REGULATION OF PHOTOSYNTHESIS IN DEVELOPING LEAVES OF SOYBEAN CHLOROPHYLL-DEFICIENT MUTANTS

In this report we examine the factors that regulate photosynthesis dur ing leaf ontogeny in y3y3 and Yllyll, two chlorophyll-deficient mutant s of soybean. Photosynthetic rates were similar during wild type and Y llyll leaf development, but the senescence decline in photosynthesis w as accelerated in y3y3. Photosynthetic rates fell more rapidly than ch lorophyll concentrations during senescence in wild type leaves, indica ting that light harvesting is not strongly limiting for photosynthesis during this phase of leaf development. Chlorophyll concentrations in Yllyll, though significantly lower than normal, were able to support n ormal photosynthetic rates throughout leaf ontogeny. Chlorophyll a/b r atios were constant during leaf development in the wild type, but in t he mutants they progressively increased (y3y3) or decreased (Yllyll). In all three sets of plants, photosynthetic rates were directly propor tional to Rubisco contents and activities, suggesting that Rubisco pla ys a dominant role in regulating photosynthesis throughout leaf ontoge ny in these plants. The expression of some photosynthetic proteins, su ch as Rubisco activase, was coordinately regulated with that of Rubisc o in all three genotypes, i.e. an early increase, coincident with leaf expansion, followed by a senescence decline in the fully-expanded lea f. On the other hand, the light harvesting chlorophyll a/b-binding pro teins of PS II (the CAB proteins), while they showed a profile similar to that of Rubisco in the wild type and y3y3, progressively increased in amount during Yllyll leaf development. We conclude that Yllyll may be defective in the accumulation of a component required for LHC II a ssembly or function, while y3y3 has more global effects and may be a r egulatory factor that controls the duration of senescence.