Carotenoids are chloroplast constituents. In vivo different carotenoids may be present at different sites within the chloroplast, and may have different functions. Especially important is a role in protecting the chloroplast and hence the plant from chlorophyll-sensitized photooxidation. If carotenoid biosynthesis can be blocked by herbicides, such protection will no longer be afforded, and rapid death of the plant is likely. In this article current knowledge of carotenoid biosynthesis is reviewed. The main characteristics of each stage in the biosynthesis are outlined and the suitability of these reactions as possible herbicide targets is assessed. Two experimental approaches to the study of the effects of herbicides on carotenoid biosynthesis are suggested.
The pigment composition of needles of Picea abies L. taken from sites in the Bavarian forest has been determined. Needles were removed from (a) healthy, control trees, (b) trees which showed visible signs of damage for either 6 months or 3.5 years, and (c) trees which had received supplemental fertilizer application. In general, needles taken from these trees could be placed into three categories; (i) green, (ii) partially chlorotic, and (iii) partially necrotic (brown). Whilst the pigment composition of needles taken from healthy trees was typical of any higher plant, substantial changes in the chlorophyll and carotenoid content of the damaged needles were recorded. Particularly noticeable was an increase in the ratio of total carotenoid/chlorophyll and losses of neoxanthin and violaxanthin. Levels of total carotene (α- and β-carotene) and, especially, of lutein remained high in damaged needles. Levels of zeaxanthin were highest in the brown, partially necrotic, needles.
The occurrence of β-carotene-5,6-epoxide in higher plant photosynthetic tissue is described. The compound is found in isolated chloroplasts, thylakoids and other subchloroplast particles but can only be detected in intact leaves or cotyledons of higher plants when these are exposed to very high light intensities or to inhibitors such as monuron or paraquat. The distribution of the epoxide within the individual pigment-protein complexes is given. It is particularly associated with the PS I reaction centres (C P I and CP la) and less so with the PS II reaction centre (CPa).
Circular dichroism shows that the β-carotene-5,6-epoxide isolated from photosynthetic tissue is optically inactive. It is therefore not produced enzymically but is a product of photooxidative events in the photosynthetic apparatus. Its presence in photosynthetic tissue is a reliable indicator of photooxidative damage to the thylakoid membrane involving oxidation of β-carotene.