Patterns of spatio-temporal distribution of winter chronic photoinhibition in leaves of three evergreen Mediterranean species with contrasting acclimation responses.

High irradiance and relatively low temperature, which characterize Mediterranean winters, cause chilling stress in plants. Downregulation of photosynthetic efficiency is a mechanism that allows plants to survive these conditions. This study aims to address whether this process shows a regular spatial pattern across leaf surface or not. Three species (Buxus sempervirens, Cistus albidus and Arctostaphylos uva-ursi) with contrasting responses to winter stress were studied. During 7 days, macro and micro Fv/Fm spatial patterns were monitored by the use of chlorophyll fluorescence imaging techniques. In the field, the strongest photoinhibition was found in B. sempervirens, while there was almost no chronic photoinhibition in C. albidus. In leaves of the first species, Fv/Fm decreased from base to tip while in C. albidus it was uniform over the leaf lamina. An intermediate behavior is shown by A. uva-ursi leaves. Spatial heterogeneity distribution of Fv/Fm was found inside the leaves, resulting in greater Fv/Fm values in the inner layers than in the outer ones. Neither xanthophyll-linked downregulation of Fv/Fm nor protein remobilization were the reasons for such spatial patterns since pigment composition and nitrogen content did not reveal tip-base differences. During recovery from winter, photoinhibition changes occurred in Fv/Fm, pigments and chloroplast ultrastructure. This work shows for the first time that irrespective of physiological mechanisms responsible for development of winter photoinhibition, there is an acclimation response with strong spatio-temporal variability at leaf level in some species. This observation should be taken into account when modeling or scaling up photosynthetic responses.

Growth, fire history, and browsing recorded in wood rings of shrubs in a mild temperate climate.

Separate effects of abiotic and biotic factors on the structure and dynamics of ecological communities may be recorded in growth rings of woody plants. We used Ceanothus cuneatus rigidus and Arctostaphylos pumila to tease apart the roles of fire, rain, and herbivores on the histories and community structure of four areas in a coastal mediterranean-type climate in central California with mild winters and mild summers. Ring widths of both species were related to rainfall in two of the areas; heavy deer browsing on Ceanothus overwhelmed the climate signal in the others. Ceanothus germination was more closely related to heavy rainfall, especially during ENSO years, than to fire events. In a related greenhouse experiment that evaluated these observations, the same proportions of new Ceanothus seeds germinated after burning and after receiving regular water for several months, but germination of old seeds responded primarily to the fire treatment. In areas where heavy browsing by mammals reduces recruitment and growth of Ceanothus and increases mortality, the continuance of the Ceanothus population must rely heavily on germination from the persistent seed bank during unusually wet years or after occasional fires. Because Arctostaphylos can produce new stems from underground roots, individual plants may survive and produce seeds until another fire.

Is shade beneficial for mediterranean shrubs experiencing periods of extreme drought and late-winter frosts?

BACKGROUND AND AIMS: Plants are naturally exposed to multiple, frequently interactive stress factors, most of which are becoming more severe due to global change. Established plants have been reported to facilitate the establishment of juvenile plants, but net effects of plant-plant interactions are difficult to assess due to complex interactions among environmental factors. An investigation was carried out in order to determine how two dominant evergreen shrubs (Quercus ilex and Arctostaphylos uva-ursi) co-occurring in continental, Mediterranean habitats respond to multiple abiotic stresses and whether the shaded understorey conditions ameliorate the negative effects of drought and winter frosts on the physiology of leaves. METHODS: Microclimate and ecophysiology of sun and shade plants were studied at a continental plateau in central Spain during 2004-2005, with 2005 being one of the driest and hottest years on record; several late-winter frosts also occurred in 2005. KEY RESULTS: Daytime air temperature and vapour pressure deficit were lower in the shade than in the sun, but soil moisture was also lower in the shade during the spring and summer of 2005, and night-time temperatures were higher in the shade. Water potential, photochemical efficiency, light-saturated photosynthesis, stomatal conductance and leaf 13C composition differed between sun and shade individuals throughout the seasons, but differences were species specific. Shade was beneficial for leaf-level physiology in Q. ilex during winter, detrimental during spring for both species, and of little consequence in summer. CONCLUSIONS: The results suggest that beneficial effects of shade can be eclipsed by reduced soil moisture during dry years, which are expected to be more frequent in the most likely climate change scenarios for the Mediterranean region.

What the towers don't see at night: nocturnal sap flow in trees and shrubs at two AmeriFlux sites in California.

At the leaf scale, it is a long-held assumption that stomata close at night in the absence of light, causing transpiration to decrease to zero. Energy balance models and evapotranspiration equations often rely on net radiation as an upper bound, and some models reduce evapotranspiration to zero at night when there is no solar radiation. Emerging research is showing, however, that transpiration can occur throughout the night in a variety of vegetation types and biomes. At the ecosystem scale, eddy covariance measurements have provided extensive data on latent heat flux for a multitude of ecosystem types globally. Nighttime eddy covariance measurements, however, are generally unreliable because of low turbulence. If significant nighttime water loss occurs, eddy flux towers may be missing key information on latent heat flux. We installed and measured rates of sap flow by the heat ratio method (Burgess et al. 2001) at two AmeriFlux (part of FLUXNET) sites in California. The heat ratio method allows measurement and quantification of low rates of sap flow, including negative rates (i.e., hydraulic lift). We measured sap flow in five Pinus ponderosa Dougl. ex Laws. trees and three Arctostaphylos manzanita Parry and two Ceanothus cordulatus A. Kellog shrubs in the Sierra Nevada Mountains, and in five Quercus douglasii Hook and Arn. trees at an oak savanna in the Central Valley of California. Nocturnal sap flow was observed in all species, and significant nighttime water loss was observed in both species of trees. Vapor pressure deficit and air temperature were both well correlated with nighttime transpiration; the influence of wind speed on nighttime transpiration was insignificant at both sites. We distinguished between storage-tissue refilling and water loss based on data from Year 2005, and calculated the percentage by which nighttime transpiration was underestimated by eddy covariance measurements at both sites.

Winter acclimation of PsbS and related proteins in the evergreen Arctostaphylos uva-ursi as influenced by altitude and light environment.

The evergreen groundcover bearberry (Arctostaphylos uva-ursi [L.] Sprengel) was characterized over two successive years (2002-2004) from both sun-exposed and shaded sites at a montane ponderosa pine and subalpine forest community of 1900- and 2800-m-high altitudes, respectively. During summer, photosynthetic capacities and pre-dawn photosystem II (PSII) efficiency were similarly high in all four populations, and in winter, only the sun-exposed and shaded populations at 2800 m exhibited complete down-regulation of photosynthetic oxygen evolution capacity and consistent sustained down-regulation of PSII efficiency. This photosynthetic down-regulation at high altitude involved a substantial decrease in PSII components [pheophytin, D1 protein, oxygen evolving complex ([OEC)], a strong up-regulation of several anti-early-light-inducible protein (Elip)- and anti-high-light-inducible protein (Hlip)-reactive bands and a warm-sustained retention of zeaxanthin and antheraxanthin (Z + A). PsbS, the protein modulating the rapid engagement and disengagement of Z +A in energy dissipation, exhibited its most pronounced winter increases in the shade at 1900 m, and thus apparently assumes a greater role in providing rapidly reversible zeaxanthin-dependent photoprotection during winter when light becomes excessive in the shaded population, which remains photosynthetically active. It is attractive to hypothesize that PsbS relatives (Elips/Hlips) may be involved in sustained zeaxanthin-dependent photoprotection under the more extreme winter conditions at 2800 m.