Physiological response of amaranth (Amaranthus caudatus L.) to plant density under deficit irrigation Somayeh Farshbaf-Jafari*, Alireza Pirzad Department of Agronomy, Faculty of Agriculture, Urmia University, (IRAN) E-mail: somayehfjafari@yahoo.com; alirezapirzad@yahoo.com FULL PAPER ABSTRACT In order to evaluate the effects of water deficit and plant density on some physiological characteristics and biological yield of amaranth (Amaranthus caudatus L.), a factorial experiment was conducted based on Randomized Complete Block Design with three replications at the University of Tabriz in 2013. Treatments were irrigation intervals (I 1 and I 2 : irrigations after 70 and 140 mm of evaporation from class A pan, respectively) and plant densities (40000, 80000 and 120000 plants/ha). The results showed significant interaction of irrigation  plant density for leaf chlorophyll a, b, total chlorophyll, carotenoid, proline, relative water content (RWC), photosystem  efficiency (Fv/Fm) and biological yield. All chlorophyll content and biological yield increased with increasing plant density under water deficit. This increase was largely attributed to increased leaf thickness. Efficiency of photosystem  has positive correlation with relative water content and biological yield. The highest (41230.60 kg/ha) and lowest (14820.03 kg/ha) biological yield were respectively obtained from 120000 and 40000 plants/ ha under deficit irrigation.  2014 Trade Science Inc. - INDIA KEYWORDS Chlorophyll content; Deficit irrigation; Photosystem  efficiency; Proline; Relative water content; Biological yield. INTRODUCTION Among the environmental stresses, drought stress is one of the most adverse factors for plant growth and productivity [29, 28] . A small decrease in the availability of water to a growing plant immedi- ately reduces its metabolic and physiological func- tions such as the decrease in chlorophyll contents, and an increase accumulation of proline in plants [32, 19] . Grain and forage amaranth ( Amaranthus BTAIJ, 10(24), 2014 [15407-15413] BioTechnology An Indian Journal Volume 10 Issue 24 BioTechnology ISSN : 0974 - 7435 caudatus), as a plant possessing C 4 -photosynthesis pathway, was also recognized as drought tolerant crop [8] . Anon [2] , reported that the C 4 plant uses about three-fifth the amount of water a C 3 plant uses to pro- duce the same amount of plant materials because of the higher carbon dioxide uptake rates and greater stomatal resistance to water loss [16] . Drought toler- ant crop can be characterized by growth response, changes in water relations of tissues exposed to low water potential, stomata conductance, ion accumu- lation, changes in fluorescence induction parameters