Regeneration of shoots was achieved from in vitro leaves of M26 at frequencies close to 100% on a medium based on MS salts and LS vitamins, containing 4.4#M BA and 0.5~M NAA. Dark and red light gave the best results in inducing shoot regeneration. White light at high intensity helped development of regenerated shoots. Inorganic nitrogen could be reduced by 75% without negative effect, and the presence of NH~ was necessary for regeneration. Leaves were able to regenerate after a 3 kR irradiation (gamma rays), not after 4 kR. Optimal dose should be between 1 and 2 kR.

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The effects of humic substances on in vitro culture of 'Golden Delicious' apple are reported. Potassium humate (KH) when used in proliferation showed a negative interaction with BA while it enhanced rooting when IBA was not present in the culture medium. In the presence of IBA, KH increased root number and reduced root growth. The highest concentration tested, 500 mg 1-i, caused a drastic reduction in root system development. 50 mg 1-1 KH hastened rooting and plants grew more rapidly when transferred to soil.

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