Response of hydroponically-grown strawberry (Fragaria × ananassa Duch.) plants to different ratios of K:Ca:Mg in the nutrient solution

Abstract

SummaryExcessive Ca2+ ion concentrations in hydroponic nutrient solutions (NS) are inevitable if the Ca2+ ion concentration of the irrigation water exceeds standard recommendations. The aim of the present study was to compare three different strategies to adjust the K+ and Mg2+ ion supply and the K+: Ca2+: Mg2+ ratio in the NS supplied to hydroponically-grown strawberry (Fragaria × ananassa Duch. cv. Camarosa) plants when the Ca2+ ion concentration in the irrigation water exceeded recommended levels.The standard NS (SNS) for strawberry that was used as a control treatment had an electrical conductivity (EC) of 1.8 dS m–1 and contained 6 mM K+, 3 mM Ca2+, and 1.4 mM Mg2+ ions. SNS was compared with three NS, all with an excessively high Ca2+ ion level (5 mM), while their K+ and Mg2+ ion levels were: (i) either not altered (“High Ca”); (ii) increased to maintain the same K+:Ca2+:Mg2+ ratio as in SNS (“High all”); (iii) or decreased to maintain the same EC as SNS (“Compensation”). The EC values of these three NS with 5 mM Ca2+ were 2.2, 2.8, and 1.8 dS m–1, respectively. The lowest plant fresh weight and fruit yield values were recorded in the “High all” treatment (78% and 57% of the values in SNS, respectively), while the two other NS with 5 mM Ca2+ did not influence plant growth or yield compared to SNS. These results indicate that for optimum crop performance the EC of the NS was more important than the K+:Ca2+:Mg2+ ratio. The reduced plant growth observed in the “High all” treatment was associated with a large increase in the EC of the drainage solution from 2.8 to 5.0 dS m–1, and significant reductions (P ≤ 0.05) in stomatal conductance, net photosynthetic rate, and transpiration rate (44%, 39%, and 43% of the values in SNS, respectively), while the intercellular CO2 concentrations in leaves were similar in all treatments. Furthermore, strawberry plants supplied with the “High all” NS exhibited synchronised reductions in leaf water and leaf osmotic potentials compared to those treated with SNS, without any alteration in leaf turgor pressure and relative water content.These results indicate that the reduced fruit yield in the “High all” treatment was associated with impaired photosynthesis, due not only to reduced stomatal conductance, but also, at the chloroplast level, caused by water stress when the EC of the NS was raised to harmful levels.