Radiation Use Efficiency,Chlorophyll Fluorescence,and Reflectance Indices Associated with Ontogenic Changes in Water-Limited Chenopodium quinoa Leaves

Net photosynthetic rate, radiation use efficiency, chlorophyll (Chl) fluorescence, photochemical reflectance index (PRI), and leaf water potential were measured during a 25-d period of progressive water deficit in quinoa plants grown in a glasshouse in order to examine effects of water stress and ontogeny. All physiological parameters except F_v/F_m were sensitive to water stress. Ontogenic variations did not exist in F_v/F_m and leaf water potential, and were moderate to high in the other parameters. The complete recovery of photosynthetic parameters after re-irrigation was related with the stability in F_v/F_m. PRI showed significant correlation with predawn leaf water potential, F_m, and midday F_v/F_m. Thus PRI and Chl fluorescence may help in assessing physiological changes in quinoa plants across different developmental stages and water status.     

Down-regulation of photosystem 2 efficiency and spectral reflectance in mango leaves under very low irradiance and varied chilling treatments

In order to elucidate the effects of chilling-stress at night on photosystem 2 (PS2) efficiency under dim irradiance (DI), mango leaves were chilled to varied extent (8–3 °C) and for varied duration (0–12 h) in growth cabinets in the dark, and then exposed to DI (20 μmol m⁻² s⁻¹ PPFD) at each chilling-temperature for 1 h. Chilling in the dark had little effect on F_v/F_m of mango leaves. But both the extent and duration of chilling pre-treatments significantly affected F_v’/F_m’ when leaves were exposed to DI. This down-regulation of PS2 efficiency was closely related to xanthophyll de-epoxidation, assessed as photochemical reflectance index (PRI) and calculated from leaf spectral reflectance [(R₅₃₁ − R₅₇₀)/(R₅₃₁ + R₅₇₀)], and non-photochemical quenching (NPQ). The down-regulation of PS2 is a defence mechanism initiated at predawn in winter to alleviate the damage of PS2 by the sudden and strong irradiation at sunrise. Mango leaves, transferred suddenly from warm and dark room to DI and chilling showed a slight down-regulation of PS2 efficiency, in spite of an increased xanthophyll de-epoxidation. This might have been due to the unavailability of some cofactors required for NPQ.     

Utilizing in situ directional hyperspectral measurements to validate bio-indicator simulations for a corn crop canopy

Two radiative transfer canopy models, SAIL and the two-layer Markov-Chain Canopy Reflectance Model (MCRM), were coupled with in situ leaf optical properties to simulate canopy-level spectral band ratio vegetation indices with the focus on the photochemical reflectance index in a cornfield. In situ hyperspectral measurements were made at both leaf and canopy levels. Leaf optical properties were obtained from both sunlit and shaded leaves. Canopy reflectance was acquired for eight different relative azimuth angles (ψ) at three different view zenith angles (θ_v), and later used to validate model outputs. Field observations of PRI for sunlit leaves exhibited lower values than shaded leaves, indicating higher light stress. Canopy PRI expressed obvious sensitivity to viewing geometry, as a function of both θ_v and ψ. Overall, simulations from MCRM exhibited better agreements with in situ values than SAIL. When using only sunlit leaves as input, the MCRM-simulated PRI values showed satisfactory correlation and RMSE, as compared to in situ values. However, the performance of the MCRM model was significantly improved after defining a lower canopy layer comprised of shaded leaves beneath the upper sunlit leaf layer. Four other widely used band ratio vegetation indices were also studied and compared with the PRI results. MCRM simulations were able to generate satisfactory simulations for these other four indices when using only sunlit leaves as input; but unlike PRI, adding shaded leaves did not improve the performance of MCRM. These results support the hypothesis that the PRI is sensitive to physiological dynamics while the others detect static factors related to canopy structure. Sensitivity analysis was performed on MCRM in order to better understand the effects of structure related parameters on the PRI simulations. LAI showed the most significant impact on MCRM-simulated PRI among the parameters studied. This research shows the importance of hyperspectral and narrow band sensor studies, and especially the necessity of including the green wavelengths (e.g., 531 nm) on satellites proposing to monitor carbon dynamics of terrestrial ecosystems.     

A proposal for universal formulas for estimating leaf water status of herbaceous and woody plants based on spectral reflectance properties

The present study deals with the relationships between water status parameters of plant leaves and reflectances (R_λ) at characteristic wavelengths, between 522 and 2450 nm, as well as reflectance ratios, R_λ/R₁₄₃₀, R_λ/R₁₆₅₀, R_λ/R₁₈₅₀, R_λ/R₁₉₂₀, and R_λ/R₁₉₅₀, based on the air-drying experimental results of soybean (Glycine max Merr.), maize (Zea mays L.), tuliptree (Liriodendron tulipifera L.) and viburnum (Viburnum awabuki K. Koch.) plants. The water status parameters include leaf water content per unit leaf area (LWC), specific leaf water content (SWC), leaf moisture percentage of fresh weight (LMP), relative leaf water content (RWC) and relative leaf moisture percentage on fresh weight basis (RMP). Effective spectral reflectances and reflectance ratios for estimating the LWC, SWC, LMP, RWC and RMP were identified. With these spectral indices, approaches to estimating LWC, RWC and RMP were discussed. Eventually, an attempt on universal formulas was made for estimating the leaf moisture conditions of both herbaceous and woody plants as mentioned above. Moreover, applicability of these formulas was checked with the field experimental results of soybean and maize grown under water and nutrient stresses. This revised version was published online in June 2006 with corrections to the Cover Date.     

Desiccation‐induced changes in photochemical processes of photosynthesis and spectral reflectance in Nostoc commune (Cyanobacteria,Nostocales) colonies from polar regions

Cyanobacterium Nostoc commune is a species highly resistant against desiccation. In this study, we investigated changes in photochemical processes of photosynthesis and spectral reflectance indices during controlled desiccation of the colonies from Antarctica. In a dehydration process, water potential (WP) reached ?3 MPa and values of potential (F _v/F _m) and effective quantum yields (ΦPSII) of photosystem II were kept to high value until 90% of water was lost from the colony, and these values decreased rapidly by further loss of water. This indicates that the colony loses water mostly from the exopolysaccharidic envelope, not from cells during the initial part of dehydration (relative water content, RWC = 100–10%). Other suggestions of inhibition of photosynthetic processes after 90% loss of water were the increase of the chlorophyll fluorescence parameter F _p/F _s. The F _m′ was higher than F _m in hydrated colonies because of state transition which change energy distribution between PS I and PS II, but decreased to same level as F _m in dehydrated colonies. The Normalized Difference Vegetation Index (NDVI) and Photochemical Reflectance Index (PRI) showed concave‐ and convex‐curvilinear relationship with RWC, respectively. The changes of NDVI values were, however, statistically insignificant. PRI values were predominantly below 0 because of phycobiliprotein involvement. These results were compared with the same species in the Arctic region. This is, according to our best knowledge, the first measurement of changes in spectral reflectance indices during desiccation of cyanobacteria.     

Assessing photosynthetic efficiency in an experimental mangrove canopy using remote sensing and chlorophyll fluorescence

This study examined the ability of the photochemical reflectance index (PRI) to track changes in effective quantum yield (Δ F/F _m ′), non-photochemical quenching (NPQ), and the xanthophyll cycle de-epoxidation (DPS) in an experimental mangrove canopy. PRI was correlated with (Δ F/F _m ′) and NPQ over the 4-week measurement period and over the diurnal cycle. The normalised difference vegetation index (NDVI) was not correlated with any aspect of photochemical efficiency measured using chlorophyll fluorescence or xanthophyll pigments. This study demonstrated that photochemical adjustments were responsible for controlling the flow of energy through the photosynthetic apparatus in this mangrove forest canopy rather than canopy structural or chlorophyll adjustments.     

Underestimated chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence measurements on <Emphasis Type="Italic">Buxus microphylla</Emphasis> red winter leaves

Leaves under stressful conditions usually show downregulated maximum quantum efficiency of photosystem II [inferred from variable to maximum chlorophyll (Chl) a fluorescence (F_v/F_m), usually lower than 0.8], indicating photoinhibition. The usual method to evaluate the degree of photoinhibition in winter red leaves is generally by measuring the F_v/F_m on the red adaxial surface. Two phenotypes of overwintering Buxus microphylla ‘Wintergreen’ red leaves, with different measuring site and leaf thickness, were investigated in order to elucidate how red pigments in the outer leaf layer affected the Chl a fluorescence (F_v/F_m) and photochemical reflectance index. Our results showed that the F_v/F_m measured on leaves with the same red surface, but different leaf thickness, exhibited a slightly lower value in half leaf (separated upper and lower layers of leaves by removing the leaf edge similarly as affected by winter freezing and thawing) than that in the intact leaf (without removing the leaf edge), and the F_v/F_m measured on the red surface was significantly lower than that on the inner or backlighted green surface of the same thickness. Our results suggest that the usual measurement of F_v/F_m on red adaxial surface overestimates the actual degree of photoinhibition compared with that of the whole leaf in the winter.     

Underestimate of PS2 efficiency in the field due to high leaf temperature resulting from leaf clipping and its amendment

Chlorophyll fluorescence parameter F_v/F_m, an indicator of the maximum efficiency of PS2, is routinely measured in the field with plant leaves darkened by leaf clips. I found that on a sunny day of subtropical summer, the F_v/F_m ratio was often underestimated because of a large F₀ value resulted from a high leaf temperature caused by clipping the leaf under high irradiance, especially for long (e.g. 20 min) duration. This phenomenon may overestimate the down-regulation of PS2 efficiency under high irradiance. When leaf temperature was lower than 40 °C, the F₀ level of rice leaves under clipping remained practically unchanged. However, F₀ increased drastically with leaf temperature rising over 40 °C. In most measurements, no significant difference in F_m was found between rice leaves dark-adapted by leaf clips for 10 min and for 20 min. Therefore, shading leaf clips to prevent a drastic increase of leaf temperature, using F₀ measured immediately after the leaf being darkened to calculate F_v/F_m, as well as shortening the duration of leaf clipping are useful means to avoid an underestimate of F_v/F_m.     

Effects of lifelong [CO2] enrichment on carboxylation and light utilization of Quercus pubescens Willd. examined with gas exchange,biochemistry and optical techniques

Lifelong exposure to elevated concentrations of atmospheric CO₂ may enhance carbon assimilation of trees with unlimited rooting volume and consequently may reduce requirements for photoprotective pigments. In early summer the effects of elevated [CO₂] on carboxylation and light utilization of mature Quercus pubescens trees growing under chronic [CO₂] enrichment at two CO₂ springs and control sites in Italy were examined. Net photosynthesis was enhanced by 36 to 77%. There was no evidence of photosynthetic downregulation early in the growing season when sink demand presumably was greatest. Specifically, maximum assimilation at saturating [CO₂], electron transport capacity, and Rubisco content, activity and carboxylation capacity were not significantly different in trees growing at the CO₂ springs and their respective control sites. Foliar biochemical content, leaf reflectance index of chlorophyll pigments (NDVI), and photochemical efficiency of PSII (ΔF/F_m′) also were not significantly affected by [CO₂] enrichment except that starch content and ΔF/F_m′ tended to be higher at one spring (42 and 15%, respectively). Contrary to expectation, prolonged elevation of [CO₂] did not reduce xanthophyll cycle pigment pools or alter mid‐day values of leaf reflectance index of xanthophyll cycle pigments (PRI), despite the enhancement of carbon assimilation. However, both these pigments and PRI were well correlated with electron transport capacity.     

The photochemical reflectance index: an optical indicator of photosynthetic radiation use efficiency across species, functional types, and nutrient levels

The photochemical reflectance index (PRI), derived from narrow-band reflectance at 531 and 570 nm, was explored as an indicator of photosynthetic radiation use efficiency for 20 species representing three functional types: annual, deciduous perennial, and evergreen perennial. Across species, top-canopy leaves in full sun at midday exhibited a strong correlation between PRI and ΔF/Fm′, a fluorescence-based index of photosystem II (PSII) photochemical efficiency. PRI was also significantly correlated with both net CO₂ uptake and radiation use efficiency measured by gas exchange. When species were examined by functional type, evergreens exhibited significantly reduced midday photosynthetic rates relative to annual and deciduous species. This midday reduction was associated with reduced radiation use efficiency, detectable as reduced net CO₂ uptake, PRI, and ΔF/Fm′ values, and increased levels of the photoprotective xanthophyll cycle pigment zeaxanthin. For each functional type, nutrient deficiency led to reductions in both PRI and ΔF/Fm′ relative to fertilized controls. Laboratory experiments exposing leaves to diurnal courses of radiation and simulated midday stomatal closure demonstrated that PRI changed rapidly with both irradiance and leaf physiological state. In these studies, PRI was closely correlated with both ΔF/Fm' and radiation use efficiency determined from gas exchange at all but the lowest light levels. Examination of the difference spectra upon exposure to increasing light levels revealed that the 531 nm Δ reflectance signal was composed of two spectral components. At low irradiance, this signal was dominated by a 545-nm component, which was not closely related to radiation use efficiency. At progressively higher light levels above 100 μmol m⁻² s⁻¹, the 531-nm signal was increasingly dominated by a 526-nm component, which was correlated with light use efficiency and with the conversion of the xanthophyll pigment violaxanthin to antheraxanthin and zeaxanthin. Further consideration of the two components composing the 531-nm signal could lead to an index of photosynthetic function applicable over a wide range of illumination. The results of this study support the use of PRI as an interspecific index of photosynthetic radiation use efficiency for leaves and canopies in full sun, but not across wide ranges in illumination from deep shade to full sun. The discovery of a consistent relationship between PRI and photosynthetic radiation use efficiency for top-canopy leaves across species, functional types, and nutrient treatments suggests that relative photosynthetic rates could be derived with the “view from above” provided by remote reflectance measurements if issues of canopy and stand structure can be resolved. Received: 6 January 1997 / Accepted: 14 July 1997     

Leaf Wilting Movement Can Protect Water-Stressed Cotton (<Emphasis Type="Italic">Gossypium hirsutum</Emphasis> L.) Plants Against Photoinhibition of Photosynthesis and Maintain Carbon Assimilation in the Field

Under severe water stress, leaf wilting is quite general in higher plants. This passive movement can reduce the energy load on a leaf. This paper reports an experimental test of the hypothesis that leaf wilting movement has a protective function that mitigates against photoinhibition of photosynthesis in the field. The experiments exposed cotton (Gossypium hirsutum L.) to two water regimes: water-stressed and well-watered. Leaf wilting movement occurred in water-stressed plants as the water potential decreased to −4.1 MPa, reducing light interception but maintaining comparable quantum yields of photosystem II (PS II; Yield for short) and the proportion of total PS II centers that were open (qP). Predrawn F _v/F _m (potential quantum yield of PS II) as an indicator of overnight recovery of PS II from photoinhibition was higher than or similar to that in well-watered plants. Compared with water-stressed cotton leaves for which wilting movement was permitted, water-stressed cotton leaves restrained from such movement had significantly increased leaf temperature and instantaneous CO₂ assimilation rates in the short term, but reduced Yield, qP, and F _v/F _m. In the long term, predrawn F _v/F _m and CO₂ assimilation capacity were reduced in water-stressed leaves restrained from wilting movement. These results suggest that, under water stress, leaf wilting movement could reduce the incident light on leaves and their heat load, alleviate damage to the photosynthetic apparatus due to photoinhibition, and maintain considerable carbon assimilation capacity in the long term despite a partial loss of instantaneous carbon assimilation in the short term.     

The geometry of light interception by shoots of Heteromeles arbutifolia: morphological and physiological consequences for individual leaves

The influence of leaf orientation and position within shoots on individual leaf light environments, carbon gain, and susceptibility to photoinhibition was studied in the California chaparral shrub Heteromeles arbutifolia with measurements of gas exchange and chlorophyll fluorescence, and by application of a three-dimensional canopy architecture model. Simulations of light absorption and photosynthesis revealed a complex pattern of leaf light environments and resulting leaf carbon gain within the shoots. Upper, south-facing leaves were potentially the most productive because they intercepted greater daily photon flux density (PFD) than leaves of any other orientation. North-facing leaves intercepted less PFD but of this, more was received on the abaxial surface because of the steep leaf angles. Leaves differed in their response to abaxial versus adaxial illumination depending on their orientation. While most had lower photosynthetic rates when illuminated on their abaxial as compared to adaxial surface, the photosynthetic rates of north-facing leaves were independent of the surface of illumination. Because of the increasing self-shading, there were strong decreases in absorbed PFD and daily carbon gain in the basipetal direction. Leaf nitrogen per unit mass also decreased in the basipetal direction but on a per unit area basis was nearly constant along the shoot. The decrease in leaf N per unit mass was accounted for by an increase in leaf mass per unit area (LMA) rather than by movement of N from older to younger leaves during shoot growth. The increased LMA of older lower leaves may have contributed directly to their lower photosynthetic capacities by increasing the limitations to diffusion of CO₂ within the leaf to the sites of carboxylation. There was no evidence for sun/shade acclimation along the shoot. Upper leaves and especially south-facing upper leaves had a potential risk for photoinhibition as demonstrated by the high PFDs received and the diurnal decreases in the fluorescence ratio F _v/F _m. Predawn F _v/F _m ratios remained high (>0.8) indicating that when in their normal orientations leaves sustained no photoinhibition. Reorientation of the leaves to horizontal induced a strong sustained decrease in F _v/F _m and CO₂ exchange that slowly recovered over the next 10–15?days. If leaves were also inverted so that the abaxial surface received the increased PFDs, then the reduction in F _v/F _m and CO₂ assimilation was much greater with no evidence for recovery. The heterogeneity of responses was due to a combination of differences between leaves of different orientation, differences between responses on their abaxial versus adaxial surfaces, and differences along the shoot due to leaf age and self-shading effects.     

Deficit irrigation for enhancing sustainable water use: Comparison of cotton nitrogen uptake and prediction of lint yield in a multivariate autoregressive state-space model

Deficit irrigation is an optimization strategy for achieving sustainability of irrigated crop production. A field-study of cotton (Gossypium hirsutum L.) response to a limited water supply was conducted in an Alfisol in the southern High Plains of Texas. The objectives were to investigate cotton N uptake, canopy temperature, plant spectral index and lint yield variation under deficit irrigation and to provide information for enhancing sustainability of the water resources and Alfisols in the semi-arid environment. The experimental treatments were two deficit-irrigation levels at 50% and 75% of cotton evapotranspiration (ET). Plant and soil variables were measured 15 m apart along the center-pivot irrigation circles. The results show that cotton plants under the 50%-ET deficit irrigation level were 21% more water stressed (P < 0.05) based on the reflectance water index ratio. The 50%-ET irrigation resulted in a 25% lint yield loss with a 33% water saving compared to the higher irrigation level (75%-ET). Plant reflectance, canopy temperature, total N uptake and lint yield were correlated with normalized difference vegetative index (NDVI), soil water content (SWC), soil NO₃-N concentrations and elevation (−0.69 < r < 0.72, P < 0.05, respectively). Future cotton lint yield is weighted on NDVI and water variation, quantified in a multivariate autoregressive state-space model. Increases in plant reflectance in the water band are signs of early plant water stress. Compared to the 12-year regional cotton lint yield obtained with full irrigation, the 75%-ET deficit irrigation would be agronomical, economical efficient in Alfisoils with only 7.8% of lint-yield loss from water stress but 25% of water saving for sustainable water use.     

Black leaf-clips increased minimum fluorescence emission in clipped leaves exposed to high solar radiation during dark adaptation

Tomato and pepper leaves were clipped with black leaf clips for dark adaptation under solar radiation in the late spring or early summer 2010 in southern Italy. The leaves showed highly variable maximum PSII quantum yield (F_v/F_m = 0.026−0.802) using a continuous-excitation fluorometer Pocket PEA. These results were confirmed using the modulated fluorometer FMS1 on tomato leaves in mid summer, with F_v/F_m as low as 0.222 ± 0.277 due to nearly equal minimum (F_o) and maximum (F_m) fluorescence emission. A significant clip effect on F_v/F_m occurred after only 12 (tomato) or 25 (pepper) min. Increasing the leaf temperature from 25 to 50°C reportedly induced an F_o increase and Fm decrease so that F_v/F_m approached zero. The hypothesis that black leaf clips overheated under intense solar irradiance was verified by shrouding the clipped leaves with aluminum foil. In clipped leaves of pepper, F_v/F_m with the black clip/Pocket-PEA was 0.769 ± 0.025 (shrouded) and as low as 0.271 ± 0.163 (nonshrouded), the latter showing a double F_o and 32% lower F_m. An 8% clip effect on F_v/F_m was observed with the white clip/FMS1. To avoid the clip effect in high irradiance environments, F_v/F_m measurements with black clip/Pocket PEA system required leaf dark adaptation with radiation-reflecting shrouds. It would be useful if manufacturing companies could develop better radiation-reflecting leaf clips for the Pocket PEA fluorometer.     

Black leaf-clips of a commercial fluorometer increased leaf temperature during dark adaptation under high solar radiation

The use of black leaf-clips for dark adaptation under high solar radiation conditions is reported to underestimate the maximum quantum yield of PSII photochemistry (F_v/F_m) measured by the continuous-excitation fluorometer Pocket PEA. The decrease in F_v/F_m was due to a rise in minimum fluorescence emission (F_o), probably resulting from increased leaf temperature (T_l). In field-grown tomato and pepper, fluorescence parameters and T_l in the region covered by the black leaf clip were measured in clipped leaves exposed to solar radiation during dark adaptation (clipped-only leaves) and in clipped leaves protected from solar radiation by aluminium foil (shrouded clipped leaves). Results confirmed significant F_v/F_m underestimates in clipped-only leaves primarily due to increased F_o. In one tomato experiment, T_l increased from 30 to 44.5°C in clipped-only leaves, with a negligible rise in shrouded clipped leaves. In two respective pepper experiments, T_l in clipped-only leaves increased from 27 to 36.2°C and 33 to 40.9°C. Based on the results of this study, a clip-effect parameter (P_CE) on fluorescence emission is proposed as the difference for F_v/F_m (or ?F_o/F_m) between shrouded clipped leaves and clipped-only leaves, which resulted to be 0.706 for tomato, and 0.241 and 0.358 for the two pepper experiments.     

Surface reflectance properties of Antarctic moss and their relationship to plant species, pigment composition and photosynthetic function

In this study the variations in surface reflectance properties and pigment concentrations of Antarctic moss over species, sites, microtopography and with water content were investigated. It was found that species had significantly different surface reflectance properties, particularly in the region of the red edge (approximately 700 nm), but this did not correlate strongly with pigment concentrations. Surface reflectance of moss also varied in the visible region and in the characteristics of the red edge over different sites. Reflectance parameters, such as the photochemical reflectance index (PRI) and cold hard band were useful discriminators of site, microtopographic position and water content. The PRI was correlated both with the concentrations of active xanthophyll‐cycle pigments and the photosynthetic light use efficiency, F_v/F_m, measured using chlorophyll fluorescence. Water content of moss strongly influenced the amplitude and position of the red‐edge as well as the PRI, and may be responsible for observed differences in reflectance properties for different species and sites. All moss showed sustained high levels of photoprotective xanthophyll pigments, especially at exposed sites, indicating moss is experiencing continual high levels of photochemical stress.     

On the Use of Leaf Spectral Indices to Assess Water Status and Photosynthetic Limitations in Olea europaea L. during Water-Stress and Recovery

Diffusional limitations to photosynthesis, relative water content (RWC), pigment concentrations and their association with reflectance indices were studied in olive (Olea europaea) saplings subjected to water-stress and re-watering. RWC decreased sharply as drought progressed. Following rewatering, RWC gradually increased to pre-stress values. Photosynthesis (A), stomatal conductance (g_s), mesophyll conductance (g_m), total conductance (g_t), photochemical reflectance index (PRI), water index (WI) and relative depth index (RDI) closely followed RWC. In contrast, carotenoid concentration, the carotenoid to chlorophyll ratio, water content reflectance index (WCRI) and structural independent pigment index (SIPI) showed an opposite trend to that of RWC. Photosynthesis scaled linearly with leaf conductance to CO₂; however, A measured under non-photorespiratory conditions (A_1%O2) was approximately two times greater than A measured at 21% [O₂], indicating that photorespiration likely increased in response to drought. A_1%O2 also significantly correlated with leaf conductance parameters. These relationships were apparent in saturation type curves, indicating that under non-photorespiratory conditions, CO₂ conductance was not the major limitations to A. PRI was significant correlated with RWC. PRI was also very sensitive to pigment concentrations and photosynthesis, and significantly tracked all CO₂ conductance parameters. WI, RDI and WCRI were all significantly correlated with RWC, and most notably to leaf transpiration. Overall, PRI correlated more closely with carotenoid concentration than SIPI; whereas WI tracked leaf transpiration more effectively than RDI and WCRI. This study clearly demonstrates that PRI and WI can be used for the fast detection of physiological traits of olive trees subjected to water-stress.     

Salt tolerance of Hibiscus hamabo seedlings: a candidate halophyte for reclamation areas

In order to evaluate the salinity tolerance of Hibiscus hamabo Siebold & Zuccarini (Malvaceae), a candidate halophyte for reclamation areas, we analyze the effects of NaCl concentration, ranging from 0 to 500?mM, on the morphological, photosynthetic and chlorophyll fluorescent traits of this species. The optimal concentration for the germination of H. hamabo was 25?mM NaCl, and the optimal concentration for the survival and growth of H. hamabo ranged from 5 to 10?mM NaCl. Growth traits of H. hamabo at 25?mM, including the plant height, canopy diameter, number of leaves and width of the largest leaf, showed no statistical differences from the control. Net photosynthetic rate, stomatal conduction, light utilization efficiency, water utilization efficiency, maximal photosynthetic rate, light saturation point and chlorophyll content were the highest at 7.5?mM NaCl. F _v/F _m and F _v/F ₀ at 5 and 7.5?mM were significantly higher than the others, while F ₀ was significantly lower. F _m and F _v at NaCl concentrations ranging from 2.5 to 10?mM were significantly higher than the others. Pearson correlation analysis showed that the chlorophyll content, maximal photosynthetic rate and light saturation point were significantly positively correlated with the number of leaves, while F ₀ was significantly negatively correlated with the width of the largest leaf. Light compensation point was significantly negatively correlated with plant height, leaf number, width of the largest leaf and canopy diameter, and might be a good indicator for the salt tolerance of H. hamabo.     

Relationships of photosynthetic capacity to PSII efficiency and to photochemical reflectance index of Pinus taiwanensis through different seasons at high and low elevations of sub-tropical Taiwan

We investigated the relationships of photosynthetic capacity (P _nsat, near light-saturated net photosynthetic rate measured at 1,200 μmol m⁻² s⁻¹ PPFD) to photosystem II efficiency (F _v/F _m) and to photochemical reflectance index [PRI = (R ₅₃₁ − R ₅₇₀)/(R ₅₃₁ + R ₅₇₀)] of Pinus taiwanensis Hay. needles at high (2,600 m a.s.l) and low-elevation (800 m a.s.l) sites through different seasons. Results indicate that at high-elevation site, P _nsat, F _v/F _m and PRI (both measured at predawn) paralleled in general with the air temperature. On the coolest measuring day with the minimum air temperature dropping to −2°C, P _nsat could decrease to ca. 15% of its highest value, which was measured in autumn. At low-elevation site, with the minimum air temperature of 10–12°C in cooler season and almost no seasonal variation of F _v/F _m, P _nsat dropped to ca. 65% of its highest value and PRI decreased ca. 0.02 in winter. Even though seasonal variation of P _nsat was affected by many factors, it was still closely related to PRI based on statistical analyses using data from both sites, through different seasons. On the contrary, seasonal variation of F _v/F _m of P. taiwanensis needles was influenced mainly by low temperature at high elevation. Therefore, the correlation of P _nsat − F _v/F _m was lower than that of P _nsat − PRI when data combined from both high- and low-elevation sites were analyzed. It is concluded that predawn PRI could be used as an indicator to estimate the seasonal potential of photosynthetic capacity of P. taiwanensis grown at low- and high-elevations of sub-tropical Taiwan.     

Enhanced antioxidant protection at the early stages of leaf expansion in ginkgo under natural environmental conditions

Photosynthetic pigments, gas exchange, chlorophyll (Chl) a fluorescence kinetics, antioxidant enzymes and chloroplast ultrastructure were investigated in ginkgo (Ginkgo biloba L.) leaves from emergence to full size. Under natural conditions, the net photosynthetic rate (P_N), contents of Chl a, Chl b and total soluble proteins and fresh and dry leaf mass gradually increased during leaf expansion. The maximum photochemical efficiency of photosystem (PS) 2 (variable to maximum fluorescence ratio, F_v/F_m) was considerably higher at the early stages of leaf development than in fully expanded leaves. During daily course, only reversible decrease in F_v/F_m was distinguished at various stages, implying that no photo-damage occurred. Absorption flux per cross section (CS) and trapped energy flux per CS were significantly lower in newly expanding leaves compared with fully expanded ones, however, dissipated energy flux per CS was only slightly lower in expanding leaves. The ratio of carotenoids (Car)/Chl decreased gradually during leaf expansion due to increasing Chl content. Moreover, activities of the antioxidant enzymes, such as superoxide dismutase, ascorbate peroxidase, catalase and peroxidase, increased at the early stages of leaf expansion. The appearance of osmiophilic granules in fully expanded leaves further proves that photo-protection is significantly strengthened at the early stages of leaf expansion.