PLANT SOIL ENVIRON., 56, 2010 (5): 209–217 209 Zinc deficiency is a serious micro-nutritional disorder, threatening world food production. Zinc plays a fundamental role in several critical func- tions in the cell such as protein metabolism, gene expression, structural and functional integrity of biomembranes and photosynthetic carbon metabo- lism (Cakmak 2000). Some of metabolic changes brought about by Zn deficiency could be well explained by the function of Zn as a structural component of a special enzyme or involvement in specific steps in particular metabolic pathways (Marschner 1995). However, there are changes in the synthesis and metabolism of Zn-deficient plants that could not be explained directly by the presence of Zn in the metabolic pathway or enzyme structure. Such responses are regarded to be rather indirect effects of Zn deficiency. Concerning the central role of Zn in stability of biomembranes and proteins (Cakmak 2000), Zn deficiency can affect the photochemical processes in the thylakoids, and thus inhibits biophysical processes of photosynthesis. The flow of electrons through PSII is indicative of the overall rate of photosynthesis and is an estimation of photosynthetic performance. Chlorophyll fluorescence measurements could be used to estimate the operating quantum efficiency of electron transport through PSII in leaves (Maxwell and Johnson 2000). Zinc deficiency can also cause a drastic decrease in chlorophyll content as well as a severe damage to the fine structure of chloroplasts (Chen et al. 2007). In addition of the expected effect on photochemi- cal processes, involvement of Zn in carbohydrate metabolism was demonstrated through its effect on net CO 2 assimilation rate via stomatal conductance (Sharma et al. 1995) and sugar transformations (Marschner 1995). Inhibition of activity of Rubisco, aldolase, sucrose synthase and starch synthetase in plant tissues due to Zn starvation has been suggested (Marschner 1995). Whether the effect of Zn nutritional status on stomatal conductance and starch or sucrose formation is a primary result of Zn deficiency still remains an open question. It was suggested that, Zn is involved in stomatal opening, possibly as a constituent of the enzyme carbonic anhydrase and/or as a factor in maintain- ing membrane integrity and K + uptake (Sharma et al. 1995). Oxidative stress is a central factor in abiotic and biotic stress phenomena that occurs when there is a serious imbalance between the production of reactive oxygen species (ROS) and antioxidant defense capacity (Apel and Hirt 2004). Growth, photosynthesis and antioxidant defense system in Zn-deficient red cabbage plants R. Hajiboland, F. Amirazad Plant Science Department, University of Tabriz, Tabriz, Iran ABSTRACT Te efect of Zn defciency was studied in red cabbage (Brassica oleracea L. var. capitata f. rubra) plants grown in nutrient solution under controlled environmental conditions. Zinc starvation afected the number (61%), surface area (72%) and biomass (62%) of leaves more than root biomass (42%). Although chlorophyll fuorescence param- eters revealed occurrence of photoinhibition following declined stomatal conductance and reduction of CO 2 avail- able at carboxylation sites, photosynthesis apparatus was not damaged seriously under Zn defciency conditions. Chlorophyll a, chlorophyll a/b ratio, soluble carbohydrates and starch declined but anthocyanins and free phenolics were accumulated under Zn defciency conditions. Activity of ascorbate peroxidase, catalase and peroxidase en- hanced under Zn defciency conditions, whereas activity of superoxide dismutase declined in leaves but not in roots of Zn-defcient plants. Maintenance of superoxide dismutase activity and malondialdehyde content in roots dem- onstrated that roots were more protected against reactive oxygen species imbalance under Zn defciency conditions compared with leaves that was correlated well with the lower sensitivity of roots to low Zn supply. Keywords: antioxidant defense system; carbohydrates; chlorophyll fluorescence; CO 2 assimilation; Brassica oleracea; Zn defciency