784 Journal of Food, Agriculture & Environment, Vol.11 (2), April 2013 www.world-food.net Journal of Food, Agriculture & Environment Vol.11 (2): 784-787. 2013 WFL Publisher Science and Technology Meri-Rastilantie 3 B, FI-00980 Helsinki, Finland e-mail: info@world-food.net Received 17 December 2012, accepted 30 April 2013. Effect of caffeine on grafted watermelon crop yields and fruit quality under greenhouse conditions Oscar Montes-Zavala, Fernando Diánez-Martínez and Francisco Camacho-Ferre* Research Group of Plant Production in Mediterranean Crop Systems, Superior Engineering School, University of Almeria, 04120 Almeria - Spain. *e-mail: fcamacho@ual.es Abstract In watermelon crops in Southeastern Spain, important thermal differences appear during the first stages of plant development that can affect the crop. This work shows the effect of applying caffeine (plant growth inducer or growth reducer and it is connected with allelopathic effects including inter- and intraspecific effects), on yield and fruit quality parameters from a crop in a greenhouse in Southeastern Spain. We assessed two treatments of caffeine, T 1 (2.25 μM), T 2 (9 μM), and a control test using an experimental design of randomized blocks with four replications. Treatment did not affect the yield variables (p>0.05) kg m - ², fruit m - ² and kg/fruit, but there were significant differences (p≤0.05) for rind thickness in the 2009 cycle. Results for soluble solids content, pH and flesh firmness did not show significant differences (p>0.05). The results showed that there is not any effect on the yield and fruit quality when applying the assessed treatments of caffeine. Key words: Alkaloid, Citrullus lanatus, 1,3,7- trimethylxanthine. Introduction Plants synthesise secondary metabolites with diverse chemical structures, such as alkaloids, flavonoids, phenols, steroids and terpenoids 1 , which play major roles in the adaptation of plants to the environment 2, 3 . Alkaloids are a group of nitrogen compounds present in plants 4 , and caffeine (1,3,7-trimethylxanthine) is an alkaloid derived from purine naturally found in approximately 100 plant species 5-7 . Caffeine plays a role as a plant growth inducer or growth reducer and it is connected with allelopathic effects including inter- and intraspecific effects 8-14 ; besides, it is an effective pesticide 15-19 . Caffeine and cytokinins are purine derivatives and their structures are resembling each other. Cytokinin-like effects of caffeine were observed in the bioassays dependent on chlorophyll synthesis in tobacco cell suspensions and cucumber cotyledons, radish cotyledon growth and elongation of etiolated soybean internode segments 20 . Cytokinins produce different biological activities in the plant, including stimulation of cell division, promotion of seed germination, growth of cotyledons and leaves, development of secondary stems, elongation of both stems and roots 21 and inhibition of the chlorophyll degradation 22 . The intact purine ring is essential for cytokinin activity in plants 23 . Cytokinins in Raphanus sativus cotyledons are deactivated by caffeine 24 . Caffeine promote stomatal opening in the epidermis of Vicia faba (L.) 25, 26 . Internodes can become etiolated in Glycine max (L) due to caffeine, and chlorophyll in Cucumis sativus L. can be increased, the positive response for caffeine in cytokinin-specific bioassays can be explained in terms of their similar structure 20 . Applied caffeine in concentrations from 50 to 2000 μM to Phaseolus aureus (Roxb) hypocotyls cuttings reduced number and length of roots 27 . Caffeine concentrations between 10,000 and 25,000 μM reduced numbers of root cells in Pisum sativum (L) seedlings 28 . Many vegetable crops are grown under protective systems such as greenhouses, mesh, etc. The profitability of these production systems is higher than that of non-protected systems. Eight crops are being used in the high-yield horticultural system in Almería, and the cultivated area used for watermelons has been kept practically constant over the last 10 years. Spain is among the world’s top 10 watermelon-producing countries 29 , and the province of Almería accounts for 60% of the national yield, which is estimated to be 721,800 t. The crop cycle carried out in the Spanish southeast for this cucurbit causes it to face thermal and/or hydric stresses, which reduce its productive potential. In an attempt to minimize this loss, various elicitors have been used to stimulate plants and overcome stress to maintain the yield and quality of harvests. This study aimed to determine the effect of caffeine on yield and fruit quality in grafted watermelon crops under greenhouse conditions. Materials and Methods The experiment was conducted during the agricultural spring season in 2009-2010 at the experimental foundation UAL- ANECOOP, located in the town of Retamar, municipality of Almeria, at 2.1708º W and 36.5177º N and 88 m altitude. The climate is Mediterranean semi-arid with less than 300 mm/year of precipitation, yearly average temperatures range from 15 to 21ºC. The winter is mild and presents significant thermal fluctuations. The minimum average temperature during January and February is approximately 6ºC 30 . The experiment was performed in an area of the experimental