Limiting Factors in Photosynthesis: II. IRON STRESS DIMINISHES PHOTOCHEMICAL CAPACITY BY REDUCING THE NUMBER OF PHOTOSYNTHETIC UNITS.

It has been proposed that Fe stress may be used in the study of limiting factors in photosynthesis as an experimental means of varying photochemical capacity in vivo (Plant Physiol 1980 65: 114-120). In this paper the effect of Fe stress on photosynthetic unit number, size, and composition was investigated by measuring P(700), cytochrome (Cyt) f, chlorophyll (Chl) a, and Chl b in sugar beet leaves. The results show that when Fe stress reduced Chl per unit area by 80% (from 60 to 12 micrograms per square centimeter), it decreased the number of P(700) molecules per unit area by 88% and Cyt f per unit area by 86%; over the same range the Chl to P(700) ratio increased by 37% but there was no significant change in the Chl to Cyt f ratio. These data suggest that Fe stress decreases photochemical capacity and Chl per unit area by diminishing the number of photosynthetic units per unit leaf area.The ratio of Chl a to Chl b did not change with Fe stress. This suggests that the proportion of light-harvesting Chl a/b-protein complex within the photosynthetic unit remained constant. Electron microscopy of chloroplasts revealed that the decrease in the number of photosynthetic units which occurred during Fe stress was accompanied by a reduction in the number of granal and stromal lamellae per chloroplast and by a reduction in the number of thylakoids per granum.