Photosynthesis of tropical tree seedlings in relation to light and nutrient supply

Are arbuscular-mycorrhizal Alnus incana seedlings more resistant to drought than ectomycorrhizal and nonmycorrhizal ones?

Arbuscular mycorrhizas (AMs) prevail in warm and dry climates and ectomycorrhizas (EMs) in cold and humid climates. We suggest that the fungal symbionts benefit their host plants especially in the corresponding conditions. The hypothesis tested was that AM plants are more drought-resistant than EM or nonmycorrhizal (NM) plants. Grey alder (Alnus incana (L.) Moench) seedlings were inoculated with two species of either AM or EM fungi or none. In one controlled-environment experiment, there was a watering and a drought treatment. Another set of seedlings were not watered until permanent wilting. The AM plants were somewhat smaller than EM and NM, and at the early stage of the drought treatment, the soil-moisture content was slightly higher in the AM pots. Shoot water potential was highest in the AM treatment during severe drought, while stomatal conductance and photosynthesis did not show a mycorrhizal effect. In the lethal-drought set, the AM plants maintained their leaves longer than EM and NM plants, and the AM seedlings survived longer than NM seedlings. Foliar phosphorus and sulfur concentrations remained higher in AM plants than EM or NM, but potassium, copper and iron increased in EM during drought. The root tannin concentration was lower in AM than EM and drought doubled it. Although the difference in drought resistance was not large, the hypothesis was supported by the better performance of AM plants during a severe short-term drought. Sustained phosphorus nutrition during drought in AM plants was a possible reason for this. Moreover, the higher foliar sulfur and lower metal-nutrient concentrations in AM may reflect differences in nutrient uptake or (re)translocation during drought, which merit further research. The much larger tannin concentrations in EM root systems than AM did not appear to protect the EM plants from drought. The differential tannin accumulation in AM and EM plants needs further attention.

Frost hardiness of mycorrhizal and non-mycorrhizal Scots pine under two fertilization treatments

Survival and functioning of mycorrhizal associations at low temperatures are not known well. In an earlier study, ectomycorrhizas did not affect the frost hardiness of Scots pine (Pinus sylvestris L.) roots, but here we studied whether differential nutrient availability would change the result and additionally, alter frost hardiness aboveground. The aim in this experiment was to compare the frost hardiness of roots and needles of mycorrhizal (Hebeloma sp.) and non-mycorrhizal Scots pine seedlings raised using two fertilization treatments and two cold-hardening regimes. The fertilization treatments were low (LF) and high (HF) application of a complete nutrient solution. Three hundred mycorrhizal and non-mycorrhizal seedlings were cultivated in growth chambers in four blocks for 16 weeks. For the first 9 weeks, the seedlings grew in long-day and high-temperature (LDHT) with low fertilization and then they were raised for 3 weeks in LDHT with either low or high fertilization. After this, half of the plants in each treatment combination remained in LDHT, and half were transferred to short-day and low-temperature (SDLT) conditions to cold acclimatize. The frost hardiness of the roots and needles was assessed using controlled freezing tests followed by electrolyte leakage tests (REL). Mycorrhizal roots were slightly more frost hardy than non-mycorrhizal roots, but only in the growing-season conditions (LDHT) in low-nutrient treatment. In LDHT and LF, the frost hardiness of the non-mycorrhizal roots was about -9 °C, and that of the non-mycorrhizal HF roots and the mycorrhizal roots in both fertilization levels was about -11 °C. However, no difference was found in the roots within the SDLT regime, and in needles, there was no difference between mycorrhizal and fertilization treatments. The frost hardiness of needles increased by SDLT treatment, being -8.5 and -14.1 °C in LDHT and SDLT, respectively. The dry mass of roots, stems, and needles was lower in LF than in HF and lower in SDLT than in LDHT. Mycorrhizal treatment did not affect the dry mass or its allocation. Although the mycorrhizal roots were slightly more frost hardy in the growing-season conditions, this is not likely to have significance in the field.

Nitrogen-addition effects on leaf traits and photosynthetic carbon gain of boreal forest understory shrubs

Boreal coniferous forests are characterized by fairly open canopies where understory vegetation is an important component of ecosystem C and N cycling. We used an ecophysiological approach to study the effects of N additions on uptake and partitioning of C and N in two dominant understory shrubs: deciduous Vaccinium myrtillus in a Picea abies stand and evergreen Vaccinium vitis-idaea in a Pinus sylvestris stand in northern Sweden. N was added to these stands for 16 and 8 years, respectively, at rates of 0, 12.5, and 50 kg N ha(-1) year(-1). N addition at the highest rate increased foliar N and chlorophyll concentrations in both understory species. Canopy cover of P. abies also increased, decreasing light availability and leaf mass per area of V. myrtillus. Among leaves of either shrub, foliar N content did not explain variation in light-saturated CO2 exchange rates. Instead photosynthetic capacity varied with stomatal conductance possibly reflecting plant hydraulic properties and within-site variation in water availability. Moreover, likely due to increased shading under P. abies and due to water limitations in the sandy soil under P. sylvestris, individuals of the two shrubs did not increase their biomass or shift their allocation between above- and belowground parts in response to N additions. Altogether, our results indicate that the understory shrubs in these systems show little response to N additions in terms of photosynthetic physiology or growth and that changes in their performance are mostly associated with responses of the tree canopy.

Analysis of the willow root system by electrical impedance spectroscopy

Information on plant roots is increasingly needed for understanding and managing plants under various environmental conditions, including climate change. Several methods have been developed to study fine roots but they are either destructive or cumbersome, or may not be suitable for studies of fine root functionality. Electrical impedance, resistance, and capacitance have been proposed as possible non-destructive measures for studying roots. Their use is limited by a lack of knowledge concerning the electrical circuit of the system. Electrical impedance spectroscopy (EIS) was used for hydroponically raised willows (Salix schwerinii) to estimate the root system size. The impedance spectra were investigated in three experimental set-ups and the corresponding appropriate lumped models were formulated. The fit of the proposed lumped models with the measured impedance spectra data was good. The model parameters were correlated with the contact area of the roots and/or stems raised in the hydroponic solution. The EIS method proved a useful non-destructive method for assessing root surface area. This work may be considered to be a new methodological contribution to understanding root systems and their functions in a non-destructive manner.

An appraisal of the electrical resistance method for assessing root surface area

Electrical resistances of roots and stems of hydroponically raised willows (Salix schwerinii) were studied and related to root morphology. Willow cuttings with and without roots were set in a constant electric field (effective voltage of 0.1 V, sine-AC, 128 Hz) in a hydroponic solution. The electrical resistance of different components in the measurement system was measured and analysed in relation to root surface area in contact with the cultivation solution. Axial resistivities of single root segments and of stems were measured. The results showed that the resistance decreased in relation to an increase in the contact surface area of the roots with the solution. The resistance depended strongly on the contact area of the stem with the solution, however, thus causing bias in the evaluation of root surface area. This work is a new contribution for the understanding of current pathways in the root system as exposed to an external electric field and for developing a non-destructive method to study plant roots accordingly. It may be concluded that the electrical resistance method is a useful non-destructive method to study roots and their physiological properties. Electrical analogues for roots and stem comprising resistors are discussed in relation to in situ measurements.

The effects of gap size on some microclimate variables during late summer and autumn in a temperate broadleaved deciduous forest

The creation of gaps can strongly influence forest regeneration and habitat diversity within forest ecosystems. However, the precise characteristics of such effects depend, to a large extent, upon the way in which gaps modify microclimate and soil water content. Hence, the aim of this study was to understand the effects of gap creation and variations in gap size on forest microclimate and soil water content. The study site, in North West England, was a mixed temperate broadleaved deciduous forest dominated by mature sessile oak (Quercus petraea), beech (Fagus sylvatica) and ash (Fraxinus excelsior) with some representatives of sycamore (Acer pseudoplatanus). Solar radiation (I), air temperature (T(A)), soil temperature (T(S)), relative humidity (h), wind speed (v) and soil water content (Psi) were measured at four natural treefall gaps created after a severe storm in 2006 and adjacent sub-canopy sites. I, T(A), T(S), and Psi increased significantly with gap size; h was consistently lower in gaps than the sub-canopy but did not vary with gap size, while the variability of v could not be explained by the presence or size of gaps. There were systematic diurnal patterns in all microclimate variables in response to gaps, but no such patterns existed for Psi. These results further our understanding of the abiotic and consequent biotic responses to gaps in broadleaved deciduous forests created by natural treefalls, and provide a useful basis for evaluating the implications of forest management practices.

The effect of leaf-level spatial variability in photosynthetic capacity on biochemical parameter estimates using the Farquhar model: a theoretical analysis

Application of the widely used Farquhar model of photosynthesis in interpretation of gas exchange data assumes that photosynthetic properties are homogeneous throughout the leaf. Previous studies showed that heterogeneity in stomatal conductance (g(s)) across a leaf could affect the shape of the measured leaf photosynthetic CO(2) uptake rate (A) versus intercellular CO(2) concentration (C(i)) response curve and, in turn, estimation of the critical biochemical parameters of this model. These are the maximum rates of carboxylation (V(c,max)), whole-chain electron transport (J(max)), and triose-P utilization (V(TPU)). The effects of spatial variation in V(c,max,) J(max), and V(TPU) on estimation of leaf averages of these parameters from A-C(i) curves measured on a whole leaf have not been investigated. A mathematical model incorporating defined degrees of spatial variability in V(c,max) and J(max) was constructed. One hundred and ten theoretical leaves were simulated, each with the same average V(c,max) and J(max), but different coefficients of variation of the mean (CV(VJ)) and varying correlation between V(c,max) and J(max) (Omega). Additionally, the interaction of variation in V(c,max) and J(max) with heterogeneity in V(TPU), g(s), and light gradients within the leaf was also investigated. Transition from V(c,max)- to J(max)-limited photosynthesis in the A-C(i) curve was smooth in the most heterogeneous leaves, in contrast to a distinct inflection in the absence of heterogeneity. Spatial variability had little effect on the accuracy of estimation of V(c,max) and J(max) from A-C(i) curves when the two varied in concert (Omega = 1.0), but resulted in underestimation of both parameters when they varied independently (up to 12.5% in V(c,max) and 17.7% in J(max) at CV(VJ) = 50%; Omega = 0.3). Heterogeneity in V(TPU) also significantly affected parameter estimates, but effects of heterogeneity in g(s) or light gradients were comparatively small. If V(c,max) and J(max) derived from such heterogeneous leaves are used in models to project leaf photosynthesis, actual A is overestimated by up to 12% at the transition between V(c,max)- and J(max)-limited photosynthesis. This could have implications for both crop production and Earth system models, including projections of the effects of atmospheric change.

The OJIP fast fluorescence rise characterizes Graptophyllum species and their stress responses

Causes for rarity in plants are poorly understood. Graptophyllum reticulatum is an endangered endemic species, and it has three close relatives with different conservation status: the vulnerable G. ilicifolium, the rare G. excelsum, and the common G. spinigerum. Applied to the chlorophyll a fluorescence transient of leaves, the JIP test provides a Performance Index (PI) which quantifies the main steps in photosystem II (PSII) photochemistry including light energy absorption, excitation energy trapping, and conversion of excitation energy into electron flow. The PI is calculated from three components which depend on the reaction center density, the trapping efficiency, and the electron transport efficiency. PI was measured in the natural habitats of the four species and under artificially imposed environmental stresses in the glasshouse to determine whether conservation status was related to stress resilience. The results showed that soil type is unlikely to restrict the endangered G. reticulatum, vulnerable G. ilicifolium, or rare G. excelsum because PI was similar in plants grown in diverse soils in the glasshouse. Photoinhibition is likely to restrict the endangered G. reticulatum to shade habitats because PI was significantly reduced when plants were exposed to more than 15% ambient light in controlled experiments. Water availability may determine the location and distribution of the vulnerable G. ilicifolium and common G. spinigerum because PI was reduced more than 60% when plants were exposed to water stress. While the characteristics of their natural habitats correspond to and explain the physiological responses, there was no obvious relationship between conservation status and environmental resilience. PI can be used to monitor vigor and health of populations of plants in the natural habitat. In cultivation experiments PI responds to key environmental variables that affect the distribution of species with conservation significance.