Photoauto-, Photohetero- and Photomixotrophic in vitro Propagation of Papaya (Carica papaya L.) and Response of Seed and Seedlings to Light-emitting Diodes

Abstract

The objective of this study was to assess whether papaya could be propagated under photoautotrophic conditions and what the response would be to light-emitting diodes (LEDs). Seeds of papaya (Carica papaya L. cv. ‘Rainbow’ and ‘Sunrise Solo’) that were soaked overnight and surface sterilized with 0.1% mercuric chloride solution + 2 drops of Tween-20 then with 80% ethanol, could germinate at 100% on Murashige and Skoog medium with 3% sucrose. Seeds were placed under different light conditions: positive control (100% white heat fluorescent lamps), negative control (darkness) and five combinations of LEDs with different red (R) and blue (B) ratios (100% R; 70% R + 30% B; 50% R + 50% B; 30% R + 70% B; 100% B) with a standard light intensity of 45 μmol/m2/s for all treatments. Seed germination was high (95%-100%) independent of the treatment, but 100% R resulted in extremely long hypocotyls while and 100% B showed highly stunted seedlings. Control seedlings germinated in light were also placed under the same light conditions and stem and root growth were stimulated or stunted by R and B LEDs, respectively. Separately, one-month-old papaya plantlets were cultured photoautotrophically, i.e., in sucrose-free medium in the presence of 3000 ppm CO2, either in non-aerated or aerated vessels. Even though leaf-drop was high, photoautotrophic in vitro propagation led to greater leaf production than control plantlets. Papaya could be propagated under photoautotrophic conditions and LEDs affected seedling growth differently.