Nitrogen absorption, growth of stock plants, and adventitious rooting of Pelargonium × hortorum cuttings as affected by the form and dosage of nitrogen

Abstract

SummaryEffective modern production of young Pelargonium hortorum L.H. Bailey plants requires a reliable rooting response of shoot tip cuttings from stock plants. This study was conducted to explore the N demand and the influence of the form and dosage of N on the growth of stock plants in aerated nutrient solutions (A, C) and thin-layer peat substrates (B). The effects of three N dosages at 1, 7, and 13 mmol N l–1 (A) or 0.5, 1.5, and 4.0 g N m–2 week–1 (B) were examined. Futhermore, at uniform dosages of 7 mmol N l–1, graduated substitutions of NO3– by NH4+ were investigated using six NH4+:NO3– ratios of 0:100, 20:80, 40:60, 60:40, 80:20, and 100:0 (C). The quality, survival and rooting capacity of cuttings were determined in response to NH4+ or NO3– alone (A), NH4+ and NO3– fertigation (B), or the NH4+:NO3– ratio (C). Natural irradiation and different shading regimes during Spring and Summer cultivation ensured adequate light [photosynthetic photon flux density (PPFD) = 314 µmol m–2 s–1], while limiting light during Autumn (PPFD = 135 µmol m–2 s–1) interfered with the N treatments. The N status, survival rate, and rooting of cuttings were examined in samples of cuttings provided by propagators worldwide. In solution-culture, NH4+:NO3– ratios of 20:80 and 40:60 provided optimised growth and quality of cuttings at 7 mmol N, regardless of light level. Compared with NO3–-only nutrient solutions, these ratios enhanced the branching of stock plant roots, NO3– absorption in shoot tissues, overall cutting yield, and adventitious root formation in cuttings. Although NH4+:NO3– ratios above 60:40 impaired fresh weight and the number of tip cuttings per plant, the rooting capacity of such cuttings was promoted further, as long as adequate light was available. We hypothesise that the ratio of N form shifts hormone levels and carbon fluxes which, in turn, influence the root and shoot morphology of stock plants, as well as the rooting of cuttings. Cultivation in peat substrate resulted in the highest yields and best rooting of cuttings at weekly N doses of 1.5 g N m–2. The high dose of N (4 g) did not enhance yield, but rather reduced the rooting of cuttings at adequate light levels.This dose also resulted in increased NO3–-N and salt accumulation in the peat. Moreover, while reduced rooting at a low N dose (i.e., 0.5 g) indicated N limitation, the impaired rooting seen at a high N dose may have been induced by osmotic stress to the stock plants.