Spectral, Physical, and Electron Transport Activities in the Photosynthetic Apparatus of Mesophyll Cells and Bundle Sheath Cells of Digitaria sanguinalis (L.) Scop

Abstract

Isolated mesophyll cells and bundle sheath cells of Digitaria sanguinalis were used to study the light-absorbing pigments and electron transport reactions of a plant which possesses the C(4)-dicarboxylic acid cycle of photosynthesis. Absorption spectra and chlorophyll determinations are presented showing that mesophyll cells have a chlorophyll a-b ratio of about 3.0 and bundle sheath cells have a chlorophyll a-b ratio of about 4.5. The absorption spectrum of bundle sheath cells has a greater absorption in the 700 nm region at liquid nitrogen temperature, and there is a relatively greater amount of a pigment absorbing at 670 nm in the bundle sheath cells compared to the mesophyll cells. Fluorescence emission spectra, at liquid nitrogen temperature, of mesophyll cells have a fluorescence 730 nm-685 nm ratio of about 0.82 and bundle sheath cells have a ratio of about 2.84. The reversible light-induced absorption change in the region of P(700) absorption is similar in both cell types but bundle sheath cells exhibit about twice as much total P(700) change as mesophyll cells on a total chlorophyll basis. The delayed light emission of bundle sheath cells is about one-half that of mesophyll cells. Both mesophyll cells and bundle sheath cells evolve oxygen in the presence of Hill oxidants with the mesophyll cells exhibiting about twice the activity of bundle sheath cells, and both activities are inhibited by 1 muM 3-(3,4-dichlorophenyl)-1, 1-dimethylurea. Ferredoxin nicotinamide adenine dinucleotide phosphate reductase is present in both cells although it is about 3- or 4-fold higher in mesophyll cells than in bundle sheath cells. Glyceraldehyde 3-P dehydrogenases, both nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate, are equally distributed in the two cell types on a chlorophyll basis. Malic enzyme is localized in the bundle sheath cells.We interpret the data as evidence for the presence of a complete chloroplast electron transport system from oxygen evolution to pyridine nucleotide reduction in both mesophyll and bundle sheath cells. However, there is a quantitative difference in the distribution of photosystem I and photosystem II components in the two photosynthetic cells with about a 3-fold higher photosystem I-II ratio in the bundle sheath cells than in the mesophyll cells. A scheme is proposed to accommodate photosynthetic CO(2) fixation and electron transport activities in the mesophyll cells via a beta-carboxylation and in the bundle sheath cells via carboxylation of ribulose-1, 5-diphosphate.