growth period could be theoretically limiting for the watermelon crop. The high growth rates (complete canopy cover) and high substrate solution temperatures (around 3ºC) in combination with ample irrigation water requirements could result in low oxygen availability because of high root respiration rates, low oxygen solubility in water and high substrate water contents (Armstrong and Drew, 22; Raviv et al., 24). During the measured diurnal period, [O 2 ] values in the substrate solution were slightly higher for the O 2 + treatment (Fig. 4), with average values of 4.5 and 3.7 mg L -1 for the O 2 + and O 2 - treatments, respectively. [O 2 ] values were above 3 mg L -1 for both treatments during most of the diurnal period, except for a short time interval in the O 2 - treatment. The minimum [O 2 ] values were detected 2-3 hours after midday in both treatments when the solution temperature was at its highest (Fig. 4), as it has been already described by Rivière et al. (1993) and Marfà and Guri (1999). A similar diurnal pattern of [O 2 ] in the substrate solution was found between oxygen treatments in previous measurement carried out 56 days after planting (data not shown), although the [O 2 ] values were higher. Slightly higher [O 2 ] in the substrate solution of a melon crop grown in rockwool slabs with oxifertigation was also found by Marfà and Guri (1999). Although there are differences between species, varieties, crop phases, etc (Armstrong and Drew, 22), oxygen deficiency or hypoxic conditions in the rhizosphere usually occur when the partial oxygen pressure is between 4 and 1% (Morard, 1995) or the [O 2 ] in the substrate solution is below 3 mg L -1 (Marfà and Guri, 1999). In general, the [O 2 ] in the substrate solution of the watermelon crop was above 3 mg L -1 for both oxygen treatments during most of the measured periods, although measured data are limited. The occurrence of hypoxic conditions that could affect watermelon growth, therefore, appears to be unlikely. 27

4 Biomass, Productivity and Fruit Quality No significant differences were found between oxygen treatments for shoot biomass or for shoot biomass partitioning at the end of the crop cycle (Table 1). Mean values of final shoot biomass were 716 and 733 g m -2 in the O 2 + and O 2 - treatments, respectively. These values are slightly lower than those measured by González (23) in an enarenado soil. Nor did the oxygen enrichment modify the crop harvest index, which was.77 and.78 g -1 g -1 for the O 2 + and O 2 - treatments, respectively (Table 1). No significant differences were found between oxygen treatments for total and marketable production of watermelon, although it was slightly higher for the O 2 + treatment (Table 2). Mean values of final marketable production were 7.2 and 6.9 kg m -2 for the O 2 + and O 2 - treatments, respectively, which are normal values for commercial greenhouses (Camacho, 1999). The same results were obtained when watermelon fruits were separated into seedless and seeded fruits (Table 2). Moreover, when yield data were analysed rearranging the experiment in order to assess the response of the irrigation sector maintained at the highest dissolved oxygen content [O 2 +(1)], there were no significant differences between oxygen treatments (data not shown), although the [O 2 +(1)] treatment presented the highest marketable yield (7.6 kg m -2 ). Finally, no significant differences between oxygen treatments were found when yield components were analysed (Table 2). None of the quality parameters (fruit morphology, dry-matter percentage, rind thickness, firmness, total soluble solids and ph) evaluated in seedless fruits was significantly affected by the oxygen enrichment (Table 3). In conclusion, growth, productivity and quality of a watermelon crop grown in two-year-old reused perlite bags were not significantly affected by the oxygen-enrichment treatment (oxifertigation). These results differ from similar greenhouse experiments carried out on the Spanish Catalonian coast, where sweet pepper, lettuce and rose crops (Marfà and Guri, 1999), grown in perlite substrates with oxifertigation yielded significantly more than the reference crop. A more comprehensive study is, therefore, required on the oxygen content in the rhizosphere of substrate-grown crops in Mediterranean greenhouses in order to identify those agrosystems where the oxygenation improvement could be of interest. ACKNOWLEDGEMENTS The authors would like to thank Sonia Guri and Oriol Marfà (IRTA of Cabrils, Barcelona) for their collaboration. They also thank the support of the company Carburos Metálicos S.A. Literature Cited Armstrong, W. and Drew, M.C. 22. Root growth and metabolism under oxygen deficiency. In: Y. Waisel, A. Eshel and U. Kafkafi (eds.), Plant roots. The hidden half. M. Dekker, New York: Baas, R Effects of oxygen deficiency on spray carnation grown in artificial sustrates. Acta Hort. 294: Bowen, P.A The effect of oxygen fumigation of sawdust medium on the yield and yield-components of greenhouse cucumbers. Scent. Hort. 2: Bunt, A.C The relationship of oxygen diffusion rate to air-filled porosity of potting substrates. Acta Hort. 294: Camacho, F El cultivo de sandía en el levante almeriense. En: Técnicas de producción de frutas y hortalizas en los cultivos protegidos, F. Camacho (ed.). Ed. Caja Rural de Almería. Almería. Vol.(2): Chun, C. and Takakura, T Rate of root respiration of lettuce under various dissolved oxygen concentrations in hydroponics. Environ. Control in Biol. 32(2): Gonzalez, A.M., 23. Programas de riego para cultivos hortícolas en invernaderos enarenados de Almería. Doctoral thesis. University of Almería, Spain. 21 pp. Goto, E., Both, A.J., Albright, L.D., Langhans, R.W. and Leed, A.R Effect of 28

5 dissolved oxygen concentration on lettuce growth in floating hydroponics. Acta Hort. 44: Marfà, O. and Guri, S Física de sustratos y oxigenación del medio radicular. En "Cultivos sin Suelo II". Fernández, M.; Cuadrado, I.M. (eds.). D.G.I.F.A., F.I.A.P.A. y Caja Rural de Almería. Almería: Morard, P Étude de l oxygenation du systeme racinaire. En: Les cultures végétales horrs-sol. Ed. SARL. Pub. Agric. Agen. France: Pérez-Parra, J. and Céspedes, A. 21. Análisis de la demanda de inputs para la producción en el sector de los cultivos protegidos de Almería. En Estudios de la demanda de inputs auxiliares: Producción y manipulación en el sistema productivo agrícola almeriense (Ed.). FIAPA, IFA, Almería: Requena, J.I Cultivo hidropónico de la sandía Citrullus lanatus. En "Cultivos sin Suelo II". Fernández, M.; Cuadrado, I.M. (eds.). D.G.I.F.A., F.I.A.P.A. y Caja Rural de Almería. Almería: Rivière, L., Charpentier, B., Jeannin, B. and Kafka, B Oxygen concentration of nutrient solution in mineral wools. Acta Hort. 342: Raviv, M., Wallach, R. and Blom, T.J. 24. The effect of physical properties of soilless media on plant perfomance. A review. Acta Hort. 644: Tables Table 1. Vegetative, generative and total shoot dry matter of a watermelon crop under two oxygen treatments: oxygen-enriched crop (O 2 +) and standard irrigated crop (O 2 ). Treatments Shoot Dry Matter (g m -2 ) Harvest index Vegetative Generative Total g g -1 O O NS NS NS NS NS: Values not significantly different (P <.5) 29

6 Table 2. Total and marketable yield (kg m -2 ) and yield components [number of fruits per square meter (NF) and mean fruit weight (FW, kg fruit -1 )] of a watermelon crop under two oxygen treatments: oxygen-enriched crop (O 2 +) and standard irrigated crop (O 2 ). Treatments Seedless and seeded fruits Seedless fruits Seeded fruits Total yield Marketable yield NF FW Marketable yield Marketable yield O O NS NS NS NS NS NS NS: Values not significantly different (P <.5) Table 3. Quality parameters [transversal (TP) and longitudinal perimeter (LP) in cm, drymatter (DM) in %, rind thickness (cm), core firmness (kg cm -2 ), soluble solids (%) and ph)] of seedless fruits. Watermelon crop under two oxygen treatments: (O 2 +) and (O 2 ). Treatments LP TP DM Rind Core Soluble ph thickness firmness solids O O NS NS NS NS NS NS NS NS: Values not significantly different (P <.5) 3

7 Figures Drainage water (%) O2+ O Days after planting Fig. 1. Time course of the drainage water (%) in a watermelon crop under two oxygen treatments: (O 2 +) and (O 2 ). Electrical conductivity (ds m -1 ) Drainage O2+ Drainage O2- Solution O2+ Solution O Days after planting Fig. 2. Time course of the electrical conductivity in the nutrient solution and the drainage water of a watermelon crop under two oxygen treatments: (O 2 +) and (O 2 ). 31

8 Dissolved oxygen content (mg L -1 ) Sector O2+(1) Sector O2+(2) O Days after planting Fig. 3. Time course of the dissolved oxygen content in the nutrient solution supplied. Watermelon crop under two oxygen treatments: (O 2 +) and (O 2 ). O 2 + data are shown separately for two irrigation sectors. Bars are twice the standard error. 35 Dissolved oxygen content (mg L -1 ) O2+ O2- Temperature Temperature (ºC) 1: 12: 14: 16: 18: Time Fig. 4. Diurnal dynamic of the dissolved oxygen content extracted form perlite bags on 28 April 21. Watermelon crop under two oxygen treatments: (O 2 +) and (O 2 ). 32