A. H. Cobb

Ultrastructural effects of the diphenyl ether herbicide acifluorfen and the experimental herbicide M&B 39279

Abstract The sequence of the herbicidal effects of acifluorfen and the novel herbicide M&B 39279 (5-amino-4-cyano-1-(2,6-dichloro-4-trifluoromethylphenyl)pyrazole) on ultrastructure in excised Galium aparine L. leaves has been examined in detail and supported by physiological measurements. Plant responses to acifluorfen and M&B 39279 were indistinguishable, the latter compound therefore being deemed to share a common mode of action with nitrodiphenyl ethers. After 3 hr in 50 μmol m −2 sec −1 photosynthetic photon flux density light, chloroplast swelling and invagination/evagination of chloroplast envelopes was observed in the presence of 100 μ M acifluorfen and M&B 39279. Tonoplast lysis and/or development of cytoplasmic vesicles occurred by 15 hr whilst plasmalemma disruption, membraneous vesicle formation, and abumdant electron opaque material were apparent between 20 and 30 hr. Chloroplasts, often fused together with highly modified envelope membranes, were the last identifiable cell structures remaining. Thylakoids were notably persistant, retaining integrity for about 20 hr. Under identical conditions, leaf photosynthetic capability diminished immediately following herbicide treatment. Leaf chlorophyll content remained constant for over 15 hr, after which a decline was accompanied by leakage of electrolytes and malondialdehyde from treated leaves. These results indicate that the diphenyl ether-type mode of action is initiated at the chloroplast envelope, but is independent of photosynthesis and involves lipid peroxidation.

A study of bentazon uptake and metabolism in the presence and the absence of cytochrome P-450 and acetyl-coenzyme A carboxylase inhibitors

Abstract The metabolism of bentazon in a resistant variety of soybean [Glycine max (L.) Harcor] was investigated. Soybean leaves produced two major metabolites under both light and dark conditions, metabolism being enhanced in the dark. No metabolism of bentazon could be demonstrated in the identically treated susceptible species, shepherds pure [Capsella bursa-pastoris (L.) Medicus], and leaf necrosis was evident after 2 days. Bentazon metabolites were separated by TLC and identified as 6-hydroxybentazon and a glycosyl conjugate, determined by comigration and β-glucosidase treatment, respectively. Pretreatment of leaves with the norbornanodiazetine tetcyclacis, the triazoles BAS 110 and BAS 111, and the pyridine metyrapone had no significant effect upon bentazon uptake but inhibited its metabolism in soybean, causing leaf necrosis, in each case, 2 days after bentazon treatment. Pretreatment of leaves with the acetyl-coenzyme A carboxylase inhibitors sethoxydim and diclofop-methyl had no significant effect upon bentazon uptake or metabolism at the concentrations used. These findings are discussed in relation to bentazon selectivity in soybean.

Photosynthesis by Codium Fragile in an Intertidal Zone Environment: A Growth Strategy

The intertidal zone environment supports a wide variety of algae which must be able to withstand considerable daily and seasonal changes in ecological and physical growth parameters. Concentrations of major nutrients show seasonal variability and become depleted when maximum algal colonization occurs and replenished by their subsequent decline and decomposition at the end of the growth phase. Water currents, wave action, tidal amplitude and light intensity also vary with time of year. Mann (1977) has reviewed the importance of seasonal growth strategies in marine algae and concluded that seasonal fluctuations in photosynthetic activity may afford an environmental advantage in the growth of Laminaria spp. Similarly Hanisak (1979) has suggested that seasonal variations in nitrogen accumulation in C. fragile may be advantageous to the growth of this alga. The aim of this study is to monitor seasonal variations in the growth of C. fragile and determine if this alga employs an ecologically advantageous photosynthetic growth strategy in its intertidal zone niche.

Pigment Composition and Stability in the Intertidal Alga Codium Fragile

Codium fragile exhibits several photosynthetic adaptations to the low intensity light fields encountered at high tide. Chloroplast photosynthesis in the alga saturates at 50–100 µE.M-2.s-1 P.A.R. (Benson, 1983) and chlorophyll b enrichment is reflected in the low chlorophyll a:b ratios of 1.5–1.8:1 of the fronds (Benson, Cobb, 1981). However when sampled during the summer C.fragile fronds are often bleached and this may suggest that this alga is susceptible to photooxidative damage when the tide recedes. Thus, despite the low light intensity adaptations demonstrated in C.fragile, photoprotective mechanisms must also be operational in this alga. The aim of this study is to investigate light adaptations in C.fragile with respect to the intertidal zone environment. This is to be achieved by characterising the pigment content of the pigment/protein complexes isolated from the alga and investigating the effects of light regimes simulating tidal exposure on frond pigment stability.

Photoinhibition in Thylakoids and Intact Chloroplasts of Codium fragile (Suringar) Hariot

There is a general acceptance in the literature that photosystem II (PSII) is the primary site of photoinhibition and that prolonged exposure to excessive photosynthetic photon flux density (PPFD) results in pigment loss and lipid breakdown due to photooxidation (as reviewed in 1). However, growth of the marine alga C. fragile is adapted to low PPFD (2,3,4) even though its intertidal habitat is regularly exposed to wide extremes of PPFD on both a seasonal and daily basis. Therefore, this alga is especially at risk of photoinhibition and photooxidation at the higher PPFD’s occurring at low tide. Experiments described in this paper illustrate that chloroplasts and thylakoids isolated from this alga photoinhibit at irradiances supra-optimal for photosynthesis (PS). As with other photosynthetic organisms, the primary lesion appears to be associated with PSII and photooxidative damage occurs with prolonged exposure to light.

Adaptations of Codium fragile (Suringar) Hariot Fronds to Photosynthesis at Varying Flux Density

The intertidal alga Codium fragile exhibits shade adaptations enabling efficient photosynthesis (PS) at the low photosynthetic photon flux densities (PPFD) experienced at high tide. Thus, chloroplasts contain high levels of siphonoxanthin, siphonein and chlorophyll b (1,2,3), reveal both low chlorophyll a:b and high photosystem II: photosystem I ratios (1,4,5) and possess 75% of the total pigment content within the light harvesting complexes (1). However, the intertidal habitat is subjected to regular extremes of PPFD on a daily and a seasonal basis so that this alga is at great risk of photoinhibition and subsequent photooxidation at the higher PPFD’s encountered at low tide. Indeed, isolated chloroplasts and thylakoids are particularly vulnerable to these photodestructive processes (6). In contrast, this paper provides evidence that intact fronds of C. fragile are able to avoid or tolerate photoinhibition and hence photooxidative damage and possess the ability to re-distribute excitation energy between the photosystems (4,5).

Relationship between Chloroplast Starch, Phosphate Content and Season in the Intertidal Algae Codium Fragile (Suringar) Hariot

C.fragile chloroplasts possess reserves of inorganic phosphate (Pi) in the form of long chain polyphosphate (poly Pi) which vary in both quantity and type throughout the life cycle of this alga (1). Acid-soluble poly Pi is considered a more readily mobilised form of storage poly Pi which may be converted in some algal species to a less readily mobilised alkali-soluble form (2). Seasonal fluctuations in the C.fragile chloroplast alkali : acid-soluble poly Pi ratio have previously been proposed as an adaptation to a fluctuating Pi environment, where a high ratio is considered to represent a mobilisation of alkali-soluble poly Pi (1). This paper provides additional evidence for seasonal variations in C.fragile chloroplast Pi content and underlines their influence on starch metabolism by this alga.

Starch Synthesis in Isolated Codium Fragile Chloroplasts in the Presence of 10mMPi, PGA and G6P

The chloroplast starch content of the siphonaceous marine alga Codium fragile (Suringar) Hariot has previously been reported to be inversely proportional to the concentration of stromal inorganic phosphate (Pi), and has been considered to reflect the gross regulation of ADP glucose pyrophosphorylase activity within the chloroplast (1). This enzyme is inhibited by high stromal Pi/PGA ratios (2,3,4), and although its presence in C.fragile chloroplasts has yet to be verified, it is conserved over a wide range of photosynthetic organisms (5). Experiments described in this paper attempt to provide supporting evidence for the regulation of ADP glucose pyrophosphorylase within C.fragile chloroplasts by investigating the effect of exogenous Pi and PGA on starch synthesis by intact isolated chloroplasts. Furthermore, as C.fragile chloroplast envelope membranes possess an additional Pi translocator facilitating the export of G6P in exchange with Pi (6), the effect of exogenous G6P on chloroplast starch synthesis is also described.