Maximum CO2 Assimilation Rate Research Articles

Interacting Effects of CO2, Temperature, and Nitrogen Supply on Photosynthetic, Root Growth, and Nitrogen Allocation of Strawberry at the Fruiting Stage

To efficiently improve the productivity of strawberries under growing environmental change, the photosynthesis, root growth, and nitrogen allocation of strawberries (Fragaria × ananassa Duch. cv. Toyonoka) were investigated in a factorial design of CO2, temperature, and nitrogen supply. Elevated CO2 decreased the maximum CO2 assimilation rate (Amax), maximum CO2 carboxylation capacity per unit leaf area (Vcmax), and maximum CO2 carboxylation capacity per unit leaf mass (Vcm-m) by 20%, 24%, and 44%, respectively. Meanwhile, it reduced the SPAD value, maximal fluorescence level in the dark-adapted state (Fm), and maximal photochemical efficiency of PSII (Fv/Fm). Moreover, root branches, root number, root dry weight, and nitrogen-use efficiency were further increased in response to elevated CO2 under low nitrogen. When elevated CO2 was applied together with nitrogen nutrients, the Vcm-m and root nitrogen concentration (RNC) declined by 32% and 12%, respectively, but the total root dry weight (TRDW) increased by 88%. If the nitrogen nutrient was individually applied, the TRDW decreased by 16%, while the RNC increased by 21%. When the high temperature was individually applied, the TRDW increased by 104%, but the RNC decreased by 5%. Overall, elevated CO2 exacerbated photosynthetic down-regulation and significantly affected nitrogen redistribution among strawberry organs, reducing leaf nitrogen concentration and accelerating leaf senescence. However, it could increase seed quantity and improve its quality as well. In other words, under nitrogen-deficient conditions, elevated CO2 could improve the survival of offspring via the cost of the mother plant’s growth capacity.

Improving C3 photosynthesis by exploiting natural genetic variation: Hirschfeldia incana as a model species

AbstractDespite research efforts toward improving crop photosynthetic energy conversion efficiencies over the past 40 years, photosynthetic efficiencies remain far from their theoretical maxima. A major challenge has been that plant photosynthesis is a complex process, controlled by many underlying genetic factors and highly dynamic in response to short‐term environmental changes. Recent approaches to improving photosynthesis involved model‐based identification of the bottlenecks in photosynthesis followed by their genetic modification (GM). While these approaches were successful and inspirational, their dependency on the use of GM techniques may restrict their implementation in some jurisdictions. We therefore suggest greater research focus on a different, yet complementary, approach to improving photosynthetic efficiency: the exploration and exploitation of natural genetic variation in photosynthesis. A substantial improvement in phenotyping and genotyping technology over the past decade has highlighted natural variation in photosynthetic sub‐traits for crop and model species. However, a comprehensive understanding of all the factors responsible for photosynthetic limitations is still lacking. We therefore propose the use of high photosynthetic capacity species as models for the exploration of the physiological and genetic basis of high photosynthetic efficiency. While most high photosynthetic capacity species are not suitable as models due to complex genetics and evolutionary distance from crops, we have identified Brassicaceae species Hirschfeldia incana (L.) Lagr.‐Foss as a promising candidate. In this perspective paper, we describe and advocate the use of H. incana as a model for the exploration of high maximum CO2 assimilation rates (Pmax) found in some C3 species. We describe the basic biology and evolutionary history of the species and report preliminary data on its photosynthetic characteristics. Our findings suggest H. incana is an excellent model species for studies aiming at understanding natural genetic variation in photosynthetic efficiency.

Modelling potential potato yields: Accounting for experimental differences in modern cultivars

Current potential yield simulations of potato crops can be lower than actual yields observed in fields and the potential yield simulations miss important cultivar differences, for instance in terms of cultivar earliness. This causes issues not only for potential yield simulations but also when estimating the limiting effects of water (and nitrogen) and yield gaps in farmers’ fields. To address this issue, we recalibrated potential yield simulations with WOFOST for five commercial Dutch potato cultivars. An experiment conducted during the 2020 season was used for model calibration while the 2019 version of the same experiment was used for evaluation. The focus of the calibration was on phenology, photosynthesis, and biomass allocation. To aid the latter, the WOFOST biomass allocation table functions were replaced with sigmoid functions and biomass reallocation from haulm to tubers was introduced. In total 19 parameters were adjusted and/or introduced. Twelve of these parameters were not cultivar-specific, e.g. the specific leaf area over the plant developmental stage (SLATB), and the maximum leaf CO2 assimilation rate as function of developmental time (AMAXTB). Seven parameters differed between the cultivars, e.g. the thermal time between emergence and tuber initiation (TSUM1), and most of these were correlated to cultivar earliness. For the calibration year, final yields were underestimated by on average 5.4%, while for the evaluation year the final yield was overestimated by 4.4%. In comparison, the final dry matter yields ranged from 16.4 to 18.5 t ha−1 in 2019 and 15.5–17.5 t ha−1 in 2020. The old WOFOST parameters for potato resulted in an underestimation of 23.7% and 28.7% for, respectively, 2019 and 2020. Improving the parameter sets greatly increased accuracy of the simulations and provided insight in which parameters are cultivar specific and related to cultivar-earliness.

Photosynthesis of Physalis peruviana under different densities of photons and saline stress

Physalis peruviana L. is a solanacea that has been gaining prominence due to its fruits presenting good acceptance in the national and international market. However, several abiotic factors, such as salinity, can cause physiological disturbances in plants, and these changes may be of greater or lesser intent according to species. Therefore, the objective of the present work was to evaluate the physiological behavior of P. peruviana submitted to different fluxes of photosynthetically active photons (PPFD) and saline stress. The experimental design was a randomized block design with three saline levels (ECw) (0.5, 2.75 and 5.00 dS m-1) with four replications. Gas exchange measurements were performed with a portable infrared gas analyzer. Liquid CO2 assimilation, stomatal conductance, internal CO2 concentration, water use efficiency and instantaneous carboxylation efficiency were measured. Data were subjected to analysis of variance by F test and in cases of significance applied to regression analysis. The increase in PPFD provided reductions in stomatal conductance up to the density of approximately 400 μmol m-2s-1, being more pronounced in ECw of 2.75 and 5.0 dS m-1. The maximum CO2 assimilation rates in the three salinities are different according to the PPFD. The salinity of irrigation water reduced the quantum efficiency of photosynthesis in P. peruviana plants.

Drought stress reduces crop carbon gain due to delayed photosynthetic induction under fluctuating light conditions.

Drought stress is a major limiting factor for crop growth and yield. Water availability in the field can cyclically change between drought and rewatering conditions, depending on precipitation patterns. Concurrently, light intensity under field conditions can fluctuate, inducing dynamic photosynthesis and transpiration during the crop growth period. The present study aimed to characterize carbon gain and water use in fluctuating light under drought and rewatering conditions in two major crops, namely rice and soybean. We conducted gas exchange measurements under fluctuating light conditions with rice and soybean plants exposed to drought treatment (9-13 days) imposed by withholding water and subsequent rewatering treatment (8-9 days). Drought stress significantly reduced the maximum CO2 assimilation rate (A) in soybean but not in rice. Under drought conditions, A increased after a step increase in light and then gradually decreased in both crops, resulting in the significant reduction of steady-state A in rice and soybean. Moreover, drought stress delayed photosynthetic induction in both crops even when it had relatively small impact on maximum A. These results suggest that the drought effects on photosynthesis should be evaluated based on induction, maximum, and steady states. The delayed photosynthetic induction under drought owing to the reduced gas diffusional conductance via stomata resulted in a substantial loss of leaf carbon gain under fluctuating light conditions. Meanwhile, rewatering, after drought, completely recovered photosynthesis under fluctuating light in both crops. Therefore, the stability of photosynthetic induction can be a promising target to improve drought tolerance during crop breeding in the future.

Plant phenomics & precision agriculture simulation of winter wheat growth by the assimilation of unmanned aerial vehicle imagery into the WOFOST model.

The aim of this study is to optimize the simulation result of the WOFOST model and explore the possibility of assimilating unmanned aerial vehicle (UAV) imagery into this model. Field images of wheat during its key growth stages are acquired with a UAV, and the corresponding leaf area index (LAI), biomass, and final yield are experimentally measured. LAI data is retrieved from the UAV imagery and assimilated into a localized WOFOST model using least squares optimization. Sensitive parameters, i.e., specific leaf area (SLATB0, SLATB0.5, SLATB2) and maximum CO2 assimilation rate (AMAXTB1, AMAXTB1.3) are adjusted to minimize the discrepancy between the LAI obtained from the model simulation and inversion of the UAV data. The results show that the assimilated model provides a better estimation of the growth and development of winter wheat in the study area. The R2, RMSE, and NRMSE of winter wheat LAI simulated with the assimilated WOFOST model are 0.8812, 0.49, and 23.5% respectively. The R2, RMSE, and NRMSE of the simulated yield are 0.9489, 327.06 kg·hm-2, and 6.5%. The accuracy in model simulation of winter wheat growth is improved, which demonstrates the feasibility of integrating UAV data into crop models.

Functional Traits Are Good Predictors of Tree Species Abundance Across 101 Subtropical Forest Species in China.

What causes variation in species abundance for a given site remains a central question in community ecology. Foundational to trait-based ecology is the expectation that functional traits determine species abundance. However, the relative success of using functional traits to predict relative abundance is questionable. One reason is that the diversity in plant function is greater than that characterized by the few most commonly and easily measurable traits. Here, we measured 10 functional traits and the stem density of 101 woody plant species in a 200,000 m2 permanent, mature, subtropical forest plot (high precipitation and high nitrogen, but generally light- and phosphorus-limited) in southern China to determine how well relative species abundance could be predicted by functional traits. We found that: (1) leaf phosphorus content, specific leaf area, maximum CO2 assimilation rate, maximum stomata conductance, and stem hydraulic conductivity were significantly and negatively associated with species abundance, (2) the ratio of leaf nitrogen content to leaf phosphorus content (N:P) and wood density were significantly positively correlated with species abundance; (3) neither leaf nitrogen content nor leaf turgor loss point were related to species abundance; (4) a combination of N:P and maximum stomata conductance accounted for 44% of the variation in species’ abundances. Taken together, our findings suggested that the combination of these functional traits are powerful predictors of species abundance. Species with a resource-conservative strategy that invest more in their tissues are dominant in the mature, subtropical, evergreen forest.

Plant functional traits along an old-field succession in the high tropical andes

The páramo, a high tropical Andean ecosystem, presents very distinct climatic, floristic and ecological features. Paramo plants have adapted to pronounced daily temperature fluctuations, freezing temperatures any night of the year, high incoming radiation and seasonal unfavorable water availability conditions. Long fallow agricultural systems are common in the páramo, permitting partial recuperation of the vegetation through a secondary succession. Different hypothesis related to plant traits (water relations and gas exchange characteristics or biomass allocation patterns) during succession have been proposed. Particular traits have been associated to the stages along successional gradients from early species (traits related to high resource acquisition abilities) to late species (traits linked to internal conservation of resources). Our purpose was to study plant functional responses of different herbaceous species in an old-field succession in the high tropical Andes. Plant traits (specific leaf area (SLA), maximum CO2 assimilation rates (Amax), leaf nitrogen content (Nleaf), leaf water relations and biomass allocation) were studied in grasses and forbs along different successional stages. Clear trends in SLA, Amax on a mass basis, Nleaf and aboveground/belowground biomass allocation were found along the successional gradient for grasses, while patterns were not as well-defined for forbs in the latter two traits. No patterns in leaf water relations characteristics were observed for either plant-growth forms. Strategies associated to plant traits significantly influence plant species dominance and community assembly during secondary succession in tropical alpine open environments like the páramos. Plant traits linked to rapid growth (SLA, Amax and Nleaf) dominate during early succession. In contrast to previous studies, drought resistance traits were not found to dominate in late succession stages for either grasses or forbs.

An extensive suite of functional traits distinguishes Hawaiian wet and dry forests and enables prediction of species vital rates

Abstract The application of functional traits to predict and explain plant species’ distributions and vital rates has been a major direction in functional ecology for decades, yet numerous physiological traits have not yet been incorporated into the approach. Using commonly measured traits such as leaf mass per area (LMA) and wood density (WD), and additional traits related to water transport, gas exchange and resource economics, including leaf vein, stomatal and wilting traits, we tested hypotheses for Hawaiian wet montane and lowland dry forests (MWF and LDF, respectively): (1) Forests would differ in a wide range of traits as expected from contrasting adaptation; (2) trait values would be more convergent among dry than wet forest species due to the stronger environmental filtering; (3) traits would be intercorrelated within “modules” supporting given functions; (4) relative growth rate (RGR) and mortality rate (m) would correlate with a number of specific traits; with (5) stronger relationships when stratifying by tree size; and (6) RGR and m can be strongly explained from trait‐based models. The MWF species’ traits were associated with adaptation to high soil moisture and nutrient supply and greater shade tolerance, whereas the LDF species’ traits were associated with drought tolerance. Thus, on average, MWF species achieved higher maximum heights than LDF species and had leaves with larger epidermal cells, higher maximum stomatal conductance and CO2 assimilation rate, lower vein lengths per area, higher saturated water content and greater shrinkage when dry, lower dry matter content, higher phosphorus concentration, lower nitrogen to phosphorus ratio, high chlorophyll to nitrogen ratio, high carbon isotope discrimination, high stomatal conductance to nitrogen ratio, less negative turgor loss point and lower WD. Functional traits were more variable in the MWF than LDF, were correlated within modules, and predicted species’ RGR and m across forests, with stronger relationships when stratifying by tree size. Models based on multiple traits predicted vital rates across forests (R2 = 0.70–0.72; p < 0.01). Our findings are consistent with a powerful role of broad suites of functional traits in contributing to forest species’ distributions, integrated plant design and vital rates. A plain language summary is available for this article.

Picea purpurea has a physiological advantage over its progenitors in alpine ecosystems due to transgressive segregation

ABSTRACTPicea purpurea originated from the homoploid hybrid speciation, which occupy novel alpine habitats in the north-east region of the Qinghai-Tibet Plateau (QTP) where its progenitors, P. wilsonii and P. likiangensis are excluded. Alpine habitats are usually characterized by low atmospheric CO2 concentrations and strong solar radiation with a large fraction of shortwave radiation (i.e. blue light). To test whether P. purpurea has the transgressive segregation in photosynthetic capacity and stomatal characteristics contributed to occupying such extreme habitats, we investigated the leaf gas exchange properties at two level of CO2 concentration (Ca: 200 and 400 μmol mol−1), light responsive curves of photosynthesis under three light qualities (red, blue and a mixture of both red and blue light at the ratio of 94:6), stomatal and other relative physiological traits for all three species. Our results show evidence of transgressive segregation in hybrid species; for instance, under blue and red light P. purpurea was demonstrated has the higher quantum yield of CO2 assimilation (φ) and maximum CO2 assimilation rate (Amax) than its progenitors. Particularly φ and Amax in P. purpurea were 2-fold higher than these in P. likiangensis. P. purpurea also exhibited higher stomatal conductance at low CO2 concentrations and instantaneous water use efficiency under all light qualities, have greater leaf pigments content, longer guard cell length and lower linear stomatal density relative to its progenitors. These traits may have enhanced the physiological fitness of P. purpurea and facilitated its colonization to alpine habitats.

Dominant Species in Subtropical Forests Could Decrease Photosynthetic N Allocation to Carboxylation and Bioenergetics and Enhance Leaf Construction Costs during Forest Succession.

It is important to understand how eco-physiological characteristics shift in forests when elucidating the mechanisms underlying species replacement and the process of succession and stabilization. In this study, the dominant species at three typical successional stages (early-, mid-, and late-succession) in the subtropical forests of China were selected. At each stage, we compared the leaf construction costs (CC), payback time (PBT), leaf area based N content (NA), maximum CO2 assimilation rate (Pmax), specific leaf area (SLA), photosynthetic nitrogen use efficiency (PNUE), and leaf N allocated to carboxylation (NC), and to bioenergetics (NB). The relationships between these leaf functional traits were also determined. The results showed that the early-succession forest is characterized with significantly lower leaf CC, PBT, NA, but higher Pmax, SLA, PNUE, NC, and NB, in relation to the late-succession forest. From the early- to the late-succession forests, the relationship between Pmax and leaf CC strengthened, whereas the relationships between NB, NC, PNUE, and leaf CC weakened. Thus, the dominant species are able to decrease the allocation of the photosynthetic N fraction to carboxylation and bioenergetics during forest succession. The shift in these leaf functional traits and their linkages might represent a fundamental physiological mechanism that occurs during forest succession and stabilization.

Diffusion limitations and metabolic factors associated with inhibition and recovery of photosynthesis following cold stress in Elymus nutans Griseb.

We studied the effects of cold stress (5°C) and re-warming (25°C) on gas exchange, photosystem II, key photosynthetic enzyme activities, gene expression, and carbohydrate metabolite concentrations in two Elymus nutans genotypes differing in cold resistance (DX, cold-tolerant and ZD, cold-sensitive). Cold stress led to irreversible reductions in photosynthetic rate. This reduction was accompanied by declining stomatal and mesophyll conductance (gs and gm), transpiration rate (Tr) and photochemical efficiency in both genotypes, however there were smaller decreases in DX than in ZD. Cold-tolerant DX maintained higher photosynthetic enzyme activities and transcript levels, as well as higher reducing sugar concentrations and sucrose accumulation. The relationship between Pn and internal leaf CO2 concentration (Pn/Ci curve) during cold and re-warming was analyzed to estimate the relative influence of stomatal and non-stomatal components on photosynthesis. Stomatal limitation, non-stomatal limitation, and CO2 compensation point (CP) increased in both genotypes under cold stress, but to a lesser extent in DX. Maximum CO2 assimilation rate (Pmax), and carboxylation efficiency (CE) declined, but DX had significantly higher levels of Pmax and CE than ZD. Following cold-stress recovery, the maximum quantum yield of PSII (Fv/Fm), apparent electron transport rate (ETR), Rubisco activity, Rubisco activation state and CE in DX resumed to the control levels. In contrast, Pn, Pmax, gs, gm, and Tr recovered only partially for DX, suggesting that incomplete recovery of photosynthesis in DX may be mainly related to diffusion limitations. Higher Rubisco large subunit (RbcL) and Rubisco activase (RCA) transcript levels, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) activity, and carbohydrate accumulation contributed to higher photosynthetic recovery in DX. These results indicate that the maintenance of higher Pn and Pmax under cold stress and recovery in cold-tolerant DX could be attributed to reduced diffusion limitations and rapid recuperation of metabolic factors.

Diurnal root zone temperature variations affect strawberry water relations, growth, and fruit quality

To test the hypothesis that the effect of daily root zone temperature (RZT) variations would affect strawberry plant performance, root systems of Control ‘Albion’ strawberry plants Treatment 1 (T1) were maintained at 20°C (no stress), while plants in other treatments were exposed to different diurnal temperature swings about the daily mean of 20°C. In treatment 2 (T2), temperature swing was 5°C (mild) which had a total variation of 10°C (Tvar-10) with a minimum of 15°C and maximum of 25°C, treatment 3 (T3) had a variation about the mean of 10°C (moderate) with a total variation of 20°C (Tvar-20) with a minimum of 10°C and maximum of 30°C, or 15°C (severe) swings about the mean in treatment 4 (T4) with a diurnal total variation of 30°C (Tvar-30) with a minimum of 5°C and maximum of 35°C. Leaf area and shoot dry weight of plants in the severe treatment were 30% lower than Controls. Root dry weight in the severe treatment was 30% lower than in the mild treatment. Fruits in the moderate and severe treatments were 22% smaller than in the mild treatment. Fruit soluble sugar content was elevated slightly by RZT fluctuation. Shoot concentrations of N, Mg, Fe, B, Zn were not affected by RZT, but there were small differences in concentrations of P, K, Ca, Mn, and Cu. Nutrient uptake of N and Ca were not affected, but P, K and Mg were about 35% less in Tvar-20 and Tvar-30. RZT variation had no effect on chlorophyll content or number of leaves, maximum carboxylation rate, photosynthetic electron transport rate, maximum CO2 assimilation rate, or fluorescence parameters (Fv/Fm and ϕPSII). There was little effect of RZT on predawn or midday stem water potential (SWP), but the SWP was substantially lowered in the severe treatment at mid-morning when RZT was about 5°C and air vapor pressure deficit was at its daily maximum. We attributed the reduced shoot and root growth to the transient reduction in SWP when RZT was near 5°C. Overall, the consequences of diurnal RZT fluctuations for strawberry production appear to be small if the minimum temperature is above 7°C.

Leaf colour polymorphisms: a balance between plant defence and photosynthesis

Summary Whether plants use leaf colour to deter herbivores remains controversial. The warning signal hypothesis predicts that red pigmentation is adaptive by reducing herbivory; plants with predominantly red foliage should have higher fitness than those with green leaves. Despite many discussions, this prediction has rarely been tested, and alternative, non‐exclusive hypotheses cannot be ruled out. We have exploited leaf colour polymorphism in Pseudowintera colorata to test the warning signal hypothesis and to address possible alternative explanations. Consistent with warning signals, redder foliage contained higher concentrations of polygodial, a sesquiterpene dialdehyde with strong antifeedant properties, and incurred less herbivory than green leaves. Redder plants hosted 22% fewer lepidopteran leafroller larvae than neighbouring green plants. However, contrary to the predictions of the hypothesis, there were no differences in fitness parameters between red and green plants. Overall leaf canopy colour was not a significant predictor of the number of seeds per fruit or of mean seed weight. This may be explained by differences in photosynthesis: green P. colorata leaves had 47% higher maximum CO2 assimilation rates than matched red leaves from neighbouring plants. These results indicate that the benefits of deterring insect herbivores by signalling may be balanced by the higher photosynthetic rate of non‐signalling plants. A balance between signalling and photosynthesis is a novel mechanism for the maintenance of leaf colour polymorphisms in nature. Synthesis. Anthocyanin pigments may simultaneously serve multiple functions within leaves, and individuals of the same plant species may use different strategies to cope with insect herbivores. Therefore, investigations into the role of these pigments in plant–insect interactions need to consider plant physiology and the diversity of plant defence mechanisms.

The functional significance of black-pigmented leaves: photosynthesis, photoprotection and productivity in Ophiopogon planiscapus 'Nigrescens'.

Black pigmented leaves are common among horticultural cultivars, yet are extremely rare across natural plant populations. We hypothesised that black pigmentation would disadvantage a plant by reducing photosynthesis and therefore shoot productivity, but that this trait might also confer protective benefits by shielding chloroplasts against photo-oxidative stress. CO2 assimilation, chlorophyll a fluorescence, shoot biomass, and pigment concentrations were compared for near isogenic green- and black-leafed Ophiopogon planiscapus ‘Nigrescens’. The black leaves had lower maximum CO2 assimilation rates, higher light saturation points and higher quantum efficiencies of photosystem II (PSII) than green leaves. Under saturating light, PSII photochemistry was inactivated less and recovered more completely in the black leaves. In full sunlight, green plants branched more abundantly and accumulated shoot biomass quicker than the black plants; in the shade, productivities of the two morphs were comparable. The data indicate a light-screening, photoprotective role of foliar anthocyanins. However, limitations to photosynthetic carbon assimilation are relatively small, insufficient to explain the natural scarcity of black-leafed plants.

Two distinct strategies of cotton and soybean differing in leaf movement to perform photosynthesis under drought in the field.

This paper reports an experimental test of the hypothesis that cotton and soybean differing in leaf movement have distinct strategies to perform photosynthesis under drought. Cotton and soybean were exposed to two water regimes: drought stressed and well watered. Drought-stressed cotton and soybean had lower maximum CO2 assimilation rates than well-watered (control) plants. Drought reduced the light saturation point and photorespiration of both species - especially in soybean. Area-based leaf nitrogen decreased in drought-stressed soybean but increased in drought-stressed cotton. Drought decreased PSII quantum yield (ΦPSII) in soybean leaves, but increased ΦPSII in cotton leaves. Drought induced an increase in light absorbed by the PSII antennae that is dissipated thermally via ΔpH- and xanthophylls-regulated processes in soybean leaves, but a decrease in cotton leaves. Soybean leaves appeared to have greater cyclic electron flow (CEF) around PSI than cotton leaves and drought further increased CEF in soybean leaves. In contrast, CEF slightly decreased in cotton under drought. These results suggest that the difference in leaf movement between cotton and soybean leaves gives rise to different strategies to perform photosynthesis and to contrasting photoprotective mechanisms for utilisation or dissipation of excess light energy. We suggest that soybean preferentially uses light-regulated non-photochemical energy dissipation, which may have been enhanced by the higher CEF in drought-stressed leaves. In contrast, cotton appears to rely on enhanced electron transport flux for light energy utilisation under drought, for example, in enhanced nitrogen assimilation.

Field assessment of photosynthetic characteristics in micropropagated and grafted wych elm (Ulmus glabra Huds.) trees

A field test of wych elm (Ulmus glabra Huds.) trees micropropagated in tissue culture was established to compare their photosynthetic characteristics with those from control grafted scions. Measurements were made at the sixth growing season after planting. The results showed similarities between the two stock types in terms of height, diameter at breast height, and leaf morphology. Grafted trees, which were markedly less rejuvenated than micropropagated trees, performed better in many of the light absorption and leaf gas exchange attributes including maximum CO2 assimilation rate at saturating photosynthetic photon flux density (Pmax), apparent quantum yield of photosynthesis (α), transpiration rate (E), and stomatal conductance (gs). In the grafts, the direct effect of photosynthesis could be clearly identified by a significantly lower specific leaf area and a higher content of leaf dry mass. Chlorophyll a fluorescence parameters showed non-stressed growth patterns in both stock types. Rates of the maximal photochemical efficiency of photosystem II (Fv/Fm), potential electron capacity (‘area’), and variable-to-initial fluorescence ratio (Fv/F0) were higher in micropropagated elms. Findings from this study indicate that micropropagated elm stock is ready for testing on a larger scale.

Some ecophysiological properties of cyanobacterial cryptobiotic crusts of tropical desiccation-tolerant vegetation

Our results indicate that the cryptogamic inselberg crusts from Tanzania and French Guyana indicate an extremely high degree of desiccation-tolerance. They are able tolerate not even a few days or weeks but 1 year (the Tanzanian sample) or 5 years (the French Guyana sample) desiccated stage. They can regain their maximum CO2 assimilation rate within 15–20 min (Tanzanian) and 45–50 min (French Guyana) after rehydration, respectively. The measured net CO2 assimilation rate of the cyanobacterial inselberg crusts from both the African and South-American habitat were similar to the rates reported on other non-inselberg and non-tropical cryptogamic cryptobiotic crust vegetations such as vegetations covered by lichens or mosses (Larcher, 2003; Juhasz et al., 2005). The success of the studied two tropical inselberg cryptogamic crusts to adaptation and CO2 assimilation on exposed rock surfaces is due to their special ecophysiological abilities. Concerning this above C-assimilation capacity of the tropical inselberg cryptogamic crusts and the significant occurrence of the tropical/sub tropical inselberg covered by cyanobacterial crusts even in a global scale (Porembski and Barthlott, 2000), it can conclude that these inselberg crusts have a considerable carbon pool and they contribute significantly to the global carbon cycling of the Earth.

Declining hydraulic efficiency as transpiring leaves desiccate: two types of response*

The conductance of transpiring leaves to liquid water (Kleaf) was measured across a range of steady-state leaf water potentials (Psileaf). Manipulating the transpiration rate in excised leaves enabled us to vary Psileaf in the range -0.1 MPa to less than -1.5 MPa while using a flowmeter to monitor the transpiration stream. Employing this technique to measure how desiccation affects Kleaf in 19 species, including lycophytes, ferns, gymnosperms and angiosperms, we found two characteristic responses. Three of the six angiosperm species sampled maintained a steady maximum Kleaf while Psileaf remained above -1.2 MPa, although desiccation of leaves beyond this point resulted in a rapid decline in Kleaf. In all other species measured, declining Psileaf led to a proportional decrease in Kleaf, such that midday Psileaf of unstressed plants in the field was sufficient to depress Kleaf by an average of 37%. It was found that maximum Kleaf was strongly correlated with maximum CO2 assimilation rate, while Kleaf = 0 occurred at a Psileaf slightly less negative than at leaf turgor loss. A strong linear correlation across species between Psileaf at turgor loss and Psileaf at Kleaf = 0 raises the possibility that declining Kleaf was related to declining cell turgor in the leaf prior to the onset of vein cavitation. The vulnerability of leaves rehydrating after desiccation was compared with vulnerability of leaves during steady-state evaporation, and differences between methods suggest that in many cases vein cavitation occurs only as Kleaf approaches zero.

Negative effects of hydroxyl radical‐generating mists (simulated dew water) on the photosynthesis and growth of Japanese apricot seedlings (Prunus mume)

AbstractThe hydroxyl (OH) radical, which is generated in polluted dew water on leaf surfaces of the Japanese apricot (Prunus mume), is known to be a potent oxidant. In order to investigate the effects of the OH radical formed in polluted dew water on the photosynthesis and growth of 3‐year‐old seedlings of P. mume, OH radical‐generating solutions simulating polluted dew water were sprayed in the early morning as a mist throughout a growing season onto the leaf surfaces of seedlings growing in experimental greenhouses. Four OH radical‐generating solutions (0, 6, 18 and 54 μM H2O2 with Fe(III) and an oxalate ion) were used in the mist treatment. Five months after the beginning of treatment, the leaves exposed to the mist containing 54 μM H2O2 showed a significantly smaller maximum CO2 assimilation rate (Amax) and stomatal conductance (gs) as compared to the leaves exposed to the mist containing 0 μM H2O2. Exposure of P. mume seedlings to the OH radical‐generating mist had caused a reduction in the dry weight and relative growth rate (RGR) of the above‐ground parts (stem + branch) at the end of the growing season. A significant positive correlation was shown between RGR and Amax. Thus, the effects of oxidants generated in polluted dew water on leaf surfaces can be considered to be a cause of the decrease in leaf photosynthesis and growth of P. mume.