Croissance juvénile et comportement physiologique d'Andropogon gayanus Kunth var. bisquamulatus en conditions de stress hydrique

Seedling growth and physiological behaviour of Andropogon gayanus Kunth cv. bisquamulatus in water stress conditions.Several studies carried on the physiology of Andropogon gayanus Kunth cv. bisquamulatus were conducted in Belgium in a wind tunnel simulating climatic conditions close to those prevailing during the rainy season in Sahelo-Sudanese region. The first experiments simulated water supply of 15, 20 and 25 mm at the outset of a caryopse sowing in a sandy soil. The observations were focused on daily evaporation losses, seedling emergencies, their growth and the mortality evolution. Results show that an initial soil water reserve of 25 mm, i.e. about 22.5 % in volume in the upper layer, is necessary to ensure the seedling survival during a period of 15 days after emergence. After 27 days, 95 % of the seedlings were still clive. With an initial soil water reserve of 13.5 or 18 % in volume (15 and 25 mm treatment respectively), mortalities occur already 4 to 6 days after sowing and increase vert' rapidly over a 10 day period. Whatever the initial water reserve mat' be, seedling losses occur systematically for a soil water reserve under or equal to 5 mm (4.5 % in volume). Two subsequent experiments were also conducted in order to analyse the photosynthetic parameters and the water status of 6 plants of Andropogon gayanus in the rainy and the dry seasons and during two drying cycles imposed in the rainy season. The first experiment shows that the hydric potentiel is -0.44 MPa during the rainy season and louver than -1 MPa during the dry season. The stomacal conductance is high (more than 1 cm . s-1) during both seasons, but the rate of photosynthesis decreases by half (from 19.8 to 10.9 graoles CO2 . m-2 . s-1) during the dry season. When the graminea endures a drought period in the rainy season, the physiological parameters evolve slowly until the soil water reserve reaches.