Levels of carboxyarabinitol 1-phosphate and related photosynthetic components during leaf development in Phaseolus vulgaris
Moore, B.D. (University of Nevada, Reno (USA)) | Kiirats, O. | Charlet, T. | Seemann, J.R.
The influence of leaf development on the levels of carboxyarabinitol 1-phosphate CA1P), carboxyarabinitol (CA), CA1P phosphatase activity, ribulose bisphosphate carboxylase (rubisco) activity, photosynthesis, and rubisco catalytic sites was examined in Phaseolus vulgaris. Leaves were sampled at mid-day or after 15 h dark over a 3 week growth period. Mid-day initial rubisco activities were comparable to net photosynthetic rates, reaching 245 mu-mol at 67-100% full size. CA1P and CA were present at substantial levels in leaves of all ages, increasing by similar amounts throughout expansion. Total rubisco catalytic sites increased 2.4-fold to 58 nmol during leaf growth to 66% full size, remaining constant with further expansion. CA1P phosphatase activity was particularly low in the youngest leaves, and increased 14-fold during leaf expansion. CA1P content of dark bean leaves was always in excess of total rubisco catalytic sites throughout development, but decreased from 2.2-fold in the youngest leaves to 1.4-fold at maturation. The number of rubisco catalytic sites bound to CA1P in the dark was measured in different aged leaves after extracting leaves with 25 mM ammonium sulfate, which stabilizes the in vivo level of CA1P bound to rubisco. These measurements demonstrated that in the youngest leaves relatively much less CA1P was bound to rubisco catalytic sites in the dark. This was in contrast to what occurred in dark-treated older leaves, in which a large proportion of catalytic sites were bound with CA1P. We suggest that in very young bean leaves either rubisco has a reduced affinity for CA1P, or much of the CA1P in dark leaves may be bound to another, unidentified cellular component(s)
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