Effect of long‐term H2S fumigation on photosynthesis in spinach. Correlation between CO2 fixation and chlorophyll <i>a</i> fluorescence

Abstract

Spinach plunts ( Spinacia oleracea L. cv. Monosa) were exposed to air with and without 0.25 μ l l ‐1 H 2 S. Effects of H 2 S exposure for up to 18 days on photosynthesis, dark respiration and on chlorophyll a fluorescence were studied. Dark respiration was not affected by H 2 S fumigation. Photosynthetic CO 2 fixation decreased linearly with time in both control and fumigated plants. The rate of decrease in CO 2 fixation was faster in the fumigated plants; after 14 days of exposure the fumigated plants showed a decrease in CO 2 fixation of 23%äs compared with the control plants. The H 2 S‐induced decrease in CO 2 fixation was accompanied by a decrease in quenching of the chlorophyll fluorescence. The most characteristic change in chlorophyll fluorescence was a decreased difference between maximum and steady‐state fluorescence [(P‐T)/P), suggesting a reduced efficiency in the use of photochemical energy in photosynthesis. Differences in CO 2 fixation were more pronounced whcn measured at high light intensity; the maximum rate of CO 2 fixation at light saturation decreased significantly with time in the H 2 S‐exposed plants; after 14 days of H 2 S exposure a decrease of more than 70% was noted. The decrease in CO 2 fixation could not be attributed to a decreased chlorophyll content; on the contrary, chlorophyll content even slightly increased during fumigation. The initial increase in CO 2 fixation rate with increasing light intensity was also reduced by prolonged H 2 S fumigation, indicating an effect of H 2 S fumigation on photosynthetic electron transport. Finally, the phytotoxicity of H 2 S is discusscd in relation to the H 2 S‐induced changes in photosynthetic CO 2 fixation and chlorophyll a fluorescence, and the effect of H 2 S on leaf development observed in earlier studies.