CONTROL OF SALINITY IN THE RHIZOSPHERE OF PLANTS GROWN IN SOILLESS MEDIA

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1 CONTROL OF SALINITY IN THE RHIZOSPHERE OF PLANTS GROWN IN SOILLESS MEDIA L. Urban, A. Jaffrin, and R. Brun Unité de recherche intégrée en horticulture Institut National de la Recherche Agronomique Sophia-Antipolis Biot France Abstract Control of plant quality and control of the amount of nutrient solution leached seem to be related in plants irrigated with a dripping system. High salinity levels tend to cause a decrease in stomatal conductivity and photosynthesis. This, in time, will obviously affect yield and it will also affect cell elongation and some metabolic reactions involved with plant quality. More precisely, the partial closing of the stomata is not, in general, caused by the average salinity of the medium but by sudden and steep increases in salinity in the roots surroundings. These increases will develop when the amount of nutrient supplied exceeds the instantaneous demand of the plants. To limit these rises, producers leach the medium by supplying more nutrient solution than necessary. But, of course, this leads to water and nutrient waste as well as to pollution of surface and underground water. Nowadays, this type of pollution is unacceptable. Therefore, it is now urgent to find ways to reduce it without affecting yield and production quality. Two technical solutions seem worth accounting for 1) supplying the exact amount of water and nutrient to meet the instantaneous plant demand. In fact, this will make leaching unnecessary and will reduce waste; 2) increasing the medium mixing rate with temporary or permanent recycling of the nutrient solution leached. This will eliminate concentration gradients and stress due to sudden increases in salinity. Index words: Photosysnthesis, quality, salinity, soilless cultivation. 1. Introduction Non-EC countries of the south Mediterranean region compete more and more with south European countries on the flower and vegetable market. These countries have milder climates and lower production costs mainly due to very cheap labor. Producers of northern Europe who must grow plants under greenhouses have shown the way to resist this competition. They can assure very high yield and a standard of quality high enough to meet the need of their demanding European consumers throughout the year because of their highly performing production tools. Therefore, it seems obvious that technology is the key to commercial and economic success in the floriculture and vegetable production. However, this level of technicality is not matched by southern European producers. Control of hydro-mineral nutrition seems to be an essential although not unique solution to improve production and quality. Increase in nitrate and phosphate levels in surface and underground water might, in time, endanger flower and vegetable production. This is a particularly complex problem in parts of southern Europe where the flower and vegetable producers are established. In these regions (specially Spain and Portugal), the low turn-over registered in these sectors of the economy makues the heavy recycling plant with full disinfection developed in the Netherlands unaffordable. Even relatively performing greenhouse productions of the south of France and Italy will no have a turn-over high enough to makes such investment profitable. In fact, in these regions, growers cannot produce quality flowers and vegetables during the summer months whereas northern producers fill the markets all year. Acta Horticulturae 408, 1995 Soilless Cultivation Technology for Protected Crops 73

2 On the other hand, pure water is needed for recycling. In southern Europe, the water available often contains mineral elements and, unlike the Netherlands, rain water is not used for irrigation purposes because the uneven rainfall distribution throughout the year. The development of techniques that will account for this situation will affect the future of these regions of southern Europe where flowers and vegetables are grown. It seems therefore essential to provide technologies aiming at reducing leaching and pollution to the producers of these regions. 2. Materials and methods 2.1. Experiment No.l. A 100m2 glasshouse was used to grow plants of Rosa hybrida cv. Vivaldi, in individual pots, with three nutrition levels: 3,5 meq/1, 7 meq/1 and 14 meg/1 N-N03. All solutions were prepared from a unique initial nitrate compound solution (KN03, C (N03)2, Mg (N03)2) with various dilution factors. Separate acid additions (H3P04 and H2S04) helped to keep ph below 6.0. Sequestrene and oligo-elements were added with equal concentration for each nutrition. Cubic PVC containers (1.3 dm3) were filled with 1.8 kg of pure silica sand. Forty-eight bare root min-graft plants were planted in February 1992 in a heated greenhouse, with la low density ( 1 plant/m2). They were distributed by blocks of three (one from each treatment) which were randomized to make 16 replications. Static feeding regime Three different nutritions were delivered by 2 1/h drippers. Irrigation was triggered when the weight loss of a model plant exceeded 70 g (15% of the retention volume). Similar volumes were delivered to all plants at the same time and ensured a minimum leaching rate of 20%. The low nutrition (L) had half the median N03-N content, and the high nutrition (H) had twice the median content. This hierarchy was kept throughout the experiment. Separate measurements of transpiration and photosynthesis rates were performed with a portable, infrared gas analyser (LCA2 from A.D.C.) and a Parkinson leaf chamber. Measurements were performed on individual leaves of rose plants, fed in their normal static regime (with 20% leaching rates). Dynamic feeding regime In early morning, a restricted (1 liter) volume of fresh nutrient solution was placed into the tank, and percolated through the medium at a rate of 1,5 1/h. This flow rate, which is roughly 20 times the maximum transpiration rate and 4 times the retention volume per hour, characterizes the dynamic feeding mode with strong mixing, by opposition with the static feeding mode which keeps the solution at rest in the medium. Weight loss measurements were performed during clear days, but ion a partially shaded, greenhouse, with PAR values below 400 ^imoles/s/cm Experiment No 2 The experiment was conducted from spring 1991 to winter 1992 on Rosa hybrida cv. Sonia plants grown in a heated greenhouse Cuttings were rooted in rockwool cubes on 2 April 1990, then planted on rockwool slabs (Cultilene) on May 10, 7 plants per slab (7.5 plants per m2). Rockwool slabs measured 74

3 100 cm length x 20 cm width x 7.5 cm depth. Fertilization followed the recommendations of Brun and Tramier (1988), with ph set at 5.6. Irrigation was scheduled using a standard computer equipped with a Kipp and zonen solar energy sensor (Systeem 9 from INDAL). Irrigation was triggered for each 280 J/cm2 increment of external horizontal solar energy. Irrigation delivered 100% of the expected maximum évapotranspiration. Cumulative drainage over 24 hours was at least 40% of the supplied volume. The electrical conductivity (EC) of the drainage water was set It 1.8 ms/cm. This EC rate was chosen following the recommendations of run and Tramier (1988) for rose cultivation in rockwool. The method devised to achieve the EC monitoring has been described by Brun et al. (1994) Experiment No 3 16 plants of rose plants cv. Sonia were grown on rockwool in two plastic gutters. The study was carried out in a climatic chamber (temperature set at ,5 C, relative humidity set at 80 ± 10%) equipped with 3 quicksilver vapour lamps (Phillips MNF ) and 3 sodium vapour lamps (Phillips SON-T 400 W) delivering 70 Watts/m2. Light was switched on at 19hOO and switched off at 19hOO. The nutrient solution (Brun and Tramier, 1982) was continuously delivered by a peristaltic pump. Three treatments were applied -T 1: Nutrient solution with a low concentration (EC -1 ms/cm) supplied at a rate of 5300 ml/h; -T21: Nutrient solution with a high concentration (EC - 4 ms/cm) supplied at a rate of 53 ml/h; -T22: Nutrient solution with a high concentration (EC - 4 ms/cm) supplied at a rate of 1100 ml/h. Gaz exchange measurements were performed with a portable infrared gas analyser (LCA 2 from A.D.C.) and a Parkinson leaf chamber. Measurements were performed on 8 mature leaves per treatment, oriented perpendicularly to the lamps. 3. Results 3.1. Experiment No 1 The results shown below (Table I) give clear indications that the nutrition level modifies the photosynthesis under strong solar radiation, when plants are fed in the normal static regime. TREATMENT PAR TRANSPIRATION PHOTOSYNTHESIS (nmol/m 2 /s) (mmol/m 2 /s) jimol/mvs) L-Nutrition 890 6,8 12 M-Nutrition 861 5,3 9,4 H-Nutrition 862 4,5 7,6 Table 1. PAR. transpiration and photosynthesis as affected by the nutrition level. Each treatment is distinguished from its neighbor by a significant 20 % difference in transpiration rate, as well as in photosynthesis rate (T-test, P = 0.05). However, measurements performed with lower irradiation levels (less than 400 (iml/cm^) do not show any significant effect on the nutrition level (data not shown). 75

The main observations for the dynamic feeding regime are the following - most transpiration/par fluctuations are closely related to radiation fluctuations (Fig.

4 The main observations for the dynamic feeding regime are the following - most transpiration/par fluctuations are closely related to radiation fluctuations (Fig. 1); - three distinct plants, belonging to different treatments, exhibit similar (nearly homothetic) transpiration/par patterns, the small gap between curves being most probably due to some leaf area excess in plant of M-treatment. One can deduce that there is no significant effect from salinity levels on plant transpiration rates in a dynamic feeding regime Experiment No 2 This monitoring helped to maintain EC drainage values in a very narrow range, very close to the target value and to the EC of the supplied nutrient solution (Fig. 20). 3.3 Experiment No 3 The Steel and Dawssn non-parametric test (P = 0.05) demonstrated that photosynthesis was significantly higher in T1 than in T21 and T22 and that photosynthesis was significantly higher in T22 than in Tl. 4. Discussion 4.1. Control of salinity, plant quality and leaching Control of plant quality and leaching of nutrient solution seem to be related in plants irrigated with dripping systems Effects of salinity on the quality of horticultural products High salinity levels have a negative effect on stomatal conductance and photosynthesis (Table I of experiment no 1). The latter affects not only yield but also cell elongation and some metabolic reactions involved in product quality as well. This was shown with roses; partial closing of stomata due to high salinity levels leads to a decrease of net photosynthesis (Urban and Langelez, 1992). Consequently, osmotic pressure and turgor pressure decrease leading to a reduction of cell elongation. This decrease in cell elongation causes a decrease in the growth of flower stems and flower buds (Urban et al ). The flower stems are then shorter which is perceived by the consumer as a decrease in plant quality. In tomatoes, it is well known that high salinity levels cause a decrease in fruit size (Sonnefeld and Welles, 1988), It can even lead to blossom-end rot problems. These make the product unsuitable for sale. On the other hand, tomato plants grown at low salinity levels can produce fruits that are not firm (Sonnefeld and Welles, 1988). The high salinity levels can reduce the yield of medicinal or favouring species (Economakis, 1990). But, for some of them, particular balances of nutrient solution at given stages of development can lead to higher contents of the wanted ingredients (A. Coudret, personal communication) Control of salinity and leaching It is important to notice that, in general, the partial closing of the stomata is not caused by the average salinity of the medium but instead by sudden and steep increases in salinity near the roots. These in creases occur when the amount of water and nutrient supplied exceeds the instantaneous demand of the plant (Jaffrin et al., 1993). To limit the effects of these concentration gradients, producers partially leach the substrate by providing more nutrient solution than necessary. In plants grown in soilless media, 76

5 leaching rates of 15% (in winter) and more than 30% (in summer) are common (Brun and Tramier, 1988). Yet this practice leads to a water and nutrient waste as well as a pollution of surface and underground water. The same occurs in plants grown in the fields and irrigated with dripping systems (Calado and al., 1992). It has been found that 2.5 tons of nitrates per hectare are wasted every year on the basis of a consumption of 1 m3 nutrient solution/ha/year, having an average concentration of 20 mmol/1 of N and an average leaching rate of 20%. Today, this type of pollution seems unacceptable and it is becoming urgent to find practical solutions to reduce it without affecting yield and production quality Different ways to control salinity Water déminéralisation The first step to avoid or limit sudden rises in salinity in roots surroundings consists in supplying a nutrient solution containing a minimum amount of useless ions. Unfortunately, the water used in the making of the nutrient solution often already contains more or less desirable ions such as chloride, sulfate ions and also sodium, calcium and magnesium in some cases. Adding fertilizers to the water can lead to a solution much too high in salinity compared to the plant needs. As we have said, the rainfall pattern being irregular in the southern regions, it seems difficult to collect rain water to make these solutions, as is done in the Netherlands. Consequently, it would be useful to find ways to treat the water by partially demineralizing it at reasonable cost (less than 5 francs m^) and to treat the large mount of water needed for irrigation even when soilless media are used. Two techniques are worth considering; ion exchange resins with non polluting regeneration and osmosis. The problems that need to be solved when treatment of water is involved are the following: - Ion exchange resins require a regeneration that introduces other mineral charges (NaCI for example) in the used water. These charges are useless, even harmful since they are themselves polluting agents and are harmful to the plants. It would be useful to evaluate a way to use regeneration salts that replace, for example, Na + by K + and CI by S04^-; - Generally, osmosis calls for a pre-treatment against membrane clogging (divalent cation exchange Ca ++ and Mg ++ ). Here, it would be useful to add acids useful to the solutions and to find ways to make these membranes work for a long time with these cations. These methods of treating water could be part of the production systems developed according to the two following ideas Meeting the actual water and nutrient demand of the plants By adjusting the water and nutrient supply to the exact demand of the plants, leaching, which permits the excess salts to be washed out of the substrate, would be considerably reduced. This would also mean a reduction of nitrate and phosphate pollution of surface and underground water (Brun and al., 1992). Control of the nutrient solution supplied can be done by monitoring its composition or its global concentration. To precisely adjust the amount of mineral elements supplied to the plants to the amount in which they will be absorbed, it seems essential to know their absorption kinetics. Unfortunately, these and especially the short-term ones (less than 24 hours) are not well known. 77

Furthermore, few studies have been done on inexpensive selective ion probes used on a continuous basis. At first, it would be useful to work on the development of inexpensive specific ion probes.

6 Furthermore, few studies have been done on inexpensive selective ion probes used on a continuous basis. At first, it would be useful to work on the development of inexpensive specific ion probes. Then, knowledge on elementary absorption kinetics for the three methods presented and on specific on probes could be used in the control systems. It would be simpler to try to control the salinity of the substrate by adjusting the global concentration of nutrient elements supplied to the plants to the total concentration of nutrient elements they absorb. The actual fertirrigation systems theoretically allow for the adjustment of this concentration as a function of received radiation. The principle of this adjustment is based on the fact that the concentration of the solution supplied must be reduced when plant transpiration increases due to higher radiation since, then, the concentration of the solution absorbed by the plants decreases. But empirical adjustments made by the growers remain incorrect. Trials to control the electrical conductivity of the supplied solution as a function of that of the leaching solution have limited concentration gradients in the substrate (see experiment no. 2) Increase of the mixing rate in the substrates with temporary or permanent recycling of the leaching solutions This is done in order to eliminate local concentration gradients and temporary soluble salt stress. High mixing rate has a positive effect on transpiration (fig. 1, experiment no. 1 ) and on photosynthesis (fig. 3, experiment no. 3). The recycled nutrient solution is unbalanced when compared to that taken up by the plants. This leads to an accumulation of more or less desirable ions and to a depletion of ions needed for plant growth and development. Consequently, this will further increase the unbalance of the recycled nutrient solution. If no adjustment is made, this will increase this unbalance in an exponential fashion. In order to reduce it, a "fresh" nutrient solution is usually mixed to the recycled one. This simple practice, used in the Netherlands, gives rather good results regarding the elements really absorbed by the plants. On the contrary, when the water used in the nutrient solution contains important quantities of elements that the plant does not absorb, then these elements can only remain in the solution. It would be interesting to define acceptable thresholds in the concentration of undesirable mineral elements (specially sulfate, chloride, and sodium) for species of horticultural plants. These thresholds should be defined as a function of yield and quality objectives for each species. Increases in the mixing rate also have disadvantages. The first one is obvious: the area around the roots is water saturated. This can even lead to root asphyxia expressed by a decrease in nutrient and water uptake. When this state lasts, it leads to fermentation and then to root rotting. This must be avoided since disease development is often associated with it. Recycling can only increase disease development. Recycling with disinfection (heat, ultraviolet, ultrafiltration, ozonization) is being developed in the Netherlands and other northern European countries as an answer to pollution problems raised by leaching in soilless media. Unfortunately, these technique are very expensive and destroy the natural antagonistic flora. It is not only wishful but also possible to try to control disease development in recycled system without involving disinfection (Waechter-Kristensen et al., 1993). To do that, the plants must not be stressed. Then, it seems possible to stimulate the natural antagonistic flora. The evolution of the pathogen population could be monitored by regular bacterial and fungal control and thus the precise moments when intervenings are needed would be determined. It is obvious that in the case of the rose plant, sanitation control of the growing conditions in soilless media with recycling can be useless if the plants are contaminated from the start. For this reason, it is essential to study the sanitary states of the rose plants before placing 78

7 them in closed systems. 5. Conclusion It appears, today, that there are a number of ways worth studying, specially on an economical standpoint, that could improve quality control of horticultural products and reduce polluting wastes. Developing new technical solutions could reduce the threatening technological gap between southern and northern Europe. References Brun, R., Un système simple pour cultiver le rosier hors sol: le bac-tranchée. P.H.M Revue horticole Brun, R. And Tramier, P.H Culture du rosier sur laine de roche. P.H.M. Revue Horticole 289: Brun, R., Morisot A. and Urban, L., Doit-on diminuer les concentrations d'engrais pour le rosier en hors-sol? Et jusqu'où? Atout-Fleurs, Bulletin d'information de l'horticulture et de la Pépinière Méditeranéenes 7: 2-3. Brun, R.,Paris, B. and I. Hamelin Management of the fertilizing irrigation of rose plants grown in greenhouses on rockwool Advances in Horticultural Science (in press). Calado, A.M., Portas, C.M., Ferreira A.G. and da Silva, M.L.P., Percolation lost of macronutrients in processing tomatoes cultivated in sandy soils. Acta Horticulturae 301: Economakis, C.D., Effect of solution conductivity on growth and yield of Origanum dictamnus L. in nutrient film technique. Acta Horticulturae 306: Jaffrin, A., Andre, J.P. and Champeroux, A., Hourly hydric and mineral absorption of 'Vivaldi' roses grown on a pure sand medium under summer conditions. Acta Horticulturae 361: Sonnefeld, C. And Welles, G.W.H., Yield and quality of rockwool-grown tomatoes as affected by variations in EC values and climatic conditions. Plant and Soil Urban, L. and Langelez, 1., Effect of high pressure mist on leaf water potential, leaf diffusive conductance, C02 fixation and production of 'Sonia' rose plants grown in rockwool. Scientia Horticulturae 50: Urban, L., Lanelez, I., Morisot, A. and Pyrrha, P., Effect of high-pressure mist and daytime continuous C02 supplementation on water status and quality of 'Sonia' rose plants grown on rockwool. Advances in Horticultural Science 6: Waechter-Kristensen, B., Gertsson, U. and Sundin, P.,1993. Prospects for microbial stabilization in the hydroponic culture of tomato using recirculating nutrient solution. Acta Horticulturae 361:

1 4 0 0 T.' TV., : i / n../xv^ w oin ui ifl o in ^ m Nui nin in m in io in N in Local time Fig.

sol. EC leachate 3000 2700 2400 2100 1800 1 500 + 1 200 900 -- 600 300-0 EC us/cm at 20'C target level 270 280 290 300 310 320 330 34 day (year 1991) Fig.

8 T.' TV., : i / n../xv^ w oin ui ifl o in ^ m Nui nin in m in io in N in Local time Fig. PAR (full line in jimol/m2s) and transpiration efficiency (in %), for dynamic feeding under moderate irradiation and three nutrition levels ' EC calculated EC nut. sol. EC leachate EC us/cm at 20'C target level day (year 1991) Fig. 2 Mean 24 hours values for the actual and the calculated EC of the drainage and the actual EC of the supplied nutrient solution, as a function of time. Fig. 3 Net photosynthesis as a function of time for the three salinity treatments 80

INFLUENCE OF ELECTRICAL CONDUCTIVITY, RELATIVE HUMIDITY AND SEASONAL VARIATIONS ON THE BEHAVIOUR OF CUT ROSES PRODUCED IN SOILLESS CULTURE

INFLUENCE OF ELECTRICAL CONDUCTIVITY, RELATIVE HUMIDITY AND SEASONAL VARIATIONS ON THE BEHAVIOUR OF CUT ROSES PRODUCED IN SOILLESS CULTURE I. Urban*, R. Brun** and L. Urban** *C.N.I.H. Station d'expérimentation