Embryonal Masses Induced at High Temperatures in Aleppo Pine: Cytokinin Profile and Cytological Characterization

Abstract: Aleppo pine (Pinus halepensis Mill.), a native species of the Mediterranean region, has been suggested as a species that when introduced in degraded areas could facilitate the long-term colonization and expansion of late-successional species. Due to climate changes, plants need to withstand extreme environmental conditions through adaptation and changings in developmental pathways. Among other paths, plants undergo changes in developmental pathways controlled by phytohormones. At the same time, somatic embryogenesis has been widely used as a model to understand the mechanisms involved in plant response to different stresses. In this study, in order to induce a strong effect of temperature stress on plants regenerated from somatic embryos, higher temperatures (40 °C for 4 h, 50 °C for 30 min, and 60 °C for 5 min) than the control (23 °C) were applied during the induction stage of somatic embryogenesis in Pinus halepensis. A morphological characterization of the embryogenic cultures showed small differences in the number of starch grains, lipid bodies, and phenolic compounds between treatments. Results showed that high temperatures (60 °C) led to higher rates at the maturation stage of somatic embryogenesis when compared to the control (23 °C), strengthening the productivity through the increase in the number of somatic embryos obtained. Finally, analysis of endogenous concentration of cytokinins showed that different conditions applied during the initiation phase of somatic embryogenesis led to different hormonal profiles; isoprenoid cytokinins showed a clear defined pattern with the higher total hormone concentration being found in embryonal masses induced at 50 °C for 30 min, while different aromatic cytokinins presented different individual responses to the treatments applied. These differences corroborate the idea that cytokinins could be potential regulators of stress– response processes during initial steps of somatic embryogenesis.