Influence of Cell Age on Chlorophyll Formation in Light-grown and Etiolated Wheat Seedlings

Abstract

A method is described for relating the age of a cereal leaf cell to its distance from the leaf base. The rates of chlorophyll synthesis per plastid in the first leaf of llght-grown and of greening etiolated seedlngs of wheat (Triicun aestivum, var. Maris Dove) increase with cel age. Normally developing plastids of light-grown wheat take over 24 hours to reach the chlorophyll a/b ratio characteristic of mature wheat chloroplasts (4.5), but mature etioplasts need only 8 hours light to achieve this a/b ratio. Plastid greening potental depends only on cell age, whereas the chlorophyl a/b ratio is influenced both by cel age and by light. This paper presents the results of a study of this kind. We have examined plastid development in wheat in order to avoid dealing with dimorphic chloroplasts, and because we already have a good background knowledge of the development of wheat seedlings (5). Methods are described for measuring the leaf tip growth rate over several days before harvest, and the growth rate at any position in the leaf relative to the growth rate at its tip, for both etiolated and light-grown wheat seedlings. Using these measurements it has been possible to convert distance from the leaf base into cell age, and so to make direct comparisons between chloroplasts and etiochloroplasts in cells of the same age in light-grown and greening etiolated seedlings.The seedlings of many cereals are particularly well suited to the study of chloroplast development, since the developing chloroplasts are in a linear series, with the youngest in cells near the base and the oldest in cells near the tip of the leaf (3, 5, 6, 8, 10-12, 14, 15). Robertson and Laetsch (14) took advantage of this arrangement to study Chl synthesis during the greening of etiolated maize seedlings, and demonstrated that the age ofdeveloping etiolated tissue had a considerable effect on the rates of Chl synthesis following illumination. They reported that the rate of greening, per g fresh weight, was greater in regions distant from the leafbase than in the younger regions near the base. Mackender (11) found a similar increase in greening potential with cell age when he measured the levels ofprotochlorophyllide/g fresh weight in sections cut at various distances from the bases of etiolated barley leaves.A greening etiolated leaf is far from being a natural phenome- MATERIALS AND METHODS Plant Material. Seeds of wheat, Triticum aestivum, var. Mans Dove (Dickson, Brown & Tait Ltd., Altrincham, U.K.) were soaked in running tap water at 20 C and surface-sterilized in sodium hypochlorite solution (13% free chlorine) after 1 h. After being soaked for an additional 16 h, the seeds were sown in Levington Universal Compost (Fisons, U.K.) at a depth of 1 cm. Light-grown seedlings were given a photoperiod of 16 h at 20 C with a 5 C night depression and 70%o RH. The light intensity at the level of the seedlings, measured with a solarimeter (Kipp & Zonen), was 4.0 mw.cm2. Dark-grown seedlings were grown in total darkness at 25 C, about 100%1o RH. After 5...

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Dynamic change in photosynthetic pigments and chlorophyll degradation elicited by cereal aphid feeding

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Ribulose Bisphosphate Carboxylase Synthesis in Barley Leaves

“…When protein synthesis inhibitors were included in the incubation solutions, the contributions of cytoplasmic and organelle protein synthesis to the total were determined. Figure 2B shows that cytoplasmic protein synthesis was maximal in region [4][5][6], while the rate of organelle protein synthesis continued to increase until region [6][7][8]. Most of the organelle protein synthesis was probably due to the activity of chloroplast ribosomes, as 25% to 40% of the ribosomes in green leaves are chloroplast ribosomes while less than 1% are mitochondrial ( 17).…”

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