Effect of different light-dark cycles on growth of fruit tree shoots cultured in vitro

Trials were carried out to test the effect of light-dark cycles different from the largely adopted 16 hours light — 8 hours dark, without altering the amount of radiation supplied to the cultures. The following treatments were compared: 16 hr light — 8 hr dark, 8 hr light — 4 hr dark, 4 hr light — 2 hr dark, 2 hr light — 1 hr dark. Light intensity at culture level was 40 μmol m⁻² sec⁻¹. Shoot tips of two peach rootstocks, the « MrS 2/5 » and the « GF 677 », a plum and a peach × almond hybrid respectively were utilized. Growth medium was MS supplemented with BA (0.6 mg/1), GA₃ (0.2 mg/1) and IBA (0.06 mg/1) for shoot proliferation; only IBA (0.4 mg/1) was used for shoot rooting. After 6-8 weeks of culture, shoot cluster fresh and dry weight and number of shoots produced from original shoot tips were much greater with 4 hours light — 2 hours dark compared to 16 hours light -8 hours dark. Also root length and average leaf area were positively influenced by this light-dark cycle. Concerning the physiological aspects involved with the enhanced growth, the hypotheses of a possible role of the phytochrome and of a higher CO₂ availability in the culture vessels are considered.