Complex dormancy in the seeds of Hypericum philonotis

Abstract

Species of the genus Hypericum have elicited great interest in pharmacological research, therefore, their propagation is necessary. However, only a low germination percentage has been achieved to date. A population of Hypericum philonotis (Cham. & Schlecht.) grows every 2 or 5 years in the Parque Ecológico de la Ciudad de México (PECM). To understand seed germination and the population dynamics of H. philonotis in the PECM, we studied their seed germination to identify the dormancy type. We assessed the effect on germination of light, constant temperatures (5–35°C) and fluctuating (25/35°C, 18/6h, with the highest temperature at noon, photoperiod 12/12), cold stratification, scarification (with HCl); gibberellins and seed immersion in acetone or hexane. We also studied the morphology and structure of the seeds, and the allelopathic potential of the pigments present in the seed coat on its own seed germination and early seedling growth. Light, gibberellins, a combination of temperature with gibberellins, and immersion in acetone increased seed germination. Alternating temperatures replaced the effect of gibberellins. Immersion in acetone eliminated a pigment from the seed coat, enhancing full seed germination. The acetone extract from H. philonotis seeds partially inhibited seed germination and early growth. The endosperm was reduced, and the main reserve in cotyledons was proteins. The H. philonotis seeds exhibited physiological dormancy. Gibberellins increased the embryo growth potential, reducing the constraint of the thick seed cover and any inhibiting effect of the acetone extract. The requirement of light and temperature fluctuation acts as an environmental cue for the successful germination of these minuscule seeds (0.5–0.8mm in length).