Photosynthetic reactions of two species of aspen (Populus alba L.) and cottonwood (Populus nigra L.) to lead increment in hydroponic medium

The effect of lead in hydroponic medium on leaf gas exchanges of two species of aspen and cottonwood, together with some other physiological indices including relative water content, maximum electrolyte conductivity and electrolyte leakage ratio of different organs and efficiency of photosystem II were investigated. One year old saplings of two poplar species were gradually subjected to chelated lead (PbCl₂) treatments with concentrations of 15, 45 and 90 part per million in medium and were harvested two weeks after reaching to the final concentration. The results revealed that maximum electrolyte conductivity of leaves in both species increased under severe treatment, but the increment of lead concentration only in cottonwood was accompanied by increment of electrolyte leakage ratio. Relative water content of organs remained constant in both species, whereas increasing lead increment up to 90 ppm diminished stomatal conductance of aspen, and not affected cottonwood. Both species were able to maintain photosystem II efficiency under stress, but net photosynthesis ratio reduced in aspen. The photosynthetic limitations in both species was observed regarding to reduction of intrinsic water use efficiency (A/g_s), that in aspen it was mostly posed by stomatal, and in cottonwood it was due to non stomatal restrictions. It can be concluded that, in cottonwood plants grown in the presence of excess Pb, plasma membrane integrity and efficiency of enzymes are related to carbon fixation which impaired due to maintenance of stomatal conductance and transpiration rate, whereas depletion of stomatal conductance in aspen prevented physiological damages.