Leaf ultrastructure, photosynthetic rate and growth of myrtle plantlets under different in vitro culture conditions

The in vitro rooting of myrtle (Myrtus communis L.) plantlets was performed in containers with gas permeable (V) and non-permeable (C) closures characterized by a different number of gas exchanges (1.4 and 0.3 h(-1), respectively). The rooting was induced on Perlite, soaked with half strength Murashige and Skoog (MS) medium with 0.5 mg dm(-3) IBA, either with and without 15 g dm(-3) of sucrose. During the rooting phase, it was demonstrated that C cultures without sucrose (C-) negatively affect the growth of myrtle plantlets. The net photosynthetic rate and the starch content showed the lowest values in C cultures with and without sucrose (C+ and C-) while chlorophyll a content did not vary among treatments, therefore it could not be considered an indicative parameter to evaluate the autotrophic metabolism in myrtle plantlets. Electron microscopy and image analysis were employed to evaluate the leaf ultrastructure at three sample dates. Plantlet rooted in vented vessels with and without sucrose (V+ and V-) showed chloroplasts with numerous starch inclusions, while several osmiophilic plastoglobules (frequently related with leaf senescence) were found in chloroplast of leaf cells of C- myrtle plantlets.