Optimization of In Vitro Culture of Pear Rootstock (Pyrodwarf)

Application of stress-tolerant rootstocks is known as one of the effective methods to enhance the productivity of fruit gardens. Pear is one of the four or five major classes of fruits that are produced worldwide. Among pear cultivars, Pyrodwarf rootstock is tolerant to alkaline soil and low temperatures of winter. In addition, many cultivars of pear with high productivity could be grafted to this rootstock. These advantages result in widespread culture of pyrodwarf rootstock in pear gardens. The increasing market demand of this rootstock presents an opportunity to investigate alternative methods for efficient production of pyrodwarf. Therefore, this study has been conducted to increase the efficiency of pyrodwarf propagation through tissue culture methods. For this experiment, sterile plantlets of pyrodwarf rootstocks in a same growth phase were used. Plantlets were cultured in different 12 media including MS, WPM, KNOP, modified KNOP, QL and modified QL basal media supplemented with 0.5 mg.L-1 BA and 0.05 mg.L-1 IAA or IBA and solidified with 8 g.L-1 agar. After that, cultures were transferred to the growth room at 25 ± 1°C under a 16/8h light/dark photoperiod (light intensity 30 μmol.m−2.s−1, cool-white fluorescent light). In the next step for rooting optimization, two separate experiments were conducted. In the first, 10 different basal media containing various nutritional compounds were examined for rooting properties of pyrodwarf plantlets. The following experiment was performed to reveal the effects of IAA and active charcoal on the efficiency of plantlets rooting. All the experiments were carried out in acompletelyrandomized design. The study of propagation efficiency of plantlets was arranged in a factorial approach with twofactors including the type of basal media (sixlevels) and auxins (twolevels) with threereplications. The both experiments of plantlet rooting were performed with sevenreplications. Data were analyzed using JMP-8 statistical software and the meanswere compared by using LSD test at 5% probability. All explants were regenerated 15 days after cultivation. The number of buds per explants indicates that nutritional compounds significantly affected bud formation. MS basal media induced the most number of adventitious buds and decreasing amount of nitrogen in the culture medium (either in its ammonium or nitrate form) leads to 50%-decrease in the number of buds induced per explant. Previous reports mentioned that macronutrients have the most significant effect on bud formation of peach hybrid rootstock GF677. In addition, high amount of nitrogen is necessary to achieve the highest height of pyrodwarfin vitro plantlets. In contrast to the basal media, neither bud formation nor height of plantlets did not affected by the type of auxins, IAA and IBA. Also weight of plantlets was similar in all treatments and shows the effect of basal media and auxins were not significant on biomass production. According to the results of rooting experiment, plantlets that were cultured in MS and DKW had higher height than QL and Knop in rooting phase. Also, 1/2 MS-cultured plantlets have the lowest height among all treatments. These results showed that macronutrients have positive effects on the growth of pyrodwarf plantlets as decreasing concentration; the growth of plantlets was inhibited. In contrast to shoot growth, number of roots per explants in 1/2 MS medium was significantly higher than the other media. These results are expected as shoots and roots of plants have different response to environmental stimuli. Data obtained from the following experiment of rooting indicated that IAA improved root induction similar toother functional traits of plants but the presence of both of those compounds in medium have inhibitory effect on the functional traits of plants. These data suggest that macronutrients and hormonal compounds have impressive impact on the regeneration of pyrodwarf plantlets. In this regard, MS medium supplemented with 0.5 mg.L-1 BA and 0.05 mg.L-1 IAA or IBA induced most number of buds and the regenerated plantlets hadthe highest height in that medium compared to the other treatments. Also, 1/2 MS medium containing 1 mg.L-1 IAA was identified as the best media in case of rooting parameters.