Relationship between photochemical reflectance index and leaf ecophysiological and biochemical parameters under two different water statuses: towards a rapid and efficient correction method using real‐time measurements

The use of the photochemical reflectance index (PRI) as a promising proxy of light use efficiency (LUE) has been extensively studied, and some issues have been identified, notably the sensitivity of PRI to leaf pigment composition and the variability in PRI response to LUE because of stress. In this study, we introduce a method that enables us to track the short‐term PRI response to LUE changes because of photosynthetically active radiation (PAR) changes. The analysis of these short‐term relationships between PRI and LUE throughout the growing season in two species (Quercus robur L. and Fagus sylvatica L.) under two different soil water statuses showed a clear change in PRI response to LUE, which is related to leaf pigment content. The use of an estimated or approximated PRI0, defined as the PRI of perfectly dark‐adapted leaves, allowed us to separate the PRI variability due to leaf pigment content changes and the physiologically related PRI variability over both daily (PAR‐related) and seasonal (soil water content‐related) scales. The corrected PRI obtained by subtracting PRI0 from the PRI measurements showed a good correlation with the LUE over both of the species, soil water statuses and over the entire growing season.     

光化学植被指数估算植物光能利用率的研究进展

 应用遥感技术可以建立光化学植被指数（Photochemical reflectance index, PRI）和光能利用效率（Light use efficiency, LUE）的关系,LUE可用来估算净初级生产力（Net primary productivity, NPP）。因而,用PRI估算植物的LUE,为估算NPP提供了新的方法,弥补了以往以经验模式通过温度和水分对最大LUE的胁迫来获取实际LUE的不足,进而可提高NPP的估算精度。研究表明：PRI和LUE在叶片、冠层和景观尺度上都有着很好的相关性,但是随着尺度的变化,很多因素会对这一关系产生干扰,如水分、氮元素含量、叶面积指数和太阳高度角等,从而削弱了PRI和LUE的关系。该文对建立PRI和LUE的关系过程中的影响因素进行了分析,并指出今后这一研究领域中可能改进的方面,主要包括526 nm 和545 nm 处的反射率对531 nm 处的反射率的作用机制、PRI随LUE的饱和现象、PRI和LUE关系的时间效应以及利用PRI估算LUE的尺度效应。     

Physiology of the seasonal relationship between the photochemical reflectance index and photosynthetic light use efficiency

The photochemical reflectance index (PRI) is regarded as a promising proxy to track the dynamics of photosynthetic light use efficiency (LUE) via remote sensing. The implementation of this approach requires the relationship between PRI and LUE to scale not only in space but also in time. The short-term relationship between PRI and LUE is well known and is based on the regulative process of non-photochemical quenching (NPQ), but at the seasonal timescale the mechanisms behind the relationship remain unclear. We examined to what extent sustained forms of NPQ, photoinhibition of reaction centres, seasonal changes in leaf pigment concentrations, or adjustments in the capacity of alternative energy sinks affect the seasonal relationship between PRI and LUE during the year in needles of boreal Scots pine. PRI and NPQ were highly correlated during most of the year but decoupled in early spring when the foliage was deeply downregulated. This phenomenon was attributed to differences in the physiological mechanisms controlling the seasonal dynamics of PRI and NPQ. Seasonal adjustments in the pool size of the xanthophyll cycle pigments, on a chlorophyll basis, controlled the dynamics of PRI, whereas the xanthophyll de-epoxidation status and other xanthophyll-independent mechanisms controlled the dynamics of NPQ at the seasonal timescale. We conclude that the PRI leads to an underestimation of NPQ, and consequently overestimation of LUE, under conditions of severe stress in overwintering Scots pine, and most likely also in species experiencing severe drought. This severe stress-induced decoupling may challenge the implementation of the PRI approach.     

Seasonal and within‐canopy variation in shoot‐scale resource‐use efficiency trade‐offs in a Norway spruce stand

Previous leaf‐scale studies of carbon assimilation describe short‐term resource‐use efficiency (RUE) trade‐offs where high use efficiency of one resource requires low RUE of another. However, varying resource availabilities may cause long‐term RUE trade‐offs to differ from the short‐term patterns. This may have important implications for understanding canopy‐scale resource use and allocation. We used continuous gas exchange measurements collected at five levels within a Norway spruce, Picea abies (L.) karst., canopy over 3 years to assess seasonal differences in the interactions between shoot‐scale resource availability (light, water and nitrogen), net photosynthesis (A_n) and the use efficiencies of light (LUE), water (WUE) and nitrogen (NUE) for carbon assimilation. The continuous data set was used to develop and evaluate multiple regression models for predicting monthly shoot‐scale A_n. These models showed that shoot‐scale A_n was strongly dependent on light availability and was generally well described with simple one‐ or two‐parameter models. WUE peaked in spring, NUE in summer and LUE in autumn. However, the relative importance of LUE for carbon assimilation increased with canopy depth at all times. Our results suggest that accounting for seasonal and within‐canopy trade‐offs may be important for RUE‐based modelling of canopy carbon uptake.     

Constitutive changes in pigment concentrations: implications for estimating isoprene emissions using the photochemical reflectance index

The photochemical reflectance index (PRI), through its relationship with light use efficiency (LUE) and xanthophyll cycle activity, has recently been shown to hold potential for tracking isoprene emissions from vegetation. However, both PRI and isoprene emissions can also be influenced by changes in carotenoid pigment concentrations. Xanthophyll cycle activity and changes in carotenoid concentrations operate over different timescales, but the importance of constitutive changes in pigment concentrations for accurately estimating isoprene emissions using PRI is unknown. To clarify the physiological mechanisms behind the PRI–isoprene relationship, the light environment of potted Salix viminalis (osier willow) trees was modified to induce acclimation in photosynthetic rates, phytopigments, isoprene emissions and PRI. Acclimation resulted in differences in pigment concentrations, isoprene emissions and PRI. Constitutive changes in carotenoid concentration were significantly correlated with both isoprene emissions and PRI, suggesting that the relationship between PRI and isoprene emissions is significantly influenced by constitutive pigment changes. Consequently knowledge regarding how isoprene emissions are affected by both longer term changes in total carotenoid concentrations and shorter term dynamic adjustments of LUE is required to facilitate interpretation of PRI for monitoring isoprene emissions.     

Relationships between the photochemical reflectance index (PRI) and chlorophyll fluorescence parameters and plant pigment indices at different leaf growth stages

This study aimed to evaluate the photochemical reflectance index (PRI) for assessing plant photosynthetic performance throughout the plant life cycle. The relationships between PRI, chlorophyll fluorescence parameters, and leaf pigment indices in Solanum melongena L. (aubergine; eggplant) were studied using photosynthetic induction curves both in short-term (diurnal) and long-term (seasonal) periods under different light intensities. We found good correlations between PRI/non-photochemical quenching (NPQ) and PRI/electron transport rate (ETR) in the short term at the same site of a single leaf but these relationships did not hold throughout the life of the plant. In general, changes in PRI owing to NPQ or ETR variations in the short term were <20?% of those that occurred with leaf aging. Results also showed that PRI was highly correlated to plant pigments, especially chlorophyll indices measured by spectral reflectance. Moreover, relationships of steady-state PRI/ETR and steady-state PRI/photochemical yield of photosystem II (Φ(PSII)) measured at uniform light intensity at different life stages proved that overall photosynthesis capacity and steady-state PRI were better correlated through chlorophyll content than NPQ and xanthophylls. The calibrated PRI index accommodated these pigments effects and gave better correlation with NPQ and ETR than PRI. Further studies of PRI indices based on pigments other than xanthophylls, and studies on PRI mechanisms in different species are recommended.     

毛乌素沙地油蒿叶性状对光能利用效率动态的影响

目前对于荒漠灌木光能利用效率（LUE）的季节变异及其调控因素,尤其是其生物调控因素的认识非常有限,导致了荒漠生态系统生产力模型的不确定性。拟验证假设：长期干旱环境下,典型荒漠灌木油蒿光能利用效率日均值（LUE_day）的动态变化与叶片性状的季节性调整有关。试验采用Li-6400便携式光合仪定期测量了油蒿生长季叶片LUE_day的季节动态及相关叶性状指标,探究叶性状对LUE_day的影响。结果表明：LUE_day的季节波动范围为0.003-0.017 mol/mol,整体变异系数（CV）为38.75%。完全展叶期LUE_day均值相比生长季平均值降低17.37%,相比展叶期和落叶期时降低30%;8个叶性状的季节变异幅度差异较大,其中总叶绿素含量（Chl）、类胡萝卜素含量（Car）和叶氮含量（LNC）均表现出较大的季节变异性（CV ≥ 20%）,叶碳含量（LCC）和叶片相对含水量（LRWC）的变异程度最低（CV<7%）。LRWC与所有叶片化学性状（Chl、Chl a/b、Car、LNC和LCC）均存在显著相关,表明其变化与叶片的养分吸收、光合色素合成以及碳同化的运输过程密切相关;油蒿LUE_day的相对变化与LRWC、Chl a/b和LNC显著正相关,而LRWC和LNC的季节动态受空气温度（T_a）和土壤含水量（VWC）的共同调节,Chl a/b的季节波动主要由浅层土壤含水量（10 cm VWC）控制。以上研究结果强调,在未来预计极端的气候事件（如极端干旱和持续热浪事件）发生更频繁的旱地场景中,时间尺度植物叶性状对于土壤干旱和高温的适应性调整应当被充分考虑到旱地生态系统的通量建模方案中。该结果将为构建叶片尺度的光合生理模型与厘清LUE的生物调控机制提供理论依据。     

Assessing leaf pigment content and activity with a reflectometer

This study explored reflectance indices sampled with a 'leaf reflectometer' as measures of pigment content for leaves of contrasting light history, developmental stage and functional type (herbaceous annual versus sclerophyllous evergreen). We employed three reflectance indices: a modified normalized difference vegetation index (NDVI), an index of chlorophyll content; the red/green reflectance ratio ( R _RED: R _GREEN), an index of anthocyanin content; and the change in photochemical reflectance index upon dark–light conversions (ΔPRI), an index of xanthophyll cycle pigment activity. In Helianthus annuus (sunflower), xanthophyll cycle pigment amounts were linearly related to growth light environment; leaves in full sun contained approximately twice the amount of xanthophyll cycle pigments as leaves in deep shade, and at midday a larger proportion of these pigments were in the photoprotective, de-epoxidized forms relative to shade leaves. Reflectance indices also revealed contrasting patterns of pigment development in leaves of contrasting structural types (annual versus evergreen). In H. annuus sun leaves, there was a remarkably rapid increase in amounts of both chlorophyll and xanthophyll cycle pigments along a leaf developmental sequence. This pattern contrasted with that of Quercus agrifolia (coast live oak, a sclerophyllous evergreen), which exhibited a gradual development of both chlorophyll and xanthophyll cycle pigments along with a pronounced peak of anthocyanin pigment content in newly expanding leaves. These temporal patterns of pigment development in Q. agrifolia leaves suggest that anthocyanins and xanthophyll cycle pigments serve complementary photoprotective roles during early leaf development. The results illustrate the use of reflectance indices for distinguishing divergent patterns of pigment activity in leaves of contrasting light history and functional type.     

基于通量和光谱观测的中亚热带人工针叶林光能利用效率的反演

利用光谱反射率测量的光化学植被指数（PRI）估算植被光合作用的光能利用效率（LUE）,能够更好地为生态系统总初级生产力的估算及尺度扩展提供重要的技术支撑.本研究以中国通量网（ChinaFLUX）千烟洲通量观测站为研究区域,2013年9月和12月在通量塔上测量了中亚热带人工针叶林的植被反射光谱,并获取了通量塔上同步观测的气象数据和涡度相关通量数据,对两者进行回归分析.结果表明： PRI-LUE相关关系（R²=0.20,P<0.001）优于NDVI LUE.在整个观测期内,土壤水分含量（SWC）与PRI组合的二元回归模型能够提高LUE的估算精度（日间观测R²=0.29,P<0.001;正午观测R²=0.30,P<0.01）,而在秋季,饱和水汽压差（VPD）与PRI组合的二元回归模型能较好地估算正午LUE（R²=0.448, P<0.001）,表明环境因子SWC和VPD是影响PRI-LUE关系的重要因素,不同季节的二元回归模型所选择的最佳环境变量有所不同.     

Tracking plant physiological properties from multi-angular tower-based remote sensing

Imaging spectroscopy is a powerful technique for monitoring the biochemical constituents of vegetation and is critical for understanding the fluxes of carbon and water between the land surface and the atmosphere. However, spectral observations are subject to the sun–observer geometry and canopy structure which impose confounding effects on spectral estimates of leaf pigments. For instance, the sun–observer geometry influences the spectral brightness measured by the sensor. Likewise, when considering pigment distribution at the stand level scale, the pigment content observed from single view angles may not necessarily be representative of stand-level conditions as some constituents vary as a function of the degree of leaf illumination and are therefore not isotropic. As an alternative to mono-angle observations, multi-angular remote sensing can describe the anisotropy of surface reflectance and yield accurate information on canopy structure. These observations can also be used to describe the bi-directional reflectance distribution which then allows the modeling of reflectance independently of the observation geometry. In this paper, we demonstrate a method for estimating pigment contents of chlorophyll and carotenoids continuously over a year from tower-based, multi-angular spectro-radiometer observations. Estimates of chlorophyll and carotenoid content were derived at two flux-tower sites in western Canada. Pigment contents derived from inversion of a CR model (PROSAIL) compared well to those estimated using a semi-analytical approach (r ² = 0.90 and r ² = 0.69, P < 0.05 for both sites, respectively). Analysis of the seasonal dynamics indicated that net ecosystem productivity was strongly related to total canopy chlorophyll content at the deciduous site (r ² = 0.70, P < 0.001), but not at the coniferous site. Similarly, spectral estimates of photosynthetic light-use efficiency showed strong seasonal patterns in the deciduous stand, but not in conifers. We conclude that multi-angular, spectral observations can play a key role in explaining seasonal dynamics of fluxes of carbon and water and provide a valuable addition to flux-tower-based networks.     

The impact of diffuse sunlight on canopy light-use efficiency, gross photosynthetic product and net ecosystem exchange in three forest biomes

For three forest canopies (a sparse, boreal needleleaf; a temperate broadleaf; and a dense, tropical, broadleaf stand) light‐use efficiency (LUE) is found to be 6–33% higher when sky radiance is dominated by diffuse rather than direct sunlight. This enhancement is much less than that reported previously for both crops (110%; Choudbury, 2001 ) and moderately dense temperate woodland (50–180%). We use the land‐surface scheme JULES to interpret the observed canopy response. Once sunflecks and leaf orientation are incorporated explicitly into the scheme, our simulations reproduce convincingly the overall level of canopy gross photosynthetic product (GPP), its enhancement with respect to diffuse sunlight and the mean 15% reduction in productivity observed during the afternoon due to stomatal closure. The LUE enhancement under diffuse sunlight can be explained by sharing of the canopy radiation‐load, which is reduced under direct sky radiance. Once sunflecks are accounted for the advantage of implementing more sophisticated calculations of stomatal conductance (e.g. Ball–Berry and SPA submodels) is less obvious even for afternoon assimilation. Empirical relations are developed between observed carbon flux and the environmental variables total downwelling shortwave radiation (SW), canopy temperature (T) and the fraction of diffuse sky radiance (fDIF). These relations allow us to gauge the impact of increased/reduced insolation on GPP and net ecosystem exchange (NEE). Overall the three stands appear to be fairly stable within global trends and typical interannual variability (SW changing by <15%). Greatest sensitivity is exhibited by the boreal site, Zotino, where NEE falls by 9±4% for a 15% reduction in SW.     

基于PROSAIL辐射传输模型的毛竹林分冠层反射率模拟研究

冠层光谱反射率直接关系到毛竹（Phyllostachys pubescens Mazel）林冠层参数的反演,对毛竹林地土壤肥力间接估测具有重要意义。以PROSPECT5、PROSAIL模型为基础,从叶片尺度和冠层尺度分析模型参数对叶片和冠层反射率的影响,构建毛竹冠层叶面积指数（LAI）-冠层反射率查找表并通过代价函数选取最优冠层反射率,从而实现毛竹林分冠层反射率的准确模拟。结果表明,在叶片尺度,PROSPECT模型参数敏感性从高到低依次为叶肉结构参数（N） > 叶绿素含量（C_ab） > 等效水厚度（EWT） > 干物质含量（C_m） > 类胡萝卜素含量（C_ar）;在冠层尺度,PROSAIL模型参数敏感性从高到低依次为LAI > C_ab > EWT > C_m > N > C_ar > ALA（平均叶倾角）;叶片尺度反射率整体大于冠层尺度反射率;在400~900 nm波长范围内,PROSAIL模型模拟的冠层光谱反射率与实测光谱反射率拟合效果较好,相对误差为6.71%。     

植被光能利用率研究进展

光能利用率是表征植物固定太阳能效率的指标,指植物通过光合作用将所截获/吸收的能量转化为有机干物质的效率,是植物光合作用的重要概念,也是区域尺度以遥感参数模型监测植被生产力的理论基础。传统的研究方法是通过生物量收获法分别确定植物生长和辐射量,求年或生长季比值;涡度相关技术作为目前直接测定植被冠层与大气间的CO2和水热交换量的唯一方法,使从冠层到景观水平的光能利用率估计成为可能。由于植被类型的差异和气候环境的综合影响使光能利用率表现出显著的空间异质性和时间动态性。在全球尺度上,利用耦合大气CO2观测、卫星遥感和大气辐射传输模型的反演模拟,发现净初级生产力的光能利用率存在明显的地理分异。影响光能利用率时空变异性的因子包括植物内在因素(如叶形、叶羧化酶含量)和外在环境因素。针对光能利用率的时空特征及其波动,建立在通量观测及模型分析基础上的跨尺度模拟,将成为今后该领域的研究重点。     

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.     

Controls on periphyton biomass in heterotrophic streams

1. Headwater streams of the Hubbard Brook Experimental Forest (HBEF) are typically characterised by a periphyton assemblage of low biomass and diversity. However, periphyton blooms have been observed following catchment deforestation experiments and occasionally during the annual spring thaw before canopy leaf‐out. 2. There is pronounced seasonal variation in both nutrient and light availability in HBEF streams. Stream water nitrogen (N) concentrations and light levels are higher before canopy leaf‐out and after leaf senescence and are lower during the growing season. Periphyton accrual rates also change seasonally; they are highest in spring prior to leaf‐out and significantly lower during summer and in autumn. 3. Periphyton biomass rarely responded positively to in‐situ experimental enrichment with nitrogen or phosphorus. In the summer, nutrient enrichment overall had no effect on periphyton biomass, while outside the growing season N enrichment had inhibitory effects on periphyton. 4. Despite these experimental results, surveys of ambient chlorophyll a concentrations in streams across the HBEF demonstrated no relationship between streamwater dissolved inorganic N or P concentrations and benthic chlorophyll a. 5. Our results suggest that HBEF periphyton communities are not closely regulated by nutrient availability, even during periods of high light availability. The inhibitory effects of nutrient enrichment outside the growing season are interesting, but further research is necessary to elucidate the mechanisms driving these responses.     

穿透雨减少下锐齿栎叶片光合色素季节动态及其反射光谱响应

通过林地穿透雨排除的方法模拟降雨减少,测定河南宝天曼自然保护区锐齿栎叶片光合色素含量与反射光谱的季节变化,对减雨处理造成的光合色素变化及其反射光谱的变化进行了定量分析,并探讨了水分控制条件下反射光谱对叶片光合色素变化的响应机制.结果表明: 锐齿栎叶片的光合色素含量和色素比率均呈现明显的季节变化.减雨样地与对照样地叶片的光合色素含量和比率在生长季的各个时期存在差异,其中,叶片叶绿素b(Chl b)含量的差异显著,说明Chl b对减雨处理的敏感性最高,叶片类胡萝卜素(Car)含量的差异较小,说明Car对减雨处理的敏感性相对较弱.550 nm处的光谱反射率对色素季节变化的响应最敏感,以其构造的简单比值指数(SR_750,550)与叶片Chl a、Chl b、总Chl和Car含量的正相关关系显著,光化学反射指数(PRI)与叶片Car/Chl的负相关关系显著.550 nm处的光谱反射率对减雨处理造成的色素变化响应最为敏感.SR_750,550对减雨处理造成的叶片Chl a、Chl b和总Chl的含量变化表现敏感(P<0.01),对Chl a/b的变化不敏感.PRI对减雨处理造成的叶片Car/Chl变化表现敏感(P<0.01).     

Factors of between- and within-plant distribution of Metopolophium dirhodum (Hom., Aphididae) on small grain cereals

Abstract: The relationship between abundance of rose‐grain aphid Metopolophium dirhodum (Walker) and leaf chlorophyll content of spring wheat, spring oats and winter barley was investigated. Within production stands of each crop 18–25 plots were established, located at places with different plant quality where aphids were counted on particular leaves of 50–100 tillers, and the leaf chlorophyll content and area were determined. In all stands aphid numbers × tiller^?1 increased exponentially with chlorophyll content (r² = 0.783–0.933). This parallel variation may be explained by increased nitrogen content and assimilate production of vigorous chlorophyll‐rich plants. The aphid numbers on leaves of particular order (within‐plant distribution) were also correlated with leaf chlorophyll content (r² = 0.373–0.827). However, in oats and barley the analysis of variance of residuals of log aphids × leaf^?1 versus leaf chlorophyll regression revealed a significant effect of leaf position (the order of the leaf from the top of the plant). The magnitude of residuals was positively related to leaf size and may be related to the intensity of phloem transport from the source leaves to sink organs.     

基于辐射传输模型的高光谱植被指数与叶绿素浓度及叶面积指数的线性关系改进

高光谱植被指数以其特有的精细光谱特征, 能够获得非常细微的植被生理状况和环境胁迫差异, 因而使遥感技术在精细农业中的应用, 尤其是在叶绿素浓度和叶面积指数的反演上面有着广阔的应用前景。然而, 现有的植被指数往往和这2个参数呈非线性关系, 且只对某一区间的数值敏感, 无法适用于其它植被覆盖程度的研究。为了寻找合适的波段位置以改善植被指数与叶绿素浓度和叶面积指数的线性关系, 去除饱和区域, 进而提高这2个参数的实际估算精度, 该文选取了叶绿素浓度和叶面积指数, 以辐射传输模型PROSPECT和SAIL为基础, 模拟了这2个参数变化对3类高光谱植被指数(归一化植被指数(NDVI)、 优化的简单比值指数(MSR)和优化的叶绿素吸收率指数(MCARI))的影响。叶绿素浓度变化敏感性分析结果表明, 对这3类植被指数而言, 750 nm 和705 nm 的叶片反射率更适合实际的叶绿素浓度反演。以750 nm 和705 nm代替 800 nm/700 nm 和670 nm成功地提高了3类植被指数与叶绿素浓度的线性相关程度, 其中MCARI705 和叶绿素浓度基本呈线性关系。叶面积指数变化敏感性分析同样显示, 以750 nm 和705 nm 组成的植被指数能够获取更可靠的叶面积指数信息, 尤其对于高植被覆盖区域。其中MCARI705 能较好地降低随叶面积指数变化的饱和程度, 相比其它植被指数, 当叶面积指数大于8时, MCARI705 才出现明显的饱和。由于冠层的尺度效应, 波段位置的选择对植被指数与叶面积指数线性关系的改善不及对叶绿素浓度明显。     

Photochemistry, remotely sensed physiological reflectance index and de-epoxidation state of the xanthophyll cycle in Quercus coccifera under intense drought

Quercus coccifera L. is a Mediterranean sclerophyllous shrub with a high capacity to resist intense drought stress. Therefore, it could be used in the study of physiological changes suffered by plants at very low water potentials. A remote sensing sensor was used to measure continuously the physiological reflectance index (PRI; defined as the changes in reflectance at 531 nm with respect to those at 570 nm; PRI = [(R531 − R570)/(R531 + R570)] at canopy level and under field conditions in an artificial carpet of seedlings of Q. coccifera during a drought cycle. Correlations between leaf level-measured chlorophyll fluorescence parameters as well as the de-epoxidation state of the xanthophyll cycle [(A + Z)/(V + A + Z)] and canopy level-measured PRI were reasonably good (R ² = 0.57–0.63, P < 0.01), and quite interesting for water stress remote sensing purposes. The instrument’s temporal resolution allowed us to follow the rapid response of PRI to changing photosynthetic active radiation, and to resolve, in response to cloud-induced changes in light intensity, a fast and a slow PRI component. We report the disappearance of the rapid one under conditions of intense drought in response to a sudden increase in light intensity. The underlying photoprotection mechanisms that Q. coccifera shows in response to intense drought stress periods seem to be related to the existence of a low intrathylakoid lumenal pH at the end of the drought cycle. Under intense drought, these mechanisms allow this species to avoid oxidative damage, which was evidenced by the maintenance of an unaltered photosynthetic pigment composition and constant photosystem II efficiency in the mornings. It is concluded that, contrary to early reports, PRI is a sensible, indirect, non-destructive water stress indicator, even in plants experiencing intense drought. Preliminary results of this work were presented at the 3rd International Workshop on Remote Sensing of Vegetation Fluorescence (February 2007, Florence, Italy).     

Responses of CO2, CH4 and N2O fluxes to livestock exclosure in an alpine steppe on the Tibetan Plateau, China

Background and aims

The photochemical reflectance index (PRI) is correlated to photosynthetic efficiency and has been successfully applied at multiple scales for remote estimation of physiological functioning. However, interpretation of the PRI signal can be confounded by many different variables including declines in photochemical pigments. Our study was aimed at investigating PRI in response to salinity stress, and evaluating physiological and pigment responses of two co-occurring shrubs, Baccharis halimifolia and Myrica cerifera in laboratory studies.

Methods

Photosynthesis, water relations, chlorophyll fluorescence, hyperspectral reflectance and leaf pigment contents were measured following salinity treatment.

Results

Physiological measurements indicated that both species exhibit adaptations which protect PSII during periods of stress. Chlorophyll fluorescence parameters were affected in both species, but indicated that other photochemical reactions (e.g. photorespiration) were important for energy dissipation in absence of chlorophyll changes. After many days of reduced photosynthesis, photochemical changes were detectable using PRI indicating chronic stress.

Conclusions

Variations in PRI were not related to changes in pigments but strongly related to tissue chlorides indicating salinity effects on the PRI signal. Thus, PRI is an indicator of salinity stress in these coastal species and may be as an early signal for increasing salt exposure associated with rising sea-level and climate change.