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The effect of pre-treatment with 200 mM NaCl on the response of four barley cultivars (Hordeum vulgare L. cv. to UV-B radiation was investigated. Salt stress as well as UV-B irradiation led to a decrease of the total chlorophyll (chl) content in all cultivars, except in Kalaycı-97. While carotenoids are almost not affected by NaCl treatment, UV-B irradiation caused an increase by 5-20% of carotenoid content of all cultivars. UV-B induced damages of photosynthetic apparatus were estimated by the rate of photosynthetic electron transport measured by chl fluorescence and the rate of oxygen evolution, the latter being more affected. Pre-treatment with NaCl alleviated harmful effect of UV-B irradiation on F v /F m and ETR, but not on oxygen evolution. UV-B-induced and UV-B-absorbing compounds with absorption at 300 and 438 nm increased as a result of UV-B treatment. The level of stress marker proline increased considerably as a result of NaCl treatment, while UV-B irradiation resulted in a pronounced increase of the level of H 2 O 2 . MDA enhanced in the seedlings subjected to salt and UV-B stress. Established cross-acclimation to UV-B as a result of salt treatment could be due to the increased free proline and the level of UV-B absorbing compounds in barley seedlings subjected to NaCl.

In the present study, low temperature-dependent physiological changes were investigated through photosynthetic activity and some endogenous mechanisms in two winter oilseed rape cultivars (Brassica napus L. ssp. oleifera cvv. Eurol and Hansen) on the basis of leaf age. Chlorophyll fluorescence measurements demonstrated that low temperature caused decreased photosynthetic activity in both cultivars. However, photosynthetic apparatus in the young leaves of Hansen is more tolerant to low temperature as demonstrated by lower F0 (minimum fluorescence yield) and 1–qp (excitation pressure of photosystem II), higher Fm (maximum fluorescence yield), Fv/Fm and non-photochemical quenching (NPQ) compared with Eurol. In addition, young leaves of Hansen represented marked increase in some antioxidant enzyme activities (superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR)) during cold exposure. In the young leaves of Eurol, however, APX and GR activity was decreased by low temperature, indicating lower efficiency of ascorbate-glutathione cycle. Lower antioxidant activity in the young leaves of Eurol may be responsible for increased malondialdehyde (MDA), H2O2 and membrane damage and decreased chlorophyll content as a result of oxidative damage during cold exposure. In the cold-stressed mature leaves, both cultivars represented similar antioxidant capacities and photosynthetic efficiency. As a consequence, coordinated increase in SOD, APX and GR activities, increased capacity to keep quinone A (QA) in an oxidised state (as indicated by lower 1–qp) and accumulation of soluble sugar and proline could be mainly attributed to higher level of tolerance of the young leaves of Hansen to low temperature when compared with Eurol.

IntroductionDuring their growth and development, plants are exposed to abiotic (high and low temperature, salinity, drought, radiation, etc.) and biotic (pathogen, fungus, etc.) stress factors, which decrease their yield and the quality of their products. To date, 20 %, 26 % and 15 % of suitable agricultural areas worldwide are affected by mineral, drought and temperature stresses respectively (Blum 1985, Ashraf 1994a, Flowers and Yeo 1995. In addition, there is a decay of 2 million ha of world agricultural lands to salinity each year (Kalaji and Pietkiewica 1993, Szabolcs 1994). Moreover, over 6 % of the world's total land area is affected by salinity and sodicity (Munns 2005). Plants are affected in several ways by increasing salt concentrations. It causes osmotic stress, specific ion toxicity and nutrient deficiencies, thereby affecting a range of physiological processes involved in cell metabolism (Munns 2002a).Salinity stress is often accompanied by temperature stress. Changes in ambient temperature occur more rapidly than changes in other stress factors and temperature extremes aggravate the adverse effects of other stresses, including salinity, on crop production and quality (Ashraf and Foolad 2007). It has been reported that environmental stresses lead to many social and economic problems in developing countries (Ashraf 1994a, Saiko andZonn 2000).Salt stress affects all the major processes such as photosynthesis, protein synthesis, and energy and lipid metabolism (Parida and Das 2005). Temperature and salt stresses directly or indirectly affect the photosynthetic functions by changing the structural organization and physicochemical properties of thylakoid membranes (Alia-Mohanty and Saradhi 1992, Karim et al. 1999, Lichtenthaler et al. 2005. The osmotic balance is essential for plants (Ashraf 2004). Plants have to maintain a high K + and a low Na + AbstractThe effect of NaCl ()0.1, )0.4 and )0.7 MPa) on some physiological parameters in six 23-day-old soya bean cultivars (Glycine max L. Merr. namely A 3935, CX-415, Mitchell, Nazlıcan, SA 88 and Türksoy) at 25, 30 and 35°C was investigated. Salt stress treatments caused a decline in the K + /Na + ratio, plant height, fresh and dry biomass of the shoot and an increase in the relative leakage ratio and the contents of proline and Na + at all temperatures. Effects of salt stress and temperature on Chl content, Chl a/b ratio (antenna size) and qN (heat dissipation in the antenna) varied greatly between cultivars and treatments; however, in all cases approximately the same qP value was observed. It indicates that the plants were able to maintain the balance between excitation pressure and electron transport activity. Pigment content and the quantum efficiency of photosystem II exhibited significant differences that depended on the cultivar, the salt concentration and temperature. The cultivars were relatively insensitive to salt stress at 30°C however they were very sensitive both at 25 and 35°C. Of the cultivars tested CX-415 and SA 88 were the best performers ...

This study was conducted to determine the tolerance of safflower (Carthamus tinctorius L.) cultivars (Dinçer, Remzibey-05, and Yenice) against salt stress based on some physiological and biochemical parameters at the vegetative stage. Eighteen-day-old plants were subjected to salt stress [0 (control), 75, 150, 225, and 300 mM NaCl concentrations] for 12 days, which led to a significant decrease in growth parameters (stem growth, fresh weight, and relative water content) and the photosynthetic pigment contents of the safflower cultivars. The photochemical activities of photosystem II of the cultivars were negatively affected by salinity, especially at the highest concentration (300 mM). Salt stress decreased K + content and K + /Na + ratio while it increased Na + content. Malondialdehyde and free proline contents in the leaves of cultivars increased gradually in proportion to the increase of NaCl concentration. Analysis of antioxidant enzyme activities showed that these enzymes responded differently to the NaCl concentrations. Dinçer, with higher antioxidant enzyme activities, had a more effective response than the other cultivars. Considering growth and biochemical and chlorophyll fluorescence parameters with the endogenous defense system, Dinçer had a higher withstanding capacity against salinity than the other cultivars.