ZAKIA I. ALI1, SABINE D. GOLOMBEK2
1Gezira Research Station, Agricultural Research Corporation (ARC), Sudan
2University of Kassel, Ecological Crop Science and Agroecosystem Research of the Tropics and Subtropics, Germany
Five Sudanese pearl millet Pennisetum glaucum L. R. Br. genotypes were cultivated in climate chambers and treated with low (N1), medium (N2) and high (N3) nitrogen supply by adding NH4NO3. Drought was induced 36 days after sowing (DAS) by withholding water until the soil reached an pF value of 4.5, which was then maintained by watering the pots to a certain weight twice a day. The plants were harvested at 45 DAS. Measurements of the gas exchange and water potential of the leaves were conducted between 43-44 DAS. For the analysis of the osmotic potential and several metabolites, the lamina of the uppermost fully expanded leaf was immediately frozen in liquid nitrogen.
The contribution of minerals, sugars and proline to the osmotic adaptation of the leaves to drought was investigated. The osmotic adaptation of the leaves to drought was mainly caused by a drastic increment of the potassium concentration. The nitrate concentrations were negligible in controls and the drought treated plants with low nitrogen supply, but rised up to 80 mM in the leaf tissue sap of the drought stressed N3 plants, where it contributed to the osmotic adaptation. Because of their high nitrate levels leaves grown under drought and high nitrogen availability are not recommendable for the use as animal fodder. Proline in the leaf tissue sap increased from very low levels in the controls (up to 2 mM) to the stressed plants with values around 10 mM in N1 and around 30 mM in N2 and N3, where it contributed slightly to the osmotic adaptation. Although the sugars contributed significantly to the osmotic potential in all treatments, they played no role in the osmotic adaptation to drought.
The degree of the decrease of the water potential of the drought stressed leaves was affected by the nitrogen supply. The decline of the net CO2-assimilation rate in response to drought was mainly due to nonstomatal factors and modified by genotype x nitrogen supply interaction. Drought had no major effect on the total nitrogen uptake and did not hamper the nitrogen translocation from the root into the shoot.
Keywords: Drought, gas exchange, osmotic adaptation, pearl millet