Photosynthesis and transpiration rate of detached leaves of pea (Pisum sativum L. cv. Itowiecki) exposed to solution of Pb(NO3)2 at 1 or 5 mmol.dm -3 concentrations were inhibited. The higher concentration of this toxicant decreased photosynthesis and transpiration rates 2 and 3 times respectively, and increased respiration by about 20 %, as measured after 24 hours of treatment. Similarly to Pb(NO3)2, glyceraldehyde solution, an inhibitor of phosphonbulokinase, at 50 mmol.dm concentration decreased the rates of photosynthesis and transpiration during introduction into pea leaves. The rate of dark respiration, however, remained unchanged during 2 hours of experiment.The potential photochemical efficiency of PS II (Fv/Fm) and the activity of Rubisco (EC 4.1.1.39) at 5 mmol.dm -3 of Pb(NO3)2 were lowered by 10 % and 20 % respectively, after 24 hours. Neither changes in the activity of PEPC (EC 4.1.1.31) or protein and pigment contents were noted in Pbtreated leaves. The photosynthetic activity of protoplasts isolated from leaves treated for 24 or 48 hours with Pb(NO3)2 at 5 mmol.dm -3 concentration was decreased 10 % or 25 %, whereas, the rate of dark respiration was stimulated by about 40 % and 75 %, respectively. The content of abscisic acid, a hormone responsible tbr stomatal closure, in detached pea leaves treated tbr 24 h with 5 mmol.dm 3 of Pb(NO3)2 solution was increased by about 3 times; a longer (48h) treatment led to further increase (by about 7 times) in the amount of this hormone. The results of our experiments provide evidences that CO2 fixation in detached pea leaves, at least up to 24 hours of Pb(NO3)2 treatment, was restricted mainly by stomatal closure. List of abbreviations."A B A = abscisic acid; F m = m a x i m a l fluorescence; Fo = initial fluorescence; Fv = variable fluorescence; F v / F m = photochemical yield o f p h o t o s y s t e m II; G = D L -g l y c e r a l d e h y d e ; F = CO2 c o m p e n s a t i o n point; P E P C = phosphoenolpyruvate carboxylase; Pn = net photosynthesis; R = dark respiration; R u b i s c o = ribulose-l,5-bisphosphate carboxylase -o x y g e n a s e ; t 1/2 = halfrise time f r o m Fo to F m ; T = transpiration

The exposure of detached leaves of C3 plants (pea, barley) and C4 plant (maize) to 5 mM Pb (NO3)2 for 24 h caused a reduction of their photosynthetic activity by 40-60%, whereas the respiratory rate was stimulated by 20-50%. Mitochondria isolated from Pb2+-treated pea leaves oxidized substrates (glycine, succinate, malate) at higher rates than mitochondria from control leaves. The respiratory control (RCR) and the ADP/O ratio were not affected. Pb2+ caused an increase in ATP content and the ATP/ADP ratio in pea and maize leaves. Rapid fractionation of barley protoplasts incubated at low and high CO2 conditions, indicated that the increased ATP/ADP ratio in Pb2+-treated leaves resulted mainly from the production of mitochondrial ATP. The measurements of membrane potential of mitochondria with a TPP+-sensitive electrode further showed that mitochondria isolated from Pb2+-treated leaves had at least as high membrane potential as mitochondria from control leaves. The activity of NAD-malate dehydrogenase in the protoplasts from barley leaves treated with Pb2+ was 3-fold higher than in protoplasts from control leaves. The activities of photorespiratory enzymes NADH-hydroxypyruvate reductase and glycolate oxidase as well as of NAD-malic enzyme were not affected. The presented data indicate that stimulation of respiration in leaves treated by lead is in a close relationship with activation of malate dehydrogenase and stimulation of the mitochondrial ATP production. Thus, respiration might fulfil a protective role during heavy metal exposure.

At concentrations in which they occur on the plant surface and retard mitosis, coumarin and xanthotoxin lowered uptake of oxygen (by 60 and 30%, respectively) by meristematic cells ofAllium cepa root tips. They caused changes in the structure of the mitochondrial matrix to become dense, and protrusions of mitochondrial membranes were visible parallelling their hypertrophy, indicating alteration in the structure and physiology of these organelles. Coumarin and, to a lesser extent, xanthotoxin increased succinate dehydrogenase production in mitochondria and also in the cytoplasm, indicating changes in membrane permeability. Changes in oxygen uptake and mitochondrial structure, in addition to the retardation of mitosis, may be the reason these compounds act as allelochemicals after they have been removed from the plant surface and reach the root meristem.

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Photosynthesis and respiration rates, pigment contents, CO2 compensation point, and carbonic anhydrase activity in Cyanidioschizon merolae cultivated in blue, red, and white light were measured. At the same light quality as during the growth, the photosynthesis of cells in blue light was significantly lowered, while under red light only slightly decreased as compared with white control. In white light, the quality of light during growth had no effect on the rate of photosynthesis at low O2 and high CO2 concentration, whereas their atmospheric level caused only slight decrease. Blue light reduced markedly photosynthesis rate of cells grown in white and red light, whereas the effect of red light was not so great. Only cells grown in the blue light showed increased respiration rate following the period of both the darkness and illumination. Cells grown in red light had the greatest amount of chlorophyll a, zeaxanthin, and β-carotene, while those in blue light had more phycocyanin. The dependence on O2 concentration of the CO2 compensation point and the rate of photosynthesis indicate that this alga possessed photorespiration. Differences in the rate of photosynthesis at different light qualities are discussed in relation to the content of pigments and transferred light energy together with the possible influence of related processes. Our data showed that blue and red light regulate photosynthesis in C. merolae for adjusting its metabolism to unfavorable for photosynthesis light conditions.