THE EFFECT OF WATER AND POTASSIUM SUPPLEMENT ON YIELD AND LYCOPENE CONTENT OF PROCESSING TOMATO

Abstract

An open field experiment was conducted to study the effects of irrigation and potassium supplementation on the yield, soluble solids and lycopene content of processing tomato fruit. Three different irrigation treatments were compared: regularly irrigated, irrigation cut-off 30 days before harvest, and unirrigated. Irrigation scheduling was based on the average daily temperature. Each irrigation treatment had two different potassium rate subplots (454 and 555 kg ha respectively). Across all potassium rates, the regularly irrigated treatments yielded higher than the irrigation cut-off and unirrigated treatments. Soluble solids content and °Brix yield were highest in the regularly irrigated + high potassium and the irrigation cut-off + high potassium treatments. All of the irrigated treatments showed significantly higher marketable yield with the high rate of potassium, while this effect did not occur in the unirrigated treatments. The high potassium rate increased the lycopene content of tomato in the irrigation cut-off and unirrigated treatments. INTRODUCTION This experiment was undertaken to investigate the effects of irrigation and potassium (K) fertigation on fruit yield and quality (color-lycopene) of drip-irrigated processing tomatoes. Tomato fruit ripening is a complex, genetically programmed process that culminates in dramatic changes in colour, texture, flavour, and aroma of the fruit flesh (Lucille and Grierson, 2002). The colour (lycopene content) and °Brix of processing tomato at harvest are an important quality criterion. Several studies have reported that lycopene constitutes 75 to 83% of the total pigment content of tomato, whereas βcarotene is only 4 to 7%. Lycopene is accumulated mainly in deep red stage and colour is an indicator of lycopene level. The higher the ratio of a*/b*, the higher the lycopene concentration (Brandt et al., 2006; Helyes et al., 2006). Tomato fruit surface temperature was a more accurate predictor of fruit lycopene content than air temperature, especially in situations where the fruit has been directly exposed to intense sunlight (Helyes et al., 2007). Main nutritional compounds of tomato can also be influenced by agricultural techniques such as the growing system, the irrigation and fertilization management as well as the soil salinity and the fruit maturity at harvest (Dorais, 2007). According to Hartz et al. (2001, 2005) potassium nutrition has been linked to tomato yield and fruit color. Potassium level is positively correlated with a good fruit shape and a reduction in ripening disorders (Dorais et al., 2001). According to Serio et al. (2007) the lycopene content increased linearly with increasing potassium level in the nutrient solution. MATERIALS AND METHODS The plots were located at Szent Istvan University Godollő. The experimental field is brown forest soil, with soil textures of sand and sandy-clay. The subsoil water is below 5 m, therefore it cannot influence the water turnover. Seeds were sown on the 2 of April a Email: Helyes.Lajos@mkk.szie.hu Proc. XI IS on the Processing Tomato Eds.: R. Pitblado and J. Routledge Acta Hort. 823, ISHS 2009 104 2007 in the greenhouse and transplanted on the 14 of May 2007. Brigade F1 variety was used. Seedlings were arranged in double (twin) rows with a distance of 1.2 and 0.4 m between the rows and 0.3 m between the plants. There were two different irrigation treatments (regularly irrigated, irrigation cut-off 30 days before harvest, and controlunirrigated) and two different K fertilisations. Drip irrigation was scheduled based on the air temperature (daily irrigation water (mm) = average daily temperature×0.2). Predicted temperatures from the National Meteorological Institute forecasts were used in the calculations. Total water applied (irrigation and rainfall) was 375, 263 and 188 mm water for the regularly irrigated, irrigation cut-off and unirrigated control respectively during the season. All treatments received Agroblen 18-8-16 (nitrogen-phosphorus-potassium) fertilizer when plants were transplanted, to deliver 266 kg ha K fertiliser. Additional potassium fertiliser was applied as potassium nitrate (KNO3) at fruit set, at two rates of 454 and 555 (+ K) kg ha per irrigation plot. Red and green fruit yield was measured at harvest on the 23 of August. Lycopene from homogenised tomato was extracted with n-hexane-methanol-acetone (2:1:1) mixture containing 0.05% BHT. Water-free Na2SO4 was used to remove water traces of the upper part. Optical density of the hexane extract was measured spectrophotometrically at 500 nm against hexane blank (Sadler et al., 1990) by UV-VIS Spectrophotometer Lambda 3B (Perkin Elmer). Concentration of lycopene was calculated using specific extinction coefficient (E1cm 3150) (Merck Index, 1989). The °Brix was measured with a refractometer. Experiments were conducted as a randomized complete block with two factors (irrigation and potassium treatment). The data were analysed by two-factor analysis of variance (ANOVA) with repetitions and the means separated using the Student’s test at p=0.05. RESULTS AND DISCUSSION Effect of Irrigation and Potassium Supplementation on Tomato Yield Temperature and precipitation conditions during the growing season (Fig. 1) have a significant impact on the effect of irrigation on yield. Treatments without irrigation has low yields, because the fruit number and size were much smaller. Irrigation treatments (cut off 30 days before harvest and regularly irrigated) increased the marketable (red and green) yield by 11 and 62% respectively. It was also observed that a higher potassium level positively correlated with yield (marketable yield increased by 29 and 21% respectively) in irrigated treatments. There were no significant yield differences due to potassium rates in the unirrigated treatment (Fig. 2). Figure 3 shows the effect of irrigation and potassium on soluble solids content (°Brix). The soluble solids content of fruits decreased with irrigation. In spite of this, the °Brix yield per hectare increased with irrigation as a result of significantly higher yield (Fig. 4). Effect of Irrigation and Potassium Supplement on Lycopene Content Many experiments have found evidence that small fruits contain more lycopene than larger ones. Figure 5 shows average lycopene content of tomato fruit across treatments. We found significant differences in average lycopene content between treatments. Unirrigated treatments (unirrigated and unirrigated +K) gave the highest average lycopene content of all (14.35 and 15.9 mg/100 g respectively). It is important to note that the average fruit weight was much smaller in unirrigated treatments than in irrigated treatments (by 40-50%). Lycopene content is fundamentally determined by the genetic nature of varieties, but it does not exclude the fact that environmental factors (temperature and light intensity), maturity and fruit weight also strongly affect it. We also found significant differences in lycopene yield between treatments. Figure 6 shows lycopene content of tomato fruits and total lycopene production in relation to average fruit weight.