Ecophysiological adaptations of black spruce (<Emphasis Type="Italic"> Picea mariana</Emphasis>) and tamarack (<Emphasis Type="Italic"> Larix laricina</Emphasis>) seedlings to flooding

Black spruce [ Picea mariana (Mill.) B.S.P.] and tamarack [ Larix laricina (Du Roi) K. Koch] are the predominant tree species in boreal peatlands. The effects of 34 days of flooding on morphological and physiological responses were investigated in the greenhouse for black spruce and tamarack seedlings in their second growing season (18 months old). Flooding resulted in reduced root hydraulic conductance, net assimilation rate and stomatal conductance and increased needle electrolyte leakage in both species. Flooded tamarack seedlings maintained a higher net assimilation rate and stomatal conductance compared to flooded black spruce. Flooded tamarack seedlings were also able to maintain higher root hydraulic conductance compared to flooded black spruce seedlings at a comparable time period of flooding. Root respiration declined in both species under flooding. Sugar concentration increased in shoots while decreasing in roots in both species under flooding. Needles of flooded black spruce appeared necrotic and electrolyte leakage increased over time with flooding and remained significantly higher than in flooded tamarack seedlings. No visible damage symptoms were observed in flooded tamarack seedlings. Flooded tamarack seedlings developed adventitious roots beginning 16 days after the start of flooding treatment. Adventitious roots exhibited significantly higher root hydraulic conductivity than similarly sized flooded tamarack roots. Flooded black spruce lacked any such morphological adaptation. These results suggest that tamarack is better able to adjust both morphologically and physiologically to prolonged soil flooding than black spruce seedlings.     

Drought response of mesophyll conductance in forest understory species – impacts on water‐use efficiency and interactions with leaf water movement

Regulation of stomatal (g_s) and mesophyll conductance (g_m) is an efficient means for optimizing the relationship between water loss and carbon uptake in plants. We assessed water‐use efficiency (WUE)‐based drought adaptation strategies with respect to mesophyll conductance of different functional plant groups of the forest understory. Moreover we aimed at assessing the mechanisms of and interactions between water and CO₂ conductance in the mesophyll. The facts that an increase in WUE was observed only in the two species that increased g_m in response to moderate drought, and that over all five species examined, changes in mesophyll conductance were significantly correlated with the drought‐induced change in WUE, proves the importance of g_m in optimizing resource use under water restriction. There was no clear correlation of mesophyll CO₂ conductance and the tortuosity of water movement in the leaf across the five species in the control and drought treatments. This points either to different main pathways for CO₂ and water in the mesophyll either to different regulation of a common pathway.     

Moss production in a black spruce Picea mariana forest with permafrost near Fairbanks,Alaska, as compared with two permafrost-free stands

During the 1975 and 1976 seasons the net primary production of five common bryophytes in different stands of mature vegetation near Fairbanks, Alaska was investigated. Overall annual moss production at the intensive black spruce site was about 120 g m^?1 yr^?1 or about twice as high as the corresponding annual spruce production. Maximum rates of net photosynthesis varied from 2.7 mg CO₂ g^?1 h^?1 in Polytrichum commune Hedw. to 0.6 mg CO₂ g^?1 h^?1 in Sphagnum nemoreum Scop. The photosynthesis of overwintered leaves early in the season was low and as a result of new growth a steady increase in net photosynthesis occurred throughout the season. Leaf water content was found to be the most important limiting factor for growth under natural conditions. There was a strong increase in growth and photosynthesis of Sphagnum nemoreum after application of N and P, indicating nutrient deficiency.     

Geographic pattern of genetic variation in photosynthetic capacity and growth in two hardwood species from British Columbia

Geographic patterns of intraspecific variations in traits related to photosynthesis and biomass were examined in two separate common garden experiments using seed collected from 26 Sitka alder (Alnus sinuata Rydb.) and 18 paper birch (Betula papyrifera Marsh.) populations from climatically diverse locations in British Columbia, Canada. Exchange rates of carbon dioxide and water vapour were measured on 2-year-old seedlings to determine the maximum net instantaneous photosynthetic rate, mesophyll conductance, stomatal conductance, and photosynthetic water use efficiency. Height, stem diameter, root and shoot dry mass and fall frost hardiness data were also obtained. Mean population maximum photosynthetic rate ranged from 10.35 to 14.57 μmol CO₂ m^–2 s^–1 in Sitka alder and from 14.76 to 17.55 μmol CO₂ m^–2 s^–1 in paper birch. Based on canonical correlation analyses, populations from locations with colder winters and shorter (but not necessarily cooler) summers had higher maximum photosynthetic rates implying the existence of an inverse relationship between leaf longevity and photosynthetic capacity. Significant canonical variates based on climatic variables derived for the seed collection sites explained 58% and 41% of variation in the rate of photosynthesis in Sitka alder and paper birch, respectively. Since growing season length is reflected in date of frost hardiness development, an intrinsic relationship was found between photosynthetic capacity and the level of fall frost hardiness. The correlation was particularly strong for paper birch (r=–0.77) and less strong for Sitka alder (r=–0.60). Mean population biomass accumulation decreased with increased climate coldness. These patterns may be consequential for evaluation of the impact of climate change and extension of the growing season on plant communities. Received: 12 July 1999 / Accepted: 24 November 1999     

Leaf anatomical properties in relation to differences in mesophyll conductance to CO2 and photosynthesis in two related Mediterranean Abies species

Abies alba and Abies pinsapo are closely related species with the same ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) large subunit (rbcL) but contrasting hydraulic traits and mesophyll structure occurring in the Iberian Peninsula under contrasting conditions. As photosynthesis and hydraulic capacities often co‐scale, we hypothesize that these species differ in mesophyll conductance to CO₂ (g_m). g_m and key anatomical traits were measured in both species. Drought‐adapted population of A. pinsapo has higher photosynthesis than the more mesic population of A. alba, in agreement with its higher hydraulic capacity. However, A. alba exhibits the largest stomatal conductance (g_s), and so water use efficiency (WUE) is much higher in A. pinsapo. The differences in photosynthesis were explained by differences in g_m, indicating a correlation between hydraulic capacity and g_m. We report a case where g_m is the main factor limiting photosynthesis in one species (A. alba) when compared with the other one (A. pinsapo). The results also highlight the discrepancy between g_m estimates based on anatomical measurements and those based on gas exchange methods, probably due to the very large resistance exerted by cell walls and the stroma in both species. Thus, the cell wall and chloroplast properties in relation to CO₂ diffusion constitute a near‐future research priority.     

Contrasting acclimation responses to elevated CO2 and warming between an evergreen and a deciduous boreal conifer

Rising atmospheric carbon dioxide (CO₂) concentrations may warm northern latitudes up to 8°C by the end of the century. Boreal forests play a large role in the global carbon cycle, and the responses of northern trees to climate change will thus impact the trajectory of future CO₂ increases. We grew two North American boreal tree species at a range of future climate conditions to assess how growth and carbon fluxes were altered by high CO₂ and warming. Black spruce (Picea mariana, an evergreen conifer) and tamarack (Larix laricina, a deciduous conifer) were grown under ambient (407 ppm) or elevated CO₂ (750 ppm) and either ambient temperatures, a 4°C warming, or an 8°C warming. In both species, the thermal optimum of net photosynthesis (T_optA) increased and maximum photosynthetic rates declined in warm‐grown seedlings, but the strength of these changes varied between species. Photosynthetic capacity (maximum rates of Rubisco carboxylation, V_cmax, and of electron transport, J_max) was reduced in warm‐grown seedlings, correlating with reductions in leaf N and chlorophyll concentrations. Warming increased the activation energy for V_cmax and J_max (E_aV and E_aJ, respectively) and the thermal optimum for J_max. In both species, the T_optA was positively correlated with both E_aV and E_aJ, but negatively correlated with the ratio of J_max/V_cmax. Respiration acclimated to elevated temperatures, but there were no treatment effects on the Q₁₀ of respiration (the increase in respiration for a 10°C increase in leaf temperature). A warming of 4°C increased biomass in tamarack, while warming reduced biomass in spruce. We show that climate change is likely to negatively affect photosynthesis and growth in black spruce more than in tamarack, and that parameters used to model photosynthesis in dynamic global vegetation models (E_aV and E_aJ) show no response to elevated CO₂.     

Variations in leaf anatomical characteristics drive the decrease of mesophyll conductance in poplar under elevated ozone

Decline in mesophyll conductance (g_m) plays a key role in limiting photosynthesis in plants exposed to elevated ozone (O₃). Leaf anatomical traits are known to influence g_m, but the potential effects of O₃-induced changes in leaf anatomy on g_m have not yet been clarified. Here, two poplar clones were exposed to elevated O₃. The effects of O₃ on the photosynthetic capacity and anatomical characteristics were assessed to investigate the leaf anatomical properties that potentially affect g_m. We also conducted global meta-analysis to explore the general response patterns of g_m and leaf anatomy to O₃ exposure. We found that the O₃-induced reduction in g_m was critical in limiting leaf photosynthesis. Changes in liquid-phase conductance rather than gas-phase conductance drive the decline in g_m under elevated O_3, and this effect was associated with thicker cell walls and smaller chloroplast sizes. The effects of O₃ on palisade and spongy mesophyll cell traits and their contributions to g_m were highly genotype-dependent. Our results suggest that, while anatomical adjustments under elevated O₃ may contribute to defense against O₃ stress, they also cause declines in g_m and photosynthesis. These results provide the first evidence of anatomical constraints on g_m under elevated O₃.     

Variable mesophyll conductance revisited: theoretical background and experimental implications

The CO₂ concentration at the site of carboxylation inside the chloroplast stroma depends not only on the stomatal conductance, but also on the conductance of CO₂ between substomatal cavities and the site of CO₂ fixation. This conductance, commonly termed mesophyll conductance (g_m), significantly constrains the rate of photosynthesis. Here we show that estimates of g_m are influenced by the amount of respiratory and photorespiratory CO₂ from the mitochondria diffusing towards the chloroplasts. This results in an apparent CO₂ and oxygen sensitivity of g_m that does not imply a change in intrinsic diffusion properties of the mesophyll, but depends on the ratio of mitochondrial CO₂ release to chloroplast CO₂ uptake. We show that this effect (1) can bias the estimation of the CO₂ photocompensation point and non‐photorespiratory respiration in the light; (2) can affect the estimates of ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) kinetic constants in vivo; and (3) results in an apparent obligatory correlation between stomatal conductance and g_m. We further show that the amount of photo(respiratory) CO₂ that is refixed by Rubisco can be directly estimated through measurements of g_m.     

Temperature response of in vivo Rubisco kinetics and mesophyll conductance in Arabidopsis thaliana: comparisons to Nicotiana tabacum

Biochemical models are used to predict and understand the response of photosynthesis to rising temperatures and CO₂ partial pressures. These models require the temperature dependency of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) kinetics and mesophyll conductance to CO₂ (g_m). However, it is not known how the temperature response of Rubisco kinetics differs between species, and comprehensive in vivo Rubisco kinetics that include g_m have only been determined in the warm‐adapted Nicotiana tabacum. Here, we measured the temperature response of Rubisco kinetics and g_m in N. tabacum and the cold‐adapted Arabidopsis thaliana using gas exchange and ¹³CO₂ isotopic discrimination on plants with genetically reduced levels of Rubisco. While the individual Rubisco kinetic parameters in N. tabacum and A. thaliana were similar across temperatures, they collectively resulted in significantly different modelled rates of photosynthesis. Additionally, g_m increased with temperature in N. tabacum but not in A. thaliana. These findings highlight the importance of considering species‐dependent differences in Rubisco kinetics and g_m when modelling the temperature response of photosynthesis.     

Implications of the mesophyll conductance to CO2 for photosynthesis and water‐use efficiency during long‐term water stress and recovery in two contrasting Eucalyptus species

Water stress (WS) slows growth and photosynthesis (A_n), but most knowledge comes from short‐time studies that do not account for longer term acclimation processes that are especially relevant in tree species. Using two Eucalyptus species that contrast in drought tolerance, we induced moderate and severe water deficits by withholding water until stomatal conductance (g_sw) decreased to two pre‐defined values for 24 d, WS was maintained at the target g_sw for 29 d and then plants were re‐watered. Additionally, we developed new equations to simulate the effect on mesophyll conductance (g_m) of accounting for the resistance to refixation of CO₂. The diffusive limitations to CO₂, dominated by the stomata, were the most important constraints to A_n. Full recovery of A_n was reached after re‐watering, characterized by quick recovery of g_m and even higher biochemical capacity, in contrast to the slower recovery of g_sw. The acclimation to long‐term WS led to decreased mesophyll and biochemical limitations, in contrast to studies in which stress was imposed more rapidly. Finally, we provide evidence that higher g_m under WS contributes to higher intrinsic water‐use efficiency (iWUE) and reduces the leaf oxidative stress, highlighting the importance of g_m as a target for breeding/genetic engineering.     

Differential tissue-specific expression of NtAQP1 in Arabidopsis thaliana reveals a role for this protein in stomatal and mesophyll conductance of CO2 under standard and salt-stress conditions

The regulation of plant hydraulic conductance and gas conductance involves a number of different morphological, physiological and molecular mechanisms working in harmony. At the molecular level, aquaporins play a key role in the transport of water, as well as CO₂, through cell membranes. Yet, their tissue-related function, which controls whole-plant gas exchange and water relations, is less understood. In this study, we examined the tissue-specific effects of the stress-induced tobacco Aquaporin1 (NtAQP1), which functions as both a water and CO₂ channel, on whole-plant behavior. In tobacco and tomato plants, constitutive overexpression of NtAQP1 increased net photosynthesis (A _N), mesophyll CO₂ conductance (g _m) and stomatal conductance (g _s) and, under stress, increased root hydraulic conductivity (L _pr) as well. Our results revealed that NtAQP1 that is specifically expressed in the mesophyll tissue plays an important role in increasing both A _N and g _m. Moreover, targeting NtAQP1 expression to the cells of the vascular envelope significantly improved the plants’ stress response. Surprisingly, NtAQP1 expression in the guard cells did not have a significant effect under any of the tested conditions. The tissue-specific involvement of NtAQP1 in hydraulic and gas conductance via the interaction between the vasculature and the stomata is discussed.     

Leaf photosynthetic rate and mesophyll cell anatomy changes during ontogenesis in backcrossed <Emphasis Type="Italic">indica</Emphasis> × <Emphasis Type="Italic">japonica</Emphasis> rice inbred lines

The high-yielding indica rice variety, ‘Takanari’, has the high rate of leaf photosynthesis compared with the commercial japonica varieties. Among backcrossed inbred lines from a cross between ‘Takanari’ and a japonica variety, ‘Koshihikari’, two lines, BTK-a and BTK-b, showed approximately 20% higher photosynthetic rate than that of ‘Takanari’ for a flag leaf at full heading. This is a highest recorded rate of rice leaf photosynthesis. Here, the timing and cause of the increased leaf photosynthesis in the BTK lines were investigated by examining the photosynthesis and related parameters, as well as mesophyll cell anatomy during ontogenesis. Their photosynthetic rate was greater than that of ‘Takanari’ in the 13th leaf, as well as the flag leaf, but there were no differences in the 7th and 10th leaves. There were no consistent differences in the stomatal conductance, or the leaf nitrogen and Rubisco contents in the 13th and flag leaves. The total surface area of mesophyll cells per leaf area (TA_mes) in the 13th and flag leaves increased significantly in the BTK lines due to the increased number and developed lobes of mesophyll cells compared with in ‘Takanari’. The mesophyll conductance (g _m) became greater in the BTK lines compared with ‘Takanari’ in the flag leaves but not in the 10th leaves. A close correlation was observed between TA_mes and g _m. We concluded that the increased mesophyll conductance through the development of mesophyll cells during the reproductive period is a probable cause of the greater photosynthetic rate in the BTK lines.     

Mesophyll conductance in land surface models: effects on photosynthesis and transpiration

The CO₂ transfer conductance within plant leaves (mesophyll conductance, g_m) is currently not considered explicitly in most land surface models (LSMs), but instead treated implicitly as an intrinsic property of the photosynthetic machinery. Here, we review approaches to overcome this model deficiency by explicitly accounting for g_m, which comprises the re‐adjustment of photosynthetic parameters and a model describing the variation of g_m in dependence of environmental conditions. An explicit representation of g_m causes changes in the response of photosynthesis to environmental factors, foremost leaf temperature, and ambient CO₂ concentration, which are most pronounced when g_m is small. These changes in leaf‐level photosynthesis translate into a stronger climate and CO₂ response of gross primary productivity (GPP) and transpiration at the global scale. The results from two independent studies show consistent latitudinal patterns of these effects with biggest differences in GPP in the boreal zone (up to ~15%). Transpiration and evapotranspiration show spatially similar, but attenuated, changes compared with GPP. These changes are indirect effects of g_m caused by the assumed strong coupling between stomatal conductance and photosynthesis in current LSMs. Key uncertainties in these simulations are the variation of g_m with light and the robustness of its temperature response across plant types and growth conditions. Future research activities focusing on the response of g_m to environmental factors and its relation to other plant traits have the potential to improve the representation of photosynthesis in LSMs and to better understand its present and future role in the Earth system.     

Drought‐introduced variability of mesophyll conductance in Gossypium and its relationship with leaf anatomy

Mesophyll conductance (g_m) is one of the major determinants of photosynthetic rate, for which it has an impact on crop yield. However, the regulatory mechanisms behind the decline in g_m of cotton (Gossypium. spp) by drought are unclear. An upland cotton (Gossypium hirsutum) genotype and a pima cotton (Gossypium barbadense) genotype were used to determine the gas exchange parameters, leaf anatomical structure as well as aquaporin and carbonic anhydrase gene expression under well‐watered and drought treatment conditions. In this study, the decrease of net photosynthetic rate (A_N) under drought conditions was related to a decline in g_m and in stomatal conductance (g_s). g_m and g_s coordinate with each other to ensure optimum state of CO₂ diffusion and achieve the balance of water and CO₂ demand in the process of photosynthesis. Meanwhile, mesophyll limitations to photosynthesis are equally important to the stomatal limitations. Considering g_m, its decline in cotton leaves under drought was mostly regulated by the chloroplast surface area exposed to leaf intercellular air spaces per leaf area (S_c/S) and might also be regulated by the expression of leaf CARBONIC ANHYDRASE (CA1). Meanwhile, cotton leaves can minimize the decrease in g_m under drought by maintaining cell wall thickness (T_cw). Our results indicated that modification of chloroplasts might be a target trait in future attempts to improve cotton drought tolerance.     

Diffusional limitations explain the lower photosynthetic capacity of ferns as compared with angiosperms in a common garden study

Ferns are thought to have lower photosynthetic rates than angiosperms and they lack fine stomatal regulation. However, no study has directly compared photosynthesis in plants of both groups grown under optimal conditions in a common environment. We present a common garden comparison of seven angiosperms and seven ferns paired by habitat preference, with the aims of (1) confirming that ferns do have lower photosynthesis capacity than angiosperms and quantifying these differences; (2) determining the importance of diffusional versus biochemical limitations; and (3) analysing the potential implication of leaf anatomical traits in setting the photosynthesis capacity in both groups. On average, the photosynthetic rate of ferns was about half that of angiosperms, and they exhibited lower stomatal and mesophyll conductance to CO₂ (g_m), maximum velocity of carboxylation and electron transport rate. A quantitative limitation analysis revealed that stomatal and mesophyll conductances were co‐responsible for the lower photosynthesis of ferns as compared with angiosperms. However, g_m alone was the most constraining factor for photosynthesis in ferns. Consistently, leaf anatomy showed important differences between angiosperms and ferns, especially in cell wall thickness and the surface of chloroplasts exposed to intercellular air spaces.     

Light-harvesting in photosystem I

Improving Rubisco catalysis is considered a promising way to enhance C₃-photosynthesis and photosynthetic water use efficiency (WUE) provided the introduced changes have little or no impact on other processes affecting photosynthesis such as leaf photochemistry or leaf CO₂ diffusion conductances. However, the extent to which the factors affecting photosynthetic capacity are co-regulated is unclear. The aim of the present study was to characterize the photochemistry and CO₂ transport processes in the leaves of three transplantomic tobacco genotypes expressing hybrid Rubisco isoforms comprising different Flaveria L-subunits that show variations in catalysis and differing trade-offs between the amount of Rubisco and its activation state. Stomatal conductance (g _s) in each transplantomic tobacco line matched wild-type, while their photochemistry showed co-regulation with the variations in Rubisco catalysis. A tight co-regulation was observed between Rubisco activity and mesophyll conductance (g _m) that was independent of g _s thus producing plants with varying g _m/g _s ratios. Since the g _m/g _s ratio has been shown to positively correlate with intrinsic WUE, the present results suggest that altering photosynthesis by modifying Rubisco catalysis may also be useful for targeting WUE.     

Photosynthetic responses of soybean (Glycine max L.) to heat‐induced electrical signalling are predominantly governed by modifications of mesophyll conductance for CO2

In recent years, the effect of heat‐induced electrical signalling on plant photosynthetic activity has been demonstrated for many plant species. However, the underlying triggers of the resulting transient inhibition of photosynthesis still remain unknown. To further investigate on this phenomenon, we focused in our present study on soybean (Glycine max L.) on the direct effect of signal transmission in the leaf mesophyll on conductance for CO₂ diffusion in the mesophyll (g_m) and detected a drastic decline in g_m following the electrical signal, whereas the photosynthetic electron transport rate (ETR) was only marginally affected. In accordance with the drop in net photosynthesis (A_N), energy dispersive X‐ray analysis (EDXA) revealed a shift of K, Mg, O and P on leaf chloroplasts. Control experiments under elevated CO₂ conditions proved the transient reduction of A_N, ETR, the chloroplast CO₂ concentration (C_c) and g_m to be independent of the external CO₂ regime, whereas the effect of the electrical signal on stomatal conductance for CO₂ (g_s) turned out much less distinctive. We therefore conclude that the effect of electrical signalling on photosynthesis in soybean is triggered by its immediate effects on g_m.     

Anatomical variation of mesophyll conductance under potassium deficiency has a vital role in determining leaf photosynthesis

Leaves exposed to potassium (K) deficiency usually present decreased mesophyll conductance (g_m) and photosynthesis (A). The relative contributions of leaf anatomical traits in determining g_m have been quantified; however, anatomical variabilities related to low g_m under K starvation remain imperfectly known. A one‐dimensional model was used to quantify anatomical controls of the entire CO₂ diffusion pathway resistance within a leaf on two Brassica napus L. cultivars in response to K deficiency. Leaf photosynthesis of both cultivars was significantly decreased under K deficiency in parallel with down‐regulated g_m. The mesophyll conductance limitation contributed to more than one‐half of A decline. The decreased internal air space in K‐starved leaves was associated with the increase of gas‐phase resistance. Potassium deficiency reduced liquid‐phase conductance by decreasing the exposed surface area of chloroplasts per unit leaf area (S_c/S), and enlarging the resistance of the cytoplasm that can be interpreted by the increasing distance of chloroplast from cell wall, and between adjacent chloroplasts. Additionally, the discrepancies of A between two cultivars were in part because of g_m variations, ascribing to an altered S_c/S. These results emphasize the important role of K on the regulation of g_m by enhancing S_c/S and reducing cytoplasm resistance.     

Abscisic Acid Induces Rapid Reductions in Mesophyll Conductance to Carbon Dioxide

The rate of photosynthesis (A) of plants exposed to water deficit is a function of stomatal (g_s) and mesophyll (g_m) conductance determining the availability of CO₂ at the site of carboxylation within the chloroplast. Mesophyll conductance often represents the greatest impediment to photosynthetic uptake of CO₂, and a crucial determinant of the photosynthetic effects of drought. Abscisic acid (ABA) plays a fundamental role in signalling and co-ordination of plant responses to drought; however, the effect of ABA on g_m is not well-defined. Rose, cherry, olive and poplar were exposed to exogenous ABA and their leaf gas exchange parameters recorded over a four hour period. Application with ABA induced reductions in values of A, g_s and g_m in all four species. Reduced g_m occurred within one hour of ABA treatment in three of the four analysed species; indicating that the effect of ABA on g_m occurs on a shorter timescale than previously considered. These declines in g_m values associated with ABA were not the result of physical changes in leaf properties due to altered turgor affecting movement of CO₂, or caused by a reduction in the sub-stomatal concentration of CO₂ (C_i). Increased [ABA] likely induces biochemical changes in the properties of the interface between the sub-stomatal air-space and mesophyll layer through the actions of cooporins to regulate the transport of CO₂. The results of this study provide further evidence that g_m is highly responsive to fluctuations in the external environment, and stress signals such as ABA induce co-ordinated modifications of both g_s and g_m in the regulation of photosynthesis.     

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.