Effects of NH4:NO3:urea ratio on cut roses yield, leaf nutrients content and proton efflux by roots in closed hydroponic system

The effects of the NH4:NO3 ratio in replenishment solution on Rosa L. flower yield and the impact of NH4 substitution by urea on plant performance and on solution EC and pH have not been studied previously in closed (no leaching) hydroponic systems. A greenhouse experiment with six NH4:NO3:urea ratios (0:100:0, 12:88:0, 25:75:0, 50:50:0, 100:0:0 and 0:50:50) and two harvest cycles (winter and spring) was carried out to investigate these relationships. In winter, total and >40 cm cut flower yields were maximal in treatment 25:75:0. At lower NH4 percentages (12.5:87.5:0 and 0:100:0), growth container solution pH varied between 7.8 and 8.5, reducing P, Ca and Mn concentration in leaves and increasing dry matter allocated to them. At higher NH4 percentages, Ca uptake was inhibited, solution pH reached 3, and %P in leaves increased. Consequently, reducing sugars concentration in leaves increased and sucrose and starch concentrations decreased. A stepwise regression analysis indicated that the optimal NH4:NO3 ratio in feed solution is 40:60, with resulting solution pH of 5.9 in the growth container. In spring the maximum yield was obtained in treatment 0:50:50 and it exceeded the winter yield despite a higher solution EC (4.3 dS m−1 vs. 3.5 dS m−1 at harvest). The beneficial effect of urea (0:50:50 vs. 50:50:0) stemmed from the relatively lower NH4 concentration in solution, that alleviated the NH4–Ca uptake competition, and higher pH. The slope of the straight line relating [H+ efflux rate] to [NH4+ uptake rate] in treatments 25:75:0, 50:50:0 and 100:0:0 was 0.44 mol H+/mol NH4. In all other treatments the proton efflux was negligible.