Analysis and interpretation of the reduction of quantum yield of photosynthesis at low irradiances

Abstract

Photosynthesis-irradiance (P-E) curves are widely used to describe photosynthetic efficiency and potential. Contemporary models assume maximal photosynthetic quantum yield (f) at low measurement irradiances. But P-E observations made with both oxygen evolution and carbon uptake techniques show that this is not always the case. Using new and published data in conjunction with modeling exercises, I demonstrate that regardless of the mechanism, there can be reductions in f at low measurement irradiances that are not readily observable using conventional P-E analyses. Yet, reductions in f can result in significant errors (>50%) in the estimation of the initial slope of the P-E curve and ultimately the maximum quantum yield of photosynthesis (fmax). Several mechanisms have been proposed to account for these reductions including S-state decay, respiration and photosystem I (PSI) limitation. Using the later two mechanisms as tools, I demonstrate the potential severity of problem. In the first example I model the effect of incorrect respiration "corrections" on P-E and f-E curves and show that fmax can be significantly positively or negatively influenced by small errors in respiration corrections. In the second example I measure f-E response curves of Dunaliella tertiolecta and Skeletonema costatum using different colors of light and demonstrate how PSI-limitation may influence P-E and f-E curve structure. Data from these models and experimental observations lead us to suggest that f max be calculated as the maximum of a f-E curve rather than using the more conventional methodology employing the initial slope of the P-E curve.