Photosynthesis by developing embryos of oilseed rape (Brassica napusL.)

Abstract

The aim of this study was to assess the photosynthetic potential of developing seeds of oilseed rape (Brassica napus L.) and to compare photosynthetic properties of embryo plastids with those of leaf chloroplasts from the same species. Measurements of CO 2 -dependent O 2 evolution show that developing seeds of B. napus are photosynthetically active in vitro. Essentially, all of the photosynthetic activity of the developing seed is accounted for by the embryo. The rate of photosynthesis by developing embryos increased until the onset of desiccation, after which it declined, so that by maturity embryos were no longer photosynthetically active. Photosynthetic activity was positively correlated with chlorophyll content throughout development. Comparison of the photosynthetic characteristics of leaf and embryo chloroplasts revealed that rates of uncoupled electron transport were 2.5-fold greater in those from the embryo. Light-saturated rates of CO 2 -dependent O 2 evolution, per unit chlorophyll, and CO 2 saturation points were similar for chloroplasts from both tissues. However, light-saturation points and chlorophyll a/b ratios were lower for embryo than for leaf choroplasts. Embryos and embryo chloroplasts also contained considerably less ribulose 1,5-bis-phosphate carboxylase/oxygenase protein per unit total protein, than leaves. Although excised embryos were capable of high rates of CO 2 -dependent O 2 evolution (90-100 μmol mg -1 chlorophyll h -1 ) under a saturating photosynthetic photon flux density (PPFD), low transmittance of light through the silique wall (30%), together with the high PPFD required to achieve light compensation points in developing seeds (500 μmol m -2 s -1 ), suggests that photosynthesis in vivo is unlikely to make a net contribution to carbon economy under normal environmental conditions.