No escape? Costs and benefits of plant de-submergence in the pasture grass Chloris gayana under different submergence regimes

STRIKER GG; KUANG X; CIFALDI JNA; CASAS C; GRIMOLDI AA

Elsinore

Congreso; 12th ISPA Conference 5-9 September 2016 in Elsinore, Denmark; 2016

International Society for Plant Anaerobiosis

Rapid leaf/shoot elongation is one of the main plant responses to cope with complete submergence by re-establishing leaf contact with the atmosphere (?escape? strategy). This response is supposed to be beneficial under long term submergence rather than under alternated short-term submergence events (e.g. fluctuating environment) as the costs of repeated plant ?adjustment? can exceed the initial benefits. Nevertheless, this is an assumption that rarely has been quantitatively addressed. In a first experiment, we examined the tolerance of 3-leaf seedlings of seven cultivars of Chloris gayana (Callide, Epica, Fine cut, Katambora, Pioneer, Tolga and Top cut) to complete submergence in clear water along 14 days. This species is a highly productive forage grass of increasing importance in grasslands, prone to flooding, in South America. All cultivars displayed an escape strategy without differences in dry mass (44-53% of drained controls at the end of submergence). However, after water subsides, the cultivars Fine cut, Pioneer, Tolga and Callide resumed their growth (dry mass 24 to 54% of their drained controls at end of recovery); contrary to Top cut, Katambora and Epica, which perished. Among the first group, Fine cut was the most tolerant/promising material. We used this cultivar in a second experiment to quantify the benefit (or cost) of exhibiting (or not allowing) the ?escape? strategy, and to assess the effects on grow of two submergence regimes. So, 3-leaf seedlings in 0.4 L pots with soil (3.3% organic carbon) and sand (1 v/v) grew in plastic containers during (i) 4 weeks at well drained conditions (i.e. field capacity), (ii) two cycles of 1-week submergence followed by 1week at well drained conditions (total: 4 weeks; repeated short-term submergence), (iii) 1 cycle of 2-week submergence followed by a 2-week period at well drained conditions (total: 4 weeks; long-term submergence). Water level was 24 cm and initial seedling height ranged between 18 and 20 cm. Plastic nettings were placed 2 cm below water over an additional set of seedlings to prevent leaf de-submergence, under the same submergence regimes commented before. Netting only reduced light by 5.5% (e.g. from 544 ± 5.1 to 514 ± 4.0 µmoles m-2s-1 when underwater) and did not affect seedlings growth in well-drained soil as they attained similar dry mass with and without netting (P=0.64). Impeding leaves from emerging above water through nettings did not compromise survival when submergence was 1-week long, but determined death of all seedlings when submergence was extended to 2 weeks. After a second 1-week submergence cycle, survival was also compromised as 50% of seedlings perished, and growth was reduced by 30% among the remaining alive seedlings (compared to those submerged without netting; P<0.05). Growth as affected by submergence regimes (without netting) at the end of experiment revealed that under one event of 2-week submergence, seedlings accumulated a 65% higher dry mass than when they experienced the same submergence duration but in two separate events of 1-week (0.73 vs. 0.25 g/seedling, respectively, P<0.01; controls: 1.73 g/seedling). Therefore, it can be concluded that the ?escape? strategy in C. gayana by which leaf contact with air is re-established is essential for its survival, and it is more beneficial under long-term submergence than under repeated short-term submergence cycles.