Prolonged exposure of tobacco to a low oxygen atmosphere to suppress photorespiration decreases net photosynthesis and results in changes in plant morphology and chloroplast structure

Air-grown tobacco (Nicotiana tabacum L.) plants were transferred for one we ek into a low oxygen atmosphere (2 kPa O-2, LO) to study both immediate and long-term effects of the suppression of photorespiration on net photosynth etic rate (P-N), plant morphology, and chloroplast ultrastructure. The P-N and the leaf conductance for CO2 increased upon exposure of attached tobacc o leaves to LO. These results may suggest that under LO, external CO2 is us ed to consume the radiant energy normally utilized in photorespiration by n et CO2 assimilation at the expense of an increased rate of transpiration. T he increase in the coefficient of nonphotochemical fluorescence quenching i ndicates that under LO, (surplus) radiant energy is also dissipated as heat . Prolonged LO-treatment of tobacco resulted in a decrease in the P-N (meas ured in air) and in a reduction in the number of starch grains in the chlor oplasts. Concomitantly, large lipid globuli appeared in the chloroplasts an d the distance between the thylakoids forming the grana decreased. These ch anges in the ultrastructure of chloroplasts may have contributed to the dec line in the P-N. The LO-treated plants were considerably smaller than the c ontrol plants maintained in air. This appears to have resulted from a reduc tion in the rate of leaf area expansion at the expense of an increase in th e specific mass of the leaves. This long-term response to LO-treatment may allow the plants to conserve water.