1. Introduction: Lettuce (Lactuca sativa L.) is one of the most important leafy vegetable. Due to importance and high demand the greenhouse and hydroponically growing of lettuce is increasing. One of the key points in effectiveness of hydroponic production is the formulation of suitable nutrient solution. Nitrogen as an essential element could be provided by two form in nutrient solution, ammonium and nitrate. Although both form provide the nitrogen for plants but they can affect the growth and physiological aspects differentiatly. In other side open systems of hydroponic in addition to to high cost for nutrient solution preparation can cause soil and water pollution, so its need to use hydroponic systems that decrease the amount of nutrient solution consumation. In this study we assessed the effect of three different NH4+:NO3- ratio in nutrient solution in an easy and very low cost closed hydroponic system so called Kratky method on lettuce production.
2. Materials and Methods: The experiment was conducted in hydroponic greenhouse as a factorial based on completely randomized design. Three nutrient solutions with the same level of total nitrogen but with different ammonium to nitrate ratio (0.5:8, 1:8 and 2:8) were used for growing the three lettuce cultivars (Local Romain, French red and Salinas) in closed hydroponic system without the exchange of nutrient solution during the growing cycle and just water were added to substition the of the consumed solution. The seeds of lettuce cultivars at first were planted in cell try that were filled with coco peat-perlite with 1:1 ratio and after emergence and until reaching to suitable size for transplating seedling were fertigated with hoglnd nutrient solution. Two lettuce seedlings were transplanted via the small plastic small baskets and styrofom plate in pots with seven liters volum until the end of growing period. The characters like leaf and root fresh and dry weight, chlorophyll, carotenoid and anthocyanin content were assessed.
3. Results and Discussion: The results indicate that the ammonium:nitrate ratio had significant effect on the lettuce leaf and roots fresh and dry weight, chlorophyll index and anthocyanin contents. The cultivars had significantly difference effects on leaf and roots fresh and dry weight, collorophyl index, carotenoid and anthocyanin content. But the interaction between the ammonium:nitrate ratio and lettuce cultivars were not significant on the investigated characters. Between the cultivars Salinas and locally Romaine respectively had the highest and lowest yield. The highest and lowest root fresh and dry weights also were obtained from the 2:8 and 0.5:8 ammonium to nitrate ratios respectively while the 0.5:8 and 1:8 ammonium to nitrate ratio were not the significantly different from each other on the leaf fresh wight. The same rule also was for the leaf dry weight. The Salinas and French red had the highest dry weight and the localy romain significantly leaf lowest dry weight. On the roots fresh and dry weight characters there was significantly difference between the cultivars that was close to the results on leaf fresh and dry weight. So that Salinas withought the significantly difference with the French red had the highest root fresh weight and lowest root fresh weight belongs to locally romain. Result showed that lettuce being a vegetative crop satisfied by increasing the ammonium:nitrate ratio in nutrient solution. This is close to the results that previously has been reported by some authors.
4. Conclusion: The highest fresh and dry weights of lettuce were obtained by the solution with the 2:8 ammoniums:nitrate ratio, which indicate the superiority of the lettuce simultaneously feeding with nitrate and ammonium. In addition to quatitative characteristic like the yield the yield, simoultaneously feeding with ammonium and nitrate affected the qualitative characteristics like the anthocyanin and cartenoeid contents. Also providing the some parts of Nitrogen needs of plant as ammonium can reduce the nitrate accumulation. All of three nutrient solution with different ammonium to nitrate ratio but with the same level of total nitrogene and without any need to aeration also without the change of nutrient solution and only by substitution of consumed water were able to feed the lettuce plants without any visible disorders. This kind of hydroponic due to the drastic decrease of production costs would be suitable method specially for small scale hydroponic vegtable production greenhouse.