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The Influence of Irrigation Method on Pot Geranium (Pelargonium peltatum L.) Grown with Saline Water S. La Fata, L. Incroccia, F. Malorgio and A. Pardossi Dip. Biologia delle Piante Agrarie, University of Pisa Pisa Italy

P. Battista, B. Rapi and L. Bacci Istituto di Biometeorologia, Consiglio Nazionale delle Ricerche Firenze Italy

Keywords: closed soilless culture, drip irrigation, pot plant, salinity, subirrigation Abstract

The paper reports a study on the influence of irrigation method on water relations and growth of pot geranium (Pelargonium peltatum L.) plants cultivated in a peat-perlite mixture. Two parallel experiments were conducted using difference sources of water for the preparation of nutrient solution: groundwater containing 10 mmol L-1 NaCl or rainwater with less than 0.5 mmol L-1 NaCl. In each experiment, six treatments, which derived from the factorial combination of three irrigation systems (open- or closed-loop drip irrigation, trough technique with recycling drain water) and two scheduling methods (timer vs. tensiometer with a water tension threshold of 50 hPa), were compared. In closed systems, the mixing tank was completely emptied whenever the EC of recycling nutrient solution exceeded 3.50 dS m-1. In both experiments, the use of tensiometer reduced significantly the water runoff in free-drain drip irrigation compared to the timer-based scheduling. The use of rainwater did not affect plant water uptake, which averaged 81.2 L m-2. In all closed systems the recycling water was never flushed out and then total water use corresponded to plant water requirement. In open drip irrigation, a huge runoff was produced, although it was substantially reduced by the use of tensiometer When groundwater was used, in closed drip irrigation the nutrient solution had to be discharged in one (tensiometer) or two occasions (timer), since its EC had reached the pre-set ceiling value; total runoff was 12.5 or 25 L m-2 , respectively. No important effects of irrigation system and scheduling method on plant growth characteristics was observed in the experiment carried out using rainwater. By contrast, the use of saline water resulted in a significant reduction of all growth parameters in closed-loop drip irrigation. Some reduction of plant fresh and dry weight, and leaf area and number of stems per plant was observed for subirrigation treatment with tensiometer-based control. The influence of irrigation technique and water quality on post-production plant performance was also investigated by simulating amateur cultivation in a patio with manual overhead irrigation. It was found that only the plants that had been subirrigated with saline water showed a rapid occurrence of leaf necrosis and abscission due to the solubilization of the salts accumulated during cultivation in the top layer of pot substrate. INTRODUCTION

The water use efficiency of greenhouse crops depends principally on the adopted irrigation technique, on the method used to estimate crop water requirements and hence to define both watering frequency and dose, and on the quality (namely, salinity) of raw water. In fact, when saline water are available, growers have to over-irrigate the plant in order to avoid the salinization of the root zone or, in closed growing systems, to discharge, more or less frequently, the recirculating nutrient solution with obvious implications from the environmental point of view (Carmassi et al., 2007). The need for leaching requirement is imperative in the production of pot ornamentals, which generally a incrocci@agr.unipi.it

Proc. IS on Prot. Cult. Mild Winter Climate Eds.: Y. Tüzel et al. Acta Hort. 807, ISHS 2009

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are quite sensitive to salinity stress. The paper reports a study on the influence of irrigation method and the salinity of

irrigation water on the water relations and the growth of potted geranium (Pelargonium peltatum L.) plants cultivated in a peat-perlite mixture. The influence of irrigation technique and water quality on post-production plant performance was also investigated. MATERIALS AND METHODS

Rooted cuttings of geranium (Pelargonium peltatum L.) were transplanted in December 2005 in a heated glasshouse (minimum temperature 10°C) using pots (diameter of 15 cm and height of 14 cm) filled a peat-perlite mixture; plant density was 8 plant m-2. The cultivation was carried out for 109 days until early April 2006.

Two parallel experiments were conducted using difference sources of water for the preparation of nutrient solution: groundwater containing 10 mmol L-1 NaCl or rainwater with less than 0.5 mmol L-1 NaCl.

In each experiment, six treatments, which derived from the factorial combination of three irrigation systems (drip irrigation with or without the recirculation of drain water, and closed subirrigation, namely trough technique) and two methods to schedule water, that is with a predefined daily watering activated by a simple timer or under the control provided by a hydraulic tensiometer placed in a reference pot. Every day at fixed time (08.00 a.m.) or whenever the tensiometer reading reached the threshold value of 50 hPa., a pre-established water dose was applied to drip-irrigated pots by means of self-compensating drippers (discharge rate of 2 L h-1) or the pots in trough system were subirrigated for 15 min with the thickness of water stream of nearly 1.5 cm. In drip irrigation system under timer control, the water dose ranged between 1.0 to 2.0 L m-2 according to the growth stage and the climate inside the greenhouse, that is the dose tended to increase during the cultivation; diversely, for tensiometer-based drip irrigation, a fixed water volume of 1.7 L m-2 was established in a preliminary experiment in order to restore the full water container capacity and produce a leaching fraction (LF, the percent ratio between supplied and drained water) not higher than 10-15%.

The composition of the nutrient solution used for free-drain irrigation or to compensate for plant water uptake in closed systems was the following (mmol L-1): 7.8 N-NO3, 0.9 P, 4.0 K, 4.0 Ca, 0.8 Mg, Na 0.5 or 10, plus microelements. The electrical conductivity (EC) was approx. 1.5 and 2.5 dS m-1, for rainwater and groundwater treatment, respectively.

Each treatment had three replicates, each consisting of 32 plants placed on a bench connected to a mixing tank containing free-drain irrigation or recycling water. Expressed on area basis, the water contained in the pot substrate and in the mixing tank was 8.0 (approx.) and 12.5 L m-2, respectively. In closed systems, the mixing tank was completely emptied whenever the EC of recycling nutrient solution exceeded 3.50 dS m-1.

The volume and the nitrogen concentration (N-NO3, as determined using the sulphur-salicylic acid method) of the nutrient solutions supplied to the plants or drained out from the systems were measured in order to determine the water and nitrogen use efficiency. Plant growth was assessed destructively at the end of cultivation. EC and Na concentration (as determined by flame photometry) were also determined in the aqueous extracts of substrate, which was obtained by means of 1 substrate: 2 water suspension method (Sonneveld and van Ende, 1971) following the horizontal division of substrate in the pot in two equal layers.

The influence of irrigation technique and water quality on post-production plant performance was also investigated by simulating plant cultivation in a patio with manual overhead irrigation for a period of four weeks.

The values reported are means (+ s.e.) of three replicates (eventually, each consisting of two plants per treatment); after ANOVA, LSD test was used for mean separation in each experiment.

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RESULTS AND DISCUSSION No important effects of irrigation system and scheduling method on plant growth

characteristics was observed in the experiment carried out using rainwater (data not shown). In these plants, total fresh and dry mass, leaf area and the number of stems averaged 301.8 g, 31.9 g, 0.40 m2 and 17.9 per plant, respectively. By contrast, the use of saline water resulted in a significant reduction in all the considered growth parameters in closed-loop drip irrigation Some reduction of plant fresh and dry weight weight, and leaf area and number of stems per plant was observed for subirrigation treatment with tensiometer-based control (Table 1).

In terms of water relations, the use of rainwater did not affect plant water uptake, which averaged 81.2 L m-2. In all closed systems the recycling water was never flushed out and then total water use corresponded to plant water requirement (plus evaporation from the substrate or from the trough, albeit not important). In open drip irrigation, a huge runoff was produced, although it was substantially reduced by the use of tensiometer; water runoff and LF were 107.3 L m-2 and 55% in timer treatment against 33.0 L m-2 and 28% in case of tensiometer.

When groundwater was used, in closed drip irrigation the nutrient solution had to be discharged in one (tensiometer) or two occasions (timer), since its EC had reached the pre-set ceiling value (3.50 dS m-1; Fig. 1). This resulted in a water or nitrogen loss, respectively, of 12.5 and 25 L m-2 or 1.6 and 4.4 g m-2 (Table 1). The rapid salinization of recycling water, which was mostly due to NaCl accumulation (as confirmed by laboratory analysis; data not shown), resulted in a reduction of both leaf area and water uptake (roughly, -20%) in closed drip-irrigated system with respect to all other treatment, including the ones conducted using rainwater (Table 1).

In both subirrigation treatments, there was no need for flushing, since the EC of recycling nutrient solution increased much more slowly than in closed drip irrigation (Fig. 1). These findings are in agreement with those reported by other authors (Morvant et al. 1997; Todd and Reed, 1998; Incrocci et al., 2006).

At the end of growing period, in all treatments under comparison a salt accumulation was observed in the top layer of pot substrate, as shown by the EC and Na concentration in the aqueous extracts (Fig. 2 for groundwater; data not shown for rainwater). This phenomenon was much more evident in case of subirrigation (roughly, top layer EC doubled the value in the bottom half) and when groundwater was used (Fig. 2), in agreement with previous reports (Incrocci et al., 2006; Santamaria et al., 2004). Indeed, in the pots subirrigated with groundwater the EC of top layer averaged 12.2 dS m-1, while a maximum EC of 6.7 dS m-1 was found in timer-controlled subirrigation with rainwater.

The influence of irrigation technique and water quality on post-production plant performance was also investigated by simulating amateur cultivation in a patio with manual overhead irrigation. It was found that only the plants that had been subirrigated with saline water showed a rapid occurrence of leaf necrosis and abscission due to the solubilization of the salts accumulated in the top layer of pot substrate during greenhouse cultivation. This phenomenon was also observed by Todd and Reed (1998) in a post-production trial with pot plants of New Guinea impatiens.

In conclusion, in closed subirrigation systems, the salts contained in excess in irrigation water tend to accumulate in the growing medium, in particular in the top layer, as a consequence of selective root mineral uptake and the ascendant water movement and this slows down the salinization of recirculating water, thus improving the overall water use efficiency of the cultivation. Incrocci et al. (2006) reported that, in comparison to drip irrigation, subirrigation increased water use efficiency and minimised the leakage of fertilisers in closed tomato culture conducted using water containing up to 10 mM NaCl. In pot ornamental production, however, the potential salinization of the pot substrate may negatively affect the post-cultivation shelf-life of the plants with evident results in term of marketing. On the other hand, the reduction of substrate salinity at the end of cultivation by ad hoc overhead watering would require plenty of (fresh) water, nullifying the water

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saved for irrigation. The only suitable practice to overcome this drawback of subirrigation with saline water is to inform the final buyer to use bottom irrigation. ACKNOWLEDGEMENTS

This work was supported by MIPA project ECO.IDRI.FLOR (2006-2008). We thank Mr. R. Pulizzi for his valuable technical support. Literature Cited Carmassi, G., Incrocci, L., Maggini, R., Malorgio, F., Tognoni, F. and Pardossi, A. 2007.

An aggregated model for water requirements of greenhouse tomato grown in closed rockwool culture with saline water. Agriculture Water Management 88:73-82.

Incrocci, L., Malorgio, F., Della Bartola, A. and Pardossi, A. 2006. The influence of drip irrigation or subirrigation on tomato grown in closed-loop substrate culture with saline water. Sci. Hort. 107:366-372.

Morvant, J.K., Dole, J.M. and Allen, E. 1997. Irrigation system alter distribution of roots, soluble salts, nitrogen, and pH in the root medium. HortTech. 7(2):156-160.

Santamaria, P., Campanile, G., Parente, A. and Elia, A. 2003. Subirrigation vs drip-irrigation: Effects on yield and quality of soilless grown cherry tomato. J. Hortic. Sci. Biotech. 78(3):290-296.

Sonneveld, C. and van den Ende, J. 1971. Soil analysis by means of a 1:2 volume extract. Plant Soil 35:505–516.

Todd, N.M. and Reed, D.W. 1998. Characterising salinity limits of new guinea impatiens in recirculating subirrigation. J. Amer. Soc. Hort. Sci. 123(1):156-160.

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Tables Table 1. Effect of irrigation system (I) and scheduling method (S) on the water relations,

nitrogen leaching and growth characteristics (determined at the end of cultivation) of potted geranium grown usingn groundwater containing 10 mmol L-1 NaCl). Mean values of three replicates (+ s.e.).

Water uptake (L m-2) Water runoff (L m-2) Timer Tensiometer Mean Timer Tensiometer Mean OD1 88.0 ± 3.6 78.7 ± 7.6 85.7 a 106 ± 4.3 16.0 ± 2.2 61.0 aCD 69.0 ± 3.6 67.7 ± 3.6 68.0 b 25.0 ± 3.2 12.5 ± 3.2 18.7 bSUB 83.7 ± 3.4 85.0 ±3.5 84.4 a 0.0 ± 0.0 0.0 ± 0.0 0.0 cMean 80.2 a1 78.5 a 43.7 a 9.5 b I S I x S

***2 ns ns

*** *** ***

Water use (L m-2) N loss (g m-2) Timer Tensiometer Mean Timer Tensiometer Mean OD 194 ± 7.9 99.5 ± 4.8 146.7 a 12.7± 0.5 1.3 ± 0.4 7.0 aCD 94 ± 4.8 79.5 ± 4.6 86.7 b 4.4 ± 0.9 1.6 ± 0.4 3.0 bSUB 83.7 ± 3.4 85.0 ± 5.9 84.4 b 0.0 ± 0 0.0 ± 0 0.0 cMean 123.9 a 88.0 b 5.7 a 1.0 b I S I x S

*** *** ***

*** *** ***

Fresh weight (g plant-1) Dry weight (g plant-1) Timer Tensiometer Mean Timer Tensiometer Mean OD 283.0 ± 19.7 314.0 ± 33.5 298.5 a 31.4 ± 3.3 33.0 ± 3.7 32.2 a CD 220.4 ± 23.8 260.9 ± 13.0 240.7 b 23.7 ± 2.5 27.9 ± 3.0 25.8 b SUB 337.4 ± 12.3 280.2 ± 44.1 308.8 a 36.0 ± 2.6 30.7 ± 5.1 33.4 a Mean 280.3 a 285.0 a 30.4 a 30.5 a I S I x S

** ns **

* ns ns

Leaf area (m2 plant-1) Number of stems (plant-1) Timer Tensiometer Mean Timer Tensiometer Mean OD 0.36 ± 0.02 0.39 ± 0.03 0.38 a 15.8 ± 2.5 18.5 ± 1.7 17.1ab CD 0.28 ± 0.02 0.29 ± 0.03 0.29 b 15.0 ± 1.9 16.8 ± 2.7 15.9 b SUB 0.42 ± 0.06 0.32 ± 0.07 0.37 a 20.5 ± 2.1 16.8 ± 2.4 18.6 a Mean 0.35 a 0.33 a 17.1 a 17.3 a I S I x S

** ns ns

* ns ns

1OD, open-loop drip irrigation, CD, closed-loop drip irrigation, SUB, subirrigation (trough system). 2 Mean separation at 5% significant level (LSD test); 3 ***, **, *, significant at P<0.001, 0.01 and 0.05 respectively; ns=not significant.

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Figures

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Fig. 1. Effect of irrigation system and scheduling method on the EC of recirculating

nutrient solution in closed soilless culture of pot geranium. The discharge of recycling nutrient solution in drip irrigation systems are indicated by arrows. Dotted line represents the EC of refill nutrient solution. See text for details. Mean values of three replicates (+ s.e.).

OD CD SUB OD CD SUB0.0

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Fig. 2. Effect of irrigation system and scheduling method on electrical conductivity (EC)

and Na concentration in the aqueous extracts of top or bottom layers of pot substrate, which were sampled at the end of cultivation of geranium. Mean values of three replicates (+ s.e.). OD, open-loop drip irrigation; CD, closed-loop drip irrigation; SUB, subirrigation (trough system).