WATER AND NUTRIENT USE EFFICIENCY OF A TOMATO CROP AS AFFECTED BY TWO REFRIGERATION METHODS: EXTERNAL MOBILE SHADING AND FOG SYSTEM

High incident radiation levels during the spring-summer cropping cycles in Mediterranean greenhouses generate microclimates of high temperature and evaporative demand. The aim of this work is to study the effect of two refrigeration methods on the water and nutrient uptake of a tomato crop in multispan greenhouse during the spring cropping season. Measurements were taken during crop ontogeny in two greenhouses equipped with (i) an external mobile shading and (ii) a fog system. Water and nutrient uptake were measured weekly by calculating the daily balance between supplied and leached nutrient solution. The adopted strategy to manage the mobile screen caused a 20% reduction of the canopy incident radiation. Water uptake was 209 and 184 L plant in the greenhouses with fog system and shade, respectively. Total nutrient uptake (N, K, Ca, Mg and P) and ion absorption rates (mmol L) were higher in the greenhouse with fog system than in the greenhouse with mobile screen. Total yield was higher in the greenhouse with fog system but no significant differences were found in marketable yield due to a higher incidence of Blossom End Rot (BER) in the greenhouse with fog system. The mobile screen increased water and nutrient use efficiency in terms of fruit yield as compared with the fog system. INTRODUCTION High incident radiation levels during the spring-summer cropping cycles in Mediterranean greenhouses generate microclimates of high temperature and evaporative demand. Passive ventilation under these conditions is insufficient to extract the excess of energy and maintain adequate conditions for crop growth (Baille, 2001), so normally it is used in combination with other climate control techniques, based on the limitation of the energetic load by decreasing the incident radiation or reducing the latent heat by evaporating water. High radiation and Vapour Pressure Deficit (VPD) values cause high transpiration rates which have a direct effect on water and nutrient uptake. In trials conducted in long periods (scale of months or weeks) it seems that water and nutrient uptake take place at the same time, while in short period trials (scale of hours or days) the ratio of water to nutrients taken up by the plants varies widely with time of the day (Andriolo et al., 1996). Plant growth ceases below critical nutrient concentrations but, if nutrient availability is excessive, nutrients accumulate in the plant with no increase of the dry matter content (Le Bot et al., 1998). Improvement of nutrient use efficiency largely depends on improving synchronisation of nutrient supply with nutrient demand (Van Noordwijk, 1990). Water and nutrient uptake are independent processes and differences in the uptake concentration are more due to water uptake than to nutrient uptake differences (Sonneveld and Bos, 1995). In some crops the concentration of the applied nutrient solution is higher than the uptake concentration to avoid possible nutrient deficiencies associates with unstable nutrient uptake concentration, which may change with the growing conditions and are generally higher in moderate climates like Central Europe than in warm areas like the Mediterranean countries (Sonneveld, 2003). The plant nutrient demand characteristics change continuously as the plant grows. Willits et al. (1992) found that there was a gradual change from week to week for chrysanthemum. Sutherland (1988) demonstrated a distinct change in nutrient uptake as cucumber development moved between growth stages. The results found by Jang and Nukaya (1997) in a rockwool muskmelon crop suggest that mineral uptake is related to the concentration of the nutrient solution during the vegetative phase, but not during the Proc. IS on Soilless Cult. and Hydroponics Ed: M. Urrestarazu Gavilán Acta Hort. 697 ISHS 2005 464 following phases. Kläring et al. (1999) related the daily ratio of nutrient to water uptake to micrometeorological conditions in the greenhouse such as radiation, temperature and VPD. The aim of this work is to study the effect of two refrigeration methods on the water and nutrient uptake of a tomato crop in multispan greenhouse during the spring cropping season. Measurements were taken during crop ontogeny in two greenhouses equipped with (i) an external mobile shading and (ii) a fog system. MATERIALS AND METHODS The experiment was conducted in two half-round roof 3-span greenhouses, 720 m each, covered with plastic-film located at CIFA (Almería, Spain, latitude 36o48’ N, longitude 2o41’W). The greenhouses were equipped with natural ventilation and heating systems operated by a climate control system. The setpoints for heating were 16/18oC for night/day conditions, respectively. The ventilation temperature was 25oC. Climate parameters inside both greenhouses and outdoors were continuously monitored: radiation (global and photosynthetically active -PAR-) and dry-wet bulb temperatures every 5 and 10 min. respectively. Tomato (Lycopersicon esculentum Mill., ‘Boludo’) seedlings were planted in 14 L pots filled with perlite A13 (particle size between 0.1 and 0.5 mm in diameter) on 6 March 2003 (28 days after sowing) at a plant density of 2 plants m. Harvesting started the 19 May 2003 and continued twice a week till the 11 July 2003 (126 days after planting). Irrigation water (EC = 1.5 dS m) and fertilizer were provided by an automatic drip irrigation system, with one dripper (3 L h) per container. The nutrient solution, with an EC value of 3.0 dS m , contained following ions on mmol L: NO3 11.5, H2PO4 2, SO4 3, K 7, Ca 4.5, Mg 2 and a concentration of sodium chloride of 8 mmol L. Irrigation was maintained high to avoid accumulation of salts in the substrate. Two refrigeration systems were conducted by (i) the installation of a mobile shading system outside one of the greenhouses (Sh treatment), an aluminised shade screen (OLS ABRI 50 %), and the combination of natural ventilation and (ii) water evaporation by means of a low pressure fog system installed in an other greenhouse (Fog treatment) to maintain VPD values under 1.5 kPa. Droplets were generated by water/air nozzles (12 L h water consumption and 6 bar air pressure) distributed in a three-section network, 240 m each. Shading was activated when outside global radiation was higher than 650-700 W m whenever the air greenhouse temperature was higher than 28 oC. When the canopy was developed (LAI > 3.5), from 10 week after planting onwards, the set-points for shading were increased to 750-800 W m and 29 oC in order to favour fruit ripening. Yield was monitored on six randomly chosen samples of six plants per treatment. Ripe fruits were harvested twice a week and were classified into marketable and unmarketable fraction and this fraction was divided into fruits with Blossom End Rot (BER) and others. Water and nutrient uptake were measured weekly by calculating the daily balance between supplied and leached nutrient solution. Water uptake of the crop was determined from the daily balance between the measures of irrigation and drain from a 6-plants section of two central rows of each treatment. Water use efficiency (g L) was calculated as the ratio between marketable yield (g m) and water uptake (L m) plus water used in Fog (L m).