Effect of phase of medium, growth regulators and nutrient supplementations on in vitro shoot-tip necrosis in pear

The incidence of shoot-tip necrosis (STN) was observed during the in vitro shoot multiplication and rooting stages of five Pyrus genotypes. During shoot multiplication, there was no incidence of STN in any of the five pear genotypes after 2 weeks of culture. However, after 6 weeks of culturing, necrotic apices were observed in all the pear genotypes, except wild pear. After 8 weeks of culture, 79.31% STN was observed in Punjab Beauty, followed by Patharnakh (49.60%), kainth (24.65%), shiara (17.68%) and wild pear (7.32%). Irrespective of the genotype, STN increased with an increase in BA concentration only in 8-week-old cultures. During in vitro rooting, the incidence of STN also varied with the genotype. In all five pear genotypes, the use of liquid media for in vitro rhizogenesis was useful in reducing the incidence of necrotic apices. The auxin type and level also influenced the occurrence of STN significantly. In wild pear, the incidence of necrotic apices increased with an increase in the concentration of auxins. A combination of Naphthalene acetic acid (NAA)+Indole-3-butyric acid (IBA) resulted in a higher incidence of STN than IBA or NAA alone at higher concentrations. Lower concentrations of IBA (0.62 to 2.46 µM L⁻¹) resulted in a higher incidence of STN compared with the lower levels of NAA (0.67 and 1.34 µM L⁻¹). In kainth, there was no significant difference in the incidence of STN at higher concentrations of IBA (5.37 and 10.74 µM L⁻¹) and NAA (4.92 and 9.84 µM L⁻¹). 2, 4-D resulted in a higher incidence of STN in comparison with IBA, NAA or NAA+IBA in the rooting medium. In Punjab Beauty, the percentage of necrotic apices at 0.67 and 1.34 µM L⁻¹ NAA (40.46% and 40.04%, respectively) was on a par with the control (37.92%), but it was significantly less than the STN observed at 0.62 and 1.23 µM L⁻¹ of IBA. In Patharnakh, IBA resulted in a significantly higher incidence of STN than NAA at all tested concentrations. In shiara, all the auxin treatments significantly increased STN. At lower levels (0.62 and 1.23 µM L⁻¹), IBA resulted in a higher incidence of STN than the lower concentration of NAA (0.67 and 1.34 µM L⁻¹), whereas at higher levels (2.46 to 9.84 µM L⁻¹) IBA was on a par with NAA (2.69 to 10.74 µM L⁻¹). Supplementation with calcium and boron significantly affected the percentage of necrotic apices in wild pear, Punjab Beauty and Patharnakh. In wild pear, raising the calcium level to 3 mM reduced the STN to 2.60%. In Punjab Beauty and Patharnakh, STN increased with an increase in calcium levels. Boron supplementations eliminated the incidence of STN in wild pear. Similarly, in Patharnakh, boron (500 µM and 1000 µM) significantly reduced the STN. Callus development was also observed at the base of pear shoots showing STN symptoms during rooting phase, but it did not show any significant trend. Higher levels of IBA, NAA+IBA and 2, 4-D resulted in the formation of large-sized calluses and STN in wild pear shoots on solid medium. There was a high incidence of STN in Punjab Beauty, Patharnakh and shiara but, the basal callus development in these genotypes was much less.