Heather Adamson

Chloroplast development and the synthesis of chlorophyll a and b and chlorophyll protein complexes I and II in the dark in Tradescantia albiflora (Kunth).

Continued synthesis of chlorophyll a and chlorophyll b occurs in Tradescantia albiflora Kunth on transfer to darkness. This synthesis continues for several days and may result in a doubling of chlorophyll content per leaf. It is accompanied by continued cell division and development of normal chloroplast ultrastructure, including stacked thylakoids.

Chloroplast development and the synthesis of chlorophyll and protochlorophyllide in Zostera transferred to darkness

Intact plants and isolated leaves of Zostera capricornii Martens ex Aschers were transferred from daylight to darkness. Substantial amounts of chloropyll a and b continued to accumulate in immature and mature tissue in the same ratio as in the light and were incorporated into chlorophyll-protein complexes in the thylakoids. A small amount of protochlorophyllide also accumulated in immature tissue in the dark. Proplastids and immature chloroplasts continued to develop into mature chloroplasts in the dark in the normal manner but prolamellar bodies, which are a conspicuous feature of immature chloroplasts, took longer to disperse than in the light. Protochlorophyllide accumulation and prolamellar-body formation were not correlated. The results indicate that Zostera has a genetic capacity for dark chlorophyll synthesis which is expressed in immature and mature leaf tissue and enables this plant to continue synthesising chlorophyll and assembling chloroplasts at night.

Photoinhibition and Recovery of Photosynthesis in Antarctic Bryophytes Under Field Conditions

Antarctic bryophytes survive frozen over winter and grow fully exposed to sunlight in summer. Preliminary observations of quantum yields, light saturated photosynthetic rates and variable fluorescence of bryophytes suggested that photoinhibition was a major factor limiting productivity of Antarctic ecosystems (1). Although the combination of low temperature and bright light are potentially damaging to the photosynthetic apparatus of plants and detrimental effects have been observed for bryophytes (2,3,4) the phenomenon of photoinhibition has not been systematically investigated in polar environments (5). This paper documents daily changes in photosynthetic capacity of an endemic Antarctic bryophyte, Grimmia antarctici Card., growing in the field over late summer. Environmental variables, light and temperature, were continuously monitored, while chlorophyll fluorescence and oxygen evolution rates were determined on samples collected every few hours from the field.

Immunodetection and photostability of NADPH-protochlorophyllide oxidoreductase in Pinus pinea L.

Antibody against the light-dependent NADPH-protochlorophyllide oxidoreductase of oat was used to detect a protein of the same molecular weight in cotyledons of 40-day-old dark-grown seedlings of Pinus pinea L. Exposure of the seedlings to light resulted in a rapid decrease in protochlorophyllide content without the concomitant decrease in 38 kDa protein which is observed on transfer of dark-grown angiosperm seedlings to light. The stability of the light-dependent NADPH-protochlorophyllide oxidoreductase in pine in the absence of accumulated substrate is consistent with either (1) a different mechanism of regulation of chlorophyll synthesis in gymnosperms or (2) a higher proportion of stable extra-plastidic protein reacting with the antibody to the light-dependent NADPH-protochlorophyllide oxidoreductase than is the case in angiosperms.

Gabaculine inhibition of chlorophyll synthesis in light and darkness in intact barley (Hordeum vulgare) seedlings

Abstract Dark chlorophyll accumulation in the first leaf of intact 5–6-day-old glasshouse-grown barley seedlings ( Hordeum vulgare ) was inhibited by gabaculine (3-amino 2,3-dihydrobenzoic acid) (GAB) supplied via the roots. The inhibitory effect of gabaculine was eliminated by the addition of 5-aminolevulinic acid. Gabaculine also inhibited chlorophyll accumulation in intact dark-grown barley seedlings transferred to light. Inhibition of dark chlorophyll accumulation by gabaculine is evidence that the aminolevulinic acid precursor for light-independent chlorophyll synthesis is formed via the C-5 pathway.

Comparison of Chlorophyll Accumulation and 14C-ala Incorporation into Chlorophyll in Dark and Light in Green Barley

Light-independent chlorophyll (chl) synthesis has been observed in angiosperm seedlings (1–8), developing and mature leaves of established plants (9, 10) and tissue culture (11). In each case, chl continued to accumulate when plants or tissues which had been exposed to light during their development were returned to darkness. The incorporation of radioactive precursors into chl in darkness has also been observed (12), however in the absence of evidence that label is in the tetrapyrrole moiety this does not necessarily indicate synthesis via a light-independent route. Recently we have demonstrated the incorporation of C-amino levulinic acid (ALA) into the tetrapyrrole nucleus as well as the ester groups of chl a in young barley seedlings returned to darkness (13). The chl accumulation and precursor incorporation experiments reported here were carried out to investigate the extent to which dark chl synthesis contributes to the total chl pool in young barley seedlings growing in a diurnal environment.

Dynamic Light Acclimation of the Photosynthetic Apparatus of Higher Plants

Higher plants have sophisticated, multiple molecular mechanisms that enable them to sense information about their light environment, and subsequently to evoke not only the appropriate developmental and physiological responses to that environment, but also to cope with momentary, daily and seasonal fluctuations in light quantity and light quality1–4.These changes in the light environment have a profound effect on the composition, structure and function of the photo synthetic apparatus. Indeed, responses of the photosynthetic apparatus to changing light quality and light quantity are amongst the most dynamic of all plant functions.

Protochlorophyllide Reduction in Anabaena

Protochlorophyllide (pchlide) reduction is an essential step in the formation of chlorophyll (chl). There are at least two enzymes which catalyse the conversion of pchlide to chlorophyllide (chlide): the light-dependent enzyme, NADPH-pchlide oxidoreductase (EC.1.6.99.1) (1) and an uncharacterised light-independent enzyme. The light-independent enzyme is apparently widely distributed since light-independent pathways are present in procaryotes (cyanobacteria) and eucaryotes, from algae to angiosperms (2).

Evidence for a Light-Independent Chlorophyll Biosynthetic Pathway in Angiosperm Seeds Germinated in Darkness

It is not generally recognised that during the course of evolution of seed plants, many angiosperms and gymnosperms have retained the capacity for light-independent (dark) chlorophyll (Chl)* synthesis exhibited by cyanobacteria, algae, bryophytes and pteridophytes (1,2,3). As a consequence the view that when greening or mature angiosperm leaves are transferred from light to darkness, Chl(ide) formation ceases immediately because of the cessation of Pch(lide) photoreduction, is still widely held (4). It is reinforced by the visual contrast between etiolated dark grown angiosperm seedlings and the bright green cotyledons of dark grown gymnosperms. This important phenotypic difference in the two groups of seed bearing plants is usually interpreted in terms of an absolute reqirement of angiosperms for light for Pch(lide) reduction, whereas in gymnosperms Chi is formed in light as well as darkness (5). In this context it is not surprising that early reports of trace amounts of Chi in dark grown angiosperm seedlings (6,7) and the accumulation of Chl by wheat embryos grafted onto pine megagametophytes (8) attracted little attention.

Chlorophyll Synthesis and Degradation in Germinating Barley Seedlings

The effect of gabaculine (GAB) on dark chlorophyll (Chl) accumulation in diurnally-grown 3.5-day-old barley seedlings was investigated under conditions in which Chl b was rapidly degrading in darkness. GAB inhibited accumulation of Chl a and promoted loss of Chl b with the net result that Chl levels declined in darkness in the presence of GAB. The inhibitory effect of GAB in darkness is evidence for the existence of light-independent Chl biosynthesis during very early leaf development.