The Effects of Fe-Chelate Type and pH on Substrate Grown Roses

Substrate grown roses appear to be susceptible to chlorosis, which indicates problems with Fe or Mn uptake and hence yield reduction. In common practice this problem is often treated by the addition of extra Fe-chelate, or the use of Fe-EDDHA instead of Fe-DTPA. In previous tests, it was shown that the pH in the root environment is a major factor in the prevention of chlorosis. Moreover, the application of FeEDDHA does not always show satisfying improvements in practice. The interaction between Fe-chelate types (EDDHA and DTPA) and pH was studied with roses cv. ‘Kiss’ and ‘Escimo’ on glasswool substrate, reusing drainage water. pH levels compared were about 7, 5.8 and 4.5. The treatments resulted in significant chlorosis and consequently yield reduction at high pH with both cultivars and both chelate types. Highest yields were obtained at low pH, especially with ‘Escimo’. The Fe uptake was clearly affected by the pH with both chelate types. At high pH the Fe-uptake was significantly higher with Fe-EDDHA; however the Mn contents in the plant were significantly lower with these treatments. The uptake of Zn and Cu was also affected by specific combinations of pH and the type of chelate. It was concluded that an optimal pH control was the best method of preventing chlorosis. The choice of the chelate type was less effective and could enhance Mn deficiency. INTRODUCTION Chlorosis is a common phenomenon in substrate grown cut-rose. It is often associated with Fe-deficiency, since there is a strong resemblance with symptoms of Fe deficiency (Winsor and Adams, 1987). This can be caused by high pH in the root environment which is a quite common phenomenon too (Bij de Vaate and RoubosHoogstraten, 1997). Actually, pH control in rose appeared to be rather difficult compared to many other substrate grown crops. This is most likely due to the rapid and sequential change in growth stage, following the method of production in flushes, which involves a quite drastic adjustment in the NH4:NO3 ratio required in the nutrient supply (Voogt, 1994). In commercial practice these requirements cannot be fulfilled: moreover many growers have a negative attitude towards this necessary high NH4 supply in the nutrient solution. Fe-DTPA is the standard recommended chelate fertilizer in nutrient solutions for substrate culture (De Kreij et al., 1993). Since chlorosis became an increasing problem, Fe-EDDHA became more and more recommended as it has a much higher stability constant at high pH than Fe-DTPA. Fe-EDDHA is also widely used to cure chlorosis in calcareous soils (AlvarezFernadez et al., 2005) and has proven to be effective. However in rose crops, the application of Fe-EDDHA has not always been effective in curing Fechlorosis. This study compares two chelate types at a range of pH levels, to investigate the effect on chlorosis, yield, stem quality and (micro-) nutrient uptake. MATERIALS AND METHODS Experimental Design The experiment consisted of three factors: two Fe-chelate types and three pH levels in the nutrient solution, and two rose cultivars. The nutrient solution treatments were laid out in four parallels in two randomized blocks in a greenhouse, and the rose cultivars were laid out as split-plots in each field. Each nutrient treatment consisted of a Proc. IS on Growing Media 2007 Eds.: W.R. Carlile et al. Acta Hort. 819, ISHS 2009 412 closed system, with one supply tank and a central drainage collection for all parallel fields. The cultivars were, ‘Escimo’ and ‘Kiss’: in commercial practice known as susceptible and moderately susceptible for chlorosis respectively. The chelate types: FeDTPA (AKZO, dissolvine D-FE-6-P, 6% Fe) and Fe-EDDHA (AKZO, dissolvine Q-Fe6, 6% Fe) were used. In all treatments, the target value was 25 μmol L Fe in the circulating nutrient solution. The Fe treatments were achieved by operating at high circulation rates as well as weekly analysis of the nutrient solutions and subsequent adjustment of the Fe supply. The pH levels were 7.0, 5.8 and 4.5 in the root environment respectively, basically regulated by the addition of 0.25, 0.75 and 1.25 mmol L NH4 respectively in the standard solution. These concentrations were slightly adjusted depending on the pH developments in time. Fine tuning was done by automatic pH control and subsequent adjustment dosing of either KHCO3 or a mixture of HNO3 and H2SO4. Growing System The experiment was conducted in a modern standard greenhouse with growing conditions in accordance with rose growing in commercial practice. Glasswool was chosen as the substrate since it is inert for both pH as well nutrients. Slabs, 20 cm in width and 10 cm in height were wrapped in polythene sheets, provided with drainage slits at 2 cm from the bottom, placed in gutters and covered with black and white polythene sheets. Each individual plant was provided with a trickle nozzle. Drainage water was collected and reused, without sterilization treatment. Irrigation frequency was adjusted to the crop demand, aiming at a drainage rate of 0.8. The base composition of the nutrient solution supplied, for the nutrients not under investigation was: K 5.0, Ca 3.5, Mg 0.75, NO3 11, SO4 1.25, P 1.25 all in mmol L , Mn 5, Zn 3.5, B, 20 m, Cu 0.75, Mo 0.5 all in μmol L. The water source was rainwater, which contained on average 2.5 μmol L of Zn, which was taken into account for the Zn supply by the fertilizer recipe. Five-week old plants, grafted on ‘Inermis’ rootstock, raised in rockwool cubes of 5 * 5 * 7.5 cm were placed on the substrate early March, The experiment was continued for 14 months. Observations Crop development was monitored throughout the experiment. Appearance of chlorosis was visually judged six times, using an index range from 0 (none) to 10 (very severe). The yield was observed 6 days a week, with determination of number, weight and length of stems. Flower stem quality, determined by flower bud height and flower colour (visually), was observed at random checks. The vase life was determined twice, using 20 stems per plot from one harvesting date. Analysis Samples from the nutrient solution were taken from the collection tank and brought to the laboratory immediately. The samples were filtered using Millipore ceramic filters; the filtrate was acidified with nitric acid to pH 1.0, then analysed by atomic absorption spectroscopy. Leaf samples were taken three times throughout the trial. “Young laminae” were sampled from shoots due to be harvested within some days, being the first two complete leaves counted from the bottom of the shoot. “Old laminae” were taken from shoots bowed-in, below the cutting zone and were not well defined of age. The leaves were kept cold and sent to the laboratory. Afterwards, the leaves were washed following the procedure of Sonneveld and van Dijk (1982) and then dried at 65-75°C for 24 h. The dry weight was determined, the dried samples were ground with a titanium rotor and after dry digestion in a muffle furnace (480°C) the ashes were dissolved using HCl. The elements Fe, Mn, Cu and Zn were determined using an atomic absorption Perkin-Elmer 4000 spectrophotometer.