NONEQUILIBRIUM RATES OF PHOTOSYNTHESIS AND RESPIRATION UNDER DYNAMIC LIGHT SUPPLY1

To identify processes that might account for differences in growth rates of rhodophytes under constant and dynamic light supply, we examined nonequilibrium gas exchange by measuring time courses of photoinduction, loss of photoinduction, and respiration rates immediately after the light–dark transition. Using the rhodophyte species Palmaria palmata (Huds.) Lamour and Lomentaria articulata (Huds.) Lyngb., we compared the effects of growth-saturating constant photon flux density (PFD) (95 μmol photons · m^?2· s^?1) to those of a dynamic light supply modeled on canopy movements in the intertidal zone (25 μmol photons · m^?2· s^?1 background PFD plus light flecks of 350 μmol photons · m^?2· s^?1, 0.1 Hz). The time required for P. palmata and L. articulata to be fully photoinduced was not affected by the dynamics of light supply. L. articulata required only 6 min of illumination with either fluctuating or constant light to be completely induced compared to 20 min for P. palmata. The latter species also lost photoinduction more rapidly than did L. articulata in the dark. There was no significant decline in photoinduction state for either species at the background PFD. The time courses of respiration after illumination with constant and fluctuating light were significantly different for P. palmata but not for L. articulata when the total photon dose was equal. In general, gas exchange of P. palmata appeared to be particularly sensitive to the temporal distribution of light supply whereas that of L. articulata was sensitive to the amplitude of variations, being photoinhibited at high PFD. These results are discussed in terms of the different mechanisms of inorganic carbon acquisition in the two species.     

Compositional changes associated with plasma membrane thickening during floral induction of spinach

Floral induction in the long-day plant spinach ( Spinacia oleracea L. cv. Nobel) was accompanied by a thickening of the plasma membrane. Densitometry analyses showed that the light space of the dark-light-dark pattern of the membrane was not changed upon photoinduction. Rather, the increase was due to an enhancement of the dark layer adjacent to the cell wall. Parallel analyses of protein and phospholipid composition revealed no marked changes in protein composition or biosynthetic rate, protein phosphorylation, glycolipids and/or phospholipids as a result of the 24 h of continuous light sufficient to induce flowering. Photoinduction, however, was accompanied by an increase in the relative amount of plasma membrane sterols which may be related to the membrane thickening.     

Changes in the polypeptide profiles during photoinduction of Xanthium strumarium

The polypeptides in the leaf blades, petioles and apices from photoinduced and noninduced Xanthium strumarium L. were compared by two dimensional gel-electrophoresis. A 15 kDa and a 16 kDa polypeptide were detected in gels of the leaf blade from noninduced, but not from induced, plants. Similarly, an acidic 9 kDa polypeptide was detected in the apices from noninduced plants, but not in apices from induced plants. Both the apices and petioles from noninduced plants showed a 34 kDa polypeptide which was absent in tissues from induced plants. Thus, the disappearence of identifiable polypeptides from photoinduced tissues may be associated with the photoinductive short-day treatment that leads to flowering.     

The photoactivated toxin cercosporin as a tool in fungal photobiology

Cercospora species are a highly successful group of fungi which pathogenize diverse species of economically important plants. Many Cercospora species produce a unique photoactivated and photoinduced polyketide toxin, cercosporin, which has been implicated as a pathogenicity factor. Illuminated cercosporin interacts with molecular oxygen to produce highly toxic singlet oxygen. Although nearly all organisms tested, including plants, mice and most fungi, are susceptible to cercosporin-mediated cell damage, Cercospora species are resistant. In general, little is known about how organisms protect themselves against singlet oxygen. Studies on how Cercospora species avoid autotoxicity are proving to be a valuable model in understanding this process in other systems. Furthermore, advances are being made in the understanding of how light regulates gene expression and cercosporin synthesis in Cercospora species. These studies are helping to elucidate mechanisms of gene regulation and light signal transduction for an environmental signal important in numerous fungal developmental processes, including secondary metabolite production.     

Constitutive and light-induced DNAseI hypersensitive sites in the rbcS genes of pea (Pisum sativum)

The chromatin structure of pea (Pisum sativum) rbcS genes in inactive (root), potentially active (dark-grown leaf), and active states (light-grown leaf) was analysed using (a) pancreatic DNAseI to detect general DNAseI sensitivity and DNAseI-hypersensitive sites, and (b) methyl-sensitive restriction endonucleases to probe for cytosine methylation within the promoter region. We showed that within the same organ individual members of the pea rbcS multigene family are differentially sensitive to DNAseI suggesting differential protection in nuclei. During light activation general DNAseI sensitivity increases in some genes, especially their 5 upstream regulatory sequences. DNAseI-hypersensitive sites are constitutively present in 5 upstream regulatory sequences around positions –335, –465, –650, and –945 (5 constitutive domain) and in the coding region around position +340, +450, +530, +640, and +810 (3 constitutive domain). One additional hypersensitive site appears after light induction (inducible site). This region is centred around position –190 and flanked by light-responsive elements (LREs). In spite of changes in the chromatin structure of rbcS genes during their transition from an inactive to an active state, their cytosine methylation at Alu I, Fnu 4HI, Hae III, Sau 3AI and Sau 96I sites in the promoter region remains uniform.     

Asparagine and regulation of photoinduced rhythm in Penicillium claviforme

A photosensitive reaction involved in the expression and regulation of the endogenous sporulation rhythm in Penicillium claviforme Bainier CBS 126-23 has been described earlier by our team. The present work shows that asparagine plays a central role in this periodic system. The 24–26 h periodicity, which becomes desynchronized over a few days, was affected by supplying asparagine (1 to 25 m M ). a) The rhythm remained synchronized for at least 3 weeks, for all the asparagine concentrations used, b) The period increased successively from one cycle to the next (from 35±3 h to 73±8 h over 6 cycles) for 1 m M asparagine. This period lengthening became less and less accentuated as the asparagine concentration was increased. At ≥10 m M , the period was stable; it was 33±5 h for 10 m M and 22±1.5 h for 25 m M asparagine. Otherwise asparagine could not elicit rhythmicity in darkness or in dim continuous light (≤5 μW m⁻²).     

Role of nitrogen in the photoinduction of protoperithecia and carotenoids in Neurospora crassa

Nitrogen, as KNO₃ or NH₄NO₃, can inhibit the photoinduction of protoperithecia in Neurospora crassa when present in the medium at a high concentration but does not inhibit the photoinduction of carotenoids. The point at which the presence of high nitrogen levels is no longer inhibitory is 5 h after illumination.Abbreviations al albino mutant - wc white-collar mutant - WM Westergaard and Mitchell (1947) medium Dedicated to Professor Wilhelm Nultsch in honour of his 60th birthday