Effect of phytoplasmal infection on photosystem II efficiency and thylakoid membrane protein changes in field grown apple (Malus pumila) leaves

We have studied the effect of the apple proliferation phytoplasmal infection on some features of the thylakoids from field grown apple (Malus pumila) leaves. Changes in photosynthetic pigments, soluble proteins, ribulose-1,5-bisphosphate carboxylase, nitrate reductase, photosynthetic activities and thylakoid membrane proteins were investigated. The level of total chlorophyll and carotenoids were reduced in phytoplasma-infected leaves. Similar results were also observed for soluble proteins and ribulose- 1,5-bisphosphate carboxylase activity. The in vivo nitrate reductase activity was significantly reduced in infected leaves. When various photosynthetic activities were followed in isolated thylakoids, phytoplasmal infection caused marked inhibition of whole chain and photosystem II activity while the inhibition of photosystem I activity was only marginal. The artificial exogenous electron donors, diphenyl carbazide and hydroxylamine significantly restored the loss of photosystem II activity in infected leaves. The same results were obtained when F_v/F_m was evaluated by chlorophyll fluorescence measurements. The marked loss of photosystem II activity in infected leaves could be due to the loss of 47, 33, 28–25, 23 and 17 kDa polypeptides. It is concluded that phytoplasmal infection inactivates the donor side of photosystem II. This conclusion was confirmed by immunological studies showing that the content of the 33 kDa protein of the water-splitting complex was diminished significantly in infected leaves.     

Phytoplasma [Stolbur-subgroup (Bois Noir-BN)] infection inhibits photosynthetic pigments, ribulose-1,5-bisphosphate carboxylase and photosynthetic activities in field grown grapevine (Vitis vinifera L. cv. Chardonnay) leaves

In this work we have studied the influence of phytoplasma-induced grapevine yellows (yellowing) on some features of the thylakoids from field grown grapevine (Vitis vinifera L. cv. Chardonnay) leaves. Changes in photosynthetic pigments, soluble proteins, ribulose-1,5-bisphosphate carboxylase, photosynthetic activities and thylakoid membrane proteins were investigated. The level of total chlorophyll and carotenoids, on a unit fresh weight basis, showed a progressive decrease in phytoplasma infected leaves. Similar results were also observed for total soluble proteins and ribulose-1,5-bisphosphate carboxylase activity. When various photosynthetic activities were followed in isolated thylakoids, phytoplasma infection caused marked inhibition of whole chain and photosystem (PS) II activity. Smaller inhibition of PSI activity was observed even in severely infected leaves. The artificial exogenous electron donors, DPC and NH₂OH significantly restored the PSII activity in both mild and severely infected leaves. The same results were obtained when F_v/ F_m was evaluated by Chl fluorescence measurements. The marked loss of PS II activity in infected leaves was evidently due to the loss of 33, 28–25, 23, 17 and 10 kDa polypeptides. This conclusion was confirmed by immunological studies showing that the content of the 33 kDa protein of the water-splitting complex was diminished significantly in infected leaves. Phytoplasma infection induced a fast degradation of LHCP II which became visible as yellowish colour in leaves.     

Genetic analysis of lethal tip necrosis induced by <Emphasis Type="Italic">Clover yellow vein virus</Emphasis> infection in pea

Clover yellow vein virus (ClYVV) elicits lethal tip necrosis in the pea line PI 118501. Pea line PI 118501 develops necrotic lesions and veinal necrosis on inoculated leaves, followed by systemic necrosis, leading to plant death. To understand the genetic basis of this lethal tip necrosis, we crossed lines PI 226564 and PI 250438, which develop mosaic symptoms in response to ClYVV inoculation. In reciprocal crosses of PI 118501 with PI 226564, all F₁ plants had mosaic symptoms with slight stem necrosis and early yellowing of upper leaves. Essentially the same symptom was manifested in PI 118501 × PI 250438 F₁ plants. In F₂ populations from the cross between PI 118501 and PI 226564, the observed ratios of necrosis, mosaic with slight stem necrosis, and mosaic fit the expected 1 : 2 : 1 ratio. These results indicate that a single incompletely dominant gene confers the induction of necrosis in PI 118501. This locus in pea, conferring necrosis induction to ClYVV infection, was designated Cyn1 (Clover yellow vein virus-induced necrosis). A linkage analysis using 100 recombinant inbred lines derived from a cross of PI 118501 and PI 226564 demonstrated that Cyn1 was located 7.5 cM from the SSR marker AD174 on linkage group III.     

Salicylic acid alleviates growth inhibition and oxidative stress caused by zucchini yellow mosaic virus infection in Cucurbita pepo leaves

The effects of zucchini yellow mosaic virus (ZYMV) infection and pretreatments with salicylic acid (SA) on biomass accumulation of pumpkin (Cucurbita pepo cv. Eskandarani) were investigated. The response of photosynthesis, transpiration and the activities of antioxidant enzymes in leaves was also considered. Significant reductions in growth parameters (i.e. leaf area, biomass and shoot height), photosynthesis and chlorophyll a and b content were detected in ZYMV-infected leaves in comparison to healthy controls. Antioxidant enzyme activities were increased up to 3-fold for peroxidase (POD), 2-fold for ascorbate peroxidase (APX) and catalase (CAT) activities and 1.3-fold for SOD activity by virus infection. ZYMV infection also caused increases in H₂O₂ and malondialdehyde (MDA) contents. These results suggest that ZYMV infection causes oxidative stress in pumpkin leaves leading to the development of epidemiological symptoms. Interestingly, spraying pumpkin leaves with SA led to recovery from the undesirable effects of ZYMV infection. Leaves treated with 100 μM SA three days before inoculation had the appearance of healthy leaves. No distinct disease symptoms were observed on the leaves treated with 100 μM SA followed by inoculation with ZYMV. In non-infected plants, SA application increased activities of POD and superoxide dismutase (SOD) and inhibited APX and CAT activities.In contrast, SA treatment followed by ZYMV inoculation stimulated SOD activity and inhibited activities of POD, APX and CAT. In addition, MDA displayed an inverse relation, indicating inhibition of lipid peroxidation in cells under SA treatment. It is suggested that the role of SA in inducing plant defense mechanisms against ZYMV infection might have occurred through the SA-antioxidant system. Such interference might occur through inhibition or activation of some antioxidant enzymes, reduction of lipid peroxidation and induction of H₂O₂ accumulation following SA application.     

Protective role of <Emphasis Type="Italic">S</Emphasis>-methylmethionine-salicylate in maize plants infected with <Emphasis Type="Italic">Maize dwarf mosaic virus</Emphasis>

This study aimed to detect the harmful effects of Maize dwarf mosaic virus (MDMV) infection, and to demonstrate the potential benefits of S-methylmethionine-salicylate (MMS) pretreatment in infected maize (Zea mays L.) plants. The results of chlorophyll a fluorescence measurements showed that in MDMV-infected plants additional quenchers of fluorescence appear, probably as the result of associations between the virus coat protein and thylakoid membranes. It is important to note that when infected plants were pretreated with MMS, such associations were not formed. MDMV infection and MMS pretreatment resulted in a decrease in ascorbate peroxidase (APX) activity in maize leaves, while infection contributed to an increase in activity in the roots. Infection raised the guaiacol peroxidase (GPX) enzyme activity level, which was reduced by MMS pretreatment. MMS contributed to a decrease in both the RNA and coat protein content of MDMV, to an equal extent in maize leaves and roots. The results showed that MMS pretreatment enhanced the stress response reactions against MDMV infection in maize plants and retarded the spreading of infection.     

The occurrence of a soluble protein (E1) in cucumber cotyledons infected with plant viruses

Analysis on polyacrylamide gels of the soluble protein fraction of cucumber cotyledons infected with tomato spotted wilt virus (TSWV) revealed a stimulated protein component, designated as protein E₁. The amount of protein E₁ was related to the size and number of local lesions produced on the inoculated cotyledons. Protein E₁ stained positive for carbohydrate and could be partly purified by gel filtration on Sephadex G 100. The molecular weight was estimated to be approximately 22000 d. Protein E₁ seems to be serologically unrelated to any of the structural proteins of TSWV. Following inoculation of cucumber cotyledons with the yellow strain of cucumber mosaic virus (CMV), a protein with identical mobility on 7.5 and 10% polycrylamide gels to protein E₁ was detected. These proteins are probably identical and their accumulation is a result of the infection process. There was a close correspondence between the amount of protein E₁ and the severity of symptom expression. A protein with a mobility differing slightly from protein E₁ occurred in the soluble protein fraction ofNicotiana rustica after infection with either TSWV or tobacco mosaic virus.     

Physiological and Biochemical Responses of Vicia Faba Plants to Foliar Application of Zinc and Iron

Iron (Fe) and zinc (Zn) are essential nutrients for plant growth and development. Therefore, a pot experiment was conducted to examine the role of Zn and Fe on vegetative growth, photosynthetic pigments, soluble sugars, soluble protein, nitrate reductase (NR), ascorbate peroxidase (APX), catalase (CAT), minerals content, amino acid composition fraction, isozymes, and protein electrophoresis in faba bean plants foliar sprayed with FeSO₄ and ZnSO₄. The results revealed that foliar spray with 1 or 3 g/L of FeSO₄ and ZnSO₄ significantly increased all the abovementioned parameters compared to control plants. The activity of esterase increased with increasing concentration of Fe and Zn as compared to control plants. The most pronounced increase was found in plants treated with 3 g/L ZnSO₄. Three bands of peroxidase isozyme were exhibited with different densities and intensities in all treatments. Three types of modification were observed in the protein patterns of faba bean leaves. Some protein bands disappeared, while other increased and synthesis of a new set of proteins was induced. It could be concluded that foliar spray with the two concentrations (1 or 3 g/L) of ZnSO₄ and FeSO₄ helped faba bean plants to overcome the deficiency in these minerals by producing antioxidant enzymes. Improved faba bean growth through adequate Fe and Zn foliar spray is likely a promising strategy to improve Vicia faba plants.     

Effects of ametryn [2-(ethylamino)-4-(isopropylamino)-6-(methylthio)-s-triazine] on nitrate reductase activity and nitrite content of wheat (Triticum aestivum L.)

The objectives of this study were to show that: (a) a herbicide, such as ametryn, which interferes with the photosynthetic electron transport system, causes nitrite to accumulate in illuminated leaves and (b) that nitrite is toxic and contributes to the herbicidal damage and death of the plant. Tests were conducted on wheat seedlings grown on 5 mM nitrate, 5 mM ammonia, and zero nitrogen. Ametryn treatment decreased in vivo and in vitro nitrate reductase activity (NRA) within a 26-hr period. In vivo NRA decreased more rapidly than in vitro NRA. Compared with control tissue, only 3% in vivo NRA remained at the end of 26 hr. The in vivo assay conducted in light confirmed the inhibition of photosynthetic electron flow by ametryn within the leaf tissue. Nitrate-grown, ametryn-treated plants accumulated nitrite and, after 10 days were the only plants that were completely desiccated and dead. Ammonia- and zero-nitrogen, ametryn-treated plants did not accumulate nitrite, were only partially chlorotic after the 10-day period, and were still living. Low levels of NO_(X) (NO₂ and/or NO) emissions were demonstrated by nitrate-grown ametryn-treated plants.     

Salicylic acid alleviates growth inhibition and oxidative stress caused by zucchini yellow mosaic virus infection in Cucurbita pepo leaves

The effects of zucchini yellow mosaic virus (ZYMV) infection and pretreatments with salicylic acid (SA) on biomass accumulation of pumpkin (Cucurbita pepo cv. Eskandarani) were investigated. The response of photosynthesis, transpiration and the activities of antioxidant enzymes in leaves was also considered. Significant reductions in growth parameters (i.e. leaf area, biomass and shoot height), photosynthesis and chlorophyll a and b content were detected in ZYMV-infected leaves in comparison to healthy controls. Antioxidant enzyme activities were increased up to 3-fold for peroxidase (POD), 2-fold for ascorbate peroxidase (APX) and catalase (CAT) activities and 1.3-fold for SOD activity by virus infection. ZYMV infection also caused increases in H₂O₂ and malondialdehyde (MDA) contents. These results suggest that ZYMV infection causes oxidative stress in pumpkin leaves leading to the development of epidemiological symptoms. Interestingly, spraying pumpkin leaves with SA led to recovery from the undesirable effects of ZYMV infection. Leaves treated with 100 μM SA three days before inoculation had the appearance of healthy leaves. No distinct disease symptoms were observed on the leaves treated with 100 μM SA followed by inoculation with ZYMV. In non-infected plants, SA application increased activities of POD and superoxide dismutase (SOD) and inhibited APX and CAT activities.In contrast, SA treatment followed by ZYMV inoculation stimulated SOD activity and inhibited activities of POD, APX and CAT. In addition, MDA displayed an inverse relation, indicating inhibition of lipid peroxidation in cells under SA treatment. It is suggested that the role of SA in inducing plant defense mechanisms against ZYMV infection might have occurred through the SA-antioxidant system. Such interference might occur through inhibition or activation of some antioxidant enzymes, reduction of lipid peroxidation and induction of H₂O₂ accumulation following SA application.     

Characterization of symptom determinants in two mutants of cucumber mosaic virus Y strain,causing distinct mild green mosaic symptoms in tobacco

Two mutants of Cucumber mosaic virus, CMV(Y/GM1) and CMV(Y/GM2), which induced mild green mosaic symptoms in tobacco, were isolated from plants regenerated from tobacco leaves with yellow mosaic symptoms originally infected with the yellow strain of CMV [CMV(Y)]. Although the appearance of mild green mosaic symptoms in tobacco infected with CMV(Y/GM2) was unstable, CMV(Y/GM3) derived from CMV(Y/GM2) reproducibly induced mild green mosaic symptoms in tobacco similar to CMV(Y/GM1). A comparison of the deduced amino acid sequences of the coat proteins of CMV(Y), CMV(Y/GM1) and CMV(Y/GM3), showed single amino acid substitutions from Thr to Ile at position 124 in the CMV(Y/GM1) coat protein and from Val to Ile at position 111 in the CMV(Y/GM3) coat protein. When the amino acid at the 124 or 111 position in the CMV(Y) coat protein was changed to Ile at the cDNA level, CMV RNA3 transcribed in vitro from each cDNA induced mild green mosaic symptoms in tobacco after inoculation with in vitro transcribed CMV(Y) RNA1 and RNA2. The results indicated that amino acids at positions 111 and 124 in the coat protein were responsible for the phenotypic changes caused by the two CMV isolates.     

Viruses and virus-like pathogens transmitted by zoosporic fungi1

The viruses and virus-like pathogens transmitted by zoosporic fungi are reviewed. The nine furoviruses (and possible members of the group), with labile rod-shaped particles, have nearly all been shown to be transmitted by plasmodiophoromycete vectors. As they have been reviewed extensively elsewhere, they are covered only briefly; important examples are beet necrotic yellow vein furovirus and potato mop-top furovirus. Five viruses with filamentous particles, tentatively recognized as poty viruses, are transmitted by Polymyxa graminis. Within this group, wheat yellow mosaic virus should be considered to include wheat spindle streak mosaic virus, while the M and NM forms of barley yellow mosaic virus, the best known members of the group, should probably be regarded as distinct viruses. Chytrids (especially Olpidium brassicae) transmit a variety of viruses in different groups (e.g. tobacco necrosis necrovirus, lettuce big-vein virus, melon necrotic spot carmovirus, red clover necrotic mosaic dianthovirus). Finally, several diseases caused by uncharacterized pathogens appear to be transmitted by O. brassicae: freesia leaf necrosis, lettuce ring necrosis, pepper yellow vein, watercress chlorotic leaf spot.     

Leaf tissue pigments and chlorophyll fluorescence parameters vary among sweet corn genotypes of differential herbicide sensitivity

Herbicide applications are meant to eliminate weed competition; however, herbicides may also impose abiotic stress on registered crops. Leaf tissue carotenoid pigments play vital roles in the photoprotection of photosynthetic membranes and contribute to non-photochemical quenching (NPQ) of excitation energy, both important to plant environmental stress tolerance. Our research objectives were to characterize leaf tissue pigments and chlorophyll fluorescence parameters following post-emergence herbicide applications (simulating an abiotic stress) to sweet corn (Zea mays var. rugosa) genotypes of differential herbicide sensitivities. Post-emergence herbicide applications of combinations of mesotrione (105 g ai/ha) and atrazine (560 g ai/ha) were applied to ‘Merit’ (sensitive), ‘Temptation’ (tolerant), and ‘Incredible’ (moderately sensitive) sweet corn genotypes. Leaf tissues were sampled after herbicide applications and measured for chlorophyll fluorescence parameters, and the same tissues were analyzed for carotenoid and chlorophyll pigments. Leaf pigments and chlorophyll fluorescence were not affected by any herbicide treatment; however, data revealed significant differences between genotypes for leaf tissue antheraxanthin, β-carotene, zeaxanthin, chlorophyll a/b ratios, and for values of F_o, F_m, F_v, and NPQ, with ‘Merit’ leaf tissue having higher values than the other two genotypes evaluated. Results demonstrate that genotypic sensitivities to certain post-emergence herbicides may be related to concentrations of photo-protective carotenoids in sweet corn leaf tissues.     

Unterschiedliche Formen der Resistenz gegen bodenbürtige Viren des Weizens

In Germany the furovirus Soil-borne cereal mosaic virus (SBCMV) and the bymovirus Wheat spindle streak mosaic virus (WSSMV) occur often together particularly in several rye production areas. Soil-borne wheat mosaic virus (SBWMV), a wheat infecting furovirus, has so far been found only in one field near Heidelberg. Each of these viruses is transmitted by Polymyxa graminis. The cultivation of resistant varieties is the only promising measure to prevent yield losses caused by soil-borne viruses. Resistance of wheat against the bymovirus WSSMV is comparable to the immunity of barley to the bymoviruses Barley yellow mosaic virus and Barley mild mosaic virus. In case of immunity no virus multiplication is observed in resistant cultivars. In contrast, all wheat cultivars are hosts of the furoviruses. All cultivars – including the resistant ones – can be infected following mechanical inoculation with SBWMV and SBCMV. Resistance to furoviruses is based on reduced levels of virus multiplication in roots and on inhibition of virus movement from roots to leaves. Because of the inhibited virus movement from roots to aerial parts of plants this type of resistance is referred to as translocation resistance. In spite of the different resistance mechanisms the absence of virus symptoms on the leaves is a common selection criterion for both immunity and translocation resistance. Therefore, the symptom free development of plants on uniformly infested fields is the best criterion for selecting wheat lines with resistance to soil-borne viruses. The limited suitability of other selection methods is discussed.     

Persistence of infectivity of three viruses in plant material dried over CaCl2 and stored under different conditions

The infectivity of three viruses in leaf material dried over CaCl₂ and stored under different conditions was tested over a period of seven years. Alfalfa mosaic virus and cucumber mosaic virus kept very well for the entire period both at 4°C and at room temperature, provided they were stored over CaCl₂ in well-closed tubes. The infectivity of bean yellow mosaic virus, however, gradually declined, especially at room temperature. In open or loosely closed containers without CaCl₂ none of the viruses survived more than three months of storage and the infectivity of bean yellow mosaic virus declined after only two weeks.     

Alteration of photosynthetic performance and source–sink relationships in wheat plants infected by Pyricularia oryzae

Two experiments were carried out to assess the changes associated with photoassimilate production and partitioning in the source–sink relationship of flag leaves and spikes of wheat plants infected with Pyricularia oryzae, the causal agent of blast. Flag leaves and spikes were inoculated at 10 and 20 days after anthesis (daa) with a conidial suspension of P. oryzae. Analysis of chlorophyll a fluorescence using maximal photosystem II quantum efficiency (F_v?F_m), fraction of energy absorbed that is used in photochemistry (YII), quantum yield of non‐regulated energy dissipation (Y(NO)) and quantum yield of regulated energy dissipation (Y(NPQ)), showed an impairment of the photosynthetic performance in both infected flag leaves and spikes, coupled with reduced concentrations of chlorophyll a  + b and carotenoids. Compared to non‐inoculated controls, there was lower capacity for CO₂ fixation by RuBisCO in the infected flag leaves. Similarly, in the infected flag leaves and grains (obtained from infected spikes), there were lower concentrations of soluble sugars, while the hexoses‐to‐sucrose ratio increased in infected flag leaves. Compared to non‐inoculated controls, infected flag leaves showed lower sucrose phosphate synthase (SPS) activity and lower expression of the sucrose synthesis (SuSy) gene, while higher expression and activity of acid invertases also occurred. At the advanced stages of fungal infection, the concentration of starch in grains decreased but remained high for the infected flag leaves. There were reductions in ADP‐glucose pyrophosphorylase activity and the expression of ADP‐glucose pyrophosphorylase genes and a down‐regulation of β‐ and α‐amylase expression at the advanced stages of fungal infection on flag leaves and spikes. In conclusion, the effect of blast on both grain quality and yield can be associated with alterations in both production and partitioning of carbohydrates during the grain filling process.     

Molecular Characterization of <Emphasis Type="Italic">Bean yellow mosaic virus </Emphasis>from Vegetatively Propagated Dwarf <Emphasis Type="Italic">Gentiana </Emphasis>Plants

The causative virus (isolate No. 4) of gentian (Gentiana spp.) mosaic, which had been identified previously as Clover yellow vein virus (C1YVV) on the basis of host range and serological reactions, was re-identified as Bean yellow mosaic virus (BYMV) on the basis of the nucleotide sequences of the gene for the coat protein (CP) and the 3′-noncoding region, as well as the predicted amino acid sequence of CP. Received 16 April 2002/ Accepted in revised form 19 June 2002     

Inhibition of photosystem II in spinach chloroplasts by trachyloban-19-oic acid

The effect of trachyloban-19-oic acid isolated from Iostephane heterophylla (Asteraceae), was investigated on several photosynthetic activities in spinach thylakoids. The results indicated that this compound inhibited ATP synthesis and uncoupled electron transport, as well as basal and phosphorylating electron flow. Therefore, trachyloban-19-oic acid acts as Hill reaction inhibitor. This compound did not affect photosystem I activity but inhibited uncoupled photosystem II electron flow from H₂O to DCPIP, and has not effect on electron flow from H₂O to SiMo, indicating that the site of inhibition of this compound is at the level of Q_A-Q_B. Chlorophyll a fluorescence measurements confirm the behavior of this diterpene as Q_A-Q_B inhibitor, and in the other hand, this results indicate that a perturbation in the thylakoid membranes at the level of LHC II occurs.     

Production of discernible disease phenotypes in Canna by five plant viruses belonging to the genera Potyvirus,Cucumovirus and Badnavirus

Cannas are tropical and subtropical flowering perennial plants. The genus contains many species but most commercially grown cultivars are interspecific hybrids selected for their attractive foliage and flowers. Canna production is so lucrative that there are farmers and nurseries dedicated solely to its production. The specific issue that the canna industry faces is virus diseases. In this study, rhizomes of 24 canna cultivars were gathered and diagnostics conducted to detect Bean yellow mosaic virus (BYMV, Potyvirus), Canna yellow mottle virus (CaYMV, Badnavirus), Canna yellow streak virus (CaYSV, Potyvirus), Cucumber mosaic virus (CMV, Cucumovirus) and Tomato aspermy virus (TAV, Cucumovirus). Visual assessment of disease symptoms and diagnostic tests were carried out to identify the prevalent diseases and describe the symptoms that are associated with virus infection. BYMV, CaYMV and CaYSV caused severe mosaic and necrosis either in the leaf lamina or veins of infected leaves. Potyvirus infection suppressed red colouration in the foliage of some varieties. CaYMV and CaYSV often appeared in the same plant, suggesting they might represent a viral complex. CMV and TAV were rarely seen in these populations. Interestingly, CaYMV but not CaYSV could be mechanically inoculated to Phaseolus vulgaris plants.     

Effect of herbicide clomazone on photosynthetic processes in primary barley (Hordeum vulgare L.) leaves

The effect of pre-emergently applied herbicide clomazone on the photosynthetic apparatus of primary barley leaves (Hordeum vulgare L.) was studied. Clomazone application caused a reduction in chlorophyll (a+b) and carotenoid levels that was accompanied by a decline in the content of light harvesting complexes as judged from the increasing chlorophyll a/b ratio. The pigment reduction also resulted in changes in 77 K chlorophyll fluorescence emission spectra indicating lower chlorophyll (Chl) fluorescence reabsorption and absence of the long-wavelength emission forms of photosystem I. The maximal photochemical yield of photosystem II (PSII) and the reoxidation kinetics of the primary quinone acceptor Q_A⁻ were not significantly influenced by clomazone. A higher initial slope of Chl fluorescence rise in the Chl fluorescence induction kinetic indicated an increased delivery of excitations to PSII. Simultaneously, analysis of the Chl fluorescence quenching revealed that clomazone reduced function of the electron transport chain behind PSII. The decrease in the saturation rates of CO₂ assimilation paralleled the decrease of the Chl content and has been suggested to be caused by a suppressed number of the electron transport chains in the thylakoid membranes or by their decreased functionality. The obtained results are discussed in view of physiological similarity of the clomazone effect with changes of photosynthetic apparatus during photoadaptation.