Epiphytes impose physical barriers to light penetration into seagrass leaves causing shading, which may decrease the production of oxygen reactive species (ROS), but also constitute a physical ...aggression that may trigger the production of ROS, leading to oxidative damage. Here we investigate the effects of epiphytes on Posidonia oceanica under both interactive perspectives, light attenuation and oxidative stress. Specifically the role of epiphytes in net photosynthesis, chlorophyll a and b, photoprotection (Violaxanthin+Anteraxanthin+Zeaxanthin cycle), soluble sugar and starch contents, enzymatic (ascorbate peroxidase (APX) and dehydroascorbate reductase (DHAR)) and global (trolox equivalent antioxidant capacity (TEAC)) and oxygen radical antioxidant capacity (ORAC)) antioxidant responses, phenolics and oxidative damage (malondialdehyde) are tested. Leaves with epiphytes showed higher chlorophyll b and lower content in VAZ cycle carotenoids. Epiphyte shading was the probable reason for the lower VAZ de-epoxidation-ratio of leaves with epiphytes. In spite of being shaded, leaves with epiphytes showed higher antioxidant levels, indicating that epiphytes trigger the production of ROS. Both ORAC and TEAC and also APX and DHAR activities were higher in leaves with epiphytes, indicating that this response was related with its presence. Malondialdehyde concentrations also suggest oxidative damage caused by epiphytes. We conclude that the epiphyte load causes oxidative stress in P. oceanica and the mechanisms to scavenge ROS were not completely effective to avoid cell damage.
The effects of a slow-imposing two-week soil drying period, and subsequent re-watering, on leaf water potential (
Ψ), gas exchange rates, chlorophyll fluorescence and on the concentrations of ...malondialdehyde (MDA) and non-structural carbohydrates (starch, hexose and sucrose) were determined in mature leaves of sunflower plants growing under controlled-environmental conditions. To assess how transient shade periods, associated with increased cloud cover, may influence drought-induced effects on carbon assimilation, measurements were carried out both in plants kept under the photosynthetic photon flux density (PPFD) prevailing during the growth period and stress imposition (750
μmol
m
−2
s
−1), and in plants subjected to a 5-h long period under a lower PPFD (200
μmol
m
−2
s
−1). In plants kept under high PPFD,
Ψ, stomatal conductance (
g), net CO
2 uptake rate (
A), the quantum yield of photosystem II electron transport (
Φ
e), the photochemical efficiency of open PSII reaction centres (
F
′
v
/
F
′
m
) and the diurnal accumulation of total non-structural carbohydrates (TNC) were significantly depressed at the end of the soil drying period, whereas non-photochemical quenching (NPQ), the concentrations of MDA and the predawn pools of soluble sugars were found to increase. Under high-light level, drought-induced effects on lipid peroxidation, chlorophyll fluorescence parameters and gas exchange rates were fully reversed upon re-watering. However, the inhibition of diurnal accumulation of TNC still persisted two days following re-watering, suggesting that carbohydrate export rates were enhanced following stress relief. An overall positive effect was found upon transferring water-stressed plants to low light level, as indicated by the increases in
Ψ, intrinsic water use efficiency (
A/
g),
Φ
e and
F
′
v
/
F
′
m
, as well as the reversal of drought-induced enhancement of both NPQ and MDA concentration. Despite
g being similar in shaded well-watered and re-watered plants, the latter exhibited net CO
2 uptake rates below those found in well-watered leaves under the same light conditions, together with a diurnal decrease in the concentration of TNC that was mainly attributable to the depletion of starch and sucrose. These results indicate that, contrasting with the positive effects of shading on water-stressed plants, low PPFD may negatively affect the recovery of net photosynthesis following stress relief.
This work represents the first contribution to (i) examine the changes in plant‐water relations of an inter‐tidal seagrass during air exposure (Zostera noltii), and (ii) compare the water status ...descriptors between inter‐tidal‐ and subtidal‐adapted species (Cymodocea nodosa, Zostera marina). Two different morphotypes of Z. noltii that develop in the highest and lowest inter‐tidal levels in the Portuguese lagoon of Ria Formosa were exposed to natural emersion periods under laboratory conditions, and the evolution of leaf water relations and osmolytes (ions, proline and non‐structural carbohydrates) was measured. Both morphotypes regulated their water potential (Ψw) by reducing the osmotic potential (Ψπ) through osmolyte accumulation, but only high inter‐tidal plants were able to do this by adjusting the turgor pressure through cell wall hardening. This is a conservative mechanism for osmotic acclimation, which occurred only after long emersion periods (7 h). After a rapid increase in ion concentration under air exposure, the high inter‐tidal morphotype replaced them by more physiologically compatible solutes (proline and non‐structural carbohydrates) to maintain the osmotic adjustment. Altered ionic homeostasis was found in low inter‐tidal plants when exposed to such unnatural, long emersion periods. Osmotic unbalances were also observed during the submerged recovery phase. Descriptors of leaf pressure–volume (P–V) curves and Höfler diagrams were derived for seagrasses for the first time. They support the divergences in water relations observed between inter‐tidal and subtidal seagrasses according to their vertical distribution. More negative water and osmotic potentials and higher rigidity of cell walls (higher elastic modulus, ε) were found to be specific osmotic adaptations of seagrasses to the inter‐tidal.
The effects of soil drying on the activity of nitrate reductase (NR; EC 1.6.6.6) were studied in Helianthus annuus L. and non‐nodulated Lupinus albus L. plants growing under two nutrient supply ...regimes. NR activity was assessed in leaf and root extracts by measuring the activity of the unphosphorylated active form (NRact), the maximal extractable activity (NRmax) and the activation state. To obtain an insight into potential signalling compounds, nitrate, free amino acids and soluble sugars were also quantified. In both species, foliar NRact and NRmax were negatively affected by soil drying and a decreased supply of nutrients, the observed changes in NR activity being linearly correlated with the depletion of nitrate. Similar results were obtained in the roots of sunflower. Conversely, in white lupin roots, NRmax was found to be independent of tissue nitrate concentration. Regardless of the species and organ, the activation state of the enzyme was unaffected by the nutrient supply regime. In well‐watered sunflower roots, only about 50% of the existing NR was unphosphorylated, but the activation state increased significantly in response to drought. In contrast, lupin roots always exhibited NR activation state values close to 80%, or even higher. At the leaf level, the NR activation state was hardly changed in response to soil drying. The observed changes in the concentrations of soluble sugars and free amino acids are discussed in terms of their possible contribution to the variations in NR activity.
The effects of light-limitation stress were investigated in natural stands of the seagrasses Zostera marina and Cymodocea nodosa in Ria Formosa coastal lagoon, southern Portugal. Three levels of ...light attenuation were imposed for 3 weeks in two adjacent meadows (2-3 m depth), each dominated by one species. The response of photosynthesis to light was determined with oxygen electrodes. Chlorophylls and carotenoids were determined by high-pressure liquid chromatography (HPLC). Soluble protein, carbohydrates, malondialdehyde and phenol contents were also analysed. Both species showed evident signs of photoacclimation. Their maximum photosynthetic rates were significantly reduced with shading. Ratios between specific light harvesting carotenoids and the epoxidation state of xanthophyll cycle carotenoids revealed significantly higher light harvesting efficiency of C. nodosa, a competitive advantage in a low light environment. The contents of both soluble sugars and starch were considerably lower in Z. marina plants, particularly in the rhizomes, decreasing even further with shading. The different carbohydrate energy storage strategies found between the two species clearly favour C. nodosa's resilience to light deprivation, a condition enhanced by its intrinsic arrangement of the pigment pool. On the other hand, Z. marina revealed a lower tolerance to light reduction, mostly due to a less plastic arrangement of the pigment pool and lower carbohydrate storage. Our findings indicate that Z. marina is close to a light-mediated ecophysiological threshold in Ria Formosa.
A resposta dos mecanismos fotossintéticos à duração e intensidade do défice hídrico e posterior rehidratação foi investigada em plantas envasadas de Helianthus annuus L. e Lupinus albus L. A rega foi ...manipulada de modo a induzir diferentes intensidades de défice hídrico e posterior rehidratação. Nas plantas de H. annuus foram analisadas comparativamente folhas jovens e folhas maduras, tendo-se também avaliado nas folhas jovens o efeito da diminuição temporária da intensidade luminosa nas plantas em défice hídrico e rehidratação. Foram também avaliados comparativamente os efeitos dos défices hídricos e da rehidratação na actividade fotossintética e no sistema antioxidante em folhas de H. annuus e L. albus, espécies com estratégias diferentes perante o défice hídrico (H. annuus é tolerante ao défice hídrico e L. albus evita o défice hídrico). Os resultados obtidos indicam que, apesar de as taxas de fotossíntese terem diminuído em resposta ao défice hídrico, não ocorreu fotoinibição crónica nem foi induzida senescência foliar nas folhas de H. annuus amostradas. Nestas plantas, em situação de défice hídrico, ocorreram limitações metabólicas à actividade fotossintética, que nas folhas maduras podem ter incluído a activação da reacção de Mehler e da fotorrespiração. As folhas jovens revelaram possuir a capacidade de reforçar a sua protecção antioxidante em resposta ao défice hídrico, através do aumento do teor foliar em carotenóides. No entanto, as diferenças registadas entre as folhas jovens e as folhas maduras nas plantas em défice hídrico não se repercutiram na sua capacidade de recuperação da actividade fotossintética após rehidratação, que foi semelhante. Os dados obtidos mostraram que o aumento do ‘quenching’ não fotoquímico nem sempre está directamente relacionado com o aumento do índice de desepoxidação do ciclo das xantofilas e confirmaram a relação de dependência entre ambos os parâmetros e a densidade de fluxo quântico. Nas plantas em défice hídrico transferidas para intensidade luminosa baixa, a ausência de stresse oxidativo pode ter implicado falhas na sinalização interna das plantas, enquanto o estímulo aparente da exportação de fotoassimilados durante a rehidratação pode ter conduzido à inibição do crescimento, pelo que não se observaram vantagens na imposição do défice hídrico e da rehidratação a baixas densidades de fluxo quântico. A manutenção de altos teores foliares de sacarose nas plantas rehidratadas sob irradiância alta não evitou que estas não recuperassem completamente a actividade fotossintética. O défice hídrico induziu senescência foliar em L. albus. Esta pode ter resultado do stresse oxidativo induzido nas plantas em défice hídrico, aparentemente devido ao desequilíbrio entre as actividades da dismutase do superóxido e da peroxidase do ascorbato. Os resultados mostram que o défice hídrico pode, por si só, induzir respostas normalmente associadas à ocorrência conjunta de défice hídrico e intensidade luminosa alta, tais como o stresse oxidativo e o aumento no teor foliar em pigmentos antioxidantes.
We present the results of a multiple organizational level analysis conceived to identify acclimative/ adaptive strategies exhibited by the seagrass Posidonia oceanica to the daily uctuations in the
...light environment, at contrasting depths. We assessed changes in photophysiological parameters,
leaf respiration, pigments, and protein and mRNA expression levels. The results show that the diel oscillations of P. oceanica photophysiological and respiratory responses were related to transcripts and proteins expression of the genes involved in those processes and that there was a response asynchrony between shallow and deep plants probably caused by the strong di erences in the light environment. The photochemical pathway of energy use was more e ective in shallow plants due to higher light availability, but these plants needed more investment in photoprotection and photorepair, requiring higher translation and protein synthesis than deep plants. The genetic di erentiation between
deep and shallow stands suggests the existence of locally adapted genotypes to contrasting light environments. The depth-speci c diel rhythms of photosynthetic and respiratory processes, from molecular to physiological levels, must be considered in the management and conservation of these key coastal ecosystems.
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