Limiting Factors in Photosynthesis

Abstract

The possibility of using Fe stress as an experimental tool in the study of limiting factors was explored. Results show that Fe stress decreased the chlorophyll (Chl) a, Chl b, carotene, and xanthophyll content of leaves of sugar beets (Beta vulgaris L.) and that the maximum rate of photosynthetic CO₂ uptake (P_max) per unit area was linearly related to Chl (a + b) per unit area. Measurements of noncyclic ATP formation by isolated chloroplasts at light saturation indicate that photosynthetic electron transport capacity decreased concomitantly with pigment content under Fe stress.

Iron stress decreased Chl per chloroplast but had no effect on the number of leaf cells per unit area, average leaf cell volume, number of chloroplasts per unit area, or leaf soluble protein per unit area. Average chloroplast volume, protein N per chloroplast, and ribulose bisphosphate carboxylase activity were diminished by Fe stress but to a lesser extent than Chl per chloroplast. The reduction in pigment concentration with Fe stress led to a relatively small decrease in light absorption, the fraction of incident light absorbed remaining high (49%) even at very low leaf Chl contents. There was no apparent change in the quantum yield of attached leaves at low irradiances, but at high irradiances, the capacity to convert absorbed light to chemical energy was greatly diminished in Fe-stressed leaves.

The results suggest: (a) that P_max per unit area are decreased linearly with Chl per unit area because of a decrease in photochemical capacity rather than a change in light absorption; and (b) that the effect of Fe stress may be sufficiently specific for it to be used as an experimental tool for the control and study of photochemical capacity in vivo.