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Nitric Oxide-Induced Tolerance in Plants under Adverse Environmental Conditions
Published in Hasanuzzaman Mirza, Nahar Kamrun, Fujita Masayuki, Oku Hirosuke, Tofazzal M. Islam, Approaches for Enhancing Abiotic Stress Tolerance in Plants, 2019
Neidiquele M. Silveira, Amedea B. Seabra, Eduardo C. Machado, John T. Hancock, Rafael V. Ribeiro
Recent studies have demonstrated that administration of exogenous NO• donor (GSNO) sprayed on leaves improves plant tolerance to water deficit, with plants presenting a significant improvement in photosynthetic rates under low water availability. Such improvement in leaf CO2 assimilation was associated, in part, with increases in relative water content, root growth and stomatal conductance under water deficit (Silveira et al., 2016). Besides the diffusive limitation imposed by stomatal closure, plants may face the biochemical limitation of photosynthesis under severe drought. In this sense, Silveira et al. (2017b) also noted that the exogenous GSNO improved CO2 uptake under water deficit by increasing Rubisco activity. Additionally, NO• induces a slow and continuous increase in the non-photochemical quenching of fluorescence, a well-known photoprotective mechanism when there is excessive light energy at PSII (Ordog et al., 2013). Improvement in plant performance under drought, when supplied with NO• donor, may be a consequence of reduced oxidative damage (Silveira et al., 2017b). Thus, NO-induced tolerance to drought is related to the alleviation of diffusive, biochemical and photochemical limitations of photosynthesis, which benefits plant growth under water deficit (Figure 21.1).
Mid-Latitude Macroalgae
Published in Donat-P. Häder, Kunshan Gao, Aquatic Ecosystems in a Changing Climate, 2018
From these fluorescence signals the fraction of absorbed light which is funnelled into photosynthesis can be determined as photochemical quenching (qP) as well as the fraction which is dissipated as heat (non-photochemical quenching, qN) according to the equations
Algal Photosynthesis and Physiology
Published in Stephen P. Slocombe, John R. Benemann, Microalgal Production, 2017
John A. Raven and John Beardall
A final point about the light-harvesting machinery is the number of light- harvesting pigment molecules per PSI or PSII reaction center (one of the uses of the term photosynthetic unit and the meaning used here). Natural selection can overendow the organism with light-harvesting machinery relative to the mean PAR in the natural environment, for example, the diel change in PAR as a function of solar altitude and variations due to entrainment in vertical water movements in the upper mixed layer. Decreasing the photosynthetic unit size would, unless there is a compensating increase in the number of reaction centers per cell, mean less pigment per cell and hence a lower photosynthetic rate at low irradiances because less PAR is absorbed. At high irradiances, a smaller photosynthetic unit size (such as occurs for photosystem II, but not photosystem I, in mutant strains of algae) decreases the potential for photodamage from excess PAR plus UVB, relative to what can be used in autotrophic carbon dioxide assimilation. This, in turn, means that there is the potential for downregulating photoprotective machinery such as photochemical quenching or the variants, using both photosystems or only photosystem II, of the water–water cycle or non-photochemical quenching, including the xanthophyll cycles (Raven 2011; Raven and Ralph 2015). This sparing of the need for photoprotection, arising from fewer light-harvesting pigments per photosynthetic unit (defined earlier), requires that there also be no decrease in the number of such photosynthetic units per cell; otherwise, the capacity for carbon dioxide assimilation would be limited on a cell basis by restrictions on interphotosystem redox reactions and proton pumping and hence on regeneration of ribulose-1,5 bisphosphate and on ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) activity. As we have noted, reduced pigment content will restrict photosynthetic rate at low irradiances. Thus, in assessing such modifications to the photosynthetic unit, it is necessary to take into account the sinusoidal variation in solar radiation through the daily photoperiod. While decreasing the photosynthetic unit size can limit photoinhibition around midday for cells near the surface of the pond or reactor, it would also decrease the capacity for photosynthesis at the lower irradiances near dawn and dusk.
Potential of halophytic plant Atriplex hortensis for phytoremediation of metal-contaminated soils in the mine of Tamra
Published in Soil and Sediment Contamination: An International Journal, 2023
S. Sai Kachout, A. Ennajah, K. Guenni, N. Ghorbel, A. Zoghlami
We measured chlorophyll fluorescence parameters of optimal (Fv/Fm) and light-adapted PSII quantum yield (ϕPSII) response to heavy metal stress (Figure 5). Under these control conditions, the Fv/Fm value was significantly lower 30 days after germination than 60 days after germination. The Fv/Fm value at sandy soil was the smallest, and values were below 0.80 for all three-determination times. The Fv/Fm value was significantly the same at polluted soil (about 0.80) for all three-determination times. It reached their respective peaks at 4 weeks, and the Fv/Fm values were >0.80. Fv/Fm was used as a sensitive indicator of the original light-energy capture efficiency of PSII reaction centers and plant health status (Wang et al. 2012; Zhang et al. 2017). High values of Fv/Fm indicated high PSII maximum light conversion efficiencies. In healthy organisms, the Fv/Fm value is about 0.8–0.84 in most C3 plant species, but the value decreases significantly when plants are exposed to stress (Da Silva Branco et al. 2017; Kalaji et al. 2012; Wang et al. 2017). The FPSII value was highest at 60 days in control and metal stress conditions, and no significant difference was noted between the two treatments (P > .05, t test). This result demonstrated that these plants despite the stress allocated more of the absorbed light energy to photochemistry than to dissipation processes (Figure 5). This allocation helped the plants to optimize photosynthesis and growth. Light energy is absorbed by PSII through three pathways: photochemistry, thermal dissipation, and no photochemical quenching. Because these processes are competitive, the responses of the photosynthetic apparatus to different environmental conditions can be indicated by complementary changes in the yield of chlorophyll fluorescence. These responses are widely described using NPQ and FPSII (Demmig-Adams et al. 1996; Kramer et al. 2004). NPQ measures the proportion of light energy lost through regulatory thermal dissipation, and FPSII measures the proportion of light energy absorbed by antenna pigments in PSII used in photochemistry. While the absolute chlorophyll fluorescence yield (F) depends on the total concentration of chlorophyll per area, F-derived ratios such as optimal PSII quantum yield (Fv/Fm), light-adapted PSII quantum yield (ϕPSII) and ϕPSII-derived electron transport rate (ETR) are leaf-area independent (Baker 2008; Kraus and Weis 1984) and therefore ideal to assess individual floret performance in the aggregate. Increased stomatal resistance (or decreased stomatal conductance) has been described in many experiments as a result of plant exposure to toxic metal concentrations. In the present study, the stomatal conductance increased first and then decreased under metal stress conditions (Figure 5), which indicated that stomatal limitation is not the main factor affecting the photosynthesis of A. hortensis (Xu et al., 2015;. Leal-Alvarado et al. 2016).