Effects of submergence on the growth of Phragmites australis seedlings

Colonisation by reed seedlings, Phragmites australis (Cav.) Trin. ex Steud. is rare and usually occurs after drawdown and when shallow water prevails. P. australis seeds have high rates of germination but successful colonisation is dependent upon subsequent water depths. We investigated the capacity of young reed plants to resist a 4 weeks submergence stress within a 5 months period, and their subsequent recovery. A pond experiment examined the interactions between submergence depth and the age of the seedlings at submergence. Four submergence treatments were used. In two partial submergence treatments, 50 and 80% of the initial leaf area was submerged. In two total submergence treatments, plants were either submerged at 125% of their initial height with possible subsequent development of emerged leaves, or the water was deepened as they grew to maintain total submergence for 4 weeks. The ages at submergence were 40, 60 and 80 days. Plants were harvested at 5 months. Shoot elongation, biomass allocations to aerial biomass, roots and rhizomes, and photosynthetic activity of aerial leaves were measured. Redox potential was measured for a subsample.Mortality (18.7%) occurred only in the permanent submergence treatment for 40-day-old seedlings. In all treatments, submerged leaves senesced, except the terminal (youngest) leaves of permanently submerged plants. Submergence differentially affected shoot length and biomass, depending upon the intensity of the treatment and the seedling age. The major differences were found between the two partial and two total submergence treatments. Partial submergence (50 and 80%) significantly enhanced biomass accumulation and growth, whereas total submergence largely decreased biomass production and growth in length, with less effect on shoot numbers. The 80-day-old seedlings tolerated submergence better but growth was poorest in medium-aged plants (60-day-old). Increased elongation of the growing internodes of up to 140% was caused by submergence, and photosynthetic activity was enhanced by 85% in emergent leaves of plants initially submerged but allowed to produce emerged leaves during the treatment period.Young P. australis plants require shallow water levels without long lasting submergence to grow and survive. Tolerance to submergence increases with age. These processes contribute to define the conditions for colonisation via seeds in P. australis.