Relationship between photosynthesis and photorespiration in field-grown wheat leaves.

Simultaneous measurements of net photosynthetic rate (P-N) and fluorescence were taken on flag leaves of field-grown wheat (Triticum aestivum L., Triticum durum L.) from anthesis to senescence. By using leaf discs maintained in saturating CO2 in the O-2 electrode we found that the electron transport measured by fluorescence, and that calculated from O-2 evolution rate were similar throughout the experimental period, which indicated that fluorescence might be used to measure the linear electron transport rate. In field measurements on intact attached leaves, the electron transport rate declined less than P-N during leaf senescence. Measurements taken in the afternoon indicated that the electron transport remained constant during the day while P-N slightly decreased. Thus, in field-grown wheat leaves photoinhibition was not a relevant phenomenon. When the alternative electron routes were negligible, the increasing discrepancy between P-N and the electron transport during leaf senescence could be explained by an increment in photorespiration rate (R(P)). The change of oxygenation to carboxylation ratio (v(o)/v(c)) might be caused by increasing resistances to CO2 diffusion in the leaf. CO2 lost through photorespiration was about half of that fixed with P-N in the sun-exposed leaves. Yet R(P) was lower in the basal part of leaves which mostly grew in shade. Fluorescence coupled with gas exchange proved to be a useful method for evaluation of the photorespiratory losses in field conditions.