NITROGEN METABOLISM IN CUCUMBER COTYLEONS AND LEAVES EXPOSED TO THE DROUGHT STRESS AND EXCESSIVE UV-B RADIATION

Plants absorb light energy for photosynthesis, and some amount of potentially damaging range of solar radiation, ultraviolet B. It accounts for less than 0.5% of the total solar radiation and the ambient current level of UV-B during the growth season provide 10 kJ.m(-2) energy per day on the Earth's surface. In the field conditions, increased UV-B radiation is often accompanied by drought, but negative effect of combined stresses is not so deleterious as the effect of one of them alone. We studied some changes in nitrogen me deficit and UV-B radiation. The stresses generally decreased the biomass and total dry matter production. Combination of water deficit and UV-B activated a stress tolerance mechanism in cucumber seedlings. The NR activity and synthesis of UV-B absorbing compounds were induced. Protection against UV-B radiation can partially involve increased production of UV-B protective pigments - slightly increased the SPAD value in cucumber cotyledons and leaves was observed. The deleterious effect of combined stresses was weaker than their additive or individual effects.