PRELIMINARY STUDY ON THE EFFECT OF LED LIGHT AND CYTOKININE ON THE GROWTH OF PEAR PLANTS IN VITRO Nataliya Dimitrova1*, Lilyana Nacheva1, Yulia Kyoseva2, Małgorzata Berova2 1 - Fruit Growing Institute, 12 Ostromila Str., 4004 Plovdiv, BULGARIA 2 - Agricultural University, 12 Mendeleev Str., 4000 Plovdiv, BULGARIA *Corresponding author: natali.dimitrova@yahoo.com Material and Methods Plant material and experimental conditions The experiment was carried out on pear rootstock (Pyrus communis L. ‘Old Home’ x ‘Farmingdale’ 333). Briefly, shoots were grown in microboxes with green filter (SacO2, Belgium) on a modified MS (Murashige and Skoog, 1962) solid medium with ½ concentration of NH4NO3 and CaCl2 and 1000 mg L-1 Ca(NO3)2, supplemented with 2.5 µM 6-benzylaminopurine (BAP), 0.05 μM IBA, 30 g L-1 sucrose, 6.5 g L-1 Phyto agar (Duchefa, The Netherlands). The cultures were incubated in the growth room at an air temperature of 22±2C with 16/8 h hours photoperiod supplied by cool-white fluorescent lamps (OSRAM 40W; 50 µmol m-2s-1 PPFD). For the purpose of the present experiment the plantlets were cultivated in vitro on above mentioned basal nutrient medium, supplemented with 2.5 µM 6-benzylaminopurine (BAP) or 2.5 µM meta-Topolin (mT) at 22±2°C using an illumination system based on Philips GreenPower LED research module (16-h photoperiod with 80-95 µmol m-2 s-1 PPFD). Three groups of LEDs emitting in white (W), blue (B), mixed (BR) (W:B:red:far-red = 1:1:1:1) lights) were applied. Data on fresh (FW) and dry (DW) mass, number and length of shoots, the length and width of the first fully developed leaf, content of photosynthetic pigments was evaluated in six passages of three weeks of culture on the corresponding light regime. For each light treatment three replications, each containing ten shoots was tested and the experiment was repeated three times. Statistical analysis of physiological parameters was performed using a one-way ANOVA using the Tukey test to validate the different significance at P ≤0.05. Fig. 2. Gas-exchange filters of the polypropylene containers allows air flow, but blocks contamination. Fig. 3. The size of the first fully developed leaf of pear plantlets, grown with BAP or mT in the nutrient medium at different light treatments. W white LEDs, B – blue LEDs, BR – mixed LEDs. Results Fig. 1. The appearance of pear plantlets, grown with BAP or mT in the nutrient medium at different light treatments.W white LEDs, B – blue LEDs, BR – mixed LEDs. Table 1. Growth indices measured in pear plantlets grown with BAP or mT in the nutrient medium at different light treatments. W white LEDs, B – blue LEDs, BR – mixed LEDs. For each column, different letters indicate significant differences at p≤0.05. LED Light Cytokinin Length of Shoot /mm/ Number of Shoots Number of Leaves FW /g/ DW /g/ White BAP mT 20.76 a 15.96 b 1.2 a 1.0 a 7.3 ab 9.2 ab 1.20 a 1.55 a 0.34 ab 0.47 a BR BAP mT 11.98 c 20.58 a 1.6 a 1.0 a 10.4 a 7.5 ab 0.67 a 0.98 a 0.15 b 0.18 b Blue BAP mT 21.96 a 15.90 b 1.6 a 1.1 a 9.1 ab 5.7 b 1.17 a 1.28 a 0.29 ab 0.34 ab Table 2. The content of photosynthetic pigments (mg g-1 FW) in the pear plantlets cultivated with BAP or mT in the nutrient medium at different light treatments. W white LEDs, B – blue LEDs, BR – mixed LEDs. For each column, different letters indicate significant differences at p≤0.05. LED Light Cytokinin Chl a Chl b Chl (a+b) Car Chl a/b Chl(a+b)/Car White BAP mT 0.52 a 0.26 cd 0.17 a 0.09 bc 0.69 a 0.35 c 0.23 a 0.14 b 3.05 ab 2.96 b 2.95 b 2.58 c BR BAP mT 0.25 cd 0.21 d 0.09 bc 0.08 c 0.34 c 0.30 c 0.15 b 0.11 b 2.58 c 2.61 c 2.31 d 2.74 bc Blue BAP mT 0.39 b 0.32 bc 0.12 b 0.10 bc 0.51 b 0.42 bc 0.16 b 0.16 b 3.15 ab 3.29 a 3.26 a 2.63 b Fig. 4. Effect of the light and cytokinin on the net photosynthetic rate (μmol CO2 m-2 s-1) and transpiration intensity (E, mmol H2O m-2 s-1) of pear plantlets; Different letters within column indicated difference at (Р<0.05). Conclusions Plantlets grown on the nu