EVALUATION OF LEAD TOLERANCE IN SOME BARLEY GENOTYPES BY MEANS OF CHLOROPHYLL A FLUORESCENCE

Abstract

Heavy metals are grouped with regard to their density. They can be found naturally in thesoil because of weathering and other processes on rocks. However, because of industrialization and a rapid population increase, production of anthropogenic biosolids and agrochemical waste has been enhancing the risk of heavy metal contamination in soils. This is one of the main environmental problems, keeping in mind that metals reach the soil and end up depreciating the whole area. In toxic concentrations, heavy metals damage plants and organisms, affecting their organs, changing their biochemical processes, organelles, cellular membranes, and causing health problems. Most of the heavy metals are persistent in soil because of their immobile nature. The main heavy metals present in soil are Cadmium (Cd), copper (Cu), lead (Pb), zinc (Zn), chrome (Cr),nickel (Ni), barium (Ba), argon (Ag), cobalt (Co), mercury (Hg), and antimony (Sb), and some of theseelements are essential for many physiological functions in plants, whereas others have noknown biological function. Lead is widespread toxic element having no role in biological metabolism. The major source of Pb in the environment includes metal smelting, agriculture, industry, and urban activities. In plants, excess Pb inhibits germination of seeds, growth of plants, synthesis of chlorophyll and photosynthesis. Photosynthesis has been reported to be one of the most sensitive process against Pb toxicity. The most modern and sensitive technique used to measure photosynthesis is chlorophyll a fluorescence. Chlorophyll a fluorescence measurements provide valuble information about the stat of photosystem II. One of the important advantages of this technique is that it enables the determination of stress effects long before the observing of visible symptoms of any stress factor. In this study, the effect of lead toxicity (1.5 mM PbNO3) in barley (Hordeum vulgare L.) genotypes (Tarm-92 and Tokak 157/37) was investigated by means of chlorophyll fluorescence technique. Root and shoot growth and total plant length were inhibited by lead treatment in the both barley genotypes. Inhibition of shoot growth was mainly responsible for the decreased total plant length, probably due to higher level of of lead accumulation in the barley leaves. Photosystem II efficiency, on the orher hand, was decreased by lead toxicity in the both barley genotypes, as evaluated by chlorophyll fluorescence measurement. Our results showed that Tarm-92 had higher level of damaged reaction centers under lead toxicity as compared to Tokak 157/37. In addition, lead treatment increased the amount of trapped energy leading to quinoneA reduction (TRo/RC) and dissipated energy as heat (DIo/RC) and decreased maximum electron transport flux further than quinoneA (ETo/RC)in Tarm-92. These results showed that Tarm-92 under lead stress can not use absorbed light energy and dissipated it as heat, resulting in the decreased photosystem II activity. In Tokak 157/37, however, less energy was dissipated as heat and higher photosystem II activity was determined as confirmed by the changes in TRo/RC and ETo/RC. As a result, it may be concluded that Tokak 157/37 is more tolerant to lead toxicity because of higher photosystem II activity under lead toxicity as compared to Tarm-92.

Bu çalışmanın amacı, iki arpa (Hordeum vulgare L.) genotipinde (Tarm-92 ve Tokak 157/37) kurşun toksisitesinin (1.5 mM PbNO3) etkilerinin klorofil a floresansı tekniği ile araştırılmasıdır. Her iki arpa genotipinde kurşun uygulaması ile kök ve gövde büyümesi ile toplam bitki boyu inhibe edilmiştir. Gövde büyümesindeki inhibisyon, muhtemelen yapraklardaki kurşun birikimi nedeniyle, toplam bitki boyundaki azalmadan sorumlu bulunmuştur. Diğer yandan, klorofil a floresansı ölçümleri ile gösterildiği üzere, her iki arpa genotipinde fotosistem II aktivitesi kurşun uygulaması sonucunda azalmıştır. Sonuçlarımız Tokak 157/37 ile karşılaştırıldığında, kurşun toksisitesi altındaki Tarm-92'deki reaksiyon merkezlerinde daha fazla hasarın oluştuğunu göstermiştir. Ayrıca kurşun uygulaması Tarm-92'de kinonA'nın indirgenmesini sağlayan yakalanan enerji miktarını ve ısı olarak dağıtılan enerji miktarını artırmış, kinonA' dan sonraki maksimum elektron taşınım hızını azaltmıştır. Bu sonuçlar kurşun stresi altındaki Tarm-92'nin absorbladığı ışığın büyük kısmını kullanamayıp ısı olarak dağıttığını ve sonuçta fotosistem II aktivitesinin azaldığını göstermektedir. Ancak Tokak 157/37'de daha az enerji ısı olarak dağıtılmakta ve TRo/RC ile ETo/RC'deki değişimlerle ispatlandığı gibi daha yüksek fotosistem II aktivitesi belirlenmiştir. Sonuç olarak, kurşun toksisitesi şartlarında daha yüksek fotosistem II aktivitesine sahip olduğu için Tokak 157/37'nin Tarm-92 ile karşılaştırıldığında kurşuna daha toleranslı olduğu söylenebilir.