| Abstract | Photosynthetic CO₂ exchange rate (CER) was determined in attached leaflets of field-grown peas (Pisum sativum L.) Three populations of genotypes were studied in different field locations and years. In 1 year, CER, leaf characteristics, and meteorological factors were measured in parallel at different times of the day and season. Differences in CER across environments or time of day were related to differences in stomatal resistance; however, changes during the season, which might be of environmental or developmental origin, were not. Genotype ranking for CER was largely independent of measuring time, field location, and year. None of the leaf characters which have been suggested as simple assays for photosynthetic ability (specific leaf weight, chlorophyll content, stomatal resistance) was consistently related to CER over several tests. In the single experiment in which they were determined, ribulose bisphosphate carboxylase activity, leaf soluble protein content, and the difference in CER at 2 and 20 kPa O₂ were also not correlated with CER. The O₂ effect expressed as a percentage of CER at 2 kPa O₂ and the slope of the light response above 50 nE cm⁻² s⁻¹ were significantly correlated to CER, but neither is suitable for predicting CER in different genotypes. A significant multiple regression of CER on stomatal resistance, total chlorophyll, and ribulose bisphosphate carboxylase activity suggests that the genetic control of CER may involve several characters. In this case, direct determination of CO₂ exchange may be the easiest and most reliable way of assessing genetic differences in photosynthetic performance. |
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