RESUMO
PREMISE: Woody plants with photosynthetic stems are common in the drylands of the world; however, we know little about the origin(s) and geographical distribution of photosynthetic stems. Therefore, we set to answer the following questions: (1) Is stem photosynthesis phylogenetically conserved? (2) Do green-stemmed and fleshy-stemmed species have identifiable climatic niches? METHODS: We mapped the photosynthetic stem trait onto a phylogeny of 228 mediterranean and desert species and calculated indices of phylogenetic signal and created climatic niche models of 28 species belonging to three groups: green, fleshy, and green-and-fleshy stemmed species. RESULTS: We found phylogenetic signal in the fleshy stem trait, but not in the green stem trait. Fleshy-stemmed species occupy areas associated with high isothermality, high precipitation seasonality, and high mean temperature of the wettest quarter, whereas green-stemmed species occupy areas associated with high precipitation of driest month, high precipitation of coldest quarter, high mean diurnal temperature range and high maximum temperature of the warmest month. CONCLUSIONS: Despite the fact that both photosynthetic stem types help cope with water shortage, having fleshy stems allows plants to cope with greater precipitation seasonality than is possible with green stems. Green stems require a lot of water to be stored in the soil to maintain net photosynthesis during the dry season, so they inhabit areas with higher and more predictable precipitation.
Assuntos
Fotossíntese , Água , California , México , FilogeniaRESUMO
Shade tolerance is a widespread strategy of rainforest understory plants. Many understory species have green young stems that may assimilate CO2 and contribute to whole-plant carbon balance. Cacao commonly grows in the shaded understory and recent emphasis has been placed on diversifying the types of trees used to shade cacao plants to achieve additional ecosystem services. We studied three agricultural cacao cultivars growing in the shade of four timber species (Cedrela odorata L., Cordia thaisiana Agostini, Swietenia macrophylla King and Tabebuia rosea (Bertol) A.D.C.) in an agroforestry system to (i) evaluate the timber species for their effect on the physiological performance of three cacao cultivars; (ii) assess the role of green stems on the carbon economy of cacao; and (iii) examine coordination between stem hydraulic conductivity and stem photosynthesis in cacao. Green young stem photosynthetic CO2 assimilation rate was positive and double leaf CO2 assimilation rate, indicating a positive contribution of green stems to the carbon economy of cacao; however, green stem area is smaller than leaf area and its relative contribution is low. Timber species showed a significant effect on leaf gas exchange traits and on stomatal conductance of cacao, and stem water-use efficiency varied among cultivars. There were no significant differences in leaf-specific hydraulic conductivity among cacao cultivars, but sapwood-specific hydraulic conductivity varied significantly among cultivars and there was an interactive effect of cacao cultivar × timber species. Hydraulic efficiency was coordinated with stem-stomatal conductance, but not with leaf-stomatal conductance or any measure of photosynthesis. We conclude that different shade regimes determined by timber species and the interaction with cacao cultivar had an important effect on most of the physiological traits and growth variables of three cacao cultivars growing in an agroforestry system. Results suggested that C. odorata is the best timber species to provide partial shade for cacao cultivars in the Barlovento region in Venezuela, regardless of cultivar origin.
Assuntos
Cacau , Árvores , Ecossistema , Fotossíntese , Folhas de Planta , ÁguaRESUMO
PREMISE: Photosynthetic stems represent a source of extra carbon in plants from hot and dry environments, but little is known about how leaves and photosynthetic stems differ in terms of photosynthetic capacity, trait coordination, and responses to seasonal drought in subtropical systems. METHODS: We studied photosynthetic, hydraulic, morphometric (specific leaf area [SLA], wood density [WD]), and biochemical (C and N isotopes) traits in leaves and photosynthetic stems of 12 plant species from a sarcocaulescent scrub in the southern Baja California Peninsula, Mexico, in wet and dry seasons. RESULTS: Leaves and stems had similar mean photosynthetic capacity, as evaluated by chlorophyll fluorescence traits, indicating similar investment in leaf and stem photosynthesis. We did not find a relationship between stem hydraulic conductivity and leaf or stem photosynthetic traits. However, we found resource allocation trade-offs, between WD and both stem hydraulic conductivity and SLA. Leaf and stem photosynthetic traits did not change with season, but specific stem area was one of the few traits that changed the most between seasons-it increased during the dry season by as much as 154% indicating substantial water storage. CONCLUSIONS: Our results indicate the same proportional investment in photosynthetic capacity and dry matter in both leaves and photosynthetic stems across all 12 species. We identified multiple strategies at this seasonal site, with species ranging from high WD, low SLA, low hydraulic conductivity, and high specific bark area on one end of the spectrum and opposite traits on the other end.
Assuntos
Fotossíntese , Folhas de Planta , México , Caules de Planta , Árvores , Água , MadeiraRESUMO
Tropical dry forests (TDFs) experience a long dry season in which plant species are subject to several months of water deficit. However, TDFs maintain a diverse group of plant life forms, growth forms and leaf phenology, and it is not clear how they vary in their mechanisms for coping with seasonal drought. We studied seasonal changes in leaf water potential (Ψleaf), gas exchange, photochemical activity and functional traits in evergreen and drought-deciduous species from a TDF to determine if leaf phenology mediates plant responses to drought. We found seasonal decreases in Ψleaf, stomatal conductance (gs) and transpiration rate (E), and increases in both intrinsic and instantaneous water-use efficiency. We did not find seasonal differences in photosynthetic rate (Aarea) and carbon isotope composition (δ13C); however, these traits differed between leaf phenology groups, with drought-deciduous plants having higher Aarea and δ13C than evergreen plants. We also found that plants with high leaf nitrogen concentration (Narea) also had low mass-based photosynthetic rate (Amass), photosynthetic-nitrogen-use efficiency and specific leaf area, contrary to the expected relationships given by the leaf economics spectrum. Despite higher Narea, sclerophyllous leaves maintained lower Amass, and this increased structural toughness of leaves may be imposing a stronger limitation for CO2 diffusion and hence photosynthesis. Overall, we found more water-conservative traits in deciduous than in evergreen plants, contrary to what is known about these two leaf phenology groups in other seasonal sites both at tropical and temperate latitudes.