Morphological and physiological traits of two closely related charophyte species: Chara virgata Kütz. and C. globularis Thuill.

Charophytes (Charophyceae) are submerged macrophytes that play an important ecological role. They provide food and shelter and consequently are often used as bioindicators for water quality. However, species identification remains challenging due to purported broad morphological plasticity. This study investigates the morphological, physiological, and pigment-based acclimation behaviour of Chara virgata Kütz. and Chara globularis Thuill. under controlled light and temperature conditions in order to clarify which traits can be used for species identification. Chara virgata exhibited relatively stable morphological traits across all conditions, with photosynthetic responses primarily driven by light availability. Depending on the prevailing light and temperature conditions, C. virgata exhibited no discernible pattern of elongation in the upper row of stipulodes. In contrast, C. globularis displayed significant morphological plasticity, elongating internodes under low-light and at higher temperatures, with pigment composition adjusting in response to temperature conditions. Moreover, the length of the upper row of stipulodes remained constant within the experimental conditions. These differences align with their respective habitat preferences. Chara virgata predominantly occurs in shallow water with large fluctuations of temperature and irradiance whereas C. globularis prefers greater depths with more stable temperature conditions. The study further demonstrates that vegetative parameters, such as stipulode length, can vary with environmental conditions, limiting their reliability for species identification. These results emphasise the need to integrate physiological traits for a robust classification of charophyte species, and reinforce the idea of including reproductive traits in the determination keys to clearly identify these two species.