Identification of large variation in the photosynthetic induction response among 37 soybean [<Emphasis Type="Italic">Glycine max</Emphasis> (L.) Merr.] genotypes that is not correlated with steady-state photosynthetic capacity

Irradiance continuously fluctuates during the day in the field. The speed of the induction response of photosynthesis in high light affects the cumulative carbon gain of the plant and could impact growth and yield. The photosynthetic induction response and its relationship with the photosynthetic capacity under steady-state conditions (P _max) were evaluated in 37 diverse soybean [Glycine max (L.) Merr.] genotypes. The induction response of leaf photosynthesis showed large variation among the soybean genotypes. After 5 min illumination with strong light, genotype NAM23 had the highest leaf photosynthetic rate of 33.8 µmol CO₂ m^?2 s^?1, while genotype NAM12 showed the lowest rate at 4.7 µmol CO₂ m^?2 s^?1. Cumulative CO₂ fixation (CCF) during the first 5 min of high light exposure ranged from 5.5 mmol CO₂ m^?2 for NAM23 to 0.81 mmol CO₂ m^?2 for NAM12. The difference in the induction response among genotypes was consistent throughout the growth season. However, there was no significant correlation between CCF and P _max among genotypes suggesting that different mechanisms regulate P _max and the induction response. The observed variation in the induction response was mainly attributed to ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activation, but soybean lines differing in the induction response did not differ in the leaf content of Rubisco activase α- and β-proteins. Future studies will be focused on identifying molecular determinants of the photosynthetic induction response and determining whether this trait could be an important breeding target to achieve improved growth of soybeans in the field.     

Photosynthetic response of Chlamydomonas reinhardtii to short-term storage on ice in darkness. Dependence of the response on growth conditions

The effect of storage of the unicellular green alga Chlamydomonas reinhardtii (strain 137+) in the pelleted state in darkness on ice (0.2–0.5°C) (further simply “SPDI-treatment”) on its photosynthetic and respiratory activities was studied. To this end, the steady-state rates of O₂ exchange in darkness (dark respiration) and under saturating light (apparent photosynthesis) as well as the induction periods (IP) of apparent photosynthesis were measured at 25°C in the SPDI-untreated and SPDI-treated for the period from ～0.5 to ～30 h algal cells. In contrast to expectations, the SPDI-treatment consistently affected the rate and IP of photosynthesis depending on the physiological state of C. reinhardtii. Dark respiration was affected by the SPDI-treatment as well. However, in absolute values the respiratory changes were much less than the photosynthetic ones, and they were insufficiently reproducible. The SPDI-treatment affected photosynthesis most significantly in high-CO₂-grown cells (cells grown at 5% CO₂ in white light). The rate of photosynthesis in these cells declined quasi-exponentially as a function of time during the SPDI-treatment with a t _1/2 ～1.5 h and finally became by about 60% lower than that before the SPDI-treatment. This decline of photosynthesis was accompanied by continuous and essential increase in the photosynthetic IP. The SPDI-induced photosynthetic changes in high-CO₂-grown cells resulted from the firm disfunction of the photosynthetic apparatus. After switch from growth at 5% CO₂ in white light to growth at ～0.03% CO₂ (air) in white, blue, or red light, the alga gradually transited to a physiological state, in which the negative effects of the SPDI-treatment on the rate and IP of photosynthesis became weak and absent, respectively. Remarkably, this transition was faster in blue (≤5 h) than in white and red light (>10 h). Similar changes in the response of the alga to the SPDI-treatment occurred when high-CO₂-grown cells (5% CO₂, white light, 26°C) were incubated in darkness (air, 24–26°C) for 20–25 h. The results of study were analyzed in the light of literature data relating to the effects of CO₂ concentration, darkness, and light quality on carbohydrates in plant organisms. The analysis led to suggestion that there is connection between the negative effect of the SPDI-treatment on C. reinhardtii and nonstructural carbohydrates presented in the alga: the more carbohydrates contain the alga, the more extensive inactivation of the photosynthetic apparatus occurs in it during its storage in the dense (pelleted) state in darkness on ice.     

Effect of temperature and light on the photosynthesis as measured by chlorophyll fluorescence of cultured Eucheuma denticulatum and Kappaphycus sp. (Sumba strain) from Indonesia

The photosynthetic performance of two Indonesian carrageenophytes (Solieriaceae), Eucheuma denticulatum and Kappaphycus sp. (so-called Sumba strain), was investigated under a variety of temperature and light conditions regarding their mariculture performance. A pulse amplitude modulated-chlorophyll fluorometer (Diving-PAM) was used to generate rapid light curves (RLCs) to provide estimates of the relative electron transport rates (rETR) for over 10 temperatures (i.e., from 16 to 34 °C) and at nine levels of photosynthetic active radiation, which ranged from 0 to 1,000?μmol photons m^?2 s^?1. Underwater irradiance in a cultivation area was also measured at the collection site in South Sulawesi, Indonesia. The initial slope (α), photoinhibition coefficient (β), and the coefficient of maximum photosynthesis assuming no photoinhibition (γ) was calculated by fitting the RLCs to a nonlinear model of the form $ {\text{rETR}} = \gamma \left( {1 - \exp \left( { - \frac{\alpha }{\gamma }{\text{PAR}}} \right)} \right)\left( {\exp \left( { - \frac{\beta }{\gamma }{\text{PAR}}} \right)} \right) $ using a two-level hierarchical Bayesian model. The experiments revealed that E. denticulatum and Kappaphycus sp. required temperatures ranging from 23 to 32 °C and 22 to 33 °C to maintain high rates of photosynthetic activity, respectively. Clearly, both species appear to be well-adapted to the natural light and temperature conditions at the cultivation site, and we expect the results of this study will be useful for the design and sustainable management of similar mariculture activity.     

Temperature-induced changes of malondialdehyde, heat-shock proteins in relation to chlorophyll fluorescence and photosynthesis in Conocarpus lancifolius (Engl.)

The effect of variable temperatures (10–50 °C) on photosynthesis and chlorophyll fluorescence in Conocarpus lancifolius was evaluated. Additionally, the ability of the species to synthesize heat-shock proteins (HSPs) to protect against high temperatures, and malondialdehyde (MDA) as a by-product of lipid peroxidation was investigated. Plants at 10 °C showed virtually no measurable growth, leaf discoloration and a few brown lesions, while high temperatures (40 and 50 °C) promoted growth and lateral branch development. Chlorophyll content index, photochemical efficiency (F _v/F _m) of PS II, electron transport rate and photosynthetic rate declined with decreasing temperature but increased significantly at higher temperatures. Heat-shock protein (HSP 70 kDa) was produced at temperatures 30–50 °C and an additional 90 kDa protein was also produced at 50 °C. Increase in the efficiency of excitation energy captured by the open PS II reaction centers (F _v/F _m) increased linearly (P ≤ 0.05) with the accumulation of HSP 70 at higher temperatures. However, at low temperatures the concentration of MDA increased significantly, indicating lipid peroxidation due to oxidative stress. The production and accumulation of HSP 70 and 90 kDa coupled with increased electron transport rate and photochemical efficiency can be used to assess survival, growth capacity and to some extent the tolerance of C. lancifolius to elevated temperatures.     

Effects of temperature on leaf hydraulic architecture of tobacco plants

Main Conclusion

Modifications in leaf anatomy of tobacco plants induced greater leaf water transport capacity, meeting greater transpirational demands and acclimating to warmer temperatures with a higher vapor pressure deficit. Temperature is one of the most important environmental factors affecting photosynthesis and growth of plants. However, it is not clear how it may alter leaf hydraulic architecture. We grew plants of tobacco (Nicotiana tabacum) ‘k326’ in separate glasshouse rooms set to different day/night temperature conditions: low (LT 24/18 °C), medium (MT 28/22 °C), or high (HT 32/26 °C). After 40 days of such treatment, their leaf anatomies, leaf hydraulics, photosynthetic rates, and instantaneous water-use efficiency (WUE_i) were measured. Compared with those under LT, plants exposed to HT or MT conditions had significantly higher values for minor vein density (MVD), stomatal density (SD), leaf area, leaf hydraulic conductance (K _leaf), and light-saturated photosynthetic rate (A _sat), but lower values for leaf water potential (ψ _l) and WUE_i. However, those parameters did not differ significantly between HT and MT conditions. Correlation analyses demonstrated that SD and K _leaf increased in parallel with MVD. Moreover, greater SD and K _leaf were partially associated with accelerated stomatal conductance. And then stomatal conductance was positively correlated with A _sat. Therefore, under well-watered, fertilized conditions, when relative humidity was optimal, changes in leaf anatomy seemed to facilitate the hydraulic acclimation to higher temperatures, meeting greater transpirational demands and contributing to the maintenance of great photosynthetic rates. Because transpiration rate increased more with temperature than photosynthetic rate, WUE_i reduced under warmer temperatures. Our results indicate that the modifications of leaf hydraulic architecture are important anatomical and physiological strategies for tobacco plants acclimating to warmer temperatures under a higher vapor pressure deficit.     

Freezing cytorrhysis and critical temperature thresholds for photosystem II in the peat moss Sphagnum capillifolium

Leaflets of Sphagnum capillifolium were exposed to temperatures from ?5°C to +60°C under controlled conditions while mounted on a microscope stage. The resultant cytological response to these temperature treatments was successfully monitored using a light and fluorescence microscope. In addition to the observable cytological changes during freezing cytorrhysis and heat exposure on the leaflets, the concomitant critical temperature thresholds for inactivation of photosystem II (PS II) were studied using a micro fibre optic and a chlorophyll fluorometer mounted to the microscope stage. Chlorophyllous cells of S. capillifolium showed extended freezing cytorrhysis immediately after ice nucleation at ?1.1°C in the water in which the leaflets were submersed during the measurement. The occurrence of freezing cytorrhysis, which was visually manifested by cell shrinkage, was highly dynamic and was completed within 2 s. A total reduction of the mean projected diameter of the chloroplast containing area during freezing cytorrhysis from 8.9 to 3.8 μm indicates a cell volume reduction of approximately ?82%. Simultaneous measurement of chlorophyll fluorescence of PS II was possible even through the frozen water in which the leaf samples were submersed. Freezing cytorrhysis was accompanied by a sudden rise of basic chlorophyll fluorescence. The critical freezing temperature threshold of PS II was identical to the ice nucleation temperature (?1.1°C). This is significantly above the temperature threshold at which frost damage to S. capillifolium leaflets occurs (?16.1°C; LT₅₀) which is higher than observed in most higher plants from the European Alps during summer. High temperature thresholds of PS II were 44.5°C which is significantly below the heat tolerance of chlorophyllous cells (49.9°C; LT₅₀). It is demonstrated that light and fluorescence microscopic techniques combined with simultaneous chlorophyll fluorescence measurements may act as a useful tool to study heat, low temperature, and ice-encasement effects on the cellular structure and primary photosynthetic processes of intact leaf tissues.     

Interactive effects of temperature and light intensity on photosynthesis and antioxidant enzyme activity in Zizania latifolia Turcz. plants

We compared the interactive effects of temperature and light intensity on growth, photosynthetic performance, and antioxidant enzyme activity in Zizania latifolia Turcz. plants in this study. Plants were grown under field (average air temperature 9.6–25°C and average light intensity 177–375 W m^?2) or greenhouse (20–32°C and 106–225 W m^?2) conditions from the spring to the early summer. The results indicated that greenhouse-grown plants (GGP) had significantly higher plant height, leaf length, and leaf width, but lower leaf thickness and total shoot mass per cluster compared with field-grown plants (FGP). Tiller emergence was almost completely suppressed in GGP. Significantly higher chlorophyll (Chl) content and lower Chl a/b ratio were observed in GGP than in FGP. From 4 to 8 weeks after treatment (WAT), net photosynthetic rate (P _N) was significantly lower in FGP than in GGP. However, from 9 to 12 WAT, P _N was lower in GGP, accompanied by a decrease in stomatal conductance (g _s) and electron transport rate (ETR) compared with FGP. Suppressed P _N in GGP under high temperature combined with low light was also indicated by photosynthetic photon flux density (PPFD) response curve and its diurnal fluctuation 10 WAT. Meanwhile, ETR in GGP was also lower than in FGP according to the ETR — photosynthetically active radiation (PAR) curve. The results also revealed that GGP had a lower light saturation point (LSP) and a higher light compensation point (LCP). From 4 to 8 WAT, effective quantum yield of PSII photochemistry (Φ_PSII), photochemical quenching (q_P), and ETR were slightly lower in FGP than in GGP. The activities of ascorbate peroxidase (APX), guaiacol peroxidase (POD), glutathione reductase (GR), superoxide dismutase (SOD), and malondialdehyde (MDA) content were significantly higher from 4 to 8 WAT, but lower from 10 to 12 WAT in FGP. However, catalase (CAT) activity was significantly lower in FGP from 4 to 8 WAT. Our results indicated that the growth and photosynthetic performance of Z. latifolia plants were substantially influenced by temperature, as well as light intensity. This is helpful to understand the physiological basis for a protected cultivation of this crop.     

Effects of heat stress on photosynthetic electron transport in a marine cyanobacterium <Emphasis Type="Italic">Arthrospira</Emphasis> sp.

Arthrospira (Spirulina) is widely used as human health food and animal feed. In cultures grown outdoors in open ponds, Arthrospira cells are subjected to various environmental stresses, such as high temperature. A better understanding of the effects of high temperature on photosynthesis may help optimize the productivity of Arthrospira cultures. In this study, the effects of heat stress on photosynthetic rate, chlorophyll a fluorescence transients, and photosystem (PS) II, PSI activities in a marine cyanobacterium Arthrospira sp. were examined. Arthrospira cells grown at 25 °C were treated for 30 min at 25 (control), 30, 34, 37, or 40 °C in the dark. Heat stress (30–37 °C) enhanced net photosynthetic O₂ evolution rate. Heat stress caused over-reduction PSII acceptor side, damage of donor side of PSII, decrease in the energetic connectivity of PSII units, and decrease in the performance of PSII. When the temperature changed from 25 to 37 °C, PSII activity decreased, while PSI activity increased, the enhancement of photosynthetic O₂ evolution was synchronized with the increase in PSI activity. When temperature was further increased to 40 °C, it induced a decrease in photosynthetic O₂ evolution rate and a more severe decrease in PSII activity, but an increase in PSI activity. These results suggest that PSI activity was the decisive factor determining the change of photosynthetic O₂ evolution when Arthrospira was exposed to a temperature from 25 to 37 °C, but then, PSII activity became the decisive factor adjusting the change of photosynthetic O₂ evolution when the temperature was increased to 40 °C.     

A variety of glycolipids in green photosynthetic bacteria

The compositions of glycolipids in the following seven strains of green photosynthetic bacteria were investigated at the molecular level using LC–MS coupled with an evaporative light scattering detector: Chlorobium (Chl.) limicola strains Larsen (30 °C as the optimal cultivation temperature) and DSM245 (30 °C), Chlorobaculum (Cba.) tepidum strain ATCC49652 (45 °C), Cba. parvum strain NCIB8327 (30 °C), Cba. limnaeum strain 1549 (30 °C), Chl. phaeovibrioides DSM269 (30 °C), and Chloroflexus (Cfl.) aurantiacus strain J-10-fl (55 °C). Dependence of the molecular structures of glycolipids including the chain-length of their acyl groups upon bacterial cultivation temperatures was clearly observed. The organisms with their optimal temperatures of 30, 45, and 55 °C dominantly accumulated glycolipids possessing the acyl chains in the range of C₁₅–C₁₆, C₁₆–C₁₇, and C₁₈–C₂₀, respectively. Cba. tepidum with an optimal temperature of 45 °C preferred the insertion of a methylene group to produce finally a C₁₇-cyclopropane chain. Cfl. aurantiacus cultured optimally at 55 °C caused a drastic increase in the chain-length. Notably, the length of such acyl groups corresponded to that of the esterifying chain in the 17-propionate residues of self-aggregative bacteriochlorophylls-c/d/e, indicating stabilization of their supramolecular structures through hydrophobic interactions among those hydrocarbon chains. Based on the detailed compositions of glycolipids, a survival strategy of green photosynthetic bacteria grown in the wide range of temperatures is discussed.     

Influence of short periods of increased water temperature on species composition and photosynthetic activity in the Baltic periphyton communities

Periphyton plays a vital ecological role in shallow, well-lit ecosystems which are vulnerable to rapidly changing environmental conditions, including raising temperature due to global warming. Nevertheless, little is known on the effect of increased temperatures on the taxonomic structure and functioning of periphytic communities. In this study, the influence of short-term temperature increase on the species composition and photosynthetic activity of the Baltic periphytic communities was investigated. The collected communities were exposed to increased temperature of 23 °C (ca. 4 °C above the summer average) for 72 h. After this time, species composition of the communities was studied under light microscope and their photosynthetic performance was evaluated using PAM fluorometry. Results showed that the biomass of cyanobacteria slightly increased. There were significant changes in the abundance of diatom species, among which Fragilaria fasciculata and Navicula ramosissima, were negatively affected by the elevated temperature and their cell number significantly decreased, whereas, Diatoma moniliformis and N. perminuta were stimulated by the increased temperature. Additionally, a shift towards higher abundance of smaller taxa was also observed. The higher quantum yield of photosystem II (PSII) (higher ΦPSII) accompanied by the lower value of non-photochemical quenching (NPQ) observed in communities kept at 23 °C showed more efficient photosynthesis. This was further confirmed by the changes in rapid light curves (higher photosynthetic capacity, rETR_max, and photoacclimation index, E_k). The obtained data constitute evidence that short periods of increased temperature significantly affect the structure and functioning of the Baltic periphyton.     

Effects of solar UV radiation and temperature on morphology and photosynthetic performance of Chaetoceros curvisetus

This study investigated the effect of solar ultraviolet radiation (UVR) and temperature on a chain length and photosynthetic performance of diatom Chaetorceros curvisetus. The cells were cultured in large quartz tubes and exposed to PAR, PAR + UV-A (PA), or PAR + UV-A + UV-B (PAB) radiation at 20°C and 28°C for six days, respectively. After recovery for 1 h, the cells were exposed again to three different radiations for 1 h. Then, a change in the photochemical efficiency (F_PSII) was examined and UVR-induced photoinhibition was calculated. The percentage of long chains (more than five single cells per chain) in C. curvisetus significantly increased from 8.2% (PAR) to 38.9% (PAB) at 20°C; while it was not notably affected at 28°C. Mycosporine-like amino acids (MAAs) concentration obviously increased by irradiance increment from PAR to PAB at 20°C. Chlorophyll (Chl) a concentration significantly declined with increasing irradiance at 20°C. Both MAAs and Chl a concentrations were not obviously changed by irradiance at 28°C. Before and after reexposure, F_PSII was significantly reduced both at 20°C and 28°C. UVR-induced photoinhibition at 20°C (39%) was higher than that at 28°C (30.9%). Solar UV radiation, especially UV-B, could significantly influence the percentage of long chains of C. curvisetus, especially at low temperature. UVR-induced photoinhibition can be alleviated by higher temperatures.     

Do fruit sunburn control measures affect leaf photosynthetic rate and stomatal conductance in ‘Royal Gala’ apple?

A field study was conducted on a 5-year-old orchard of ‘Royal Gala’ apple (Malus domestica Borkh.) in Stellenbosch, South Africa, to investigate whether the measures employed to control sunburn in fruit, viz., evaporative cooling, Surround WP and 20% black shade net affect leaf photosynthetic gas exchange attributes in comparison to untreated control during the 2003/2004 season. Shade net significantly reduced midday leaf net photosynthetic rate (A) compared to evaporative cooling. Furthermore, shade net and Surround WP significantly reduced midday leaf stomatal conductance (g_s) compared to evaporative cooling and control. Evaporative cooling increased light saturated photosynthetic rate by 27 and 24% compared to shade net and Surround WP, respectively. Light compensation point and dark respiration of shaded leaves were about a third of the other treatments and about 50% less than the control leaves, respectively. Shade net down-regulated photosynthetic capacity of the leaves as evidenced by lower maximum rate of carboxylation and light saturated rate of electron transport compared to control leaves. Sunburn control treatments reduced day respiration by 60–70% compared to the control. Response of A and g_s to increasing temperature showed only slight increase in both A and g_s with increasing temperature from 20 to 30 °C. A declined at 35 °C in Surround WP and shade net leaves while it declined at 40 °C in evaporatively cooled and control leaves. Evaporative cooling and control had higher g_s than shade net and Surround WP at all leaf temperatures. In conclusion, shade net down-regulated photosynthetic reactions and Surround WP and shade net reduced leaf g_s and increased the vulnerability of leaf A and g_s to high temperature compared to evaporative cooling and control.     

Time-dependent upregulation of electron transport with concomitant induction of regulated excitation dissipation in <Emphasis Type="Italic">Haslea</Emphasis> diatoms

Photoacclimation by strains of Haslea “blue” diatom species H. ostrearia and H. silbo sp. nov. ined. was investigated with rapid light curves and induction–recovery curves using fast repetition rate fluorescence. Cultures were grown to exponential phase under 50 µmol m^?2 s^?1 photosynthetic available radiation (PAR) and then exposed to non-sequential rapid light curves where, once electron transport rate (ETR) had reached saturation, light intensity was decreased and then further increased prior to returning to near growth light intensity. The non-sequential rapid light curve revealed that ETR was not proportional to the instantaneously applied light intensity, due to rapid photoacclimation. Changes in the effective absorption cross sections for open PSII reaction centres (σ_PSII′) or reaction centre connectivity (ρ) did not account for the observed increases in ETR under extended high light. σ_PSII′ in fact decreased as a function of a time-dependent induction of regulated excitation dissipation Y(NPQ), once cells were at or above a PAR coinciding with saturation of ETR. Instead, the observed increases in ETR under extended high light were explained by an increase in the rate of PSII reopening, i.e. Q_A^? oxidation. This acceleration of electron transport was strictly light dependent and relaxed within seconds after a return to low light or darkness. The time-dependent nature of ETR upregulation and regulated NPQ induction was verified using induction–recovery curves. Our findings show a time-dependent induction of excitation dissipation, in parallel with very rapid photoacclimation of electron transport, which combine to make ETR independent of short-term changes in PAR. This supports a selective advantage for these diatoms when exposed to fluctuating light in their environment.     

Photosynthetic characteristics of female vegetative tissues of Porphyra katadai var. hemiphylla (Bangiales, Rhodophyta) in culture

In this study, chlorophyll fluorescence parameters (?F/F _m′, F _v/F _m) and oxygen evolution of female vegetative tissues of Porphyra katadai var. hemiphylla in unisexual culture (FV) and in mixed culture with male vegetative tissues (FV-M) were followed at 5–20 °C, 10 and 80 μmol photons m^?2 s^?1. The formation of reproductive tissues was closely correlated with decreasing photosynthetic activities. At the same temperature the tissues cultured under 80 μmol photons m^?2 s^?1 showed a greater extent of maturation than those under 10 μmol photons m^?2 s^?1, and their decrease in photosynthesis was also larger. Under the same light intensity the extent of maturation increased with increasing temperature, and both cultures showed higher values of ?F/F _m′ and F _v/F _m at 10 and 15 °C, while their oxygen evolution became negative at 15–20 °C during the later period. Under the same culture condition the maturation of FV-M culture was relatively faster than that of FV culture, while their photosynthetic activity, especially ?F/F _m′, was lower.     

Effects of low nocturnal temperature on photosynthetic characteristics and chloroplast ultrastructure of winter rapeseed

We investigated the effects of low nocturnal temperature on photosynthetic apparatus of winter rapeseed (Brassica campestris L.). An artificial climate chamber was used to simulate the effects of low nocturnal temperature on seedling and stomatal morphology, chloroplast ultrastructure, photosynthetic parameters, and dry matter distribution and accumulation in two winter rapeseed cultivars, Longyou-7 (ultra coldresistant) and Tianyou-2 (weak cold resistance). Compared with those at diurnal/nocturnal temperatures of 20°/10°C (control), rapeseed seedlings at 20°/5°C had increased leaf chlorophyll content, deepened green leaf color, decreased stomatal conductance (G_s), intercellular CO₂ concentration (C_i), and photosynthetic rate (P_n), and improved root/shoot ratio; the majority of stomata remained open in Longyou-7 while those in Tianyou-2 were mostly closed or semi-closed. At diurnal/nocturnal temperatures of 20°/–5°C, rapeseed seedlings had decreased leaf chlorophyll content with increased C_i but decreased G_s and P_n; Tianyou-2 exhibited ruptured chloroplast membrane, dissolved grana, broken stroma lamella, and decreased root/shoot ratio, whereas Longyou-7 had chloroplasts retaining partial structure of grana with a small amount of starch granules in guard cells. Low nocturnal temperature damaged the photosynthetic membrane of chloroplasts and reduced P_n in the leaves of winter rapeseed influencing photosynthetic processes in this crop. The reduction of P_n was mainly related to stomatal limitation at diurnal/nocturnal temperatures of 20°/5°C and non-stomatal limitation at diurnal/nocturnal temperatures of 20°/–5°C.     

Using leaf optical properties to detect ozone effects on foliar biochemistry

Efficient methods for accurate and meaningful high-throughput plant phenotyping are limiting the development and breeding of stress-tolerant crops. A number of emerging techniques, specifically remote sensing methods, have been identified as promising tools for plant phenotyping. These remote sensing methods can be used to accurately and rapidly relate variations in leaf optical properties with important plant characteristics, such as chemistry, morphology, and photosynthetic properties at the leaf and canopy scales. In this study, we explored the potential to utilize optical (λ = 500–2,400 nm) near-surface remote sensing reflectance spectroscopy to evaluate the effects of ozone pollution on photosynthetic capacity of soybean (Glycine max Merr.). The research was conducted at the Soybean Free Air Concentration Enrichment (SoyFACE) facility where we subjected plants to ambient (44 nL L^?1) and elevated ozone (79–82 nL L^?1 target) concentrations throughout the growing season. Exposure to elevated ozone resulted in a significant loss of productivity, with the ozone-treated plants displaying a ~30 % average decrease in seed yield. From leaf reflectance data, it was also clear that elevated ozone decreased leaf nitrogen and chlorophyll content as well as the photochemical reflectance index (PRI), an optical indicator of the epoxidation state of xanthophyll cycle pigments and thus physiological status. We assessed the potential to use leaf reflectance properties and partial least-squares regression (PLSR) modeling as an alternative, rapid approach to standard gas exchange for the estimation of the maximum rates of RuBP carboxylation (V _c,max), an important parameter describing plant photosynthetic capacity. While we did not find a significant impact of ozone fumigation on V _c,max, standardized to a reference temperature of 25 °C, the PLSR approach provided accurate and precise estimates of V _c,max across ambient plots and ozone treatments (r ² = 0.88 and RMSE = 13.4 μmol m^?2 s^?1) based only on the variation in leaf optical properties and despite significant variability in leaf nutritional status. The results of this study illustrate the potential for combining the phenotyping methods used here with high-throughput genotyping methods as a promising approach for elucidating the basis for ozone tolerance in sensitive crops.     

Steady-state and dynamic photosynthetic performance and nitrogen partitioning in the shade-demanding plant Panax notoginseng under different levels of growth irradiance

In this study, we examined steady-state and dynamic photosynthetic performance and leaf nitrogen (N) partitioning in the typical shade-demanding herb Panax notoginseng grown along a light gradient. Gas exchange on a leaf area basis was significantly reduced under low irradiance, with gas exchange on a leaf mass basis reaching a maximum value and then decreasing along the light gradient. Specific leaf area significantly increased with decreasing irradiance levels (P < 0.001), whereas carboxylation efficiency was decreased (P < 0.001). In addition, decreasing growth irradiance levels led to declines in maximum carboxylation rate (V _cmax) and maximum electron transport rate (J _max), although V _cmax/mass and J _max/mass were relatively less affected than V _cmax/area and J _max/area. Slow photosynthetic response to simulated sunflecks was observed under low levels of growth irradiance, with stomatal limitations only detected in leaves grown under low-light conditions. Chlorophyll content increased significantly with decreasing irradiance levels. N content on a leaf mass basis apparently increased, while N content on a leaf area basis markedly decreased. The fraction of leaf N allocated to light-harvesting components increased significantly with decreasing growth irradiance levels, whereas the fraction allocated to carboxylation and bioenergetics was significantly reduced. As an adaptation strategy to growth irradiance, we conclude that adjustments in specific leaf area may be more important than changes in leaf physiology and biochemistry in typical shade-demanding species such as P. notoginseng.     

Prolonging the hydration and active metabolism from light periods into nights substantially enhances lichen growth

This study investigates how hydration during light and dark periods influences growth in two epiphytic old forest lichens, the green algal Lobaria pulmonaria and the cyanobacterial L. scrobiculata. The lichens were cultivated in growth chambers for 14 days (200 μmol m^?1 s^?2; 12 h photoperiod) at four temperature regimes (25/20 °C, 21/16 °C, 13/8 °C, and 6/1 °C; day/night temperatures) and two hydration regimes (12 h day-time hydration; 12 h day-time + 12 h night-time hydration). Growth was highly dynamic, showing that short-term growth experiments in growth cabinets have a high, but largely unexplored potential in functional lichen studies. The highest measured growth rates were not far from the maximal dry matter gain estimated from published net photosynthetic CO₂ uptake data. For the entire data set, photobiont type, temperature, hydration regime and specific thallus mass accounted for 46.6 % of the variation in relative growth rate (RGR). Both species showed substantially higher relative growth rates based on both biomass (RGR) and thallus area (RT_AGR) when they were hydrated day and night compared to hydration in light only. Chronic photoinhibition was substantial in thalli hydrated only during the day time and kept at the highest and lowest temperature regimes, resulting in exponential increases in RGR with increasing maximal PSII efficiency (F _v/F _m) in both species. However, the depression in F _v/F _m was stronger for the cyanolichen than for the cephalolichen at extreme temperatures. The growth-stimulating effect of night-time hydration suggests that nocturnal metabolic activity improves recovery of photoinhibition and/or enhances the conversion rate of photosynthates into thallus extension.