Effect of chilling on growth and nitrogen assimilation in Azolla caroliniana

Abstract

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.

The oxidative stress caused by NaCl inAzolla carolinianais mitigated by nitrate

“…Many authors reported that Azolla plants grew significantly better in presence of nitrate in comparison to those entirely dependent upon symbiotically fixed nitrogen (Rai and Rai 2003;Mostafa and Hassan 2006). In our study, addition of different concentrations of NaCl (0, 50, 100, and 150 mM) for 5, 10, and 15 days in presence of 4 mM nitrate exhibited a reduction in biomass and RGR, and this reduction resulted in an increase in DT over the control.…”

“…in the presence of nitrate, the contribution of cyanobacterial fixed nitrogen is then negligible (Reddy et al 1989;Rai and Abraham 1995). In contrary, Azolla-Anabaena relationship is tolerant to repression by combined nitrogen (Peters 1978;Mostafa and Hassan 2006). It has been reported reduction in the NaCl-induced growth inhibition of cyanobacteria by nitrate and the reason given is that NO À 3 and Na ' share a common transport system (Reddy et al 1989;Rai and Tiwari 1999).…”

Changes of Enzymatic Antioxidants and Minerals Content of Broad Bean ( Vicia faba L.) Plants in Response to Chilling Stress and Reacclimation

“…Cold temperature stress (0-10 o C) has a broad spectrum effects on cellular components including plasma membranes, chloroplasts and mitochondria as well as physiological and biochemical processes of plants Hassan, 2006 andYin et al, 2009). Moreover, low temperature has an inhibitory effect on uptake of mineral elements such as P, K, Ca, Mg, Fe, Zn and B and their allocation between different plant organs, and finally depress the growth (Taspinar et al, 2009 andNxawe et al, 2010).…”