Genetic variation in soybean photosynthetic electron transport capacity is related to plastocyanin concentration in the chloroplast.

Fifteen ancestral genotypes of United States soybean cultivars were screened for differences in photosynthetic electron transport capacity using isolated thylakoid membranes. Plants were grown in controlled environment chambers under high or low irradiance conditions. Thylakoid membranes were isolated from mature leaves. Photosynthetic electron transport was assayed as uncoupled Hill activity using 2,6-dichlorophenolindophenol (DCIP). Soybean electron transport activity was dependent on genotype and growth irradiance and ranged from 6 to 91 mmol DCIP reduced mol chlorophyll s(-1). Soybean plastocyanin pool size ranged from 0.1 to 1.3 mol plastocyanin mol Photosystem I. In contrast, barley and spinach electron transport activities were 140 and 170 mmol DCIP reduced mol chlorophyll s(-1), respectively, with plastocyanin pool sizes of 3 to 4 mol plastocyanin mol Photosystem I. No significant differences in the concentrations of Photosystem II, plastoquinone, cytochrome b6f complexes, or Photosystem I were observed. Thus, genetic differences in electron transport activity were correlated with plastocyanin pool size. The results suggested that plastocyanin pool size can vary significantly and may limit photosynthetic electron transport capacity in certain species such as soybean. Soybean plastocyanin consisted of two isoforms with apparent molecular masses of 14 and 11 kDa, whereas barley and spinach plastocyanins each consisted of single polypeptides of 8 and 12 kDa, respectively.