Leaf CO2 Assimilation Research Articles

Magnesium deficiency affects secondary lignification of the vascular system in Citrus sinensis seedlings

Mg deficiency affected only the differentiation zone in Citrus sinensis roots. Phloem impairment in Mg-deficient leaves resulted in cell wall lignifications of both vascular cambium and spongy parenchyma cells. Decrease of Mg content in the roots hampered secondary lignification of endodermal and protoxylem cell walls. Magnesium (Mg) is a vital nutrient for plant photosynthesis and biochemical reactions. Mg deficiency is a widespread problem in citrus production, directly affecting yield and quality. With the objective of investigating the relationship among root and leaf anatomy, photosynthetic efficiency and mineral nutrient uptake and transport in Mg-deficient citrus, the effects of Mg deficiency in ‘Xuegan’ (Citrus sinensis) seedlings on plant biomass, leaf gas exchange and chlorophyll, nutrient contents, root and leaf anatomy and lignin biosynthesis were studied. Mg deficiency decreased root and shoot dry weight, leaf CO2 assimilation, chlorophyll and Mg, phosphorus (P), boron (B), cupper (Cu) and iron (Fe) concentration in roots, stems and leaves, and increased potassium (K) and calcium (Ca) concentration. Mg deficiency in the leaves resulted in phloem disorder that blocked phloem transport, leading to carbohydrate accumulation in chloroplasts and secondary lignifications of both vascular cambium and spongy parenchyma cell walls. In contrast, decrease of Mg content in the roots hampered lignin deposition on the protoxylem and endodermal cell walls at the root differentiation zone. Poorly lignified Casparian strip and xylem cell walls in Mg-deficient root tips might decrease xylem loading efficiency of nutrients, lowering their contents in the above ground parts, which ultimately inhibited whole plant growth.

Physiological Effect of Kinetin on the Photosynthetic Apparatus and Antioxidant Enzymes Activities During Production of Anthurium

The results observed in the literature raise the hypothesis according to which cytokinin plays important roles in photosynthetic metabolisms and antioxidant enzymes. Thus, the study aimed to evaluate the effect of foliar application of the isolated cytokinin kinetin at the production cycle, seeking to analyze its effects on enzyme activity and photosynthetic parameters. The plants treated with CK presented reduction of leaf CO2 assimilation rate (Pn) and stomatal conductance (Gs), while that transpiration rate (Tr) was unaffected. The internal CO2 concentrations decreased with the increase in cytokinin levels, but were maintained under CK 50 mg·L−1. The plants treated with CK 75 mg·L−1 was verified higher carboxylation efficiency (Pn/Ci), which was associated to values of CO2 assimilation and transpiration unaltered. Apparent electron transport rate showed variations in the concentration of 25 mg·L−1. Considering the study of enzyme activity, on the other hand, it cannot be stated that kinetin has an effective action in delaying oxidative damage. It presents mixed results, since an efficiency in the application of cytokinin was not observed, presenting induction levels of ascorbate peroxidase activity. Thus, further research is needed to determine more precisely the effects of kinetin on gas exchange and antioxidant enzymes in anthurium plants.

Rhizobacteria improve sugarcane growth and photosynthesis under well‐watered conditions

AbstractMorpho‐physiological changes caused by particular plant growth‐promoting rhizobacteria were evaluated in sugarcane plants under varying water availability. Under well‐watered conditions, we have found one rhizobacteria isolate (IAC‐RBcr5) able to enhance root dry matter and photosynthesis of sugarcane plants. The IAC‐RBcr5 genome was sequenced and high similarity was found with Pseudomonas putida GB‐1. Based on increased root system size of inoculated plants, we hypothesised that sugarcane plants inoculated with IAC‐RBcr5 would have improved performance under water deficit. Although IAC‐RBcr5 had improved plant leaf CO2 assimilation under water shortage, inoculation caused reduction of biomass accumulation in sugarcane. The negative influence of water deficit on shoot growth rate and root traits such as volume, area, diameter, length and specific root area was higher in plants treated with IAC‐RBcr5 as compared to non‐inoculated ones. However, rhizobacteria‐induced improvements in leaf and root proline contents would represent a strategy for storing carbon and nitrogen during low water availability and helping both organisms to resume their metabolism after rehydration. In conclusion, we found and identified a rhizobacterium able to improve growth and photosynthesis of sugarcane plants. Such benefit for plant growth was lost under low water availability as a likely consequence of increased carbon‐energy demand by rhizobacteria and their sensitivity to drought.

Dynamic modelling of limitations on improving leaf CO2 assimilation under fluctuating irradiance.

A dynamic model of leaf CO2 assimilation was developed as an extension of the canonical steady-state model, by adding the effects of energy-dependent non-photochemical quenching (qE), chloroplast movement, photoinhibition, regulation of enzyme activity in the Calvin cycle, metabolite concentrations, and dynamic CO2 diffusion. The model was calibrated and tested successfully using published measurements of gas exchange and chlorophyll fluorescence on Arabidopsis thaliana ecotype Col-0 and several photosynthetic mutants and transformants affecting the regulation of Rubisco activity (rca-2 and rwt43), non-photochemical quenching (npq4-1 and npq1-2), and sucrose synthesis (spsa1). The potential improvements on CO2 assimilation under fluctuating irradiance that can be achieved by removing the kinetic limitations on the regulation of enzyme activities, electron transport, and stomatal conductance were calculated in silico for different scenarios. The model predicted that the rates of activation of enzymes in the Calvin cycle and stomatal opening were the most limiting (up to 17% improvement) and that effects varied with the frequency of fluctuations. On the other hand, relaxation of qE and chloroplast movement had a strong effect on average low-irradiance CO2 assimilation (up to 10% improvement). Strong synergies among processes were found, such that removing all kinetic limitations simultaneously resulted in improvements of up to 32%.

Comparison of Growth, Development, and Photosynthesis of Petunia Grown Under White or Red-blue LED lights

Red-blue (RB) LED lighting systems are widely used for plant cultivation because red and blue light are effectively absorbed by photosynthetic pigments. However, numerous studies have shown that RB LED light is either comparable to or less effective than white light in supporting plant growth. In this study, we compared the effects of RB or white LED light on growth, development, and photosynthesis of petunia (Petunia × hybrida Vilm.) plants. The plants in both treatments had similar shoot fresh weights, but the plants grown under white LED light had significantly higher shoot dry weight than those grown under RB LED light. Petunia plants grown continuously under RB light exhibited the higher single-leaf CO2 assimilation rate at 9 weeks after sowing but the lower maximum quantum efficiency and operating efficiency of PSII than those grown under white LED light. In another experiment, petunia plants were grown continuously under white LED light before measurements of single leaf CO2 assimilation rate and canopy apparent photosynthesis (CAP) under RB or white LED light. It was found that single leaf CO2 assimilation rates under RB light were higher than those under white light. However, CAP measured under RB light was either similar to or lower than that in plants measured under white LED light. White LED light had higher percentages of light transmission through the leaves than RB LED light. Results from this study suggest that white LED light could be more effective at supporting petunia plant growth than RB LED light because of its greater ability to transmit through leaves and drive photosynthesis at the canopy level.

Effects of bamboo biochar on soybean root nodulation in multi-elements contaminated soils

Improvements in plant physiological performance by means of biochar application in soils contaminated by multi-elements are determinants of agroecosystem functioning. This study analyzed the effects of bamboo-derived biochar on root nodulation and plant growth in a moderately acidic Andosol (pH = 5.56) contaminated with multi-elements during a 70-day investigation of soybean growth.Bamboo biochar that had been pyrolyzed at a temperature below 500°C was applied to soils at three different and moderately high rates (5%, 10%, and 15%, w/w). Biochar amendment beyond 5% stimulated root nodulation as well as soybean growth. The nodule weight per root system was significantly enhanced by 186% and 243% over the control at the 10% and 15% addition rates, respectively. The primary explanation for these stimulatory effects was attributed to an increase in the K and Mo supplies for plant uptake that was induced by the biochar application, whereas the increased availability of P contributed to a lesser extent. Leaf CO2 assimilation rate was slightly enhanced at the highest application rate, but this enhancement was not associated with an increase in biomass. The incorporation of biochar into the soil reduced extractable-NH4NO3 Cd, Cu, Mn, Ni, and Zn, but not Pb, regardless of the application dose. This change was accompanied by a significant (P < 0.05) suppression of the uptake od trace elements in soybean shoots at the optimum application rate (10%); the degree of reduction followed this order: Pb>Mn>Cd>Zn>Cu>Ni. The increase in soil pH and the diffusion/adsorption of trace elements onto the biochar may have contributed to the lowering of the concentration of trace elements in the soil as well as in soybean shoots.

Phenylalanine as a nitrogen source induces root growth and nitrogen-use efficiency in Populus × canescens.

To investigate the physiological responses of poplars to amino acids as sole nitrogen (N) sources, Populus × canescens (Ait.) Smith plants were supplied with one of three nitrogen fertilizers (NH4NO3, phenylalanine (Phe) or the mixture of NH4NO3 and Phe) in sand culture. A larger root system, and decreased leaf size and CO2 assimilation rate was observed in Phe- versus NH4NO3-treated poplars. Consistently, a greater root biomass and a decreased shoot growth were detected in Phe-supplied poplars. Decreased enzymatic activities of nitrate reductase (NR), glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH) and elevated activities of nitrite reductase (NiR), phenylalanine ammonia lyase (PAL), glutamine synthetase (GS) and asparagine synthase (AS) were found in Phe-treated roots. Accordingly, reduced concentrations of NH4+, NO3- and total N, and enhanced N-use efficiencies (NUEs) were detected in Phe-supplied poplars. Moreover, the transcript levels of putative Phe transporters ANT1 and ANT3 were upregulated, and the mRNA levels of NR, glutamine synthetase 2 (GS2), NADH-dependent glutamate synthase (NADH-GOGAT), GDH and asparagine synthetase 2 (ASN2) were downexpressed in Phe-treated roots and/or leaves. The 15N-labeled Phe was mainly allocated in the roots and only a small amount of 15N-Phe was translocated to poplar aerial parts. These results indicate that poplar roots can acquire Phe as an N source to support plant growth and that Phe-induced NUEs in the poplars are probably associated with NH4+ re-utilization after Phe deamination and the carbon bonus simultaneously obtained during Phe uptake.

Slow induction of photosynthesis on shade to sun transitions in wheat may cost at least 21% of productivity.

Wheat is the second most important direct source of food calories in the world. After considerable improvement during the Green Revolution, increase in genetic yield potential appears to have stalled. Improvement of photosynthetic efficiency now appears a major opportunity in addressing the sustainable yield increases needed to meet future food demand. Effort, however, has focused on increasing efficiency under steady-state conditions. In the field, the light environment at the level of individual leaves is constantly changing. The speed of adjustment of photosynthetic efficiency can have a profound effect on crop carbon gain and yield. Flag leaves of wheat are the major photosynthetic organs supplying the grain of wheat, and will be intermittently shaded throughout a typical day. Here, the speed of adjustment to a shade to sun transition in these leaves was analysed. On transfer to sun conditions, the leaf required about 15 min to regain maximum photosynthetic efficiency. In vivo analysis based on the responses of leaf CO2 assimilation (A) to intercellular CO2 concentration (ci) implied that the major limitation throughout this induction was activation of the primary carboxylase of C3 photosynthesis, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). This was followed in importance by stomata, which accounted for about 20% of the limitation. Except during the first few seconds, photosynthetic electron transport and regeneration of the CO2 acceptor molecule, ribulose-1,5-bisphosphate (RubP), did not affect the speed of induction. The measured kinetics of Rubisco activation in the sun and de-activation in the shade were predicted from the measurements. These were combined with a canopy ray tracing model that predicted intermittent shading of flag leaves over the course of a June day. This indicated that the slow adjustment in shade to sun transitions could cost 21% of potential assimilation.This article is part of the themed issue ‘Enhancing photosynthesis in crop plants: targets for improvement’.

Photosynthetic and Antioxidant Responses of Jatropha curcas Plants to Heat Stress: On the Relative Sensitivity of Shoots and Roots

This study aimed to evaluate the photosynthetic and antioxidant responses of Jatropha curcas plants exposed to heat stress applied simultaneously in both shoots and roots and solely in each tissue. Thirty-day-old plants were transferred to two growth chambers, where they were subjected to 27 or 42 °C. In each growth chamber, the root system was isolated and then subjected to 27 or 42 °C. Then, four groups of plants were formed according to shoot/root temperature: 27/27; 27/42; 42/27; and 42/42 °C. Plants under 27/27 °C were considered as control and plants were exposed to each thermal treatment for 12 h. The leaf CO2 assimilation rate, stomatal conductance, and mesophyll conductance were decreased by high temperature, regardless of plant tissue. However, primary photochemistry given by effective quantum yield of PSII, electron transport rate, photochemical and non-photochemical quenchings, and maximum electron transport rate driving RuBP regeneration revealed that high shoot temperature was more deleterious for photosynthesis than high root temperature. J. curcas presented efficient mechanisms to avoid photo-damage and photo-inhibition through increases in non-photochemical quenching, maintenance of the maximum quantum yield of PSII, and increases in total superoxide dismutase, ascorbate peroxidase, and catalase activities when shoots were exposed to high temperature, regardless of root temperature. In general, our data suggest that J. curcas present an efficient antioxidant protection system in photosynthetic tissues and acclimation of PSII avoiding photo-damage and photo-inhibition under heat stress. On the other hand, high root temperature inhibits antioxidant metabolism, leading to higher oxidative damage in root tissues and affecting primary photochemistry in leaf tissues.

Effects of Low pH on Photosynthesis, Related Physiological Parameters, and Nutrient Profiles of Citrus.

Seedlings of “Xuegan” (Citrus sinensis) and “Sour pummelo” (Citrus grandis) were irrigated daily with a nutrient solution at a pH of 2.5, 3, 4, 5, or 6 for 9 months. Thereafter, the following responses were investigated: seedling growth; root, stem, and leaf concentrations of nutrient elements; leaf gas exchange, pigment concentration, ribulose-1,5-bisphosphate carboxylase/oxygenase activity and chlorophyll a fluorescence; relative water content, total soluble protein level, H2O2 production and electrolyte leakage in roots and leaves. This was done (a) to determine how low pH affects photosynthesis, related physiological parameters, and mineral nutrient profiles; and (b) to understand the mechanisms by which low pH may cause a decrease in leaf CO2 assimilation. The pH 2.5 greatly inhibited seedling growth, and many physiological parameters were altered only at pH 2.5; pH 3 slightly inhibited seedling growth; pH 4 had almost no influence on seedling growth; and seedling growth and many physiological parameters reached their maximum at pH 5. No seedlings died at any given pH. These results demonstrate that citrus survival is insensitive to low pH. H+-toxicity may directly damage citrus roots, thus affecting the uptake of mineral nutrients and water. H+-toxicity and a decreased uptake of nutrients (i.e., nitrogen, phosphorus, potassium, calcium, and magnesium) and water were likely responsible for the low pH-induced inhibition of growth. Leaf CO2 assimilation was inhibited only at pH 2.5. The combinations of an impaired photosynthetic electron transport chain, increased production of reactive oxygen species, and decreased uptake of nutrients and water might account for the pH 2.5-induced decrease in CO2 assimilation. Mottled bleached leaves only occurred in the pH 2.5-treated C. grandis seedlings. Furthermore, the pH 2.5-induced alterations of leaf CO2 assimilation, water-use efficiency, chlorophylls, polyphasic chlorophyll a fluorescence (OJIP) transients and many fluorescence parameters, root and leaf total soluble proteins, H2O2 production, and electrolyte leakage were all slightly greater in C. grandis than in C. sinensis seedlings. Hence, C. sinensis was slightly more tolerant to low pH than C. grandis. In conclusion, our findings provide novel insight into the causes of low pH-induced inhibition of seedling growth and leaf CO2 assimilation.

Agronomical, physiological and fruit quality responses of two Italian long-storage tomato landraces under rain-fed and full irrigation conditions

Drought is the major environmental stress that adversely affects crop productivity in the Mediterranean region. Adopting water saving strategies, such as deficit irrigation or even no irrigation (rain-fed) and using drought-tolerant genotypes and/or landraces may represent effective tools to save water without substantial reduction of yield. An experiment was conducted in two consecutive growing seasons (2013 and 2014), to assess soil water content and matric potential of soil, physiological parameters, growth, yield and fruit quality of two Italian long-storage tomato landraces: “Locale di Salina 6” (LS; 2013 and 2014) and “Piennolo del Vesuvio” (PV; 2014) under rain-fed (RF) and full irrigation (FI) conditions. Leaf water potential, CO2 assimilation, stomatal conductance, photosynthetic efficiency and growth were moderately impaired under rain-fed conditions, while intrinsic water use efficiency slightly increased. The marketable yield of LS in both growing seasons, and PV in 2014 under RF conditions was slightly reduced (by 6%) as compared with the FI treatment, indicating a drought tolerance of both landraces. In the 2014 experiment, the marketable yield was significantly higher by 55% in PV than in LS landrace. When averaged over landraces, the fruit quality traits in particular fruit dry matter, total soluble solids and total ascorbic acid contents increased by 21, 33 and 55%, respectively under RF compared to FI. The results can play an important role in selecting tolerant genotypes for use under limited water supply in order to save water and improve fruit quality without affecting the crop productivity.

The influence of elevated CO2 on the photosynthesis, carbohydrate status, and plastochron of young peach (Prunuspersica) trees

The plastochron, defined as the time interval between the initiations of two successive leaves, can also indicate the development of successive phytomers along a shoot. Previous work has shown that crop load impacts the plastochron in field-grown peach (Prunuspersica) trees, which led us to hypothesize that the plastochron of peach trees may be sensitive to the carbon status of the tree. To testthis hypothesis, a 38-day growth chamber study was conducted to determine if elevated CO2 (800 μmol·mol-1) speeds up the plastochron of young peach trees relative to their growth in ambient (400 μmol·mol-1) CO2. The leaf lamina lengths were measured every other day to generate leaf growth rate curves that were fitted against a classic Gompertz growth curve to estimate the time of the initiation of each leaf, which in turn, was used to estimate the plastochron. Additionally, in order to non-destructively gauge the effects of CO2 concentration on plant performance during the experiment, net leaf CO2 assimilation and stomatal conductance measurements were taken approximately half way through and at the end of the 38-day experiment. Doubling the ambient CO2 concentration had no effect on the plastochron, even though the leaf CO2 assimilation rates, leaf starch and total nonstructural carbohydrate concentrations were greater in trees grown in elevated CO2. In addition, there were no significant treatment differences in incremental shoot growth or the number of lateral syleptic shoots.

Altitudinal changes in leaf hydraulic conductance across fiveRhododendronspecies in eastern Nepal

This study investigated altitudinal changes in leaf-lamina hydraulic conductance (KL) and leaf morphological traits related to KL using five Rhododendron species growing at different altitudes (2500-4500 m above sea level) in Jaljale Himal region in eastern Nepal. Sun leaves were collected from the highest and the lowest altitude populations of each species, and KL was measured with a high pressure flow meter method. Leaf-lamina hydraulic conductance ranged from 7.7 to 19.3 mmol m-2 s-1 MPa-1 and was significantly positively correlated with altitude. The systematic increase with altitude was also found in KL, leaf nitrogen content and stomatal pore index. These relationships suggest that plants from higher-altitude habitats had a large CO2 supply to the intercellular space in a leaf and high CO2 assimilation capacity, which enables efficient photosynthesis at high altitude. The variation in KL was associated with the variation in several leaf morphological traits. High KL was found in leaves with small leaf area and round shape, both of which result in shorter major veins. These results suggest that the short major veins were important for efficient water transport in unlobed leaves of Rhododendron species. The extent of lignification in bundle sheaths and bundle sheath extension was associated with KL Lignified compound primary walls inhibit water conduction along apoplastic routes. All species analyzed had heterobaric leaves, in which bundle sheath extensions developed from minor veins, but strongly lignified compound primary walls were found in Rhododendron species with low KL It is still unclear why cell walls in bundle sheath at minor veins were markedly lignified in Rhododendron species growing at lower altitude. The lignified cell wall provides a high pathogenic resistance to infection and increases the mechanical strength of cell wall. The data imply that lignified bundle sheath may provide a trade-off between leaf hydraulic efficiency and leaf mechanical toughness or longevity.

Reduced crown root number improves water acquisition under water deficit stress in maize (Zea mays L.).

In this study we test the hypothesis that maize genotypes with reduced crown root number (CN) will have greater root depth and improved water acquisition from drying soil. Maize recombinant inbred lines with contrasting CN were evaluated under water stress in greenhouse mesocosms and field rainout shelters. CN varied from 25 to 62 among genotypes. Under water stress in the mesocosms, genotypes with low CN had 31% fewer crown roots, 30% deeper rooting, 56% greater stomatal conductance, 45% greater leaf CO2 assimilation, 61% net canopy CO2 assimilation, and 55% greater shoot biomass than genotypes with high CN at 35 days after planting. Under water stress in the field, genotypes with low CN had 21% fewer crown roots, 41% deeper rooting, 48% lighter stem water oxygen isotope enrichment (δ(18)O) signature signifying deeper water capture, 13% greater leaf relative water content, 33% greater shoot biomass at anthesis, and 57% greater yield than genotypes with high CN. These results support the hypothesis that low CN improves drought tolerance by increasing rooting depth and water acquisition from the subsoil.

Seleção de genótipos de tomateiro submetidos ao estresse hídrico em função da expressão de características fisiológicas

The objective of this study was to evaluate the physiological behavior of different tomato genotypes to assist in the selection of plants with tolerance to drought stress. It was used three blocks random and ten treatments: eight genotypes F2RC1 [UFU80- F2RC1 #1 (3.5); UFU102- F2RC1 #7 (13.4); UFU102- F2RC1 #7 (13.3); UFU102- F2RC1 #7 (16.8); UFU102- F2RC1 #3 (2.7); UFU80- F2RC1 #1 (11.8); UFU102- F2RC1 #7 (16.7); UFU102- F2RC1 #3 (14.5)], UFU-650 and LA-716. The experiment was conducted in a greenhouse with monitoring the flux density (W m-2), solar radiation (Qg), air temperature (°C), relative humidity (%) and the matric potential in the substrate (kPa). The collected physiological characteristics were: leaf temperature (Tleaf), internal CO2 (Ci), transpiration (E), stomatal conductance (gs) and CO2 assimilation (A), determined by portable gas analyzer infrared - IRGA. It was observed that the wild tomato S. pennellii was 6.96 times lower than the recurrent parent UFU-650 (pre-commercial line, susceptible to drought). The stomatal conductance (gs) showed significant values among genotypes. The results contribute to physiological characterization access S. pennellii (drought tolerant) and can assist in the selection of F2RC1 plants resistant to drought.

Leaf turgor pressure in maize plants under water stress

Maize grown is affected by water stress reducing photosynthetic rate and availability of water in its tissues, decreasing plant yield. Monitoring plant water potential is an important indicator of the degree of water stress. With the new magnetic probe for determining leaf turgidity, it is possible to evaluate the water status of the plant and, in some cases, to indicate the optimal relative tolerance to water stress. The union of new and old approaches gives us a better knowledge of water relations in plats. Therefore, the aim of this study was to understand the behavior of maize plants subjected to water stress, using novel and conventional approaches. Maize plants were grown in pots in a greenhouse for 45 days. After this period, plants were subjected to water stress, where turgor measurements expressed by the variable Pp (patch pressure) were monitored. In addition, the leaf water potential, stomatal conductance, CO2 assimilation, transpiration rate, and variable growth (height, leaf area and dry weight) had been monitored for 30 days. Two treatments were conducted, one in which the plant was irrigated and the other one in which irrigation was fully suspended for a period of time and monitored the water status. As the days passed, the plants showed the first visual signs of stress like leaf rolling. During this period, we observed fluctuating Pp values throughout the day, but with a recovery of turgor at night. There were significant differences between treatments for stomatal conductance, water potential, photosynthesis, and Pp, mainly before irrigation. After each irrigation, there has been a rapid recovery in all parameters. There was five periods of stress and it is possible to see a pattern of decreasing and increasing the Pp as the advance of stress, mainly in the last two. Maize plants had a big resilience in water stress conditions, due to mechanisms of water loss mitigation, like leaf rolling and possibly osmotic adjustment. Thus, it was concluded that Pp introduced a new approach to study plants subjected to water stress and it is a complement to other variables as CO2 assimilation rate, stomatal conductance, transpiration rate, and leaf water potential.

A novel mechanistic interpretation of instantaneous temperature responses of leaf net photosynthesis.

Steady-state rates of leaf CO2 assimilation (A) in response to incubation temperature (T) are often symmetrical around an optimum temperature. A/T curves of C3 plants can thus be fitted to a modified Arrhenius equation, where the activation energy of A close to a low reference temperature is strongly correlated with the dynamic change of activation energy to increasing incubation temperature. We tested how [CO2]<current atmospheric levels and saturating light, or [CO2] at 800µmolmol(-1) and variable light affect parameters that describe A/T curves, and how these parameters are related to known properties of temperature-dependent thylakoid electron transport. Variation of light intensity and substomatal [CO2] had no influence on the symmetry of A/T curves, but significantly affected their breadth. Thermodynamic and kinetic (physiological) factors responsible for (i) the curvature in Arrhenius plots and (ii) the correlation between parameters of a modified Arrhenius equation are discussed. We argue that the shape of A/T curves cannot satisfactorily be explained via classical concepts assuming temperature-dependent shifts between rate-limiting processes. Instead the present results indicate that any given A/T curve appears to reflect a distinct flux mode, set by the balance between linear and cyclic electron transport, and emerging from the anabolic demand for ATP relative to that for NADPH.

Alterations of physiology and gene expression due to long-term magnesium-deficiency differ between leaves and roots of Citrus reticulata

Seedlings of Ponkan (Citrus reticulata) were irrigated with nutrient solution containing 0 (Mg-deficiency) or 1mM MgSO4 (control) every two day for 16 weeks. Thereafter, we examined magnesium (Mg)-deficiency-induced changes in leaf and root gas exchange, total soluble proteins and gene expression. Mg-deficiency lowered leaf CO2 assimilation, and increased leaf dark respiration. However, Mg-deficient roots had lower respiration. Total soluble protein level was not significantly altered by Mg-deficiency in roots, but was lower in Mg-deficient leaves than in controls. Using cDNA-AFLP, we obtained 70 and 71 differentially expressed genes from leaves and roots. These genes mainly functioned in signal transduction, stress response, carbohydrate and energy metabolism, cell transport, cell wall and cytoskeleton metabolism, nucleic acid, and protein metabolisms. Lipid metabolism (Ca2+ signals)-related Mg-deficiency-responsive genes were isolated only from roots (leaves). Although little difference existed in the number of Mg-deficiency-responsive genes between them both, most of these genes only presented in Mg-deficient leaves or roots, and only four genes were shared by them both. Our data clearly demonstrated that Mg-deficiency-induced alterations of physiology and gene expression greatly differed between leaves and roots. In addition, we focused our discussion on the causes for photosynthetic decline in Mg-deficient leaves and the responses of roots to Mg-deficiency.

Seasonal stability of chlorophyll fluorescence quantified from airborne hyperspectral imagery as an indicator of net photosynthesis in the context of precision agriculture

Abstract The seasonal stability of solar-induced chlorophyll fluorescence (SIF) vs field-measured leaf CO2 assimilation (A) was assessed over a period of 2 years by means of airborne flights performed at midday and diurnally over a citrus (evergreen) crop canopy. The orchard was cultivated under a control treatment (ET) that received 100% of its water requirements and two regulated deficit irrigation (RDI) treatments with water supply reduced to 37% and 50% of the control level during the summer. Field measurements consisted of assimilation rate, stomatal conductance, stem water potential, leaf fluorescence and leaf reflectance. The airborne campaigns took place in 2012 and 2013, and were flown on the solar plane in order to acquire hyperspectral imagery at 40 cm resolution, 260 spectral bands and 1.85 nm/pixels in the 400–885 nm spectral region. A thermal camera was installed in tandem in all flights, acquiring imagery in the 7.5–13 μm spectral region at 640 × 480 pixel resolution, yielding a 50 cm pixel size. The robustness of the SIF quantification through the Fraunhofer Line Depth (FLD) principle based on three spectral bands (FLD3), as well as the performance of physiological and structural hyperspectral indices, was evaluated in order to understand their ability to track photosynthesis at different phenological and stress stages throughout the season. Solar induced fluorescence quantified as FLD3 was the most robust indicator of photosynthesis in all the airborne campaigns performed in the course of the two-year experiment, which comprised seven midday flights and two diurnals. The relationships between fluorescence (FLD3) and assimilation rates yielded correlation coefficients (R) between 0.64 and 0.82 across all dates, these being statistically significant with p-values between p

YIELD PERFORMANCE, CARBON ASSIMILATION AND SPECTRAL RESPONSE OF TRITICALE TO WATER STRESS

SUMMARYWater stress is arguably the most limiting factor affecting cereal productivity in the world and its effects are likely to increase due to climate change. It is therefore imperative to have a wide-ranging understanding of water stress effects on crop physiological processes so as to better manage, improve and adapt crops to future climates. A field study was carried out to investigate the influence of four moisture levels on the following: (1) flag leaf CO2 assimilation and flag leaf carbon content; (2) the utility of flag leaf spectral reflectance to monitor leaf water status and as an indicator of biomass and grain yield; and (3) biomass and grain yield performance of four spring triticale genotypes in a dry winter environment (steppe, arid climate). The experiment was carried out in a factorial arrangement of four moisture levels and four spring type triticale genotypes). Soil moisture level significantly influenced biomass accumulation, grain yield, CO2 assimilation, flag leaf carbon content and spectral reflectance. Grain yield levels ranged from 0.8 to 3.5 t ha−1 in 2013 and 1.8 to 4.9 t ha−1 in 2014. CO2 assimilation was significantly higher under well-watered (WW) conditions (9.92 µmol m−2 s−1 in 2013; 11.64 µmol m−2 s−1 in 2014) and decreased gradually with moisture level to 1.82 and 4.74 µmol m−2 s−1 under severe stress (SS) in 2013 and 2014, respectively. Flag leaf carbon content was significantly higher under water limited conditions compared to WW. Normalised Difference Vegetation Index (NDVI), Normalised Difference Water Index (NDWI) and Water Index (WI) were significant and positively correlated to biomass and grain yield. WI was particularly strongly correlated to biomass (0.72***) and grain yield (0.55***). However, no clear varietal effects were detected. This study revealed that carbon tends to accumulate in flag leaves under water stress and that flag leaf carbon content is influenced more by the export capacity of the flag leaves than on CO2 assimilation rate. WI was found to be superior index in monitoring water stress in triticale compared to NDVI and NDWI. Above all, spring triticale proved to be adaptable to steppe (dry) climate of Limpopo and that livestock farmers in the province can successfully grow triticale for silage under MS conditions.