Effect of Putrescine and Paclobutrazol on Growth, Physiochemical Parameters, and Nutrient Acquisition of Salt-sensitive Citrus Rootstock Karna khatta (Citrus karna Raf.) under NaCl Stress Dew Kumari Sharma • A. K. Dubey • Manish Srivastav • A. K. Singh • R. K. Sairam • R. N. Pandey • Anil Dahuja • Charanjeet Kaur Received: 29 July 2010 / Accepted: 18 November 2010 / Published online: 3 February 2011 Ó Springer Science+Business Media, LLC 2011 Abstract Salinity is a serious problem in arid and semi- arid areas and citrus trees are classified as salt-sensitive. Because putrescine (Put) and paclobutrazol (PBZ) are known to act as plant protectants under environmental stresses, we examined the effect of Put and PBZ on the physiochemical parameters of the salt-susceptible citrus rootstock Karna khatta under NaCl stress. PBZ was applied at 0, 250, and 500 mg L -1 as a soil drench 1 week prior to salinization. A computed amount of NaCl salt to develop soil salinity of 3 dS m -1 (3 g NaCl kg -1 soil) and foliar spray of Put at 0 or 50 mg L -1 were applied. The electrical conductivity (EC) of the garden soil (0.35 dS m -1 ) was used as control. Application of PBZ and/or Put reduced the membrane injury index and increased relative water content, photosynthetic rate, and pigments content under saline conditions compared to what occurred in plants exposed to NaCl in the absence of PBZ or Put. Application of PBZ or Put alone or in combination also improved the activities of SOD and peroxidase and proline content under saline conditions. Application of PBZ and/or Put also increased K ? and reduced Na ? and Cl - concen- trations in leaf tissues. It is proposed that PBZ and/or Put could improve the tolerance of salt-susceptible Karna khatta by regulating absorption and accumulation of ions and improving antioxidant enzyme activities. Keywords Citrus  Karna khatta  PBZ  Physiochemical parameters  Putrescine  Nutrient acquisition  Salinity Introduction Citrus species are classified as salt-sensitive and are grown preferentially in semiarid areas where irrigation is required to produce a maximum yield. In these areas, soil and water often contain excessive concentrations of soluble salts. Soil salinity is the most important factor responsible for poor productivity of citrus trees. Salinization is one of the serious problems confronting sustainable agriculture in irrigated production systems in arid and semiarid regions (Marschner 1995; Ravindran and others 2007). Salt induces various biochemical and physiological responses in plants and affects almost all metabolic processes (Nemoto and Sasakuma 2002). One of the biochemical changes that occurs when plants are subjected to salt stress is the pro- duction of reactive oxygen species (ROS) such as the superoxide radical (O 2 - ), hydrogen peroxide (H 2 O 2 ), and the hydroxyl radical (OH - ). ROS can have detrimental effects on normal metabolism through oxidative damage to lipids, proteins, and nucleic acids (Mittler 2002). Most halophytes react to environmental stresses with an effective ROS-scavenging system involving antioxidant enzymes D. K. Sharma  A. K. Dubey (&)  M. Srivastav  A. K. Singh Division of Fruits and Horticultural Technology, Indian Agricultural Research Institute, New Delhi 110012, India e-mail: akd67@rediffmail.com R. K. Sairam Division of Plant Physiology, Indian Agricultural Research Institute, New Delhi 110012, India R. N. Pandey Division of Soil Science and Agricultural Chemistry, Indian Agricultural Research Institute, New Delhi 110012, India A. Dahuja Division of Biochemistry, Indian Agricultural Research Institute, New Delhi 110012, India C. Kaur Division of Post Harvest Technology, Indian Agricultural Research Institute, New Delhi 110012, India 123 J Plant Growth Regul (2011) 30:301–311 DOI 10.1007/s00344-011-9192-1