Water availability is a major abiotic factor limiting plant growth and productivity. Grasses are major contributors to global primary productivity. Many C3 and C4 grass species are intolerant to drought stress and are often limited by water availability in terms of their distribution and productivity. However, Some C3 and C4 grass species, such as Festuca arundinaccea (C3) and Cynodon dactylon (C4) are adapted to a wide range of climatic zones in Israel, with an annual rainfall of 30 to 1000 mm. The overall goal of the proposed research is to examine mechanisms of adaptation to drought stress of dryland C3 and C4 grasses. The specific objectives of the study are: 1) to compare drought tolerance between ecotypes of C3 and C4 grass species collected from different climatic zones; 2) to determine mechanisms regulating drought survival of dryland perennial C3, C4 and C3-C4 intermediate grasses in terms of water use and carbon budget and to link between water and carbon relations;3) to create a wide germplasm bank of drought-tolerant grasses for future research and breeding;. C3, C4 and C3-C4 intermediate grasses will be collected from a wide range of climatic zones and studied for drought tolerance. A number of physiological parameters, including growth rate and photosynthetic rate, will be measured and compared for the evaluation of drought adaptability. Carbon metabolism and water use characteristics will be examined to determine drought tolerance strategies of dryland C3 and C4 grass species. Carbon benefits and costs associated with water relations will be assessed for different grass species adapted to different climatic conditions. Examination of C3 and C4 mechanisms that link water use and carbon expenditure will provide further insights into plant adaptation to long-term drought stress and is expected to contribute significantly to key areas such as sustainable agriculture, desertification, C3/C4 competition and global carbon cycle under drought.