Growth and Developmental Parameters, Quality and Productivity of Cucumber as Affected by K: Ca Ratios of Nutrient Solution in Soilless System

Cucumber (Cucumis sativus L.) is an important fruit crops and cultivation in soilless condition may help to improve the quality and productivity. Due to susceptibility of cucumber to climate condition, the cultivation should be done under precise consideration. Nutrient uptake by plant is greatly affected by the concentration, as well as by the elements ration, especially the cations. Calcium and potassium play an important role in crops biology, functions, quality, and productivity. This experiment was conducted to evaluate the effect of different K: Ca ratios on quality and productivity of cucumber cv. ‘Negin’ to determine the best K: Ca ratio.
Material and This experiment was done in a hydroponic greenhouse with polyethylene cover. Day/night temperatures were 26±2 and 19±2, respectively. Relative humidity was adjusted at 55-60%. A pot experiment was conducted based on completely randomized design with three replications (9 plants/rep). Treatments included K: Ca ratios (2, 1.5, 1, 0.7, and 0.5). Cucumber cv. ‘Negin’ was treated with above-mentioned K: Ca ratios in an open soilless system equipped with drippers and plants were fertigated basis on Steiner nutrition formula. The culture medium was coco-fibre: perlite (1:1 v/v). Morpho-physiological, as well as biochemical parameters of cultivated plants including, plant fresh and dry weigh, leaf area, root volume, fruit length and weight, total carbohydrates, total phenol, vitamin C content, and fruit yield were determined. Results indicated that morpho-physiological and biochemical parameters, also, plant fresh and dry weight of cucumber cv. ʽNegin’ were significantly affected by application of different K: Ca ratios in nutrient solution (P<.0.05). The highest plant fresh weight (184.3 g) and root fresh and dry weight 29.36 g and 1.88 g, respectively) were obtained in K: Ca (1). Probably, the better growth condition such as proper relative water content and higher leaf area in K: Ca (1) caused the higher plant fresh weight and root fresh and dry weight. Moreover, the highest relative water content (RWC, 57.7 %) was recorded in K: Ca (1). The higher RWC in K: Ca (1) reveals the importance of K in maintaining plant water status through assimilates transportation, enzymes activation and stomata movement control. Additionally, fruits length, fruits weight, and fruit firmness were significantly affected by K: Ca ratios, where K: Ca (1) was the best. The enhanced parameters of cucumber cv. ‘Negin’ under K: Ca (1), reveals the importance of this ratio in nutrient solution. Calcium is a determinative element in fruit firmness, which enhances fruit firmness through producing crosslinks with cell wall components, as well as with pectin compounds in middle lamella. The highest and lowest content of carbohydrates were recorded in K: Ca ratios 1.5 and 0.5, respectively. Probably, K: Ca (1.5) shows positive effects on sugar synthesis. This increase might be due to over-activation of invertase enzyme and ATP production in K: Ca (1.5). The highest content of total phenol was produced in K: Ca ratio (2). Phenolic compounds are produced under stress (biotic and abiotic) condition. It seems that K: Ca (2) has led to unfavorable condition in nutrient solution, which has adversely affected the plant. Plants in order to deal with this condition have increased phenol production. The highest content of vitamin C was produced in K: Ca (1). Changes in this ratio resulted in reduced vitamin C content. Fruits with higher vitamin C content have higher nutritional value. Some researchers have reported that there is appositive and direct relation between calcium and vitamin C content in tomato, however it seems that the ratio of elements, especially K: Ca ratio is more important than the element concentration. Results indicated that fruit yield was significantly affected by treatments, where K: Ca (1) and (2) produced the highest (8.7 kg/plant) and lowest (5.5 kg/plant) fruit yield, respectively. K: Ca (1) resulted in highest fruit length and weight. These parameters are features of fruit yield and increasing in these parameters directly effect on fruit productivity. Taken together, results revealed that the ratio of elements are more important than the elements concentration in nutrient solution for producing high quality and yield of cucumber cv. ‘Negin’. K: Ca (1) was the best ration in most parameters. Although, K: Ca (2) and (1.5) were the best ratios for producing total phenol and carbohydrate content, respectively. Considering the antioxidant activity of phenolic compound, when producing cucumber fruits with high antioxidant activity is the goal, the K: Ca (2) in nutrient solution is a proper ratio.