Trends in culture medium nitrogen requirements for in vitro shoot growth of diverse pear germplasm

A major problem limiting in vitro germplasm collections is that many species or cultivars have suboptimal growth on standard growth media. A series of studies determined that increased amounts of mesos components (CaCl₂, MgSO₄, KH₂PO₄) and changes in the nitrogen (N) were required for producing high quality shoots for five pear genotypes. Many species required unique combinations of N ions (NO₃^- and NH₄⁺) and total N for optimal growth. To make these results applicable to germplasm collections, this study evaluated 16 genotypes from five species maintained at the USDA-ARS National Clonal Germplasm Repository in Corvallis Oregon. Growth responses were determined for quality rating, multiplication, shoot length and leaf characters. Four patterns of N requirements were observed for shoots grown on increased mesos nutrients. Most of the pears responded well to NH₄⁺ at moderate concentrations (~20 mM) and NO₃^- at low to moderate concentrations (20-40 mM). Pyrus communis cultivars 'Horner 51', 'NY 10353', 'OH×F 87R'O, 'Pyrodwarf', 'bileen Gift' and 'Winter Nelis'; P. pyrifolia 'Scion SzuMi'; and P. ussuriensis 'Hang Pa Li' grew well on these concentrations when combined with increased mesos (1.5-2.0×). Five genotypes responded best on moderate to high NH₄⁺ and low NO₃^- concentrations (P. communis 'Bartlett' and 'Luscious'; a P. pyrifolia wild selection and 'Seuri Li'; and P. ussuriensis 'Harbin'). High concentrations of both N ions were required for P. koehnei and P. pyrifolia 'Hosui', while only P. cordata grew best with high NH₄⁺ and any level of NO₃^-. It is now possible to culture the entire range of pear genotypes by using a combination of ≥1.5× mesos and one or more of these four N formulations.