Relationship between thylakoid electron transport and photosynthetic CO2 uptake in leaves of three maize (Zea mays L.) hybrids

The introduction of a more efficient means of measuring leaf photosynthetic rates under field conditions may help to clarify the relationship between single leaf photosynthesis and crop growth rates of commercial maize hybrid s. A large body of evidence suggests that gross photosynthesis (AG) of maiz e leaves can be accurately estimated from measurements of thylakoid electro n transport rates (ETR) using chlorophyll fluorescence techniques. However, before this technique can be adopted, it will first be necessary to determ ine how the relationship between chlorophyll fluorescence and CO2 assimilat ion is affected by the non-steady state PPFD conditions which predominate i n the field. Also, it must be determined if the relationship is stable acro ss different maize genotypes, and across phenological stages. In the presen t work, the relationship between ETR and AG was examined in leaves of three maize hybrids by making simultaneous measurements of leaf gas exchange and chlorophyll fluorescence, both under controlled environment conditions and in the field. Under steady-state conditions, a linear relationship between ETR and AG was observed, although a slight deviation from linearity was ap parent at low AG. This deviation may arise from an error in the assumption that respiration in illuminated leaves is equivalent to respiration in dark ened leaves. The relationship between chlorophyll fluorescence and photosyn thetic CO2 assimilation was not stable during fluctuations in incident PPFD . Since even minor (e.g. 20%) fluctuations in incident PPFD can produce sus tained( > 20 s) departures from the mean relationship between ETR and AG, c hlorophyll fluorometry can only provide an accurate estimate of actual CO2 assimilation rates under relatively stable PPFD conditions. In the field, t he mean value of ETR / AG during the early part of the season (4.70 +/- 0.0 7) was very similar to that observed in indoor-grown plants in the vegetati ve stage (4.60 +/- 0.09); however, ETR / AG increased significantly over th e growing season, reaching 5.00 +/- 0.09 by the late grain-filling stage. D ifferences in ETR / AG among the three genotypes examined were small (less than 1% of the mean) and not statistically significant, suggesting that chl orophyll fluorometry can be used as the basis of a fair comparison of leaf photosynthetic rates among different maize cultivars.