Seedling growth and physiological behaviour of Andropogon gayanus Kunth cv. bisquamulatus in water stress conditions | Croissance juvenile et comportement physiologique d'Andropogon gayanus Kunth var. bisquamulatus en conditions de stress hydrique

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 per cent 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 per cent of the seedlings were still alive. With an initial soil water reserve of 13.5 or 18 per cent in volume (15 and 25 mm treatment respectively), mortalities occur already 4 to 6 days after sowing and increase very rapidly over a 10-day period. Whatever the initial water reserve may be, seedling losses occur systematically for a soil water reserve under or equal to 5 mm (4.5 per cent 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 potential is -0.44 MPa during the rainy season and lower than -1 MPa during the dry season. The stomatal conductance is high (more than 1 cm per sec) during both seasons, but the rate of photosynthesis decreases by half (from 19.8 to 10.9 micromoles CO2 per square metre and second) 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 7-10 per cent in volume. Then leaf water potential drops rapidly while stomatal conductance and rate of photosynthesis remain relatively high, even when soil water content reaches 5 per cent in volume. The second experiment demonstrated that the graminea was able to perform active osmotic adjustments, mainly through K+ and glucose accumulation in the symplasmic liquid. Experiments carried out with 12 strains cultivated in field conditions in Senegal showed large differences between the capacity of osmotic adjustment of the strains. These differences were related to the leaf size for each of the strains. A drought resistance index, based on measures of osmolality in leaf saps and dry matter contents in leaves, was also proposed in order to facilitate the selection of drought resistant strains.