2018 ASHS Annual Conference

A study was established to re-evaluate the relationships of rose leaf nitrogen (N) status - expressed both on leaf weight and area basis - and flower productivity/quality in own-rooted and grafted (Peach Avalanche® on ‘Natal Briar’) rose plants. The plants were irrigated with six complete nutrient solutions (based on a modified 0.5X Hoagland solution) differing in total N concentration: 2, 4, 6, 8, 10 and 12 mM; tap water added an extra 1.5 mM of N to all treatments. Nitrogen was applied with an average 12% ammonium fraction, using sulfate salts as substitute for nitrate salts in the lower N treatments, thus producing iso-equivalent solutions with electrical conductivities averaging 1.74 dS/m. While no statistical differences in flower yield parameters were detected after four flowering flushes, chlorosis symptoms, supported by chlorophyll index readings, started to be noticeable in the foliage of plants receiving the lowest N concentrations. Leaf tissue analyses pointed out to a progressing manganese (Mn) deficiency (< 30 mg/kg), along with relatively low concentrations of iron (Fe). Micronutrient levels in the nutrient solutions were increased to 1.25X Hoagland solution concentrations and the study continued for another four flowering flushes. The flower yield and quality responses displayed thereafter a classical asymptotic behavior, which were not or minimally correlated with leaf N status. Leaf N concentrations remained similar across all N treatments, averaging 30.8 g/Kg and 1.48 g/m2 on a dry mass and leaf area basis, respectively, pointing to a remarkable control of plant N uptake and partitioning. Leaf tissue concentrations of Mn, and Fe as well, provided significantly high correlations with the observed flower yield and foliage quality responses. Evaluation of the applied nutrient solutions and drainage (leachate) solutions collected at several points through the study suggest that despite initial pH adjustments across all N solution treatments, nitrification activity increased with total applied N concentrations, leading to concomitant reductions in pH values. While the soil solution pH differences were within 1 pH unit across all treatments, the range in which it occurred (5.8 to 6.9) was deemed critical for a differential availability of micronutrients in solution, more acutely for the non-chelated Mn supply, despite increases in their application rate during the second half of the study. As this particular scenario is a first occurrence in our rose nutrition studies, a specific cultivar (varietal) response is strongly suspected to be in play as well.