Structural modifications in the mesophyll associated with cryopreservation of seven Hypericum species

Abstract

The present work examined seven Hypericum species (H. perforatum, H. humifusum, H. kalmianum, H. annulatum, H. tomentosum, H. pulchrum, and H. rumeliacum) produced in vitro and regenerated after cryopreservation. The aim of the study was to assess, by means of light microscopy (LM) and transmission electron microscopy (TEM), the effect of freezing temperature on leaf histological organization and mesophyll chloroplast ultrastructure. Histological analysis showed a negative effect of ultralow temperatures on leaf tissue structure in H. pulchrum and a positive effect in H. perforatum. The TEM analysis showed that chloroplasts from ultralow temperature treated H. annulatum, H. tomentosum, and H. rumeliacum had a typically structured internal membrane system without destruction of thylakoid membranes, however, those of H. humifusum had very high grana, and in H. perforatum, chloroplast thylakoid destruction occurred. The chloroplast internal membrane system of in vitro cultured control plants and in vitro cultured cryopreserved plants of H. kalmianum and H. pulchrum had a specific spatial orientation without any destruction of the membranes. The results show a specific response of each species to these experimental conditions.

Keywords

• chloroplast ultrastructure
• in vitro culture
• leaf histology