Light Spectrum Affects Growth and Endogenous GAs Content of In Vitro Grown Apple Shoots

'Golden Delicious' fruit grown at moderate altitudes on hillsides are less sensitive to russeting than fruit grown in lower lands. Fruit grown on hillsides are also more elongated. Treatments with exogenous gibberellins in the low valley locations produce the same effects on russet and elongation as high altitude, while treating apple trees with GA inhibitors stimulates the opposite response. The hypothesis is made that the different characteristics shown by apples grown at different altitudes are due to their different endogenous gibberellin contents, and that these are, in turn, related to the different solar spectra in the two environments. To investigate if the light spectrum can affect GAs biosynthesis, an in vitro model was studied: propagules of different 'Golden Delicious' clones, cultivated in vitro on substrates with and without GA3, were illuminated with different light spectra, and the growth of plantlets and their gibberellin content were analyzed. Shoot growth, and GA4 and GA7 content were affected by light spectrum, differently in clones with different sensitivity to skin russeting.