American-Eurasian J. Agric. & Environ. Sci., 11 (3): 371-384, 2011 ISSN 1818-6769 © IDOSI Publications, 2011 Corresponding Author: El-Refaey F.A. El-Dengawy, Department of Pomology, Faculty of Agriculture, Damietta Branch, Mansoura University, Damietta, Egypt. 371 Improving Growth and Salinity Tolerance of Carob Seedlings (Ceratonia siliqua L.) by Azospirillum Inoculation El-Refaey F.A. El-Dengawy, Ahmed A. Hussein and Saad A. Alamri 1,2 2 2 Department of Pomology, Faculty of Agriculture, 1 Damietta Branch, Mansoura University, Damietta, Egypt Department of Biology, Faculty of Science, King Khalid University, Abha, Saudi Arabia 2 Abstract: Carob has been neglected with respect to both cultural practices and research and development. The current study was carried out for two successive seasons to find a practical method for extension of carob cultivation under saline water irrigation and to evaluate the efficacy of the Azospirillum inoculation in the development of salinity tolerance of the carob seedlings under salinity stress (4.69EC, 9.38EC and 14.07EC of seawater). The results showed that the seedling growth characteristics particularly seedling dry weight, the new leaf area and root characters showed significant progressive reductions with increasing salinity levels. But these reductions were significantly ameliorated by Azospirillum inoculation. Regarding the leaf mineral and proline content, the saline irrigated seedlings either inoculated or non-inoculated with Azospirillum contained significantly higher Na , Cl and proline concentrations and significantly lower K and N concentrations in leaf + - + than that of the normal irrigated control. However, applying Azospirillum inoculum in combination with saline water significantly decreased Na+ and Cl- and increased K+, N and proline concentrations in the seedling leaves under all salinity levels. Also, K/Na ratio was significantly increased by Azospirillum inoculum under the most tested levels of salinity. Applying Azospirillum inoculum gave significantly higher total phenols concentration than uninoculated seedlings. The SCC percentage was significantly higher in saline water irrigated seedlings at 14.07EC with or without Azospirillum inoculum than the other tested treatments. Chlorophyll concentration in leaves of the treated seedling was significantly decreased with rising levels of salinity. Applying Azospirillum inoculum increased chlorophyll concentration in seedling leaves under all salinity levels compared to the uninoculated ones. Moreover, applying Azospirillum inoculum in combination with saline water significantly increased survival ratio of the seedlings and decreased injured leaves (%) compared to the uninoculated ones under the same level of salinity. It can be conclude that application of Azospirillum inoculum can initiate systemic resistance of carob seedling to salt stress and improve growth. Key words:Carob seedlings Salinity stress Azospirillum Seedling characteristics Physiological parameters INTRODUCTION Azospirillum was discovered as a growth promoter of The carob (Ceratonia siliqua L.) is found in a range conducted on cereal, the perception of Azospirillum as a of the wild habitat of many Mediterranean countries plant-growth-promoting bacterium (PGPB) for cereals is including Cyprus, Egypt, Italy, Lebanon, Libya, southern still widespread. This ignores the multitude of studies in Jordan, Syria, Tunisia and Turkey, representing different which Azospirillum affected and promoted the growth of genetic resources [1]. The origin of carob is not clear as it numerous other species, including trees, cacti, vegetables, has undergone extensive cultivation since ancient times fruit, flowers, medicinal plants and spices [5]. This review [2]. Carob is also widely planted as an ornamental and proposed that Azospirillum be considered a non-specific shade tree in some countries as USA and Australia [3]. PGPB. Successful applications and research in these areas Besides, all parts of the carob tree, e.g. pods, seeds and are progressing slowly and most studies are conducted woods have great economic aspects [4]. under in vitro conditions. Research in these areas is cereals. Because much Azospirillum research is still