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

Bar-Yosef, B., Mattson, N.S., Lieth, H.J.
Scientia horticulturae 2009 v.122 no.4 pp. 610-619
Rosa, nitrogen, plant nutrition, urea, ammonium nitrogen, nitrate nitrogen, crop yield, leaves, nutrient content, protons, ion transport, root exudates, greenhouse experimentation, hydroponics, electrical conductivity, pH, phosphorus, calcium, manganese, dry matter partitioning, carbohydrate metabolism
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 >40cm 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 (4.3dSm⁻¹ vs. 3.5dSm⁻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 0.44molH⁺/mol NH₄. In all other treatments the proton efflux was negligible.