Effects of high electrical conductivity of nutrient solution and its application timing on lycopene, chlorophyll and sugar concentrations of hydroponic tomatoes during ripening

Abstract

Tomato (cv. Durinta) plants were grown hydroponically under two electrical conductivities (EC, 2.3 and 4.5 dS m −1) of nutrient solution inside a greenhouse. The high EC treatment was initiated either immediately after anthesis (high EC treatment) or 4 weeks after anthesis (delayed high EC treatment). Fruits were harvested weekly beginning 2 weeks after anthesis, until all fruits reached the red stage (8 weeks after anthesis). Lycopene, chlorophyll, sugar and total soluble solid (TSS) concentrations of fruits were measured every week for all harvested tomatoes from the different ripeness stages. The results showed that lycopene concentration, fructose and glucose concentrations and TSS of red ripe tomato fruits were enhanced by both high EC and delayed high EC treatments compared to those in the low EC treatment. The lycopene concentration of red ripe tomato fruits in the high EC and the delayed high EC treatments showed an increase of 30–40% (1.29–1.39 mg g −1 dry matter) compared to those in the low EC treatment (0.99 mg g −1); however, there was no significant difference in the lycopene concentration between the high EC and delayed high EC treatments. TSS of red ripe tomato fruits grown in the high EC treatment was 6.1%, significantly greater than those grown in the delayed high EC treatment (5.7%). Weekly change in lycopene concentration indicated that lycopene synthesis was enhanced by the high EC treatment, regardless of the application timing. Regardless of EC treatment, chlorophyll concentration in fruit declined linearly during fruit development and ripening and reached non-detectable levels 7 weeks after anthesis. Our results indicated that: (1) accumulation of sugars and TSS in fruit was due to reduced water flux to the fruit under high EC as previously reported, and (2) lycopene synthesis was promoted by, but chlorophyll degradation was independent from, the osmotic and/or salt stress caused by the high EC.