Evaluation of Rapid Methods for Nitrate Plant Sap Analysis of Globe Artichoke Grown in Sand Culture

Artichoke has a high nitrogen requirements and often farmers apply excessive amounts of fertiliser that contributes to groundwater pollution by nitrate. Artichoke nitrogen fertilisation can be made more efficient by plant analysis. Petiole sap nitrate concentrations in recently mature leaves was tested as an indicator of crop nitrogen status in artichoke grown in sand culture. Two rapid methods were assessed: a selective ion meter, Cardy Meter and a hand-held reflectometer RQflex. Artichoke plants were grown in containers filled with sand and subjected to five treatments ranging from deficient to excess nitrogen supply. The five treatments were applied in a randomised block design with four replicates, and with four plants per experimental unit distributed under unheated greenhouse. Nitrate concentration in petiole sap measured by the Cardy Meter and the hand-held reflectometer (RQflex) was highly correlated with the laboratory method, giving a correlation coefficient (r) of 0.91 and 0.92, respectively. Petiole sap nitrate levels reflected different nitrogen applications rates satisfactorily along the growth cycle. Petiole sap nitrate determined either by the Cardy Meter or RQflex can be a quick and useful tool to assess artichoke nitrogen status. INTRODUCTION An inadequate management of nitrogen fertilisers has caused aquifer pollution by nitrate in intensive horticulture areas (European Commission, 1999; ITGME 1998) as in “L’Horta de Valencia”. Nitrogen is one of the most important plant nutrients due to its role in vegetative growth, yield and crop quality. Furthermore it is one of the most difficult elements to optimise due to its susceptibility to leaching, denitrification and volatilisation. Therefore appropriate management of nitrogen fertilisers is necessary to control input costs and reduce the risk of surface and ground-water nitrate pollution (Jemison and Fox, 1988). The difficulties in assessing in nitrogen availability in soil prompted the development of other techniques such as plant tissue analysis which have been used as a tool guide to manage crop N fertilisation (Reuter and Robison, 1997). A quick alternative to soil and plant standard analysis has been petiole sap tests. The development of equipments such as hand-held reflectometers or selective ion meters that can be easily performed by growers which have increased and spread its use. These equipments have been tested with satisfactory results in several crops (Prasad and Spiers, 1984; Hartz et al., 1993 and 1994; Hochmuth, 1994; Errebhi et al., 1998; Coulombe et al., 1999). Sap analysis had proved to be faster and a more sensitive indicator than standard plant analysis of crop nutrient status (Matthäus and Gysy, 2001; Villeneuve et al., 2002). Artichoke is one of the most relevant crops in the Valencian Community. It is regarded for its high nitrogen requirements and it is prone to contribute to nitrate leaching and ground-water pollution (Ramos et al., 2002, Khayyo, 2004). The lack of data about sap nitrate analysis in artichoke and the need to improve nitrogen fertilisation had lead to develop this technique. The aim of this study was to assess two rapid methods of nitrate sap analysis for artichoke plants grown in sand culture under controlled conditions. MATERIAL AND METHODS Artichoke (Cynara cardunculus var scolymus L.) ‘Blanca de Tudela’ plants were Proc. IS on Soilless Cult. and Hydroponics Ed: M. Urrestarazu Gavilán Acta Hort. 697 ISHS 2005 394 grown in containers filled with a sand mixture (1:1 AF:AG) under unheated greenhouse and subjected to five N fertilisation treatments: 1 (50 kg N·ha -1 ); 2 (125 kg N·ha ); 3 (200 kg N·ha -1 ); 4 (275 kg N·ha) and 5 (350 kg N·ha) to obtain a range of different available mineral nitrogen levels in the growing media. The equivalent nitrogen rates per container and plant were calculated assuming a normal planting density in the field of 7.000 plants · ha. Nitrogen and the other nutrients were applied weekly as a complete nutrient solution during all the growing period. The experiment was carried out at IVIA Moncada (Valencia). One artichoke plant per container was transplanted and distributed in four rows along the greenhouse, rows were separated 1.8 m each and plants were separated 1m within rows to allow satisfactory canopy development. The experimental design was four randomised blocks with five treatments and four plants per experimental unit. Plant leaf samples were taken fortnightly from all plots early in the morning: for each treatment sixteen recently matured leaves were excised, placed in plastic bags and stored at 46 oC until processed. In the laboratory, prior to processing, each sample was divided in two subsamples. In one subsample was oven-dried (65 oC) for later analysis. On the other one, the midrib was separated from the leave blade, which was discarded, and then was cut and squeezed in a stainless steel garlic crusher for determining NO3-N content by three methods: 1) a portable selective ion meter Cardy Meter (Horiba, Kyoto, Japan) in which two to three drops of non-diluted fresh sap were placed on the sensor pad and a nitrate reading (mg·l) was recorded 2) a hand-held reflectometer RQflex (Merck, Darmstadt, Germany) sap samples were centrifuged during 10 minutes and diluted with deionised water, prior to reading the samples were mechanically agitated to homogenise the sample. After that a reflectoquant strip was dipped during two seconds in the sample and waited until complete colour development occurred (60 s), than it was placed in RQflex for reading. Both methods 1 and 2 were regarded as rapid methods, and the third method was a laboratory method (Sempere et al., 1993). It was used for comparison purposes. Weather data as well as growth and production measurements were also recorded (data not presented). Data were statistically analysed by statistical package SAS (SAS Institute v. 8) using a 5 % significance level. RESULTS AND DISCUSSION Nitrate concentration in petiole sap measured by the Cardy Meter and the hand-held reflectometer (RQflex) was highly correlated with the laboratory method, giving a correlation coefficient (r) of 0.91 and 0.92, respectively (Fig. 1). The results obtained in artichoke plants grown under controlled conditions are in accordance with those achieved in artichoke plants grown in experimental fields (Rodrigo et al., 2004), however Cardy Meter readings were much more accurate than in the previous experiment. Nitrate sap concentration varied with time following the same pattern in all treatments: an increase until the beginning of the harvesting period (January-February 04), followed by a slight but steady decrease till the end of the experiment (Fig. 2 & 3). This decline for all nitrogen application levels corresponded with the harvesting period. These results contrasted with those obtained in experimental fields where a steady decrease in nitrate sap content during the growing period was observed with the three methods (Rodrigo et al., 2004), and this may be due to the need to supply nutrients along the trial due to the growing media characteristics. Changes in nitrate sap during growth have been found in other species (Hochmuth, 1995; Huett and Rose, 1989) with constant nitrogen supply. Petiole sap nitrate concentration was very sensitive to the different nitrogen treatments (Fig. 2 and 3). The concentration of petiole sap nitrate was significatively different at rosette of leaves and shoot formation period (October 2003), and the starting of head emission (December 2003, future artichoke production) from plants, and these differences remained along the whole productive period which lasted until May 2004. Significant differences between the lowest (deficient nitrogen supply –treatment 1) and the highest (extra nitrogen supplytreatment 5) were observed all along the experiment with the three methods (Fig. 2 and 3). The range in petiolar sap nitrate concentrations in response to applied nitrogen was in agreement with rates reported in other species such as cabbage (Huett and Rose, 1989), pepper (Olsen and Lyons, 1994) and potato (Huett & White, 1992).