Light-induced development of photosystem (PS)-II activity was followed during irradiance of etiolated Helianthus annuus (sunflower) cotyledons using chlorophyll a fluorescence. Cotyledons from seedlings grown in continuous darkness for 6 d were exposed to 100 μmol photons·m(-2)·s(-1) for time periods of 1, 3, 6, and 12 h. Associated with increased time of irradiance exposure were significant: (1) increases in concentration of PS II, (2) increases in quantum efficiency of PS II, (3) decreases in the ratio of PS-II quinone(B) (Q(B))-nonreducing centers to total PS-II centers (PS-II Q(B)-nonreducing centers + PS-II Q(B)-reducing centers), and (4) decreases in the ratio of slow PS-II Q(B)-reducing centers to total PS-II Q(B)-reducing centers (fast PS-II Q(B)-reducing centers + slow PS-II Q(B)-reducing centers). The results support the hypotheses that development of PS II involves assembly of complexes which initially cannot reduce Q(B) and that heterogeneous aspects of PS-II pools during chloroplast maturation may represent different developmental states.
The chlorophyllous spores of Equisetum survive desiccation, yet cannot tolerate this quiescent state for more than ~2 wk. The hypothesis that spore viability of Equisetum hyemale L. is limited by inhibition of photosynthetic recovery was tested using chlorophyll a fluorescence and oxygen-exchange analyses. Experimental spores were desiccated at 2% relative humidity and 25C for time periods of 24 h, 1 wk, and 2 wk, and then rehydrated at 200 mmol photons/m2s (PAR) and 25C for up to 24 h. Spores desiccated for 24 h recovered photosynthetic competence very rapidly during rehydration, reaching the O2 compensation point in 6.3 ~ 0.3 (mean +/- SE) min. Recovery of photosynthetic performance of spores desiccated for 1 wk was slower, as judged by significantly slower increases of (1) photochemical efficiency of photosystem (PS) II, (2) PS II quinoneB-reducing center concentration, (3) quinoneB concentration, (4) water-oxidation activity, (5) rate of light-induced O2 evolution, and (6) apparent quantum yield of net O2 exchange. Photosystem-II and whole-spore photosynthetic competence of 2-wk desiccated spores was increasingly impaired, and did not recover during rehydration. Origin fluorescence yield and dark respiration were not affected by desiccation time following rehydration. The results suggest that the extremely short viability of disseminated spores of Equisetum hyemale is due to the inability to recover losses of water oxidation and photosystem II-core function following 2 wk of desiccation.
The stems of the resurrection plant Selaginella lepidophylla, a poikilohydrous desert pteridophyte, curl dramatically as the plants dry and uncurl when rewetted. We tested the hypothesis that stem curling is a morphological feature that may serve to limit bright-light and/or thermal damage in resurrection plants with a field experiment at a Chihuahuan desert site in west Texas. Experimental plants were irrigated for 3d and then permitted to dry. The plants were either allowed to curl normally or mechanically restrained to prevent curling and either shaded (l 0% of ambient irradiance) or left fully exposed during desiccation. Nine days after complete desiccation, the plants were harvested and returned to the laboratory. When rehydrated, plants that had been unshaded and restrained from curling during field desiccation had significantly lower photosystem II electron transport rates, higher intrinsic chlorophyll fluorescence (F0 ) yields, lower variable to maximum chlorophyll fluorescence (F/ F m) yield ratios, lower chlorophyll contents, and both lower whole-plant photosynthetic quantum efficiencies and light-saturated C02 assimilation rates relative to plants from the other desiccation treatments. The results of both low-irradiance and high-irradiance hydrations in the laboratory suggest that the curling during desiccation and the uncurling during rehydration of S. lepidophylla in its natural habitat serves to protect the plant from damage due to high irradiance, high temperature, or perhaps a synergistic combination of the two. This protection permits the rapid recovery of photosynthetic competence when next the plants are wetted, thereby helping to maximize net carbon balance over time.
Knowledge of the effects of nutrient concentration on the composition and structure of photoautotrophic periphyton is essential to understand the impact of eutrophication on shallow lotic systems. Reaches of Sulphur Fork Creek upstream and downstream of effluent from Springfield Wastewater Treatment Plant in Middle Tennessee were sampled to assess the effects of trophic state on characteristics of photoautotrophic periphyton, including composition of diatom and soft-algae assemblages. Pearson's correlation coefficient (r) for log 10 -transformed concentrations of soluble reactive phosphorus (log 10 [SRP]) to percent composition was significant for 4 of 63 soft-algae taxa sampled from cobbles. Five algae trophic indices (ATIs) to assess the effects of trophic state on soft-algae assemblages were developed using different taxon-trophic indicators which included: (1) r values for log 10 [SRP] to percent composition (ATI r ), (2) abundance-weighted averages of [SRP] (ATI A-WA [SRP] ), (3) abundance-weighted averages of log 10 [SRP] (ATI A-WA log [SRP] ), (4) weighted averages of log 10 [SRP] where the taxa occur, and (5) abundance-weighted ranks to phosphorus tolerance listed by the National Water-Quality Assessment Program. Eutrophication-induced impairment of Sulphur Fork Creek downstream of the effluent from the wastewater treatment plant was indicated by: (1) high concentrations of photoautotrophic periphyton, (2) low values for the pollution tolerance index of diatom assemblages, (3) positive values for the ATI r , and (4) high values for both the ATI A-WA [SRP] and ATI A-WA log [SRP] . Of the indices using soft-algae taxa evaluated, the ATI r exhibits the strongest and significant correlations to [SRP], [NO 2 C NO 3 nitrogen], and the pollution tolerance index of diatom assemblages. The ATI r accurately reflects the trophic state of the sites studied and provides a novel additional tool to evaluate the effects of nutrient concentration on the structure of photoautotrophic-periphyton assemblages.Keywords: periphyton; algae; diatoms; trophic state; nutrient enrichment; stream monitoring; indices of biotic integrity Introduction Nutrient enrichment is a primary basis of biological impairment of aquatic habitats worldwide (Irvine & Murphy 2009). Quantification of the impacts of eutrophication is required to monitor the efficacy of best management practices designed to improve integrity of nutrient-impaired waters (Smucker & Vis 2009). Increased concentration of chlorophyll a is a hallmark of eutrophication (Khan & Ansari 2005). Because chlorophyll a concentration is influenced by many abiotic and biotic characters of a stream reach, measurements of chlorophyll a concentration alone may not be adequate to demonstrate impairment by nutrient enrichment (Kurle & Cardinale 2011 Ecology, 2015 Vol. 30, No. 3, 349À376, http://dx.doi.org/10.1080/02705060.2014.951883 Ó 2014 Diatoms are the focus of most studies characterizing impacts of eutrophication on periphyton composition because more autecological informat...
Selaginella lepidophylla, the resurrection plant, curls dramatically during desiccation and the hypothesis that curling may help limit bright light-induced damage during desiccation and rehydration was tested under laboratory conditions. Restraint of curling during desiccation at 25° C and a constant irradiance of 2000 μmol m s significantly decreased PSII and whole-chain electron transport and the F/F fluorescence yield ratio following rehydration relative to unrestrained plants. Normal curling during desiccation at 37.5°C and 200 μmol m s irradiance did not fully protect against photoinhibition or chlorophyll photooxidation indicating that some light-induced damage occurred early in the desiccation process before substantial curling. Photosystem I electron transport was less inhibited by high-temperature, high-irradiance desiccation than either PSII or whole-chain electron transport and PSI was not significantly affected by restraint of curling during desiccation at 25°C and high irradiance. Previous curling also helped prevent photoinhibition of PSII electron transport and loss of whole-plant photosynthetic capacity as the plants uncurled during rehydration at high light. These results demonstrate that high-temperature desiccation exacerbated photoinhibition, PSI was less photoinhibited than PSII or whole-chain electron transport, and stem curling ameliorated bright light-induced damage helping to make rapid recovery of photosynthetic competence possible when the plants are next wetted.
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