Impact of polyethylene glycol-induced water stress on growth and development of shoot tip cultures from different banana (Musa spp.) cultivars

Shoot tip explants of the Egyptian banana cultivars Maghraby, Valery, Grand Nain and Hindy were tested for their tolerance to water stress. Shoot survival, shoot growth and root growth stimulation in presence of polyethylene glycol (PEG) was strongest in cultivar Hindy followed by Grand Nain, Maghraby and Valery. The accumulation of soluble sugars and proline in shoots was positively correlated with the applied polyethylene glycol concentration, while the reverse was true for N, P and K content. The cultivar Hindy exhibited higher metabolite accumulation response and cultivar Maghraby the least. The effects were most clear on liquid medium whereas solid (agar) medium exerted some additional effects increasing the osmotic stress at low PEG concentrations and alleviating the PEG effect at high PEG concentrations. In conclusion, the cultivar Hindy appeared to be the most tolerant to water stress because of strong accumulation of compatible solutes and greater stimulation of root development. (2000-2002). The cultivars were Maghraby, Valery, Grand Nain, which belong to the semi-dwarf Cavendish group and Hindy is a strain of dwarf Cavendish. All are of triploid Acuminata type. Aseptic cultures were established from shoot tips which were surface-sterilized in 3 % NaOCl solution (contained 0.1 % Tween 20 as a wetting agent) for 20 min. Thereafter, the tips were rinsed several times in sterilized distilled water to remove all traces of chlorine. After removal of the outside tissues, apical meristems were vertically cultured for 4 weeks on Murashige-Skoog basal medium (Murashige and Skoog, 1962) supplemented with benzyladenine (3 mg/l) and agar (6 g/l). The growing explants were recultured, at 4 week intervals, on fresh media until the onset of proliferation (ca. 2 months). This culture period was called starting phase. In order to obtain sufficient number of explants, the produced shoots were subcultured four times on solid Murashige-Skoog basal media supplemented with benzyladenine (5 mg/l) in a so-called multiplication phase. The obtained plantlets were used for the following experiments (Fig.1). Incubation with polyethylene glycol: To determine the lethal concentration of polyethylene glycol for each cultivar, PEG-6000 was added to solid basal media at levels of 0, 5, 15, 25, 35 and 45 g/l. The produced shoots were cultured for 4 weeks on these media and the percentage of survival was determined. Polyethylene glycol (0, 10 and 20 g/l) was added to basal media which were either supplemented with benzyladeneine (5 mg/l) to produce shoots or α-naphthalene acetic acid (1 mg/l) to produce roots. After 4 weeks, shoot or root growth and development were determined. This procedure was applied in liquid and in solid media (0.6 % agar). Shoot strength (or vigour) was determined according to Pottino (1981) on a rating scale: 1 = no growth, 2 = below average, 3 = average, 4 = above average, 5 = excellent.