INTERACTIONS OF IRRADIANCE LEVEL AND IRON CHELATE SOURCE DURING SHOOT-TIP CULTURE OF CARICA-PAPAYA L

Interactions between irradiance levels (5-40 mu mol.m(-2).s(-1)) and i ron chelate sources (FeEDTA and FeEDDHA) were observed for Carica papa ya shoot tip cultures during both the establishment and proliferation stages of microculture. Reduced levels of irradiance (5 mu mol.m(-2).s (-1)) favored shoot tip establishment regardless of the source or leve l of iron. However, the highest percentage of successful explant estab lishment (100%), and significantly greater leaf length (1.16 cm; over double the size attained in any other treatment), resulted when a low concentration of FeEDTA alone was used at low irradiance. During the s ubsequent shoot proliferation stage, however, higher irradiance levels (30 and 40 pmol.m(-2).s(-1)) were required, and FeEDTA failed to supp ort culture growth when used as the sole iron source. The highest mult iplication rates (3.6 shoots per explant) and leaf chlorophyll concent rations (0.22 mg/g fresh mass), and significantly improved shoot quali ty were achieved at 30 mu mol.m(-2).s(-1) irradiance when both iron ch elate formulations were combined (each at a 100 mu M concentration) in the proliferation medium.