Midday Depression Of Photosynthesis Research Articles

Heat and Drought Have Exacerbated the Midday Depression Observed in a Subtropical Fir Forest by a Geostationary Satellite

Recently, increasing heat and drought events have threatened the resilience of Chinese fir forests. Trees primarily respond to these threats by downregulating photosynthesis including through stomatal limitation that causes a drop in productivity at noon (known as the midday depression). However, the effects of these events on midday and afternoon GPP inhibition are rarely analyzed on a fine timescale. This may result in negligence of critical responses. Here, we investigated the impact of climatic events on the midday depression of photosynthesis at a subtropical fir forest in Huitong from 2016 to 2022 using data from the Himawari 8 meteorological satellite and flux tower. Our results indicated that the highest number of midday depression occurred in 2022 (126 times) with the highest average temperature (29.1 °C). A higher incidence of midday depression occurred in summer and autumn, with 48 and 34 occurrences, respectively. Compound drought, heat, and drought events induced increases in midday depression at 74.3%, 66.0%, and 47.5%. Thus, trees are more likely to adopt midday depression as an adaptive strategy during compound drought and heat events. This study can inform forest management and lead to improvements in Earth system models.

Evaluate the photosynthesis and chlorophyll fluorescence of Epimedium brevicornu Maxim

The diurnal variation of photosynthesis, light response curve and CO2 response curve in Epimedium brevicornu Maxim leaves were determined with Li-6400 photosynthesis system to evaluate the photosynthesis of E. brevicornu. Fluorescence of chlorophyll in the leaves were determined with PAM-2500 portable chlorophyll fluorescence apparatus in the study. The results showed that the midday depression of photosynthesis was very obvious in the E. brevicornu leaves. The light compensation point of E. brevicornu leaves was about 15 µmol m−2 s−1. The light saturation point of E. brevicornu leaves was below 800 µmol m−2 s−1, which was lower than the general sunlight intensity at noon in summer. The CO2 saturation point of E. brevicornu leaves was much higher than the content of CO2 in general air. E. brevicornu was a typical shade plant and could survive in very low sunlight. E. brevicornu could not endure strong sunlight and high air temperature. The net photosynthetic rate of E. brevicornu leaves linearly correlated with the content of CO2 in the leaf chamber when the content was below CO2 saturation point. E. brevicornu possessed great potential of photosynthesis.

Analysis of photosynthetic adaptability and shade tolerance of lianas in riverside

This investigation aimed to study the photosynthetic adaptability and shade tolerance of liana in riverside green space. For this purpose, the illumination intensity gradient is set to 2000, 1800, 1500, 1200, 1000, 800, 600, 400, 200, 100, 0 μmol·m-2·s-1, and the corresponding net photosynthetic rate (Pn) value is determined. After the data on the photosynthetic tester is stable, read the value and repeat the measurement 3 times for each gradient; Net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Cond), and intercellular CO2 concentration (Ci) were measured. Instantaneous water use efficiency (WUE) was calculated by Pn/Tr; Typha under different shading conditions was marked, and the leaf number, plant height, crown width, and other growth and development indicators of Typha were observed and recorded regularly. The data were analyzed by software to find out the law of the influence of shading on the growth of Typha. Results showed that the diurnal variation of net photosynthetic rate (Pn) of Typha and Canna showed a "single peak" curve in May and August, and there was no midday depression of photosynthesis, indicating that the two plants had strong adaptability to different light intensities. Shading has a significant impact on the growth of Typha. With the increase of shading degree, the plant height, leaf width and other indicators of Typha have a significant increasing trend, and there is a positive correlation. Still, it is not that the stronger the shading, the better. It is concluded that appropriate light and shade can promote the growth of liana in riverside green space.

Alleviatory effect of rare earth micro-fertilizer on photosystem II (PSII) photoinhibition in Pseudostellaria heterophylla leaves at photosynthetic midday depression

In order to explore the mitigatory mechanism of rare earths on the midday depression of photosynthesis, this study used fast-phase chlorophyll fluorescence technology combined with an antioxidant enzyme system to analyze the effects of lanthanum and cerium on the photosystem II (PSII) function of Pseudostellaria heterophylla leaves at noon. The results show that the maximum photochemical efficiency (Fv/Fm) and the photosynthetic performance index (PIABS) of P. heterophylla leaves during the photosynthetic noon-break are only 0.73 and 0.86, respectively. The leaves of P. heterophylla show obvious photoinhibition. The application of two rare earth fertilizers promotes the light energy absorption, capture, conversion, and electron transfer efficiencies of PSII in the P. heterophylla leaves and alleviates the photoinhibition caused by excess excitation energy. Additionally, the effect of lanthanum is better than that of cerium. In the fertilization groups, P. heterophylla leaves show higher PSII photochemical activity. Fertilization protects the oxygen-evolving complex (OEC) activity of P. heterophylla, thus reducing oxidative damage to the chloroplasts and preventing the dissociation of the thylakoids to maintain the normal physiological functioning of PSII, thereby reducing the degree of photoinhibition. Additionally, fertilization increases the electron transfer ability from QA to QB on the PSII electron receptor side of P. heterophylla. Correlation analysis shows that the fluorescence parameters VL, VK, and VJ are significantly negatively correlated with antioxidant enzyme activity and are significantly positively correlated with malondialdehyde content, relative electric conductivity and reactive oxygen species (ROS), which further indicates that rare earths can enhance the ROS scavenging system in plants and reduce the degree of membrane lipid peroxidation and plasma membrane damage, thereby maintaining the structure and function of the OEC on the donor side of PSII, increasing the electron receptor pool, and contributing to the defense against photoinhibition.

Effects of shading and micro‐spray of canopy on photosynthetic characteristics, quality and yield of drip‐irrigated grapes*

AbstractGrapes are prone to midday depression of photosynthesis (MDP) under the action of high temperature and strong light, which can reduce the accumulation of organic matter, resulting in a decline in yield and quality. In this study, the grapes in Shanshan County, Xinjiang, China, were taken as an example to implement shading and micro‐spray measures. Shading degrees of 0, 15 and 30% were combined with micro‐spray flow rates of 20, 30 and 40 L h−1. Nine treatments and one control treatment (the control treatment is without shading and mist spraying) were designed through a combination of the shading and micro‐spray conditions. The photosynthetic prediction model of the multivariate nonlinear model was constructed and had high simulation accuracy. The decline of air temperature at the top of the canopy increased as the degree of shading and the amount of micro‐spray increased; MDP could be eliminated under the combined conditions of shading and micro‐spray. The mean daily net CO2 assimilation rate (An) increased by more than 0.53 μmol m−2 s−1. The combination of shading and micro‐spray conditions significantly increased fruit size and promoted greenness. The WP1 (15% and 40 L h−1) treatment had the highest mean daily An and the largest fruit volume. The vitamin C (VC) content of WP1 treatment was the highest, 4.13 mg kg−1, which was 1.2 times that of the control (CK) treatment. The yield of the WP1 treatment was the highest, which was 19.7% higher than that of the CK treatment. Thus, the WP1 treatment was the optimal treatment for promoting the An of grape leaves and improving the fruit quality of grapes. This research could provide a reference for improving the photosynthesis, fruit quality and yield of grapes and alleviating the photosynthetic MDP of crops.

PHOTO-SENSITIVE LEAF ROLLING 1 encodes a polygalacturonase that modifies cell wall structure and drought tolerance in rice.

The biosynthesis and modification of cell wall composition and structure are controlled by hundreds of enzymes and have a direct consequence on plant growth and development. However, the majority of these enzymes has not been functionally characterised. Rice mutants with leaf-rolling phenotypes were screened in a field. Phenotypic analysis under controlled conditions was performed for the selected mutant and the relevant gene was identified by map-based cloning. Cell wall composition was analysed by glycome profiling assay. We identified a photo-sensitive leaf rolling 1 (psl1) mutant with 'napping' (midday depression of photosynthesis) phenotype and reduced growth. The PSL1 gene encodes a cell wall-localised polygalacturonase (PG), a pectin-degrading enzyme. psl1 with a 260-bp deletion in its gene displayed leaf rolling in response to high light intensity and/or low humidity. Biochemical assays revealed PG activity of recombinant PSL1 protein. Significant modifications to cell wall composition in the psl1 mutant compared with the wild-type plants were identified. Such modifications enhanced drought tolerance of the mutant plants by reducing water loss under osmotic stress and drought conditions. Taken together, PSL1 functions as a PG that modifies cell wall biosynthesis, plant development and drought tolerance in rice.

Photosynthetic and Chlorophyll Fluorescence Characteristics of Isodon rubescens (Hemsley) H. Hara

The ecological and economic cultivation of Isodon rubescens is currently being carried out. The demand of I. rubescens for light intensity should be made clear to estimate whether the environmental conditions of an area are suitable for cultivating I. Rubescens and improve cultivation techniques. The photosynthetic and chlorophyll fluorescence characteristics of I. rubescens were determined with a Li-6400 photosynthesis system and PAM-2500 portable chlorophyll fluorescence apparatus. The results showed that there was no obvious midday depression of photosynthesis in I. rubescens leaves. The light compensation point and light saturation point of I. rubescens leaves were 21.83482 µmol·m−2·s−1 and 802.262 µmol·m−2·s−1, respectively. The CO2 compensation point and CO2 saturation point of I. rubescens leaves were 101.7199 µmol·mol−1 and 1674.514 µmol·mol−1, respectively. The maximal photochemical efficiency of photosystem II ((Fm-Fo)/Fm) in I. rubescens leaves reached 0.7. The electron transport rate of photosystem II in I. rubescens leaves reached 20 μmol electrons/(m2·s). I. rubescens can tolerate intense light above the light compensation point and utilize low light. I. rubescens leaves have a strong photoprotective capacity. I. rubescens can grow in both sunny and shady places. The most important factor affecting photosynthetic efficiency in I. rubescens leaves is the concentration of CO2 in air.

Light stress-induced chloroplast movement and midday depression of photosynthesis in sorghum leaves

ABSTRACTPlants are exposed to high light intensity, high leaf temperatures and high air-to-leaf water vapor pressure deficit (ALVPD) during the day. These environmental stresses cause stomatal closure and photoinhibitory damage, leading to midday depression of photosynthesis. Chloroplast positioning is essential for the efficient operation of photosynthesis. However, chloroplast behavior before, during, and even after the midday depression of photosynthesis remains unknown. We investigated changes in the intracellular positioning of chloroplasts and photosynthetic traits under a diurnal pattern of light. Sorghum leaves were exposed to a 12-h regime of light mimicking the natural light environment, with constant leaf temperature and ALVPD. Net photosynthetic rate (Pn) showed a diurnal pattern, and midday depression in Pn was observed at 3.8 h of irradiation. Depression in Pn was attributed to stomatal limitation because the decrease in Pn was in accordance with the decrease in stomatal conductance. The maximum efficiency of photosystem II decreased with the increase in light intensity and remained low after 12 h of irradiation. Bundle sheath chloroplasts swelled after 8 h of irradiation, representing the accumulation of starch. Conversely, mesophyll chloroplasts exhibited avoidance response after 4 h of irradiation, and the avoidance position was maintained during the remainder of the daytime. These data suggest that chloroplasts are subject to light stress during and after the midday depression of photosynthesis. The intensity of natural light is excessive for most of the day and this light stress induces chloroplast avoidance response and depression of photosynthesis.

Improving leaf area index retrieval using spectral characteristic parameters and data splitting

ABSTRACT Spectral variables such as spectral characteristic parameters (SCPs) commonly change with intraday phenology. Empirical retrieval methods, which are generally used in leaf area index (LAI) retrieval due to their simplicity and computational efficiency, typically relate the biophysical parameter of interest to the spectral variable during the whole observation period. Whilst information regarding diurnal changes in spectral variables is necessary and useful in applied contexts. We analysed the diurnal change characteristics of canopy spectral reflectance and SCPs of winter wheat in the jointing stage based on field data collected at fixed sampling points with different vegetation canopies, and validated the effectiveness of data splitting strategy with field data collected in random sample pattern. The key results are as follows: (i) Canopy spectral reflectance of winter wheat in the jointing stage exhibited clear intraday variability, typically presenting a double-peak characteristic occurring from 11:35 to 12:34, where the reflectance changed substantively during this period. (ii) The SCPs of winter wheat in the jointing stage exhibited different diurnal patterns. Specifically, the blue edge position presented ‘blue shifts’, the yellow edge position generally exhibited steady fluctuations, and the red edge position followed divergent trends between the two sampling points due to differences in the vegetation canopy. Amplitude and area parameters exhibited a double-peak characteristic but there were slight differences between them. (iii) By dividing the whole observation period into sub-periods, the coefficient of variation (CV) of each spectral characteristic parameter can be greatly reduced, whilst the coefficient of determination (R 2) of LAI retrieval can be greatly increased. Optimal spectral parameters and sub-periods for LAI retrieval were confirmed based on the diurnal variation of SCPs. To optimize LAI retrieval the suggested spectral parameters are blue edge amplitude, red edge amplitude, and red edge area, and the sub-periods are 09:50–11:35, 11:35–12:34, 12:34–13:50, and 13:50–15:00, respectively. The 11:35–12:34 sub-period should be carefully considered due to possible midday depression of photosynthesis.

GASEOUS CO2 EXCHANGE IN SILVER BIRCH SAPLINGS GROWING UNDER DIFFERENT LEVELS OF NITROGEN SUPPLY

A comparative assessment of gaseous CO 2 exchange in leaves of 5–6 years old saplings o f two forms of silver birch ( Betula pendula and Betula pendula var. сarelica ) and downy birch ( Betula pubescens Ehrh., 1789) growing under different levels of nitrogen nutrition was done. The study was carried out in five plots (control sector, three sectors fertilized with different doses of NH 4 NO 3 , and a sector with NPK fertilization) at the Agrobiological Research Station of the Karelian Research Centre RAS located at the southern outskirts of Petrozavodsk. Both in the control sector (where the content of nitrogen was 0.23 ± 0.04 %) and in most of the fertilized sectors, there was no significant difference between the mean values of photosynthesis in B. pendula and B. pendula var. сarelica either for the entire PhAR range or for PhAR > 1400 μmol m- 2 с -1 . Nitrogen fertilization had a beneficial effect on the growth and the magnitude of gaseous CO 2 exchange of the studied objects. The calculated parameters of light response curves for net photosynthesis have shown an improved performance of the photosynthetic apparatus upon fertilization. Where PhAR values were low, its efficiency in Karelian birch was much higher compared with silver birch ( B. pendula ), and increased with the elevation of the nitrogen dosage. In the conditions of soil moisture shortage, there was no significant difference between chlorophyll content in leaves of fertilized and non-fertilized plants. The carotenoid content in the leaves of non-fertilized plants was 1.3–1.5‑fold that of fertilized plants. Even a small dose of nitrogen fertilizer (20 g per tree) had a favorable effect on the water content of leaves in a dry period. In the second year of fertilization, the midday depression of photosynthesis increased considerably, especially in the sectors with high doses of nitrogen fertilizer. In the control sector, the decrease in photosynthesis in the absence of air drought and with sufficient soil moisture content was 20–25 %, and in fertilized plots there was a more than three-fold decrease in photosynthesis from 13:00 to 16:00. As a result, on sunny days in the second part of the growing season daily photosynthetic production in fertilized plants was lower compared to non-fertilized ones. In the first year of fertilization, there was no strong midday depression of photosynthesis, and nitrogen fertilization promoted CO 2 uptake on both cloudy and sunny days.

Photosynthetic Properties and Potentials for Improvement of Photosynthesis in Pale Green Leaf Rice under High Light Conditions.

Light is the driving force of plant growth, providing the energy required for photosynthesis. However, photosynthesis is also vulnerable to light-induced damage caused by the production of reactive oxygen species (ROS). Plants have therefore evolved various protective mechanisms such as non-photochemical quenching (NPQ) to dissipate excessively absorbed solar energy as heat; however, photoinhibition and NPQ represent a significant loss in solar energy and photosynthetic efficiency, which lowers the yield potential in crops. To estimate light capture and light energy conversion in rice, a genotype with pale green leaves (pgl) and a normally pigmented control (Z802) were subjected to high (HL) and low light (LL). Chlorophyll content, light absorption, chloroplast micrographs, abundance of light-harvesting complex (LHC) binding proteins, electron transport rates (ETR), photochemical and non-photochemical quenching, and generation of ROS were subsequently examined. Pgl had a smaller size of light-harvesting chlorophyll antenna and absorbed less photons than Z802. NPQ and the generation of ROS were also low, while photosystem II efficiency and ETR were high, resulting in improved photosynthesis and less photoinhibition in pgl than Z802. Chlorophyll synthesis and solar conversion efficiency were higher in pgl under HL compared to LL treatment, while Z802 showed an opposite trend due to the high level of photoinhibition under HL. In Z802, excessive absorption of solar energy not only increased the generation of ROS and NPQ, but also exacerbated the effects of increases in temperature, causing midday depression in photosynthesis. These results suggest that photosynthesis and yield potential in rice could be enhanced by truncated light-harvesting chlorophyll antenna size.

Stomatal Conductance Is Essential for Higher Yield Potential of C3 Crops

The present review of gas-exchange studies conducted in rainfed or irrigated field conditions clearly indicates that stomatal conductance (gs) of major crops has been inadvertently increased in the last 50 to 80 years as higher yields were being reached through conventional plant breeding. These findings suggest that high gs rates are critical to optimum growth and yield of modern crops, in particular to crop growth rate (CGR) during seed formation and filling. Several hypotheses are presented that may account for this increase. This review also includes studies in which increases in gs were documented for genetically-engineered plants. However, field evaluations of their gas-exchange performance remain sparse. The main finding of this review is that higher rates of gs should be sought, as we forego the usual consideration of crop plants as single organisms losing water, but rather acknowledge that water does exit a crop canopy along a Soil Plant Atmosphere Continuum (SPAC). In non-limiting or moderately-limiting soil water conditions, high yields are generated by an active SPAC that requires sustained levels of gs. In conditions of high evaporative demand, as in summer afternoons, sustained gs allows for cooler canopies with no mid-day depression in photosynthesis as CO2 remains able to diffuse in the leaf through stomata.

Study on the Growth and the Photosynthetic Characteristics of Low Energy C+ Ion Implantation on Peanut

Employing the Nonghua 5 peanut as experimental material, the effects of low energy C+ ion implantation on caulis height, root length, dry weight, photosynthetic characteristics and leaf water use efficiency (WUE) of Peanut Ml Generation were studied. Four fluences were observed in the experiment. The results showed that ion implantation harmed the peanut seeds because caulis height, root length and dry weight all were lower in the treatments than in CK, and the harm was aggravated with the increase of ion fluence. Both Pn and Tr show a saddle-shape curve due to midday depression of photosynthesis. Low fluence of low energy C+ ion implantation could increase the diurnal average Pn of peanut. The diurnal variation of Tr did not change as significantly as Pn. The light saturation point (LSP) was restrained by the ions. After low energy C+ ion implantation, WUE was enhanced. When the fluence increased to a certain level, the WUE began to decrease.

Effect of limited irrigation on diurnal variation in flag-leaf photosynthesis and yield of dryland wheat

在大田条件下, 以“青麦6 号”小麦为试验材料, 研究了5 个灌水处理下旱地冬小麦灌浆期旗叶光合特性日变化和产量的变化。结果表明, 5 个灌水处理净光合速率日变化曲线均为双峰曲线, 但随着灌水次数的增加, “午休”明显减弱。在灌溉低于3 水的情况下, 旗叶的净光合速率、气孔导度都随灌溉量的增加呈上升趋势,均以灌3 水(W3, 拔节水60 mm+孕穗水60 mm+灌浆水60 mm)处理最高, 而胞间CO₂ 浓度和气孔限制值随着灌溉量的增加呈下降趋势; 而灌溉4 水(W4, 起身水60 mm+拔节水60 mm+孕穗水60 mm+灌浆水60 mm)处理的净光合速率明显小于W3 处理, 表明过量灌溉对旱地小麦灌浆期旗叶的光合作用有消极作用。小麦产量以灌2 水(W2, 拔节水60 mm+孕穗水60 mm)处理为最高, 且在灌溉2 水以下随着灌溉次数增加而增加, 但灌1 水(W1, 拔节水60 mm)和W2 处理间产量差异不显著, 同时灌溉3 水(W3)和4 水(W4)会使产量下降, 反而低于旱地处理, 因此过多灌水不利于旱地小麦的高产。试验结果还表明, 拔节期、孕穗期和灌浆期灌溉对旱地小麦旗叶灌浆中期达到较高光合作用至关重要, 而起身水对旱地小麦光合作用无显著影响。拔节期是旱地小麦达到高产最重要的灌溉时期, 拔节~孕穗期为冬小麦需水关键期, 拔节~孕穗水对冬小麦产量和水分利用效率有重要影响。水分生产效率也随着灌水量的增加呈下降趋势。综合产量和水分利用效率因素, 以灌溉拔节水60 mm(W1)处理为达到旱地冬小麦高产的最佳灌溉模式。

Effects of controlling apple orchard productivity on soil moisture and photosynthetic characteristics

Aims Our objective is to explore the effects of controlling productivity on resource use in an apple orchard in order to improve sustainability and efficiency. Methods We investigated the effects of different productivities (3.6 × 105,3.15 × 105,2.7 × 105,2.25 × 105 and 1.8 × 105 ind.hm–2) on soil moisture and photosynthetic characteristics in 'Fuji' apple trees during the fruit-bearing period using a neutron probe (503DR) and Li-6400 portable photosynthetic analysis system. Important findings Soil moisture at every soil layer increased with decreased productivity,with the maximum increment of 31.02% at the 60 cm depth. Within the 600 cm profile,water storage capacity could increase 15.41%. With decreased productivity,midday depression of photosynthesis was abated,net photosynthetic rate (Pn) increased,the highest Pn increased 25.71% compared with the control treatment (3.6 × 105 ind.hm–2),the transpiration rate (Tr) was decreased in the afternoon and the optimum water use efficiency (WUE) was improved by 34.12% compared with the control. There were significant correlations between water storage capacity (WSC) and Pn,Tr and WUE (Pearson correlation coefficients of 0.973**,–0.543* and 0.992**,respectively). The regression model of WSC (x) and WUE (y) was y = 0.002 3x – 1.480 6,R2 = 0.984 4**. Controlling productivity improved soil moisture and photosynthetic characteristics and increased WUE,improving harmonious,sustainable development of nature,the economy and society in the apple production area.

Midday depression of photosynthesis is related with carboxylation efficiency decrease and D1 degradation in bayberry ( Myrica rubra) plants

Diurnal patterns of photosynthesis in two-year-old Myrica rubra young trees under natural conditions were studied by measuring gas exchange, chlorophyll fluorescence parameters and D1 protein. When measured on a clear day, the diurnal changes of net photosynthetic rate ( P n), stomatal conductance ( G s) and apparent quantum yield (AQY) show two daily maxima; the maximum value occurred at about 09:00 h, then declined, and reached the lowest values at about 13:00 h then increased to reach the second maxima at about 15:00 h. However, with the consistent decline of P n and G s in the afternoon, the ratio of intercellular CO 2 concentration ( C i) to atmospheric CO 2 concentration ( C a) increased. In addition, carboxylation efficiency (CE) and RUBP regeneration declined as the afternoon progressed. In the morning, the maximum yield of fluorescence after dark adaptation ( F m) and maximal photochemical efficiency of PSII ( F v/ F m) decreased continuously until it reached its minimum at 13:00 h, while the reverse occurred in the later afternoon. The quantum yield of PSII ( Φ PSII) declined after 09:00 h; in contrast, initial fluorescence ( F o) increased. A decrease in the rate of Q A reduction and an increase in inactive PSII reaction centers was observed as the day progressed. Non-photochemical quenching (qN) and its slow-relaxing (qS) component increased at about midday, while the fast-relaxing component (qF) declined. The amount of inactive PSII centers was significantly enhanced, while F v/ F m, Φ PSII, qS and rate of Q A reduction were significantly reduced by DTT (dithiothreitol), an inhibitor of the xanthophyll cycle. The D1 protein was significantly degraded at 13:00 h relative to that at 09:00 h during the course of the day. These results suggest that stomatal and non-stomatal limitations, which decreased carboxylation and photochemical efficiency, may cause the midday depression; and that non-stomatal limitations may be due to the decrease in RuBPCase activity and degradation of D1 protein, which causes decreased photoprotection at midday.

Contribution of diffusional and non-diffusional limitations to midday depression of photosynthesis in Arbutus unedo L.

It is still unknown whether the midday depression of photosynthesis under severe water stress, frequently observed in plants growing in a Mediterranean-type climate, is primarily a consequence of diffusional or non-diffusional limitations. We carried out combined measurements of gas exchanges and chlorophyll fluorescence in field-grown Arbutus unedo L. trees during late spring and mid summer, and a quantitative limitation analysis was performed to distinguish between the different limitations to photosynthesis, i.e., diffusional [DL = stomatal (SL) + mesophyll (MCL)] and non-diffusional (carboxylation capacity and electron transport, BL) limitations. Light-saturated assimilation at ambient CO2 (Amax), stomatal conductance to water vapour (gsw) and maximum carboxylation rate (VcmaxCi) showed a marked midday depression during both periods. The total limitations tended to increase during the day and were remarkably similar in June and July (50 and 48%, respectively); on a daily basis, DL was similar to BL (about 23%) in June; whereas, in July the former was predominant (38 and 4%, respectively). We concluded that the midday depression in photosynthesis was largely caused by diffusional limitations, with non-diffusional limitations playing a smaller role. Although stomatal closure was the main diffusional limitation, the decline in mesophyll conductance was not negligible during the hottest and driest period.

Effect of nitrogen-deficiency on midday photoinhibition in flag leaves of different rice (Oryza sativa L.) cultivars

Effects of nitrogen (N)-deficiency on midday photoinhibition in flag leaves were compared between two contrastive Japanese rice cultivars, a traditional japonica cultivar with low yield, cv. Shirobeniya (SRB), and a japonica-indica intermediate type with high yield, cv. Akenohoshi (AKN). Both cultivars were grown under high-N and low-N conditions. At midday, low-N supply resulted in more intensive reductions in net photosynthetic rate, stomatal conductance, maximal quantum yield of photosystem II (PSII) and quantum yield of PSII electron transport in SRB than in AKN, indicating that SRB was more strongly photoinhibited than AKN under low-N condition. At midday, the low-N plants of two cultivars showed higher superoxide dismutase (SOD) activities than the high-N plants. However, ascorbate peroxidase (APX) activity was maintained in AKN but significantly decreased in SRB under low-N condition (N-deficiency). In contrast, hydrogen peroxide (H2O2) content in SRB significantly increased under low-N condition, indicating that the susceptibility to midday photoinhibition in the low-N plants of SRB is related to the increased H2O2 accumulation. It is suggested that the midday depression in photosynthesis may be a result of oxidative stress occurring in the low-N plants in which antioxidant capacity is not enough to cope with the generation of H2O2. Therefore, H2O2-scavenging capacity could be an important factor in determining the cultivar difference of midday photoinhibition in flag leaves of rice under low-N condition.

Photosynthetic characteristics of spring ephemerals in the desert ecosystem of Dzungaria Basin, northwest China

Spring ephemerals have the highest photosynthetic rates in higher plants. Gas exchanges and related environmental factors of four typical spring ephemerals, Eremopyrum orientale, Malcolmia scorpioides, Tetracme quadricormis and Arnebia decumbens were measured in their natural environments in the Gurbantunggut Desert of the Dzungaria Basin, northwestern China. Diurnal variations in net photosynthetic rate (P N), transpiration rate (E), stomatal conductance (gs), and water use efficiency (WUE) were analyzed at individual leaf level for these species. P N of E. orientale and M. scorpioides showed a bimodal diurnal pattern, whereas that of T. quadricormis and A. decumbens showed a unimodal pattern, with the maximum value occurred at noon (around 12:00). E of all four species showed a unimodal pattern. Higher WUE in the morning was related to higher P N and lower E. The maximum value of gs appeared in early morning, and then decreased gradually during the daytime. The highest photosynthetically active radiation (PAR) occurred at 14:00, but there was a 2 h lag between the highest temperatures of air and leaf and the highest PAR. The maximum net photosynthetic rate (P Nmax) of the four spring ephemerals were 18.44, 32.03, 33.17, and 20.04 μmolCO2 m−2 s−1, respectively. The present study revealed that the midday depression of photosynthesis of E. orientale and M. scorpioides was mainly due to non-stomatal limitation, whereas that of T. quadricormis and A. decumbens was mainly due to stomatal limitation during 14:00–16:00 followed by non-stomatal limitation after 16:00. Our results illustrated that the change of E was consistent with the diurnal fluctuation of air temperature, but the diurnal change of P N was affected by the whole microclimate, i.e. temperature, relative humidity (RH), PAR, etc.

Physiological responses of different olive genotypes to drought conditions

Gas exchange rates, chlorophyll fluorescence, pressure–volume relationships, photosynthetic pigments, total soluble sugars, starch, soluble proteins and proline concentrations were investigated in five Olea europaea L. cultivars with different geographical origins (Arbequina, Blanqueta, Cobrancosa, Manzanilla and Negrinha) grown under Mediterranean field conditions. We found considerable genotypic differences among the cultivars. Comparing the diurnal gas exchange rates, we observed that Cobrancosa, Manzanilla and Negrinha had high photosynthetic rate than Arbequina and Blanqueta. The first group reveals to be better acclimated to drought conditions, and appears to employ a prodigal water-use strategy, whereas Blanqueta and Arbequina, with high water-use efficiency, appear to employ a conservative water-use strategy. The degree of midday depression in photosynthesis was genotype dependent, with a maximum in Arbequina and a minimum in Negrinha. The reductions in the photosynthetic rate were dependent from both stomatal and non-stomatal limitations. Elastic adjustment plays an important role as drought tolerance mechanism. The group of cultivars that employ a prodigal water-use strategy revealed high tissue elasticity, whereas Arbequina and Blanqueta revealed high tissue rigidity. We also identified the existence of drought tolerance mechanisms associated with soluble proteins accumulation in the foliage. The high levels of soluble proteins in Arbequina may represent an increased activity of oxidative stress defence enzymes and may also represent a reserve for post stress recovery. In all cultivars, especially in Manzanilla, free proline was accumulated in the foliage. The discussed aspects of drought stress metabolism may have an adaptative meaning, supporting the hypothesis that olive cultivars native to dry regions, such as Cobrancosa, Manzanilla and Negrinha, have more capability to acclimate to drought conditions than cultivars originated in regions with a more temperate climate, like Arbequina and Blanqueta.