Light-saturated CO2 Assimilation Rate Research Articles

Photosynthetic gas exchange, plant water relations and osmotic adjustment of three tropical perennials during drought stress and re-watering.

Planting vegetation on slopes is an effective way of improving slope stability while enhancing the aesthetic appearance of the landscape. However, plants growing on slopes are susceptible to natural drought stress (DS) conditions which commonly lead to water deficit in plant tissues that affect plant health and growth. This study investigated the photosynthetic gas exchange, plant water status and proline accumulation of three tropical perennials namely Clerodendrum paniculatum, Ipomoea pes-caprae and Melastoma malabathricum after being subjected to DS and re-watering (RW). During DS, there was a significant decrease in light-saturated photosynthetic CO2 assimilation rate (Asat), stomatal conductance (gs sat), and transpiration rate (Tr) for all three plant species. Leaf relative water content, shoot water potential, and leaf, stem and root water content also declined during DS. Proline concentration increased for all three species during DS, reaching especially high levels for C. paniculatum, suggesting that it heavily relies on the accumulation of proline to cope with DS. Most of the parameters recovered almost completely to levels similar to well-watered plants after RW, apart from M. malabathricum. Strong linear correlations were found between Asat and gs sat and between gs sat and Tr. Ultimately, C. paniculatum and I. pes-caprae had better drought tolerance than M. malabathricum.

Leaf and tree water-use efficiencies of Populus deltoides × P. nigra in mixed forest and agroforestry plantations.

In a global context where water will become a scarce resource under temperate latitudes, managing tree plantations with species associations, i.e., forest mixture or agroforestry, could play a major role in optimizing the sustainable use of this resource. Conceptual frameworks in community ecology suggest that, in mixed plantations, environmental resources such as water may be more efficiently used for carbon acquisition and tree growth thanks to niche complementarity among species. To test the hypotheses behind these conceptual frameworks, we estimated water-use efficiency (WUE) for poplar trees grown in a monoculture, in association with alder trees (forest mixture) and in association with clover leys (agroforestry) in an experimental plantation located in northeastern France. Water-use efficiency was estimated (i) at leaf level through gas exchange measurements and analysis of carbon isotope composition, (ii) at wood level through carbon isotope composition and (iii) at tree level with sap flow sensors and growth increment data. We hypothesized that species interactions would increase WUE of poplars in mixtures due to a reduction in competition and/or facilitation effects due to the presence of the N2-fixing species in mixtures. Poplar trees in both mixture types showed higher WUE than those in the monoculture. The differences we found in WUE between the monoculture and the agroforestry treatment were associated to differences in stomatal conductance and light-saturated net CO2 assimilation rate (at the leaf level) and transpiration (at the tree level), while the differences between the monoculture and the forest mixture were more likely due to differences in stomatal conductance at the leaf level and both transpiration and biomass accumulation at the tree level. Moreover, the more WUE was integrated in time (instantaneous gas exchanges < leaf life span < seasonal wood core < whole tree), the more the differences among treatments were marked.

Ozone uptake at night is more damaging to plants than equivalent day-time flux

Main conclusionPlants exposed to equivalent ozone fluxes administered during day-time versus night-time exhibited greater losses in biomass at night and this finding is attributed to night-time depletion of cell wall-localised ascorbate.The present study employed Lactuca sativa and its closest wild relative, L. serriola, to explore the relative sensitivity of plants to ozone-induced oxidative stress during day-time versus night-time. By controlling atmospheric ozone concentration and measuring stomatal conductance, equivalent ozone uptake into leaves was engineered during day and night, and consequences on productivity and net CO2 assimilation rate were determined. Biomass losses attributable to ozone were significantly greater when an equivalent dose of ozone was taken-up by foliage at night compared to the day. Linkages between ozone impacts and ascorbic acid (AA) content, redox status and cellular compartmentation were probed in both species. Leaf AA pools were depleted by exposure of plants to darkness, and then AA levels in the apoplast and symplast were monitored on subsequent transfer of plants to the light. Apoplast AA appeared to be more affected by light–dark transition than the symplast pool. Moreover, equivalent ozone fluxes administered to leaves with contrasting AA levels resulted in contrasting effects on the light-saturated rate of CO2 assimilation (Asat) in both species. Once apoplast AA content recovered to pre-treatment levels, the same ozone flux resulted in no impacts on Asat. The results of the present investigation reveal that plants are significantly more sensitive to equivalent ozone fluxes taken-up at night compared with those during the day and were consistent with diel shifts in apoplast AA content and/or redox status. Furthermore, findings suggest that some thought should be given to weighing regional models of ozone impacts for extraordinary night-time ozone impacts.

Effects of elevated ozone and nitrogen addition on leaf nitrogen metabolism in poplar

AbstractAimsOzone (O3) pollution and nitrogen (N) deposition/fertilization often simultaneously affect plant growth. However, research of their interactive effects on leaf N metabolism is still scarce. We investigated their interactive effects, aiming to better understand plant N metabolism processes and biogeochemical cycles under high O3 pollution and N deposition/fertilization.MethodsPoplar saplings were exposed to two O3 levels (NF, non-filtered ambient air; NF60, NF + 60 ppb O3) and four N treatments (N0, no N added; N50, N0 + 50 kg N ha−1 yr−1; N100, N0 + 100 kg N ha−1 yr−1; N200, N0 + 200 kg N ha−1 yr−1) in open-top chambers for 95 days. The indicators related to leaf N metabolism were analyzed, including the activities of N-metabolizing enzymes and the contents of total N, NO3−-N, NH4+-N, total amino acid (TAA) and total soluble protein (TSP) in the leaves.Important FindingsNF60 stimulated the activities of nitrate reductase (NR) by 47.2% at August relative to NF, and stimulated glutamine synthetase (GS) by 57.3% when averaged across all N treatments and sampling times. In contrast, O3 did not significantly affect TSP and even reduced TAA content in August. Relative to N0, N200 significantly increased light-saturated rate of CO2 assimilation (Asat) by 24%, and increased total N content by 70.3% and 43.3% in August and September, respectively, while it reduced photosynthetic N-use efficiency by 26.1% in August. These results suggest that the increase in Asat and total N content are uncoupled, and that the surplus N is not used to optimize the capacity for carbon assimilation under high N treatment. Simultaneously, high N treatment significantly promoted leaf N metabolism by increasing NO3−-N contents, NH4+-N contents, TAA contents and the activities of NR and GS. There was no significant interaction between O3 and N for all variables.

Quantity of supplementary LED lightings regulates photosynthetic apparatus, improves photosynthetic capacity and enhances productivity of Cos lettuce grown in a tropical greenhouse.

Although cooling their rootzone allows year-round (temperate) vegetable production in Singapore's warm climate, these crops have frequently experienced increasingly unpredictable cloudy and hazy weather. Supplementary lighting with light-emitting diodes (LEDs) could be used to reduce the impacts of low light intensity. This study investigated the responses of temperate Cos lettuce (Lactuca sativa L.) to different quantities (photosynthetic photon flux density, PPFD of 0, 150, 300µmolm-2s-1) of supplementary LED lightings in the tropical greenhouse. Increasing light intensity significantly increased total leaf area, shoot and root fresh weight (FW) and dry weight (DW), total chlorophyll (Chl) and carotenoids (Car) contents, light-saturated photosynthetic CO2 assimilation rate (Asat) and transpiration rate (Tr). There were no significant differences in Fv/Fm ratio, total reduced nitrogen, specific leaf area (SLA) and PSII concentration among the three light treatments. However, there was an increasing trend with increasing light intensity for Chl a/b ratio, net photosynthetic O2 evolution rate (PN), cytochrome b6f (Cyt b6f), leaf total soluble protein and Rubisco concentrations. This study provides the basic understanding of photosynthetic apparatus and capacity of temperate crops grown under different supplementary LED lightings in the tropical greenhouse.

Interactive Effect of Elevated CO2 and Reduced Summer Precipitation on Photosynthesis is Species-Specific: The Case Study with Soil-Planted Norway Spruce and Sessile Oak in a Mountainous Forest Plot

We investigated how reduced summer precipitation modifies photosynthetic responses of two model tree species—coniferous Norway spruce and broadleaved sessile oak—to changes in atmospheric CO2 concentration. Saplings were grown under mountainous conditions for two growing seasons at ambient (400 μmol CO2 mol–1) and elevated (700 μmol CO2 mol–1) CO2 concentration. Half were not exposed to precipitation during the summer (June–August). After two seasons of cultivation under modified conditions, basic photosynthetic characteristics including light-saturated rate of CO2 assimilation (Amax), stomatal conductance (GSmax), and water use efficiency (WUE) were measured under their growth CO2 concentrations together with in vivo carboxylation rate (VC) and electron transport rate (J) derived from CO2-response curves at saturating light. An increase in Amax under elevated CO2 was observed in oak saplings, whereas it remained unchanged or slightly declined in Norway spruce, indicating a down-regulation of photosynthesis. Such acclimation was associated with an acclimation of both J and VC. Both species had increased WUE under elevated CO2 although, in well-watered oaks, WUE remained unchanged. Significant interactive effects of tree species, CO2 concentration, and water availability on gas-exchange parameters (Amax, GSmax, WUE) were observed, while there was no effect on biochemical (VC, J) and chlorophyll fluorescence parameters. The assimilation capacity (Asat; CO2 assimilation rate at saturating light intensity and CO2 concentration) was substantially reduced in spruce under the combined conditions of water deficiency and elevated CO2, but not in oak. In addition, the stimulatory effect of elevated CO2 on Amax persisted in oak, but completely diminished in water-limited spruce saplings. Our results suggest a strong species-specific response of trees to reduced summer precipitation under future conditions of elevated CO2 and a limited compensatory effect of elevated CO2 on CO2 uptake under water-limited conditions in coniferous spruce.

Hydrogen peroxide mediates spermidine-induced autophagy to alleviate salt stress in cucumber

ABSTRACT Autophagy, an evolutionally conserved cellular degradation process, plays critical roles in plant development and stress response. Despite the wealth of information on the vital role of autophagy in responses to environmental stresses, little is known about the regulation of autophagy. In this study, we demonstrated that spermidine (Spd), a kind of polyamine, was involved in the regulation of salt tolerance through activating the expression of ATG (autophagy-related) genes and the formation of autophagosomes in cucumber under salt stress. Furthermore, NADPH oxidase-derived apoplastic H2O2-mediated Spd-induced salt tolerance and autophagy. Exogenous Spd significantly increased the tolerance to salt stress and inhibited the accumulation and ubiquitination of insoluble proteins. Foliar application of Spd promoted the transcript levels of ATG genes and autophagosomes formation. Besides, Spd induced the expression of RBOH (respiratory burst oxidase homolog), and the accumulation of H2O2 both in leaves and roots. However, either pretreatment with dimethylthiourea (DMTU, an H2O2 scavenger) or diphenyleneiodonium chloride (DPI, an inhibitor of NADPH oxidase) reduced Spd-induced accumulation of apoplastic H2O2. Importantly, Spd-induced salt tolerance and autophagy were compromised when plants were pretreated with DMTU or DPI. Furthermore, the silencing of ATG4 and ATG7 reduced Spd-induced salt tolerance and autophagosomes formation. Taken together, these results revealed that RBOH-dependent H2O2 mediated the Spd-induced autophagy and salt tolerance in cucumber. Abbreviations: Asat: light-saturated rate of CO2 assimilation; ATG: autophagy-related; DCF-DA: 2, 7-dichlorofluorescein diacetate; DMTU: dimethylthiourea; DPI: diphenyleneiodonium chloride; DW: dry weight; EL: electrolyte leakage; FW: fresh weight; Fv/Fm: the maximum quantum yield of photosystem II; GFP: green fluorescent protein; MDC: monodansylcadaverine; PDS: phytoene desaturase; PE: phosphatidylethanolamine; PLD: phospholipase D; RBOH: respiratory burst oxidase homolog; ROS: reactive oxygen species; SDS-PAGE: sodium dodecyl sulfate-polyacrylamide gel electrophoresis; SIN1: salt induced NAC1; Spd: spermidine; TOR: target of rapamycin; VIGS: virus-induced gene silencing.

Elevated CO2 and soil mercury stress affect photosynthetic characteristics and mercury accumulation of rice

This is a novel study about responses of leaf photosynthetic traits and plant mercury (Hg) accumulation of rice grown in Hg polluted soils to elevated CO2 (ECO2). The aim of this study was to provide basic information on the acclimation capacity of photosynthesis and Hg accumulation in rice grown in Hg polluted soil under ECO2 at day, night, and full day. For this purpose, we analyzed leaf photosynthetic traits of rice at flowering and grain filling. In addition, chlorophyll content, soluble sugar and Malondialdehyde (MDA) of rice leaves were measured at flowering. Seed yield, ear number, grain number per ear, 1000-grain weight, total mercury (THg) and methylmercury (MeHg) contents were determined after harvest. Our results showed that Hg polluted soil and ECO2 had no significant effect on leaf chlorophyll content and leaf mass per area (LMA) in rice. The contents of soluble sugar and MDA in leaves increased significantly under ECO2. Mercury polluted soil treatment significantly reduced the light saturated CO2 assimilation rate (Asat) of rice leaves only at flowering, but not at grain filling. Night ECO2 greatly improved rice leaf water use efficiency (WUE). ECO2 greatly increased seed yield and ear number. In addition, ECO2 did not affect THg accumulation in rice organs, but ECO2 and Hg treatment had a significant interaction on MeHg in seeds, husks and roots.

Functional traits indicate faster resource acquisition for alien herbs than native shrubs in an urban Mediterranean shrubland

In urban Mediterranean ecosystems, invasive alien plants should have characteristics that enable faster resource acquisition and utilization than native species. We tested this hypothesis by comparing stem and leaf functional traits from five abundant native woody shrubs and five of the most abundant alien species within an urban coastal scrub community in Ensenada, Baja California, Mexico. Twelve anatomical and ecophysiological traits were studied: light-saturated CO2 assimilation rate, stomatal conductance, stem and leaf water potentials during transpiration, % stem allocation to bark, xylem and pith, stem xylem vessel diameter and density, leaf size, and % allocation to spongy and palisade mesophyll. Alien species varied more in these traits, and their trait combinations reflected less investment in long-lived tissues; they had higher CO2 assimilation and stomatal conductance rates, higher values of leaf and stem water potential, and higher allocation to pith, larger stem xylem vessels, and larger leaf areas. In contrast, native woody shrub species were relatively convergent in leaf and stem traits reflecting more conservative resource use. Within disturbed urban environments, alien species may outcompete the native stress-tolerant species through rapid resource acquisition, enhanced by a broader set of functional trait combinations across species.

Plant Growth and Photosynthetic Characteristics of Soybean Seedlings Under Different LED Lighting Quality Conditions

Light-emitting diodes (LEDs) have recently been widely used as light sources for the cultivation of plants in controlled environments. However, before LEDs can be used as the sole light source, it is imperative that soybean (Glycine max) response to light quality be well characterized. The objective of the present work was to investigate how soybean seedlings respond to different blue (B) and red (R) photon flux ratios (PFR) using LEDs. Light treatments were comprised of 170 ± 5 μmol m−2 s−1 photosynthetic photon flux density with B:R ratios of 0B:100R%, 10B:90R%, 25B:75R%, 50B:50R%, 80B:20R%, 90B:10R%, and 100B:0R%. Our results suggest that with the increase in B PFR from 0 to 90%, both plant height and specific leaf area inclined to decline while stem diameter gradually increased. The plants exposed to 80B:20R% showed unexpectedly greater total root length, root surface area, root volume, number of root tip, plant dry mass, leaf dry mass, root dry mass, leaf mass fraction and root mass fraction than plants exposed to all other treatments. Plants grown with 0B:100R% had the highest content of chlorophyll b (Chl b) and content of total chlorophyll [Chl (a + b)] but possessed the lowest ratio of the content of chlorophyll a to the content of chlorophyll b (Chl a/b). Plants grown with R alone also showed the lowest chlorophyll fluorescent parameters. The light-saturated CO2 assimilation rate (Asat) and the light-saturated stomatal conductance (gssat) for 0B:100R% were, respectively, 64.85% and 203.23% lower than those for 80B:20R%. However, plants grown with sole R accumulated higher soluble sugar and starch content during the daytime. Collectively, our results suggest that compared to R- or B-LED alone, an appropriate combination of R and B promotes plant growth and photosynthetic performance for soybean.

The seasonal regulation of gas exchange and water relations of field grown macadamia

Macadamia F. Muell is a recently domesticated nut crop characterized by a certain amount of drought tolerance. Whilst stomatal regulation of gas exchange and water relations has been documented in macadamia, there are no reports on non-stomatal limitations to photosynthesis and if changes occur over a season in relation to different phenological stages. It was hypothesised that in order for macadamias to be adapted to a seasonally dry native environment the trees would be characterised by strong stomatal control over gas exchange, indicating an isohydric water use strategy that is related to a hydraulic limtation within the tree. However, due to high assimilate demand during nut filling, the level of stomatal control would vary between fruiting and non-fruiting phenological stages. Gas exchange and water relations measurements were made for 18 months on irrigated mature macadamia trees (cv. HAES 695, ‘Beaumont’) in a subtropical region of South Africa. Results confirmed that macadamias had relatively low light-saturated net CO2 assimilation rates (Amax) (8.34 ± 1.21 μmol CO2 m−2 s-1). The low Amax values resulted from relatively high stomatal and non-stomatal limitations and decreasing stomatal conductance (gs) in response to increasing leaf-to-air vapour pressure deficit (VPDleaf) above ca. 2 kPa. Strict stomatal control and nearly constant midday leaf water potential (ψleaf = -1.16 ± 0.43 MPa) confirmed the predominantly isohydric nature of the crop, which seemed to be a result of low hydraulic conductance in the stem to leaf pathway. Significant differences in leaf gas exchange capacity were, however, observed between the fruiting and non-fruiting phenological stages. The presence of fruit resulted in significantly higher Amax (10.27 ± 2.23 μmol CO2 m−2 s−1) compared to non-fruiting periods (Amax = 6.58 ± 2.00 μmol CO2 m−2 s−1). Increases in Amax were mediated by increased rates of electron transport (Jmax) and triose phosphate use (TPU). Fruiting stages were also characterized by variable responses of gs to increases in VPDleaf, which indicated varying degrees of isohydricity. This study reaffirms that macadamias are inherently adapted to seasonally dry environments, characterized by strict stomatal control, yet under humid conditions or in the presence of developing fruit, macadamias are proposed to deviate from a purely isohydric water management strategy.

Effects of Combined CO2 and O3 Exposures on Net CO2 Assimilation and Biomass Allocation in Seedlings of the Late-Successional Fagus Crenata

We examined the effects of elevated CO2 and elevated O3 concentrations on net CO2 assimilation and growth of Fagus crenata in a screen-aided free-air concentration-enrichment (FACE) system. Seedlings were exposed to ambient air (control), elevated CO2 (550 µmol mol−1 CO2, +CO2), elevated O3 (double the control, +O3), and the combination of elevated CO2 and O3 (+CO2+O3) for two growing seasons. The responses in light-saturated net CO2 assimilation rates per leaf area (Agrowth-CO2) at each ambient CO2 concentration to the elevated CO2 and/or O3 treatments varied widely with leaf age. In older leaves, Agrowth-CO2 was lower in the presence of +O3 than in untreated controls, but +CO2+O3 treatment had no effect on Agrowth-CO2 compared with the +CO2 treatment. Total plant biomass increased under conditions of elevated CO2 and was largest in the +CO2+O3 treatment. Biomass allocation to roots decreased with elevated CO2 and with elevated O3. Elongation of second-flush shoots also increased in the presence of elevated CO2 and was largest in the +CO2+O3 treatment. Collectively, these results suggest that conditions of elevated CO2 and O3 contribute to enhanced plant growth; reflecting changes in biomass allocation and mitigation of the negative impacts of O3 on net CO2 assimilation.

The ozone sensitivity of five poplar clones is not related to stomatal conductance, constitutive antioxidant levels and morphology of leaves

Ground-level ozone (O3) is an important phytotoxic air pollutant in China. In order to compare the sensitivity of common poplar clones to O3 in China and explore the possible mechanism, five poplar clones, clone DQ (Populus cathayana), clone 84 K (P. alba × P. glandulosa), clone WQ156 (P. deltoids × P. cathayana), clone 546 (P. deltoides cv. ‘55/56’ × P. deltoides cv. ‘Imperial’) and clone 107 (P. euramericana cv. ‘74/76’) were exposed to four O3 treatments. According to the date of the initial visible O3 symptom and the slopes of O3 exposure-response relationships with the relative light-saturated rate of CO2 assimilation, we found that clone DQ and clone 546 were the most sensitive to O3, clone 84 K and clone WQ156 were the less sensitive, and clone 107 was the most tolerant, which could provide a basis to select O3 tolerant clones for poplar planting at areas with serious O3 pollution. Elevated O3 significantly reduced photosynthetic parameters, total phenols content, potential antioxidant capacity, leaf mass per area and biomass of five poplar clones, and there were significant interactions between O3 and clones for most photosynthetic parameters. Elevated O3 also significantly increased malondialdehyde content and total ascorbate content. The responses of total antioxidant capacity for poplar clones to elevated O3 were different, as indicated by the increase for clone 107 and reduction for other clones under elevated O3 treatment. Our results on the sensitivity of different poplar clones to O3 are not related to leaf stomatal conductance, leaf constitutive antioxidant levels or leaf morphology of plant grown in clean air. The possible reason is little difference in leaf traits among clones within close species, suggesting that more properties of plants should be considered for exploring the sensitivity mechanism of close species, such as mesophyll conductance, antioxidant enzyme activity and apoplastic antioxidants.

Effects of elevated ozone and water deficit on poplar saplings: Changes in carbon and nitrogen stocks and their allocation to different organs

Ozone (O3) pollution and drought are frequent in summer, which adversely affects plant growth. In this study, we investigated the interactive effects of O3 and water deficit on growth parameters, and carbon (C) and nitrogen (N) stocks and their allocation to different organs in poplar saplings in the summer of 2017. The saplings were subjected singularly and in combination to two chronic O3 treatments (charcoal-filtered air [CF] and ambient air + 40 ppb of O3 [E-O3]) and two watering regimens (well-watered [WW] and reduced watering [RW]). The results showed that both O3 and drought inhibited poplar growth, whereas the effects of drought were greater than those of O3. Moreover, drought stress protected the saplings from O3 damage, suggesting that some previous studies on well-watered plants might have overestimated the damage caused by O3. Inconsistent with the general trend, RW significantly increased the light-saturated rate of CO2 assimilation (Asat) in this study, whereas it reduced the total leaf area. E-O3 significantly increased the N concentration and reduced the C/N ratio of each organ as well as the total C stock (CStotal), while it did not significantly affect the total N stock (NStotal). Drought significantly increased the C and N concentrations and reduced the C/N ratio in most organs, and significantly reduced the C stock of each organ, CStotal, and NStotal. CStotal was positively correlated with the total leaf area and the photosynthetic rate of the whole plants (Aplant). However, there was no significant relationship between CStotal and Asat, suggesting that Aplant can better reflect the growth of the whole plant than Asat. In addition, NStotal was positively correlated with the root biomass. Both O3 and drought had no significant effect on the ratio between the root and shoot N stocks, and there were also no interactive effects of O3 and drought on most C and N parameters. These findings will facilitate better understandings of C and N metabolisms future environment with higher O3 and more frequent droughts.

Impacts of LED spectral quality on leafy vegetables: Productivity closely linked to photosynthetic performance or associated with leaf traits?

The success of growing vegetables indoors requires the most appropriate selection of lighting spectrum. This mini review discusses the impacts of LED spectral quality on different leafy vegetables with a focus on the studies of Chinese broccoli (Brassica alboglabra), ice plants (Mesembryanthem crystallinum) and lettuce (Lactuca sativa L. cv. Canasta). For each species, plants exposed to different spectral LED lights were all under the same light intensity and same photoperiod. Chinese broccoli grown under red(R):blue(B)-LED ratio of 84:16 (16B) had the highest light-saturated photosynthetic CO2 assimilation rate (Asat) and stomatal conductance (gs sat) compared to plants grown under other R:B-LED ratios. It was also shown that 16B is the most appropriate selection for Chinese broccoli to achieve the highest shoot productivity with a rapid leaf number and leaf area development. The highest concentrations of photosynthetic pigments, soluble and Rubisco protein on a leaf area basis were also observed in 16B plants. The results conclusively affirmed that the highest productivity of Chinese broccoli grown under 16B is closely linked to the highest photosynthetic performance on a leaf area basis. For ice plants grown under R:B-LED ratios of 90:10 (10B), they had the highest shoot biomass with a faster leaf development compared to plants grown under other RB-LED combinations. However, there were no differences in Asat, gs sat, photosynthetic pigments, soluble and Rubisco proteins on a leaf area basis. In the case of lettuce plants, it was a surprise to observe that plants grown under 0B and 20G (20% green (G)-LED and 80% R-LED) had the highest shoot biomass, and largest total leaf area and light interception area but the lowest net maximal photosynthetic rate on a leaf area basis, compared to other plants. The combined RB-LED enhanced other photosynthetic parameters while 0B and 20G conditions had inhibitory effects on maximum quantum efficiency of PS II with lower photosynthetic pigments, total soluble protein and Rubisco protein. These results suggest that impacts of LED light quality on productivity of lettuce (L. sativa L. cv. Canasta) is closely linked to leaf traits not associated with photosynthetic performance on a leaf area basis. Keywords: leafy vegetable, leaf traits, LED spectral quality, photosynthetic performance, productivity DOI: 10.25165/j.ijabe.20191206.5178 Citation: He J, Qin L, Chow W S. Impacts of LED spectral quality on leafy vegetables: Productivity closely linked to photosynthetic performance or associated with leaf traits? Int J Agric & Biol Eng, 2019; 12(6): 16–25.

Photosynthetic acclimation of Grammatophyllum speciosum to growth irradiance under natural conditions in Singapore

Grammatophyllum speciosum, a native species to Singapore, have become extinct mainly due to habitat loss. Recently, Singapore has reintroduced G. speciosum into the natural environment under the orchid conservation programme. In this study, leaves of G. speciosum grown under low light (LL) under natural conditions had faster expansion rate and higher specific leaf area than leaves grown under intermediate light (IL) and high light (HL). All leaves had more than 95% midday relative water content. Although midday Fv/Fm ratios were lower in HL leaves than in IL and LL leaves, none of them exhibited chronic photoinhibition. HL leaves had upregulated their light utilization through higher photochemical quantum yield (ΔF/Fm′) and greater electron transport rate. HL leaves also had higher non-photochemical quenching, indicating that they had higher capability to dissipate excess light as heat, which was supported by their lower chlorophyll but higher carotenoids content. Although there was a linear correction between leaf temperature and photosynthetic photon flux density (PPFD), no correlations were found between stomatal conductance (gs) and PPFD, gs and leaf temperature. Light-saturated photosynthetic CO2 assimilation rate (Asat) was significantly higher in HL leaves than those of IL and LL leaves. However, all leaves had similar light-saturated stomatal conductance. Although LL leaves had higher leaf total reduced nitrogen that those of IL and HL leaves, none of them seemed to suffer from nitrogen deficiency during the experimental period. To conclude, G. speciosum is able to survive under different growth irradiances without watering and adding fertilizers.

Growth and photosynthesis characteristics of invading larch saplings in an occasionally flooded dry stream bed in cool-temperate Japan

ABSTRACTSemi-naturally occurring Japanese larch (Larix kaempferi) saplings originating from planted trees are often observed in riparian habitats around Nikko National Park, Japan, and they may be affecting the structure and functioning of endemic riparian vegetation. To evaluate possible larch establishment in riparian habitats, we examined the aboveground structure and leaf (needle)-level ecophysiological characteristics of larch saplings (~3 m in height and ~8 years old) grown in a dry-wet stream bed (site DW, well-drained but floods after heavy rains) and in neighboring non-riparian vegetation (site M, stably mesic) for comparison. DW saplings were generally shorter than M saplings for their ages, and the total length of current-year stems in DW saplings was about a half that of M saplings. DW saplings produced a larger number of short shoots (shoots without apparent stem elongation), resulting in a similar amount of leaves to those of the M saplings. Leaves of DW saplings had lower nitrogen contents (N) and photosynthetic capacity (light-saturated net CO2 assimilation rate and electron transport rate measured on excised branches) than those of M saplings. These results suggest that larch saplings were subject to some environmental stressor, but were able to grow slowly yet steadily in riparian environments for a relatively long period, implying considerable potential for colonization of riparian ecosystems. Further studies on the structure, functioning and management of cool-temperate Japanese ecosystems should consider the unintended expansion of introduced larch into surrounding habitats.

Water stress mitigates the negative effects of ozone on photosynthesis and biomass in poplar plants

Tropospheric ozone (O3) pollution frequently overlaps with drought episodes but the combined effects are not yet understood. We investigated the physiological and biomass responses of an O3 sensitive hybrid poplar clone (‘546’) under three O3 levels (charcoal-filtered ambient air, non-filtered ambient air (NF), and NF plus 40 ppb) and two watering regimes (well-watered (WW) and reduced watering (RW), i.e. 40% irrigation) for one growing season. Water stress increased chlorophyll and carotenoid contents, protecting leaves from pigment degradation by O3. Impairment of photosynthesis by O3 was also reduced by stomatal closure due to water stress, which preserved light-saturated CO2 assimilation rate, and the maximum carboxylation efficiency. Water stress increased water use efficiency of the leaves while O3 decreased it, showing significant interactions. Effects were more evident in older leaves than in younger leaves. Water stress reduced biomass production, but the negative effects of O3 were less in RW than in WW for total biomass per plant. A stomatal O3 flux-based dose-response relationship was parameterized considering water stress effects, which explained biomass losses much better than a concentration-based approach. The O3 critical level of Phytotoxic Ozone Dose over a threshold of 7 nmol O3.m−2.s−1 (POD7) for a 4% biomass loss in this poplar clone under different water regimes was 4.1 mmol m−2. Our results suggest that current O3 levels in most parts of China threaten poplar growth and that interaction with water availability is a key factor for O3 risk assessment.

Plant Growth and Photosynthetic Characteristics of Mesembryanthemum crystallinum Grown Aeroponically under Different Blue- and Red-LEDs.

Mesembryanthemum crystallinum is a succulent, facultative crassulacean acid metabolism (CAM) plant. Plant growth and photosynthetic characteristics were studied when M. crystallinum plants were grown indoor under light emitting diodes (LED)-lighting with adequate water supply. Plants were cultured aeroponically for a 16-h photoperiod at an equal photosynthetic photon flux density of 350 μmol m-2 s-1 under different red:blue LED ratios: (1) 100:0 (0B); (2) 90:10 (10B); (3) 80:20 (20B); (4) 70:30 (30B); (5) 50:50 (50B); and (6)100:0 (100B). M. crystallinum grown under 10B condition had the highest shoot and root biomass and shoot/root ratio while those grown under 0B condition exhibited the lowest values. Compared to plants grown under 0B condition, all other plants had similar but higher total chlorophyll (Chl) and carotenoids (Car) contents and higher Chl a/b ratios. However, there were no significant differences in Chl/Car ratio among all plants grown under different red- and blue-LEDs. Photosynthetic light use efficiency measured by photochemical quenching, non-photochemical quenching, and electron transport rate, demonstrated that plants grown under high blue-LED utilized more light energy and had more effective heat dissipation mechanism compared to plants grown under 0B or lower blue-LED. Statistically, there were no differences in photosynthetic O2 evolution rate, light-saturated CO2 assimilation rate (Asat), and light-saturated stomatal conductance (gssat) among plants grown under different combined red- and blue-LEDs but they were significantly higher than those of 0B plants. No statistically differences in total reduced nitrogen content were found among all plants. For the total soluble protein, all plants grown under different combined red- and blue-LEDs had similar values but they were significantly higher than that of plants grown under 0B condition. However, plants grown under higher blue-LEDs had significant higher ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) protein than those plants grown under lower blue-LED. High Asat and gssat but very low CAM acidity of all M. crystallinum plants during light period, imply that this facultative CAM plant performed C3 photosynthesis when supplied with adequate water. Results of this study suggest that compared to red- or blue-LED alone, appropriate combination of red- and blue-LED lighting enhanced plant growth and photosynthetic capacities of M. crystallinum.

Distinct growth and physiological responses of Arabidopsis thaliana natural accessions to drought stress and their detection using spectral reflectance and thermal imaging.

Reduced growth and stomatal closure are the two main responses of plants to drought stress. The extent to which these processes are connected and whether different genotypes prefer one over the other remains unclear. To understand the genotype-specific interconnections of these two processes and evaluate potential utilisation of this knowledge for drought tolerance phenotyping, six natural accessions of Arabidopsis thaliana (L.) Heynh. were exposed to drought stress for 10 days. Projected leaf area of rosette, light-saturated CO2 assimilation rate (Amax), relative water content (RWC), leaf temperature (thermal imaging), and spectral reflectance were measured through the course of induced drought stress. Three types of acclimation were identified: (i) growth not affected but Amax significantly reduced, (ii) both growth and Amax significantly reduced, and (iii) growth significantly reduced but only small decrease in Amax. Within the last type, the smallest decline in RWC was evident. These results show that a substantial reduction in leaf area may cause a decline in transpiration that enables maintenance of both RWC and physiological processes. Both non-invasive thermal imaging and spectral reflectance measurements proved reliable tools for tracking drought-induced changes in Amax and RWC across all accessions tested and thus are effective tools for phenotyping stress tolerance.