Non-stomatal Limitations Research Articles

Photosynthetic activities, C<sub>3</sub> and C<sub>4</sub> indicative enzymes and the role of photoperiod in dormancy induction in 'Chunjie' peach

Our study examined the relationship between photosynthetic performance and activities of key photosynthetic enzymes to understand the photosynthetic variation and reasons for the variation during dormancy induction under different photoperiods in peach (Prunus persica L. cv. Chunjie). Furthermore, the study explained the changes in the key enzymes from the viewpoint of differential proteomics. The results showed that the leaf net photosynthetic rate (PN) and stomatal conductance tended to decrease, while the intercellular CO2 concentration rose, which indicated that the reduced PN resulted from nonstomatal limitation. During the dormancy induction period, the activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) declined, which was the main reason for the reduced PN. Two-dimensional electrophoresis maps and differential protein identification demonstrated that the decrease in activity of the photosynthetic enzymes was mainly due to enzymatic degradation. The enzyme degradation by a long-day treatment occurred later and to a lesser degree than that of the short-day treatment. In the long-day treatment, the carboxylation activity of Rubisco was higher than that of the control treatment, and the PEPC activity and the ratio of the PEPC/Rubisco activity were lower than the corresponding activities during the control treatment. These differences under long-day conditions were significant but did not occur in the short-day treatment, suggesting that the C4 pathway might be more active under short-day conditions.

Effect of NaCl Tolerance on Photosynthetic Physiology and Growth of Seedlings of Three Poplar Clones

We studied the response of photosynthetic physiological characteristics of three poplar clone seedlings to Na Cl stress with three kinds of poplar( Populus L.) clones of poplar( Populus L.) clone seedlings including‘A5'( Populus pseudo-simonii × P. nigra) ×( P. simonii × P. nigra),‘J7'( P. deltoidis × P. catyana CL.‘J7') and ‘1344'( P. deltoids × P. cathayana CL.‘1344') in Heilongjiang Province. Three Na Cl stress treatments were established at the contents of 0. 3%,0. 5% and 0. 7%( percentage of Na Cl in total dry soil mass). The salt injury symptoms first appeared in ‘1344 'under Na Cl stress,followed by ‘J7 'and ‘A5 '.Furthermore,under the same Na Cl content,the maximum damage due to Na Cl stress also occurred first in‘1344',followed by ‘J7'and ‘A5'. The mean tree diameter at breast height and total tree height differed signi? cantly under three Na Cl stress treatments. Moreover,the difference between three Na Cl stress treatments gradually become bigger,which was 10%- 35% lower than the control treatment during the late period of this experiment. With the increase of Na Cl,the chlorophyll content decreased,with the decrease in ‘J7 'being much more obvious than in ‘A5'and ‘1344'. With the increase of Na Cl,the content of carotenoid in ‘A5'increased,and ‘J7 'and ‘1344 'decreased. Chlorophyll b content under 0. 3% Na Cl stress in ‘A5 'and‘1344'was higher than that in the control,while the‘A5'chlorophyll b content under the 0. 5% and 0. 7%Na Cl stress levels were in contrast not significantly different,but ‘1344'under 0. 3% and 0. 5% Na Cl stress levels was significantly higher than that in the control and the 0. 7% Na Cl stress level. ‘J7 ' showed no significant difference when compared with the 0. 3% Na Cl stress,but was higher than the 0. 5% and 0. 7%levels. The photosynthesis of the clone seedlings reduced due to the Na Cl stress. The Gsand Ciof the leaves decreased with the increase of the salt,while the Lsmainly decreased. One possible explanation for the phenomenon in this study may be that the photosynthetic physiology was greatly influenced by stomatal limitation and non-stomatal limitation. The WUE of three poplar seedlings under Na Cl was maintained at a higher level,which may suggest that these poplar seedlings have a strong resistance to Na Cl stress. Therefore,‘A5'had the strongest salt-tolerant ability among these three clones.

Modification of non-stomatal limitation and photoprotection due to K and Na nutrition of olive trees

Potassium (K) is an essential macronutrient shown to play a fundamental role in photosynthetic processes and may facilitate photoinhibition resistance. In some plant species, sodium (Na) can partially substitute for K. Although photosynthetic enhancement has been well established, the mechanisms by which K or Na affects photosynthesis are not fully understood. Olive (Olea europaea L.) trees were previously shown to benefit from Na nutrition when K is limiting. In order to study the effect of K and Na on photosynthetic performance, we measured gas exchange and chlorophyll fluorescence in young olive trees supplied with either K, Na or no fertilizer, and subjected to manipulated levels of CO2, O2 and radiation. Light and CO2 response curves indicate substantially superior photosynthetic capacity of K-sufficient trees, while Na substitution generated intermediate results. The enhanced performance of K, and to a lesser extent, Na-supplied trees was found to be related mainly to modification of non-stomatal limitation. This indicates that K deficiency promotes inhibition of enzymatic-photochemical processes. Results indicate lower chlorophyll content and altered Rubisco activity as probable causes of photosynthetic impairment. Potassium deficiency was found to diminish photoprotection mechanisms due to reduced photosynthetic and photorespiratory capacity. The lower CO2 and O2 assimilation rate in K-deficient trees caused elevated levels of exited energy. Consequently, non-photochemical quenching, an alternative energy dispersion pathway, was increased. Nonetheless, K-deficient trees were shown to suffer from photodamage to photosystem-II. Sodium replacement considerably diminished the negative effect of K deficiency on photoprotection mechanisms. The overall impact of K and Na nutrition plays down any indirect effect on stomatal limitation and rather demonstrates the centrality of these elements in photochemical processes of photosynthesis and photoprotection.

Effect of Combined Pollution of Cd and B[a]P on Photosynthesis and Chlorophyll Fluorescence Characteristics of Wheat

The single and joint effects of cadmium(Cd) and benzo (a) pyrene (B(a)P) on the seedling growth and antioxidant enzyme activities of wheat (Triticum aestivum L.) were investigated after 4, 8, and 11 days of exposure under10 mg·L -1 Cd and 54 mg·L -1 B(a)P combined stress. In comparison with the control, the reduc- tions under Cd + B(a)P stress treatment of Chl a, Chl b, Chl a+b and Chl a/b ratio were 53%, 44%, 49%, and 16% after 11-d exposure, the reductions under combination stress of Pn, Gs, Tr, and Ci were 61%, 72%, 67%, and 9%, the reductions of Fv/Fm, ΦPSII and qP under combination stress treatment were 24%, 23%, and 7% after 11 days, while the increases of WUE and qN under Cd + B(a)P were 19% and 81% after 11-d exposure. The nonstomatal limitation is the major reason for the decrease of Pn under the cadmium and B(a)P treatment in wheat leaves. The higher Tr value in wheat leaves is probably a positive adaptation response to the cadmi- um and B(a)P. This adaption response may play a protective role in the photosystem, resulting in a higher Pn. B(a)P may enhance the toxicity of the cadmium because they can penetrate into the perforated cells more eas- ily. The toxicity of combined stress to photosynthesis and chlorophyll fluorescence parameters is stronger than the toxicity of single cadmium or single B(a)P, while cadmium had stronger toxic effects than B(a)P. The joint action of cadmium and B(a)P was a significant synergistic effect.

Effects of Drought Stress on Photosynthetic Characteristics of Maize Hybrids at Seedling Stage

Maintaining active photosynthesis is very important for crops to adapt drought stress. The electron transport chain and stomatal conductance are important in photosynthesis. Limited data are available on their combined responses to drought stress in maize. In this paper, the effects of drought stress on photosynthetic characteristics in maize hybrids Shaandan 609 and Zhengdan 958 at seedling stage were investigated. The results showed that the CO2 assimilation(Pn), stomatal conductance(Gs) and intercellular CO2 concentration(Ci) decreased during drought stress, showing the decreased photosynthesis due to stomatal factor. During moderate and severe drought stress, down-regulation in net CO2 fixation(Pn) was primarily mediated through non-stomatal limitation. The OJIP transients and other associated biophysical parameters elucidated the events of photoacclimatory changes in photosystem II(PSII) with progressive increase of drought stress, and the K-bands appeared in the stage of extreme drought severity indicating the imbalance between the electrons at the acceptor and donor sides of PSII, which suggested the oxygen-evolving complex(OEC) and electron transport were inhibited. In conclusion, the moderate and severe drought stress mainly damages the electron transfer from the plastoquinone(PQ) pool to the PSI terminal acceptors; this, along with constraints to both stomatal and non-stomatal components of photosynthesis, limits carbon assimilation. There are differences in cultivars with the response of PSII activity to drought stress.

Effect of Mercury Stress on Photosynthetic Characteristics of Two Kinds of Warm Season Turf Grass

In this paper, we adopted the pot simulation test method and took the plants that had been polluted by heavy metals as the research materials, to reveal the effect of mercury stress on photosynthetic characteristics and material production of turf grass. The results showed that, with the increase of the mercury stress intensity and the extension of time, the net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) of leaf of the two kinds of grass continued to decline, while the intercellular CO₂ concentration (Ci) continued to rise. The influence of mercury stress on photosynthetic characteristics of the two kinds of grass was shifting from stomatal limitation to non-stomatal limitation. The results of this study provide a theoretical basis for exploring the mechanism of mercury stress on the photosynthetic characteristics of turf grass.

Effect of exogenous 24-epibrassinolide on chlorophyll fluorescence, leaf surface morphology and cellular ultrastructure of grape seedlings (Vitis vinifera L.) under water stress

The effects of 24-epibrassinolide (EBR) on chlorophyll fluorescence, leaf surface morphology and cellular ultrastructure of grape seedlings (Vitis vinifera L.) under water stress were investigated. The grape seedlings were subjected to 10 % (w/v) polyethylene glycol (PEG-6000) and treated with 0.05, 0.10 or 0.20 mg L−1 EBR, respectively. EBR application increased chlorophyll contents, the effective photochemical quantum yield of PSII, maximum photochemical efficiency of PSII, maximal fluorescence and non-photochemical quenching coefficient under water stress in each concentration. Compared with water stress control, higher stomatal density and stomatal length were observed in young leaves under EBR treatments, but not in mature leaves. In-depth analysis of the ultrastructure of leaves indicated that water stress induced disappearance of nucleus, chloroplast swelling, fractured mitochondrial cristae and disorder of thylakoid arrangement both in young leaves and mature leaves. However, EBR application counteracted the detrimental effects of water stress on the structure of the photosynthetic apparatus better in young leaves than in mature leaves. Compared to the other treatments, treatment of 0.10 mg L−1 EBR had best ameliorative effect against water stress. These results suggested that exogenous EBR could alleviate water stress-induced inhibition of photosynthesis on grape possibly through increasing chlorophyll content, lessening the stomatal and non-stomatal limitation of photosynthesis performance.

Early growth promotion and leaf level physiology changes in Burkholderia phytofirmans strain PsJN inoculated switchgrass

Switchgrass (SG) is one of the most promising next generation biofuel crops in North America. Inoculation with bacterial endophytes has improved growth of several plant species. Our study demonstrated that Burkholderia phytofirmans strain PsJN, a well-studied plant growth promoting rhizo-bacterium (PGPR) significantly increased both aboveground and belowground biomass (DW) and promoted elongation of root, stem and leaf within 17 days following inoculation. Furthermore, the enhanced root growth in PsJN inoculated plants lagged behind the shoot response, resulting in greater allocation to aboveground growth (p = 0.0041). Lower specific root length (SRL, p = 0.0158) and higher specific leaf weight (SLW, p = 0.0029) were also observed in PsJN inoculated seedlings, indicating changes in development. Photosynthetic rates (Ps) were also significantly higher in PsJN inoculated seedlings after 17 days (54%, p = 0.0016), and this occurred initially without increases in stomatal conductance resulting in significantly greater water use efficiency (WUE, 37.7%, p = 0.0467) and lower non-stomatal limitation (LNS, 29.6%, p = 0.0222). These rapid changes in leaf level physiology are at least partially responsible for the growth enhancement due to PsJN.

The effect of atmospheric carbon dioxide concentrations on the performance of the mangrove Avicennia germinans over a range of salinities.

By increasing water use efficiency and carbon assimilation, increasing atmospheric CO2 concentrations could potentially improve plant productivity and growth at high salinities. To assess the effect of elevated CO2 on the salinity response of a woody halophyte, we grew seedlings of the mangrove Avicennia germinans under a combination of five salinity treatments [from 5 to 65 parts per thousand (ppt)] and three CO2 concentrations (280, 400 and 800 ppm). We measured survivorship, growth rate, photosynthetic gas exchange, root architecture and foliar nutrient and ion concentrations. The salinity optima for growth shifted higher with increasing concentrations of CO2 , from 0 ppt at 280 ppm to 35 ppt at 800 ppm. At optimal salinity conditions, carbon assimilation rates were significantly higher under elevated CO2 concentrations. However, at salinities above the salinity optima, salinity had an expected negative effect on mangrove growth and carbon assimilation, which was not alleviated by elevated CO2 , despite a significant improvement in photosynthetic water use efficiency. This is likely due to non-stomatal limitations to growth at high salinities, as indicated by our measurements of foliar ion concentrations that show a displacement of K(+) by Na(+) at elevated salinities that is not affected by CO2 . The observed shift in the optimal salinity for growth with increasing CO2 concentrations changes the fundamental niche of this species and could have significant effects on future mangrove distribution patterns and interspecific interactions.

Effects of drought stress on leaf gas exchange and chlorophyll fluorescence of Glycyrrhiza uralensis

Leaf gas exchange and chlorophyll fluorescence of Glycyrrhiza uralensis Fisch at two water supplies (equivalent to 240 mm for present precipitation and to 120 mm for future decreasing precipitation) were compared. There was a midday depression in net photosynthetic rate (PN) for either treatment, but stomatal limitation was dominant for present precipitation treatment and non-stomatal limitation for the drought treatment. Compared with present precipitation treatment, PN, stomatal conductance (gs) and transpiration rate (E) were always lower at drought stress, but stomatal limitation value (Ls) and water use efficiency (WUE) were higher in the morning. The decrease in carboxylation efficiency (CE), apparent quantum yield (AQY), light-saturated photosynthetic rate and maximal photochemical efficiency of photosystem 2 (PS 2) (Fv/Fm) suggested that rubisco activity reduced and PS 2 partially inactivated during the day under drought. However, part of this inactivation of PS 2 might be alleviated due to increased thermal dissipation under drought.

Physiology, anatomy, and cell membrane thermostability selection of leafy radish (Raphanus sativus var. oleiformis Pers.) with different tolerance under heat stress

Leafy radish (Raphanus sativus L. var. oleiformis Pers.), a temperate crop, is usually heat-sensitive. Line 9911-9 is a heat-tolerant line bred for summer vegetables in Taiwan. Understanding the heat tolerance mechanism of Line 9911-9 may facilitate future breeding programs. We conducted a comparative study on growing plants of heat-tolerant Line 9911-9, moderately tolerant Line 9611, and heat-sensitive ‘Taichung No. 1’ at 40/35°C and 25/20°C to elucidate the heat tolerance mechanism of leafy radish. After 28-day of acclimation, the maintenance of net photosynthetic rate (Pn) and chlorophyll fluorescence (Fv/Fm) accompanying a double increase of stomatal conductance (gs) and transpiration rate (E) in Line 9911-9 revealed that the heat-tolerant line had adjusted physiologically to be more efficient in heat dispersing. Morphologically, Line 9911-9 developed more xylem vessels, an enlarged stomatal aperture by 5-fold, and an increased stomatal opening to 93%, which contributed 6-fold increase in the occupancy of total stomata aperture on leaf without altering stomata density under high temperature stress. Furthermore, the occupancy was correlated with gs and E with a coefficient of 0.87 and 0.98, respectively, confirming that stomatal factors play a crucial role in heat tolerance in leafy radish. Regarding the heat shock response, Line 9911-9 could have sustained over 9 days at 40/35°C, whereas Line 9611 was maintained over 3 days, with ‘Taichung No. 1’ for only 1 day. Line 9911-9 was able to maintain its cellular membrane stability by inducing the activities of superoxide dismutase (SOD) and catalase (CAT) to scavenge reactive oxygen species (ROS) within the first 24h and detoxify ROS unbalance under high temperature stress. These results indicated that Line 9911-9 was able to lower the non-stomatal limitation effect on Pn. A high correlation (R2=0.89) between cotyledon relative injury at 50°C for 30min in 21 leafy radish accessions and their yield reduction ratio under high temperature stress suggests that the CMT of cotyledon may be used as a reliable index for selecting heat-tolerant leafy radish.

The type of competition modulates the ecophysiological response of grassland species to elevated CO2 and drought.

The effects of elevated CO2 and drought on ecophysiological parameters in grassland species have been examined, but few studies have investigated the effect of competition on those parameters under climate change conditions. The objective of this study was to determine the effect of elevated CO2 and drought on the response of plant water relations, gas exchange, chlorophyll a fluorescence and aboveground biomass in four grassland species, as well as to assess whether the type of competition modulates that response. Elevated CO2 in well-watered conditions increased aboveground biomass by augmenting CO2 assimilation. Drought reduced biomass by reducing CO2 assimilation rate via stomatal limitation and, when drought was more severe, also non-stomatal limitation. When plants were grown under the combined conditions of elevated CO2 and drought, drought limitation observed under ambient CO2 was reduced, permitting higher CO2 assimilation and consequently reducing the observed decrease in aboveground biomass. The response to climate change was species-specific and dependent on the type of competition. Thus, the response to elevated CO2 in well-watered grasses was higher in monoculture than in mixture, while it was higher in mixture compared to monoculture for forbs. On the other hand, forbs were more affected than grasses by drought in monoculture, while in mixture the negative effect of drought was higher in grasses than in forbs, due to a lower capacity to acquire water and mineral nutrients. These differences in species-level growth responses to CO2 and drought may lead to changes in the composition and biodiversity of the grassland plant community in future climate conditions.

Comparative analysis of drought stress effects on photosynthesis of Eurasian and North African genotypes of wild barley

The impact of drought stress (DS) on eight Eurasian and North African genotypes of wild barley (Hordeum spontaneum) was evaluated by analysis of chlorophyll (Chl) a fluorescence fast induction curves using the JIP-test. Three-week-old, pot-grown plants were exposed to a DS treatment by withholding water for nine days. The genotype-specific impairment of the functionality of the photosynthetic electron transport chain was quantified using the relative decline of the performance indices (PIabs and PItot), two key parameters of the JIP-test. The genotypes showing the highest (HOR10164) and lowest (HOR10710) relative PIs under DS were subjected to additional experiments, including measurements of leaf gas exchange, water status, pigment content, key enzyme activity, and protein abundance. The genotypes showed a specific profile of DS-mediated inhibition of photosynthesis, associated with higher relative leaf water contents in HOR10164 at the end of the treatment. Whereas decreased photosynthetic rate in HOR10164 was mainly caused by stomatal closure, nonstomatal limitations (decreased Rubisco content and activity) were detected in HOR10710. Additional genotype specific features were the upregulation of the NADP-malate dehydrogenase in HOR10164 and a decreased fraction of QA-reducing reaction centers in HOR10710.

Mitochondrial alternative oxidase maintains respiration and preserves photosynthetic capacity during moderate drought in Nicotiana tabacum.

The mitochondrial electron transport chain includes an alternative oxidase (AOX) that is hypothesized to aid photosynthetic metabolism, perhaps by acting as an additional electron sink for photogenerated reductant or by dampening the generation of reactive oxygen species. Gas exchange, chlorophyll fluorescence, photosystem I (PSI) absorbance, and biochemical and protein analyses were used to compare respiration and photosynthesis of Nicotiana tabacum 'Petit Havana SR1' wild-type plants with that of transgenic AOX knockdown (RNA interference) and overexpression lines, under both well-watered and moderate drought-stressed conditions. During drought, AOX knockdown lines displayed a lower rate of respiration in the light than the wild type, as confirmed by two independent methods. Furthermore, CO2 and light response curves indicated a nonstomatal limitation of photosynthesis in the knockdowns during drought, relative to the wild type. Also relative to the wild type, the knockdowns under drought maintained PSI and PSII in a more reduced redox state, showed greater regulated nonphotochemical energy quenching by PSII, and displayed a higher relative rate of cyclic electron transport around PSI. The origin of these differences may lie in the chloroplast ATP synthase amount, which declined dramatically in the knockdowns in response to drought. None of these effects were seen in plants overexpressing AOX. The results show that AOX is necessary to maintain mitochondrial respiration during moderate drought. In its absence, respiration rate slows and the lack of this electron sink feeds back on the photosynthetic apparatus, resulting in a loss of chloroplast ATP synthase that then limits photosynthetic capacity.

Flooding and Phytophthora cinnamomi: Effects on photosynthesis and chlorophyll fluorescence in shoots of non-grafted Persea americana (Mill.) rootstocks differing in tolerance to Phytophthora root rot

Losses in the production of avocado (Persea americana (Mill.)) are incurred due to Phytophthora root rot (PRR), a disease of the feeder roots that results in tree-dieback and eventual tree death. Avocado is also a flood-sensitive species and flooding exacerbates the effects of PRR. The avocado industry relies on the use of rootstocks tolerant to PRR to minimise losses. The present study compared the gas exchange and chlorophyll fluorescence responses of avocado rootstock plants of ‘Dusa™’, the current South African industry standard, with ‘Duke 7’, and the selections R0.12 and R0.06 which show reduced and superior tolerance to PRR, respectively. A decline in stomatal conductance (gs) and net CO2 assimilation (PN) over the 30day evaluation period were early responses to flooding. ‘Dusa™’, the more tolerant rootstock plants, demonstrated a better recovery in PN and gs in response to inoculation; however, both rootstocks performed poorly under flooded conditions. A decline in PN in infected ‘Duke 7’ plants appeared to be associated with stomatal limitations due to reduced stomatal conductance. The decline in PN and gs was not apparent in infected ‘Dusa™’ plants. Non-stomatal limitations to PN in rootstock plants exposed to flooding were also evident as indicated by increases in the ratio of internal to atmospheric CO2 concentrations (Ci/Ca). Impaired photosynthetic capacity in flooded rootstock plants was reflected by reduced photosystem II efficiency and photochemical quenching. In comparison to ‘Dusa™’, R0.12 rootstock plants showed reduced PN and gs following inoculation with Phytophthora cinnamomi whereas the more tolerant R0.06 rootstock plants revealed sustained photosynthetic activity. Interestingly R0.06 was the only rootstock able to maintain PN and gs in non-inoculated, flooded plants.

Short-term water stress impacts on stomatal, mesophyll and biochemical limitations to photosynthesis differ consistently among tree species from contrasting climates.

Predicting the large-scale consequences of drought in contrasting environments requires that we understand how drought effects differ among species originating from those environments. A previous meta-analysis of published experiments suggested that the effects of drought on both stomatal and non-stomatal limitations to photosynthesis may vary consistently among species from different hydroclimates. Here, we explicitly tested this hypothesis with two short-term water stress experiments on congeneric mesic and xeric species. One experiment was run in Australia using Eucalyptus species and the second was run in Spain using Quercus species as well as two more mesic species. In each experiment, plants were grown under moist conditions in a glasshouse, then deprived of water, and gas exchange was monitored. The stomatal response was analysed with a recently developed stomatal model, whose single parameter g1 represents the slope of the relationship between stomatal conductance and photosynthesis. The non-stomatal response was partitioned into effects on mesophyll conductance (gm), the maximum Rubisco activity (Vcmax) and the maximum electron transport rate (Jmax). We found consistency among the drought responses of g1, gm, Vcmax and Jmax, suggesting that drought imposes limitations on Rubisco activity and RuBP regeneration capacity concurrently with declines in stomatal and mesophyll conductance. Within each experiment, the more xeric species showed relatively high g1 under moist conditions, low drought sensitivity of g1, gm, Vcmax and Jmax, and more negative values of the critical pre-dawn water potential at which Vcmax declines most steeply, compared with the more mesic species. These results indicate adaptive interspecific differences in drought responses that allow xeric tree species to continue transpiration and photosynthesis for longer during periods without rain.

Severe water deficit-induced ethylene production decreases photosynthesis and photochemical efficiency in flag leaves of wheat

Wheat (Triticum aestivum L.) cv. Jimai22 was used to evaluate the effect of ethylene evolution rate (EER) and 1-aminocyclopropane-1-carboxylic acid (ACC) and their relations with photosynthesis and photochemical efficiency in plants well-watered (WW) and under a severe water deficit (SWD). SWD caused a noticeable reduction in the grain mass. The marked increases in both EER and the ACC concentration were observed under SWD; it was reversed effectively by exogenous spermidine (Spd) or amino-ethoxyvinylglycine (AVG). Thermal images indicated that SWD increased obviously the temperature of flag leaves, mainly due to the decrease in transpiration rate under SWD. Exogenous Spd or AVG decreased to some extent the temperature of the flag leaves. The strong decline in photosynthetic rate (P N) and stomatal conductance as well as the photodamage of PSII were also observed under SWD after 14 and 21 days after anthesis (DAA). Intercellular CO2 concentration was reduced at 7 DAA, but slightly increased at 14 and 21 DAA under SWD, indicating that the decreased P N at 7 DAA might result from stomatal limitations, while the decline after 14 and 21 DAA might be attributed to nonstomatal limitations. Correlation analysis suggested that EER and ACC showed negative relations to photosynthesis and photochemical efficiency. Data obtained suggested that the effects of SWD were mediated predominantly by the increase in EER and ACC concentration, which greatly decreased the leaf photosynthesis and photochemical efficiency, and, therefore, reduced the grain mass. Application of Spd or AVG reduced the EER and ACC, and thus positively influenced photosynthesis and photochemical efficiency under SWD.

Impacts of Salinity and Nitrogen on the Photosynthetic Rate and Growth of Sunflowers (Helianthus annuus L.)

Understanding the interactions between salinity and fertilizers is of significant importance for enhancing crop yield and fertilizeruse efficiency. In this study a complete block design experiment was performed in the Hetao Irrigation District of Inner Mongolia, China, to evaluate the effects of interactions between soil salinity and nitrogen (N) application rate on sunflower photosynthesis and growth and to determine the optimum N application rate for sunflower growth in the district. Four levels of soil salinity expressed as electrical conductivity (0.33–0.60, 0.60–1.22, 1.2–2.44, and 2.44–3.95 dS m−1) and three application rates of N fertilization (90, 135, and 180 kg ha−1) were applied to 36 micro-plots. Soil salinity inhibited the photosynthetic rate, stomatal conductance, transpiration rate, plant height, leaf area, and aboveground dry matter of sunflowers. The intercellular CO2 concentration first decreased and then increased with increasing soil salinity in the seedling stage, and the instantaneous leaf water-use efficiency fluctuated with soil salinity. The stomatal and non-stomatal limitations of sunflowers alternated in the seedling stage; however, in the bud, blooming, and mature stages, the stomatal limitation was prevalent when the salinity level was lower than 2.44 dS m−1, whereas the non-stomatal limitation was predominant above the salinity level. The application of N fertilizer alleviated the adverse effects of salinity on sunflower photosynthesis and growth to some extent. During some key growth periods, such as the seedling and bud stages, a moderate N application rate (135 kg ha−1) resulted in the maximum photosynthetic rate and yielded the maximum dry matter. We suggest a moderate N application rate (135 kg ha−1) for the Hetao Irrigation District and other sunflower-growing areas with similar ecological conditions.

Gas exchange and chlorophyll fluorescence of pea (Pisum sativum L.) plants in response to ambient ozone at a rural site in Egypt

Egyptian pea cultivars (Pisum sativum L. cultivars Little Marvel, Perfection and Victory) grown in open-top chambers were exposed to either charcoal-filtered (FA) or non-filtered air (NF) for five consecutive years (2009–2013) at a rural site in northern Egypt. Net photosynthetic rates (PN), stomatal conductance (gs), intercellular CO2 (Ci) and chlorophyll fluorescence were measured.Ozone (O3) was found to be the most prevalent pollutant common at the rural site and is suspected to be involved in the alteration of the physiological parameters measured in the present investigation.PN of different cultivars were found to respond similarly; decreases of 23, 29 and 39% were observed in the cultivars Perfection, Little Marvel and Victory, respectively (averaged over the five years) due to ambient O3. The maximum impairment in PN was recorded in the cultivar Victory (46%) in 2013 when the highest O3 levels were recorded (90nLL−1). The average stomatal conductance decreased by 20 and 18% in the cultivars Little Marvel and Perfection, respectively, while the average stomatal conductance increased on average by 27% in the cultivar Victory. A significant correlation was found between PN and Ci, indicating the importance of non-stomatal limitations of photosynthesis, especially in the cultivar Victory. The PN vs. Ci curves were fitted to a non-rectangular hyperbolic model.The actual quantum yield (ΦPSII) and photochemical quenching coefficient (qP) were significantly decreased in the leaves of plants exposed to NF air. Non-photochemical quenching (NPQ) was increased in all cultivars.Exposure to NF air caused reductions in chlorophyll (Chl a) of 19, 16 and 30% in the Little Marvel, Perfection and Victory cultivars, respectively.

Non-stomatal limitation to photosynthesis in Cinnamomum camphora seedings exposed to elevated O3.

Ozone (O3) is the most phytotoxic air pollutant for global forests, with decreased photosynthesis widely regarded as one of its most common effects. However, controversy exists concerning the mechanism that underlies the depressing effects of O3 on CO2 assimilation. In the present study, seedlings of Cinnamomum camphora, a subtropical evergreen tree species that has rarely been studied, were exposed to ambient air (AA), ambient air plus 60 [ppb] O3 (AA+60), or ambient air plus 120 [ppb] O3 (AA+120) in open-top chambers (OTCs) for 2 years. Photosynthetic CO2 exchange and chlorophyll a fluorescence were investigated in the second growing season (2010). We aim to determine whether stomatal or non-stomatal limitation is responsible for the photosynthesis reduction and to explore the potential implications for forest ecosystem functions. Results indicate that elevated O3 (E-O3) reduced the net photosynthetic rates (P N) by 6.0-32.2%, with significant differences between AA+60 and AA+120 and across the four measurement campaigns (MCs). The actual photochemical efficiency of photosystem II (PSII) in saturated light (Fv ′/Fm ′) was also significantly decreased by E-O3, as was the effective quantum yield of PSII photochemistry (ΦPSII). Moreover, E-O3 significantly and negatively impacted the maximum rates of carboxylation (V cmax) and electron transport (J max). Although neither the stomatal conductance (g s) nor the intercellular CO2 concentration (C i) was decreased by E-O3, P N/g s was significantly reduced. Therefore, the observed reduction in P N in the present study should not be attributed to the unavailability of CO2 due to stomatal limitation, but rather to the O3-induced damage to Ribulose-1,5-bisphosphate carboxylase/oxygenase and the photochemical apparatus. This suggests that the down-regulation of stomatal conductance could fail to occur, and the biochemical processes in protoplasts would become more susceptible to injuries under long-term O3 exposure, which may have important consequences for forest carbon and water budget.