Exposure of Azolla plants to UV-B radiation for 6 h resulted in a decrease in biomass and relative growth rate (RGR), which coincided with an increase in doubling time (DT) as compared with the control. Also, the protein content decreased. On the other hand, hydrogen peroxyde (H 2 O 2 ) and malondialdehyde (MDA) accumulated significantly in UV-treated Azolla plants. Conversely, the addition of selenium (Se) at 1 ppm resulted in a significant increase in biomass and protein content of untreated and UV-treated Azolla plants, and a significant reduction in both H 2 O 2 and MDA. Moreover, the addition of Se to UV-treated and untreated Azolla plants resulted in a significant increase in total ascorbate and total glutathione (GSH) contents compared with the control and UV-stressed Azolla plants. Also, glutathione redox potential (GSH/TG) increased significantly in UV-treated Azolla plants in the presence of Se. There also was a significant increase (38%) in ascorbate peroxidase (APX) activity in UV-treated plants compared with the control. APX activity in the presence of Se did not change significantly compared with the control. Glutathione reductase (GR) activity increased significantly in UV-treated Azolla, while glutathione peroxidase (GSH-PX) activity did not. On the other hand, both GSH-PX and GR activity in untreated and UV-treated Azolla plants were significantly enhanced by the application of Se to the nutrient media at a concentration of 1 ppm. Therefore, we can conclude that Se protects Azolla plants from UV-B stress.

T HE AIM of this study is evaluating the effects of nutrients deprivation in growth media of Azolla caroliniana plant on growth and production of various phenolic components and antioxidant activity. Nutrient-deficient cultures (0.2, 0.1, 0.05 strength Hoagland solution) of Azolla plants resulted in a significant reduction in Azolla biomass, particularly with 0.05 strength compared to those grown on nutrient-sufficient culture (control). The doubling time (DT) and relative growth rate (RGR) were significantly affected with different Hoagland strength and that was accompanied with a significant accumulation of H 2 O 2 and MDA. Anthocyanins, total flavonoids and total phenolics were enhanced in starved-Azolla. HPLC analysis of phenolic acids revealed a significant increase in caffeic, o-coumaric and t-cinnamic acids in Azolla grown on 0.2 strength Hoagland culture, whereas coumaric acid was markedly accumulated in Azolla plants grown on 0.1 strength media. There was a significant increase in phenylalanine ammonia lyase (PAL) and antioxidant activities in starved Azolla. The results indicated that, the antioxidant activity might depend mainly on phenolic compounds.

Azolla caroliniana was exposed to 5 °C in darkness for 1, 2, 3, 5 or 7 d and then recovered for 7 d. Plants previously chilled for 2 or 3 d exhibited higher growth rates when transferred to normal temperature than either the control plants or those previously chilled for 5 or 7 d. Increased plant growth may be related to increased contents of chlorophyll, sucrose, and reducing sugars, due to increased photosynthetic capacity. In another experiment Azolla plants were chilled at 5 °C for 7 d and then transferred for 0, 4, 8, 12, or 16 d recovery to the N-free Hoagland solution or Hoagland solution containing 5 mM KNO 3 . In previously chilled plants, the growth rate was decreased. In the medium supplemented with nitrogen, the growth rate was greater than in the N-free medium in both chilled and nonchilled plants. In chilled plants the decrease in growth rate may be related to the disturbance of Anabaena azollae cells where the protecting envelope of the heterocysts was deorganized. During the recovery the rate of N 2 -fixation increased in both chilled and nonchilled plants up to 12 d after which both rates were similar. However, during the first 4 d the rate of the nonchilled plants was approximately 4-fold that of the previously chilled plants. Nitrate reductase and nitrite reductase activities in control plants were higher than in those previously chilled for 7 d. Both activities increased in nonchilled and previously chilled plants up to 12 d then decreased. The total protein content increased up to 12 d in chilled and nonchilled plants after which it decreased. Under all treatments, the values were higher in nonchilled plants than in those previously chilled ones and were also higher in presence of N than in its absence. Thus the presence of N-source in the medium counteracts the effect of chilling injury particularly during prolonged recovery.Additional key words: nitrate reductase, nitrite reductase, rate of nitrogen fixation.

A rapid, simple, selective, and sensitive method for trace determination of Zr (IV) has been developed based on the reaction of 1,4-dichloro-2,5-dihydroxyquinone in a hydrochloric acid medium to form a colored complex which is rendered water soluble by the micellar action of cetylpyridinium chloride (CPC) and measured at λmax 331 nm. Beer's law is obeyed in the concentration range 0-5 µg.ml -1 . The molar absorptivity of the complex, є was found to be 1.6 ×10 4 L.mol -1 cm -1 and Sandell's sensitivity was found to be 5.7 ng.cm -2 . A method for determination of Zr (IV) by second derivative spectrophtometry has also been proposed. The detection limit of Zr (IV) in the derivative method is 6 times lower than that of the zero order method, which shows the higher sensitivity of the derivative method. The method has been applied for determination of Zr (IV) in various samples, and satisfactory results have been obtained.

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