José Miguel Olano

Plastic bimodal xylogenesis in conifers from continental Mediterranean climates

*Seasonal radial-increment and xylogenesis data can help to elucidate how climate modulates wood formation in conifers. Few xylogenesis studies have assessed how plastic xylogenesis is in sympatric conifer species from continental Mediterranean areas, where low winter temperatures and summer drought constrain growth. *Here, we analysed intra-annual patterns of secondary growth in sympatric conifer species (Juniperus thurifera, Pinus halepensis and Pinus sylvestris). Two field sites (xeric and mesic) were evaluated using dendrometers, microcores and climatic data. *A bimodal pattern of xylogenesis characterized by spring and autumn precipitation and subsequent cambial reactivation was detected in J. thurifera at both study sites and in P. halepensis at the xeric site, but was absent in P. sylvestris where growth was largely controlled by day length. In the xeric site J. thurifera exhibited an increased response to water availability in autumn relative to P. halepensis and summer cambial suppression was more marked in J. thurifera than in P. halepensis. *Juniperus thurifera exhibited increased plasticity in its xylogenesis pattern compared with sympatric pines, enabling this species to occupy sites with more variable climatic conditions. The plastic xylogenesis patterns of junipers in drought-stressed areas may also provide them with a competitive advantage against co-occurring pines.

Sex-specific, age-dependent sensitivity of tree-ring growth to climate in the dioecious tree Juniperus thurifera

Tree features may modulate the sensitivity of radial growth to climate, leading to a nonuniform response. Age-related increases in climatic sensitivity have been observed repeatedly. Sex-related climatic sensitivity is also possible because of the long-term differential reproductive cost between the sexes. This study analysed the simultaneous effects of age and sex on the sensitivity of tree-ring growth to climate. Ring widths were measured from 50 female and 50 male Juniperus thurifera trees, 50-350 yr old, growing under a Mediterranean continental climate. Response functions were calculated based on tree-ring chronologies and monthly climatic records. Climatic sensitivity decreased with increasing age. Young trees (50-100 yr) were the most climatically sensitive to June-July precipitation, which affected growth positively. We found a significant interaction between age and sex in the climatic response of J. thurifera, with young females the most sensitive to summer water stress. Our results suggest that age-dependent climatic sensitivity can be determined by site-specific limiting environmental conditions and species-specific architectural and physiological adjustments during ontogeny. This study supports that the different ontogenetic stages of J. thurifera differ in their root structural traits and that sex-related sensitivity to summer drought may be attributable to less efficient water use by females.

The operation of the lutein epoxide cycle correlates with energy dissipation

A new xanthophyll cycle involving de-epoxidation of lutein epoxide (Lx) into lutein in the light and epoxidation back in the dark has been recently described in parasitic plants and in trees from the genus Quercus. To explore the role of the Lx cycle in photoprotection, shade leaves of red oak (Q. rubra), with a relatively high Lx pool, were exposed to different light intensities. Both violaxanthin and Lx were de-epoxidised to the same extent, although the initial kinetics differed, with a rate proportional to the light intensity. De-epoxidation of violaxanthin and Lx was inhibited by dithiothreitol, suggesting that the same enzyme, violaxanthin de-epoxidase (VDE), catalyses both reactions. Dark recovery lagged in the case of Lx, and after 5 h in darkness, the Lx cycle was much more de-epoxidised than the violaxanthin cycle. The different rates of epoxidation of the violaxanthin and Lx cycles were used to study the role of the Lx cycle in photoprotection. Statistical approaches (partial correlation and multiple regression) indicate that in these leaves, maximal photochemical efficiency of PSII (Fv/Fm) and non-photochemical quenching are correlated with the level of Lx de-epoxidation. The potential implications of this finding for the understanding of the photosynthetic process in shaded and understorey leaves are discussed.

Quantitative Tracheid Anatomy Reveals a Complex Environmental Control of Wood Structure in Continental Mediterranean Climate

A detailed understanding of how the climate models the hydraulic system of trees is still lacking, in spite of the need to understand tree response to the ongoing process of climatic change. A systematic correlation of tracheid anatomical features with climate has not been implemented in Mediterranean ecosystems, where climatic change is expected to be particularly intense. We measured ring width and cell number—in addition to intraring position, lumen size, and wall thickness of tracheids—in 10 Juniperus thurifera individuals from north-central Spain. We used this information to perform an exploratory analysis of how these parameters correlated with climatic variables in 1965–2004. Cell number and ring width shared a relatively similar climatic signal, whereas the anatomical variables provided differentiated and diverse signals about climatic conditions during their formation. Earlywood and latewood tracheids differed in controlling factors, with earlywood tracheid size and wall thickness being positively related to rainfall during early summer and latewood tracheid size being positively related to August temperature. Tracheid anatomical variables improved our understanding of climate effects on tree growth and wood formation under harsh environmental conditions, as those experienced in continental Mediterranean climates, where limiting factors show multiple shifts across the year.

New star on the stage: amount of ray parenchyma in tree rings shows a link to climate

Tree-ring anatomy reflects the year-by-year impact of environmental factors on tree growth. Up to now, research in this field has mainly focused on the hydraulic architecture, with ray parenchyma neglected despite the growing recognition of its relevance for xylem function. Our aim was to address this gap by exploring the potential of the annual patterns of xylem parenchyma as a climate proxy. We constructed ring-width and ray-parenchyma chronologies from 1965 to 2004 for 20 Juniperus thurifera trees growing in a Mediterranean continental climate. Chronologies were related to climate records by means of correlation, multiple regression and partial correlation analyses. Ray parenchyma responded to climatic conditions at critical stages during the xylogenetic process; namely, at the end of the previous years xylogenesis (October) and at the onset of earlywood (May) and latewood formation (August). Ray parenchyma-based chronologies have potential to complement ring-width chronologies as a tool for climate reconstructions. Furthermore, medium- and low-frequency signals in the variation of ray parenchyma may improve our understanding of how trees respond to environmental fluctuations and to global change.

Distribution and evolutionary trends of photoprotective isoprenoids (xanthophylls and tocopherols) within the plant kingdom.

The earliest land photosynthesis would have increased the risk of photo-oxidations and the demand of anti-oxidative protection. In this work, we aimed to determine the evolutionary trends in photoprotection across a wide representation of the plant kingdom and to verify whether the non-ubiquitous lutein-epoxide (Lx) cycle is a polyphyletic or an ancient character. Carotenoids and alpha-tocopherol (alpha-toc) were analysed by HPLC in 266 species. Phylogenetic analyses of the presence of photoprotective compounds and zeaxanthin-epoxidase (ZE) sequences were performed. Violaxanthin-cycle pigments (VAZ) and alpha-toc were taxonomically ubiquitous. Ancient groups showed higher contents of VAZ than vascular plants, while alpha-toc showed the opposite pattern. Lutein-epoxide was present in 45% of the species. It showed a remarkable variation across groups but with a clear increasing trend from algae to basal angiosperms. Lutein-epoxide was also related to woody trait and leaf longevity. No correlation between the presence of Lx and recurrent mutations in ZE sequences, including the duplications, was found. Thus, there is an evolutionary trend to increase the content of alpha-toc and to decrease the total amount of VAZ pigments. Absence of Lx in algae discards an ancestral origin. Present results are also inconsistent with a polyphyletic origin of Lx in angiosperms.

Seed bank spatial structure in semi-arid environments: beyond the patch-bare area dichotomy

The prevalence of patchy structures in vegetation is a common feature in semi-arid ecosystems. Although the effect of patches on seed density is widely known, we still lack information on how patch features affect seed bank density and composition. Our aim was to answer two basic questions: (1) How do seed bank density and composition vary within and outside patch aboveground physical limits? and (2) Do patch characteristics affect soil seed bank density and composition? We sampled 50 shrub patches in a semi-arid gypsum system in Central Spain, measuring patch size, composition and structure, and seed bank at three locations per shrub (centre, edge and outside). We calculated the effect of interior patch location, patch composition and structure on seed density and composition. Patches acted both as seed sources, increasing seed density in neighbouring areas and as seed sinks by trapping seeds from bare areas. Patch structure (erect perennial cover) had the greatest effect on seed bank density, whereas patch size and microslope had the greatest influence on bare area density. Patch structure, composition and interior location explained the variation in seed bank composition. Patch effect extends to the surrounding bare matrix creating a seed bank gradient in density and composition. This effect is modulated by patch structure and composition and affects seed bank composition. Our results suggest that the spatial structure of gypsum community seed banks may act as a mechanism for a spatial storage effect contributing to the maintenance of high levels of diversity in semi-arid environments

Under pressure: how a Mediterranean high‐mountain forb coordinates growth and hydraulic xylem anatomy in response to temperature and water constraints

Summary 1. Plant growth in Mediterranean high mountains is limited by the double climatic stress of low winter temperatures and summer drought. Elevational shifts in response to climate change may be complex for species whose distribution is constrained by several climatic factors. 2. We used herb-chronology, that is, the analysis of annual rings in the secondary root xylem of perennial forbs, to evaluate life-long factors constraining secondary growth and xylem hydraulic anatomy along an elevational gradient from the upper to the lower distribution limits of the alpine forb Silene ciliata at its southernmost distribution range. 3. Generalized additive models (GAM) showed that annual ring width (RW) in S. ciliata was greatest at intermediate elevations and smallest at the upper and lower limits of its elevational range. In contrast, maximal vessel area (MVA) was greatest at lower elevations. RW responded to climatic conditions in early spring and late summer, suggesting the presence of a bimodal xylogenesis. Structural equation modelling (SEM) indicated a positive influence of MVA on RW in the same year; improved hydraulic efficiency seemed to promote higher secondary growth. 4. The observed greatest secondary growth (RW) and maximal vessel area (MVA) at intermediate and intermediate-low elevations, respectively, contrasts with previous evidence of an improvement in plant reproduction and recruitment with increasing elevation for S. ciliata. However, our results are in agreement with other indicators suggesting that best conditions occur at intermediate elevations, such as better seed quality or larger genome size. 5. This study reinforces the evidence that the response of high-mountain plants to climatic change under simultaneous temperature and drought stress is complex and that models that simply assume an increase in elevation as a response to higher temperatures may fail to predict future responses to climate change.

Prediction of plant cover from seed bank analysis in a semi-arid plant community on gypsum

Abstract Question: Does the seed bank filter annual plant composition and determine cover at the species level? Location: 510 m a.s.l., central Spain. Methods: Seven transects and 136 quadrats were established in a semi-arid gypsum system. Seed bank samples were collected in each quadrat in September. The community was sampled the following April. For each quadrat we measured slope, microslope, landform, elevation, perennial cover and crust cover. Seed bank was estimated using the direct emergence method in glasshouse. Relationship among seed bank and annual community was assessed by Mantel correlations. Above-ground cover for the five most abundant species was modelled with GLMs. Results: Seed bank density was the best predictor for annual community cover; perennial cover and landform were also included in the model. Species composition between September seed bank and April annual community cover was also highly related according to the Mantel test. This relationship was constant, even when the effect due to other abiotic (landform, microslope) or biotic (perennial cover, crust cover) parameters were partialled out. Microslope, elevation and seed bank density were the best parameters to predict spring cover of the five most abundant species. Conclusions: Above-ground and below-ground community compartments are strongly related in terms of abundance and species composition. This relationship is filtered by several environmental factors (e.g. perennial cover, landform, microslope) that exert a strong control at community and individual levels. Our results support the hypothesis that annual community performance is affected by seed bank pattern. Nomenclature: Tutin et al. (1964–1980) and Castroviejo et al. (1986–2005), except for Chaenorhinum reyesii (C. Vicioso & Pau) Benedí. Abbreviation: GLM = General linearized model.

The lutein epoxide cycle in vegetative buds of woody plants

Recent works have shown that two xanthophyll cycles operate simultaneously in several plant species: the ubiquitous violaxanthin + anteraxanthin + zeaxanthin (VAZ) cycle and the lutein epoxide (Lx) cycle. In the present work we tested for the presence of the Lx cycle in vegetative buds of woody plants. After an extensive screening of 130 species, we have shown that Lx is present in buds of many different plant species and leaf primordia are enriched in Lx compared with leaves. As a result, the Lx pool was higher than the violaxanthin (V) pool in several species. Although Lx can be potentially de-epoxidised in buds, light attenuation by scales inhibited the daily operation of the Lx cycle. This finding would imply that the Lx cycle is not involved in short-term reversible photoprotection in buds. However, a light-induced decrease in Lx was observed through the winter. An extensive screening of 130 species in 49 families conducted to elucidate the taxonomic extension of this cycle showed a widespread presence of the Lx cycle. The presence of Lx has a high fidelity at the family level, but its presence in unrelated taxa suggests that this character has appeared independently in several different groups of plants.