Physiological and anatomical adaptations induced by flooding in Cotula coronopifolia

Cotula coronopifolia is a wild annual Asteraceae that grows in periodically-flooded prone environments and seems highly tolerant to periodic flooding. Seedlings of about 15 cm were collected directly from the edge of Soliman sabkha (N-E Tunisia, semi-arid stage) and grown under greenhouse conditions. Two treatments were considered: drainage and flooding. After 56 days of treatment, flooded plants showed a pronounced growth increase. This performance was essentially associated with significant increment in biomass production of both shoots and roots (about 220% of the control). The appropriate response to flooding was also characterized by the ability of the species to maintain its water status under such conditions. Neither water content nor water potential showed a significant variation as compared to those of non-flooded plants. However, transpiration rate decreased slightly but significantly in flooded plants (from 0.86 to 0.64 mmol H2O m-2 s-1). Na+ and K+ concentrations were practically maintained under waterlogging conditions, except a significant increase of Na+ content in roots of flooded plants (157% of the control). These responses were concomitant with maintenance of photosynthetic rate. However, the contents of chlorophylls a and b increased to 167% and 295%, respectively. It seems that the enhancement in these photosynthetic pigments together with a significant improvement in water use efficiency (from 4.66 to 6.07 mmol CO2 mol-1 H2O) allowed to the species to compensate the decrease in photosynthetic rate. At the anatomical level, this species responded to flooding by a significant development of its root aerenchyma (+63%) and an increase in the lignification of its stem xylem tissues (+37%). Based on the presented data, the plant fitness under flooding conditions was a result of dynamic readjustment of several morphological, physiological, and anatomical adaptive traits. Flood requirement together with salt tolerance are responsible for the predominance of C. coronopifolia in a large area in its natural biotope where most plants cannot tolerate interactive effects of flooding and salinity.