Stimulating effect of low-dose stressors on the development of maize and bean chloroplasts

Abstract

Different kinds of harmful compounds, as heavy metals and herbicides used in low concentration (low dose stressors) have a beneficial effect on plants. This stimulation can be shown in the development of seedlings and delaying or inhibition of senescence and rejuvenation of scenescing plants as well. Our model system to investigate the effect of these low dose stressors (Cd, Pb, Ni, Ti and DCMU in 5.10-8 ¿ 10-6 M range treated in nutrient solution or spraying the leaves) was the chloroplast in greening maize and bean seedlings and in their detached senescencing leaves. Chlorophyll (Chl) content of maize leaves (up to four weeks) increased considerably: in the case of Cd 120-150%, Pb and Ni 115-130%, Ti 115-150% and DCMU 120-200%, and in bean leaves: Pb, Ti and DCMU 120-140%, Cd and Ni 120-180%. Chl a/b ratio altered (decreased) only in the case of DCMU treatment. CO2 fixation capacity decreased in all treatments. Electron micrographs did not show significant difference between the lamellar system of control and treated chloroplasts, except a slight increase in the proportion of grana after DCMU treatment. The starch content seemed to change in a compound-specific manner. We used detached leaves standing or floating on nutrient solution as a model to investigate the effect of stressors on senescence. Every treatment decreased the loss of Chl and sometimes even increased the Chl content (rejuvenation). CO2 fixation increased considerably in all cases. Leaves developed roots during three-week treatment. The number of plastoglobuli was reduced in the treated samples. Ni and Pb treatment caused large accumulation of starch. It is assumed, that these low dose stressors generate specific or non-specific alarm reactions in plants, which may involve changes of the hormonal balance.