J. S. Pereira

Understanding plant responses to drought — from genes to the whole plant

In the last decade, our understanding of the processes underlying plant response to drought, at the molecular and whole-plant levels, has rapidly progressed. Here, we review that progress. We draw attention to the perception and signalling processes (chemical and hydraulic) of water deficits. Knowledge of these processes is essential for a holistic understanding of plant resistance to stress, which is needed to improve crop management and breeding techniques. Hundreds of genes that are induced under drought have been identified. A range of tools, from gene expression patterns to the use of transgenic plants, is being used to study the specific function of these genes and their role in plant acclimation or adaptation to water deficit. However, because plant responses to stress are complex, the functions of many of the genes are still unknown. Many of the traits that explain plant adaptation to drought - such as phenology, root size and depth, hydraulic conductivity and the storage of reserves - are those associated with plant development and structure, and are constitutive rather than stress induced. But a large part of plant resistance to drought is the ability to get rid of excess radiation, a concomitant stress under natural conditions. The nature of the mechanisms responsible for leaf photoprotection, especially those related to thermal dissipation, and oxidative stress are being actively researched. The new tools that operate at molecular, plant and ecosystem levels are revolutionising our understanding of plant response to drought, and our ability to monitor it. Techniques such as genome-wide tools, proteomics, stable isotopes and thermal or fluorescence imaging may allow the genotype-phenotype gap to be bridged, which is essential for faster progress in stress biology research.

Ecosystem effects of biodiversity manipulations in European grasslands.

We present a multisite analysis of the relationship between plant diversity and ecosystem functioning within the European BIODEPTH network of plant-diversity manipulation experiments. We report results of the analysis of 11 variables addressing several aspects of key ecosystem processes like biomass production, resource use (space, light, and nitrogen), and decomposition, measured across three years in plots of varying plant species richness at eight different European grassland field sites. Differences among sites explained substantial and significant amounts of the variation of most of the ecosystem processes examined. However, against this background of geographic variation, all the aspects of plant diversity and composition we examined (i.e., both numbers and types of species and functional groups) produced significant, mostly positive impacts on ecosystem processes. Analyses using the additive partitioning method revealed that complementarity effects (greater net yields than predicted from monocultures due to resource partitioning, positive interactions, etc.) were stronger and more consistent than selection effects (the covariance between monoculture yield and change in yield in mixtures) caused by dominance of species with particular traits. In general, communities with a higher diversity of species and functional groups were more productive and utilized resources more completely by intercepting more light, taking up more nitrogen, and occupying more of the available space. Diversity had significant effects through both increased vegetation cover and greater nitrogen retention by plants when this resource was more abundant through N2 fixation by legumes. However, additional positive diversity effects remained even after controlling for differences in vegetation cover and for the presence of legumes in communities. Diversity effects were stronger on above- than belowground processes. In particular, clear diversity effects on decomposition were only observed at one of the eight sites. The ecosystem effects of plant diversity also varied between sites and years. In general, diversity effects were lowest in the first year and stronger later in the experiment, indicating that they were not transitional due to community establishment. These analyses of our complete ecosystem process data set largely reinforce our previous results, and those from comparable biodiversity experiments, and extend the generality of diversity–ecosystem functioning relationships to multiple sites, years, and processes.

Mediterranean cork oak savannas require human use to sustain biodiversity and ecosystem services

Mediterranean cork oak savannas, which are found only in southwestern Europe and northwestern Africa, are ecosystems of high socioeconomic and conservation value. Characterized by sparse tree cover and a diversity of understory vegetation – ranging from shrub formations to grasslands – that support high levels of biodiversity, these ecosystems require active management and use by humans to ensure their continued existence. The most important product of these savannas is cork, a non-timber forest product that is periodically harvested without requiring tree felling. Market devaluation of, and lower demand for, cork are causing a decline in management, or even abandonment, of southwestern Europes cork oak savannas. Subsequent shrub encroachment into the savannas grassland components reduces biodiversity and degrades the services provided by these ecosystems. In contrast, poverty-driven overuse is degrading cork oak savannas in northwestern Africa. “Payment for ecosystem services” schemes, such as Forest S...

Photochemical efficiency of photosystem II, photon yield of O2 evolution, photosynthetic capacity, and carotenoid composition during the midday depression of net CO2 uptake in Arbutus unedo growing in Portugal

During the “midday depression” of net CO2 exchange in the mediterranean sclerophyllous shrub Arbutus unedo, examined in the field in Portugal during August of 1987, several parameters indicative of photosynthetic competence were strongly and reversibly affected. These were the photochemical efficiency of photosystem (PS) II, measured as the ratio of variable to maximum chlorophyll fluorescence, as well as the photon yield and the capacity of photosynthetic O2 evolution at 10% CO2, of which the apparent photon yield of O2 evolution was most depressed. Furthermore, there was a strong and reversible increase in the content of the carotenoid zeaxanthin in the leaves that occurred at the expense of both violaxanthin and β-carotene. Diurnal changes in fluorescence characteristics were interpreted to indicate three concurrent effects on the photochemical system. First, an increase in the rate of radiationless energy dissipation in the antenna chlorophyll, reflected by changes in 77K fluorescence of PSII and PSI as well as in chlorophyll a fluorescence at ambient temperature. Second, a state shift characterized by an increase in the proportion of energy distributed to PSI as reflected by changes in PSI fluorescence. Third, an effect lowering the photon yield of O2 evolution and PSII fluorescence at ambient temperature without affecting PSII fluorescence at 77K which would be expected from a decrease in the activity of the water splitting enzyme system, i.e. a donor side limitation.

Partial rootzone drying: effects on growth and fruit quality of field-grown grapevines (Vitis vinifera)

A study to assess the effects of the Partial Rootzone Drying (PRD) irrigation strategy in comparison to other irrigation systems was carried out in southern Portugal in two field-grown grapevines varieties, Moscatel and Castelão. We addressed the question of whether by regulating growth and plant water use, the PRD system would enable an equilibrated vegetative development, leading to a favourable capture of solar radiation for photoassimilate production and, at the same time to provide an optimum environment for fruit maturation. Three irrigation schemes were applied in addition to the non-irrigated (NI) vines: partial root drying (PRD), 50% of crop evapotranspiration (ETc), supplied to only one side of the root system while the other one was allowed to dry, alternating sides every 15 days; deficit irrigated (DI), 50% ETc supplied, half to each side of the root system and full irrigated (FI, 100% ETc). During the whole season FI plants of both varieties exhibited a high leaf predawn water potential (ψpd , ca-0.2MPa) while a progressive decline was observed in NI plants, reaching ψpd values near -0.7 MPa at the end of August. PRD and DI presented intermediate values. PRD vines exhibited a stronger control over vegetative growth as compared with DI and FI plants. This was expressed by lower values of total leaf area at harvest, leaf layer number, canopy wideness and water shoots number, allowing a higher light interception at the cluster zone that induced an improvement in some berry quality characteristics. Watering had no significant effects on sugar accumulation in the berries but led to a favourable increase in the must titratable acidity, mainly in Castelão. Whereas in DI and FI treatments berry skin anthocyanins and phenols content were always lower than in NI, in PRD there was either no reduction or the reduction was much lower than in the other irrigation treatments. Water use efficiency (WUE) was increased by about 80% in PRD and DI when compared with FI, as a result of almost similar yields in the three treatments. Yield gains of irrigated plants in relation to NI were modest, explained by the rainy spring in both years.

Transpiration of a 64-year old maritime pine stand in Portugal

The transpiration, sap flow, stomatal conductance and water relations ofPinus pinaster were determined during spring and summer in a 64-year-old stand in Ribatejo (Portugal). The transpiration of the pine canopy was determined from sap flow or eddy covariance techniques. Canopy conductance values (gc) were estimated from inversion methods using eddy covariance or sap flow data, respectively, and from scaling-up methods using stomatal conductance values measured in the field and leaf area index (LAI) values. The transpiration was closely controlled by the stomatal conductance of pines (Ω was 0.05–0.15). For wet soil conditions, the various estimates ofgc showed reasonable agreement.gc peaked in the morning at 0.01 m×s-1, exhibited a midday depression and showed a secondary peak in late afternoon. This behaviour could be predicted simply on the basis of the stomatal sensitivity to air vapour pressure deficit. On a seasonal basis, monthly average values ofgc decreased from 4×10-3 m×s-1 in spring to 1.7×10-3 m·s-1 in late summer. Accordingly, the transpiration peaked at 3 mm×d-1 on wet soil in May. It decreased progressively during the summer drought to 0.8 mm×d-1 at the end of August. The minimal value of needle water potential was maintained at -1.9 MPa but predawn values decreased from -0.6 MPa in May to -0.9 MPa in July. It may have reached lower values in August. The amount of water stored in the trunk accounted for a 12% (10 kg×tree-1×day-1) of the daily transpiration in spring. The storage capacity of the canopy was within the same order of magnitude. The trunk storage increased to 25% (13 kg×tree-1×day-1) of the daily transpiration at the end of summer under drought conditions. The sap flow beneath the crown lagged accordingly behind transpiration with a time constant estimated between 26 min in spring and 40 min at the end of summer.

Implications of the carbon cycle steady state assumption for biogeochemical modeling performance and inverse parameter retrieval

We analyze the impacts of the steady state assumption on inverse model parameter retrieval from biogeochemical models. An inverse model parameterization study using eddy covariance CO 2 flux data was performed with the Carnegie Ames Stanford Approach (CASA) model under conditions of strict and relaxed carbon cycle steady state assumption (CCSSA) in order to evaluate both the robustness of the models structure for the simulation of net ecosystem carbon fluxes and the assessment of the CCSSA effects on simulations and parameter estimation. Net ecosystem production (NEP) measurements from several eddy covariance sites were compared with NEP estimates from the CASA model driven by local weather station climate inputs as well as by remotely sensed fraction of photosynthetically active radiation absorbed by vegetation and leaf area index. The parameters considered for optimization are directly related to aboveground and belowground modeled responses to temperature and water availability, as well as a parameter (η) that relaxed the CCSSA in the model, allowing for site level simulations to be initialized either as net sinks or sources. A robust relationship was observed between NEP observations and predictions for most of the sites through the range of temporal scales considered (daily, weekly, biweekly, and monthly), supporting the conclusion that the model structure is able to capture the main processes explaining NEP variability. Overall, relaxing CCSSA increased model efficiency (21%) and decreased normalized average error (-92%). Intersite variability was a major source of variance in model performance differences between fixed (CCSSA f ) and relaxed (CCSSA r ) CCSSA conditions. These differences were correlated with mean annual NEP observations, where an average increase in modeling efficiency of 0.06 per 100 g Cm -2 a -1 (where a is years) of NEP is observed (α < 0.003). The parameter η was found to be a key parameter in the optimization exercise, generating significant model efficiency losses when removed from the initial parameter set and parameter uncertainties were significantly lower under CCSSA r . Moreover, modeled soil carbon stocks were generally closer to observations once the steady state assumption was relaxed. Finally, we also show that estimates of individual parameters are affected by the steady state assumption. For example, estimates of radiation-use efficiency were strongly affected by the CCSSA f indicating compensation effects for the inadequate steady state assumption, leading to effective and thus biased parameters. Overall, the importance of model structural evaluation in data assimilation approaches is thus emphasized.

Transpiration from a mature Eucalyptus globulus plantation in Portugal during a spring-summer period of progressively higher water deficit

Abstract The rates of transpiration from a mature Eucalyptus globulus Labill. stand in Portugal were evaluated during a drying period of the spring-summer 1994. Transpiration was measured by the Granier sap flow method and estimated by the Penman-Monteith model. During the experimental period daily transpiration varied between 3.64 and 0.50 mm day−1. For high-transpiration days, a good agreement was observed between Penman-Monteith estimates and sap flow measurements, both on a daily and on an hourly basis. However, for low-transpiration days, the Penman-Monteith model overestimated transpiration in comparison with the sap flow method. The diurnal variation of sap flow was then smoother and lagged behind the estimates of the Penman-Monteith model. E. globulus showed an efficient control of transpiration losses during dry periods through a progressive stomatal closure. As soil moisture deficit increased, the daily maximum stomatal conductance decreased from 0.46 to 0.14 cm s−1. The results also show that, on a seasonal basis, stomatal conductance and daily transpiration were mainly related to predawn leaf water potential and, thus, to soil moisture content.

On the differential advantages of evergreenness and deciduousness in mediterranean oak woodlands: a flux perspective

We assessed the differential advantages of deciduousness and evergreenness by examining 26 site-years of carbon dioxide, water vapor, and energy flux measurements from five comparable oak woodlands in France, Italy, Portugal, and California (USA). On average, the evergreen and deciduous oak woodlands assimilated and respired similar amounts of carbon while using similar amounts of water. These results suggest that evergreen and deciduous woodlands have specific, and similar, ecological costs in mediterranean climates, and that both leaf habits are able to meet these costs. What are the mechanisms behind these findings? Deciduous oaks compensated for having a shorter growing season by attaining a greater capacity to assimilate carbon for a given amount of intercepted solar radiation during the well-watered spring period; at saturating light levels, deciduous oaks gained carbon at six times the rate of evergreen oaks. Otherwise, the two leaf habits experienced similar efficiencies in carbon use (the change in carbon respired per change in carbon assimilated), water use (the change in carbon assimilation per change in water evaporated), and rainfall use (the change in evaporation per change in rainfall). Overall, leaf area index, rather than leaf habit, was the significant factor in determining the absolute magnitude of carbon gained and water lost by each evergreen and deciduous oak woodland over an annual interval; the closed canopies assimilated and respired more carbon and transpired more water than the open canopies. Both deciduous and evergreen mediterranean oaks survive in their seasonally hot/dry, wet/ cool native range by ensuring that actual evaporation is less than the supply of water. This feat is accomplished by adjusting the leaf area index to reduce total water loss at the landscape scale, by down-regulating photosynthesis, respiration, and stomatal conductance with progressive seasonal soil water deficits, and by extending their root systems to tap groundwater.

Hydraulic and chemical signalling in the regulation of stomatal conductance and plant water use in field grapevines growing under deficit irrigation

Effects of irrigation strategies on stomata and plant water use were studied in field-grown grapevines (Vitis vinifera L.). We assessed the importance of root-derived chemical signals vs. hydraulic signalling in stomatal regulation. The experiment included two treatments with the same water added to the soil (50% ETc) applied either to the whole root system (DI) or to half of the roots, alternating irrigation side every 15 days (PRD). Well-watered plants (FI) (100% ETc) and non-irrigated grapevines (NI) were also studied. Partial stomata closure occurred in both PRD and DI plants. [ABA] of xylem sap remained constant during the day and was maintained throughout the season, with higher values in NI plants. Xylem sap pH was not affected by soil water availability. A positive correlation between ψpd and maximum g s was found, indicating that grapevine stomata strongly respond to plant water status. In contrast, ABA did not explain stomatal control at veraison. At mid-ripening g s was significantly correlated with ABA, apparently interacting with the rise in xylem sap pH. Therefore, our data suggest that hydraulic feedback and feed-forward root-to-shoot chemical signalling mechanisms might be involved in the control of stomata in response to decreased soil water availability, hydraulic signals playing the dominant role.