Mechanical basis for thermonastic movements of cold-hardy Rhododendron leaves.

The profusion of rhododendrons in cold climates is as remarkable as the beauty of their blooms. The cold-hardiness of some of the montane species is in part due to reversible leaf movements triggered under frigid conditions wherein the leaves droop at the leaf stalks (petioles) and their margins roll up around the midrib. We probe the mechanics of these movements using leaf dissection studies that reveal that the through-thickness differential expansion necessary for leaf rolling is anisotropically distributed transverse to and along the midrib. Numerical simulations and theoretical analyses of bilayer laminae show that the longitudinal expansion amplifies the transverse rolling extent. The curvature diversion scales with the in-plane Poisson's ratio, suitably aided by the stiff midrib that serves as a symmetry breaking constraint that controls the competition between the longitudinal and transverse rolling. Comparison of leaf rolling with and without the petiole indicates that the petiole flexibility and leaf rolling are in part mechanically coupled responses, implicating the hydraulic pathways that maintain the critical level of midrib stiffness necessary to support the longitudinal expansion. The study highlights the importance of curvature diversion for efficient nastic and tropic leaf movements that enhance cold-hardiness and drought resistance, and for morphing more general hinged laminae.

The role of idioblasts in leaf water relations of tropical Rhododendron.

PREMISE OF THE STUDY: Although much is known about the anatomy of idioblasts, the relationship of leaf idioblasts to leaf physiology is poorly known. Our goal was to understand the relationship between idioblast abundance and leaf water relations. Tropical epiphytic and shrub Rhododendron species were the model system. METHODS: Leaf succulence and idioblast metrics of 61 plants representing 17 species were compared with stomatal and water relation metrics. Correlation, ANOVA, and regression were used to understand which water relation traits were best aligned with the proportion of the leaf occupied by idioblasts for epiphytic and shrub species. KEY RESULTS: Idioblast volume per leaf area varied from 1.4-9.5 mm3·cm-2 among accessions, and an index of stomatal area per leaf area varied from 0.08-3.3. Succulence, stomatal, and water relation metrics varied significantly among species. Total idioblast volume was negatively correlated with leaf succulence metrics, and positively correlated with relative water content at the turgor loss point. Idioblasts were better related to capacitance in thin leaves. All idioblast metrics were significantly greater in epiphytic species. CONCLUSIONS: Leaf idioblasts can have a significant effect on leaf-lamina water relations. Idioblasts likely function as a water buffering system for thin leaves in tropical Rhododendron species. The association of idioblast prominence with leaf water buffering is greatest for epiphytic species with thin leaves. Thus, the proliferation of leaf idioblasts may have been an important innovation for Rhododendron species diversification into tropical epiphytic habitats.

Plant Invasions Associated with Change in Root-Zone Microbial Community Structure and Diversity.

The importance of plant-microbe associations for the invasion of plant species have not been often tested under field conditions. The research sought to determine patterns of change in microbial communities associated with the establishment of invasive plants with different taxonomic and phenetic traits. Three independent locations in Virginia, USA were selected. One site was invaded by a grass (Microstegium vimineum), another by a shrub (Rhamnus davurica), and the third by a tree (Ailanthus altissima). The native vegetation from these sites was used as reference. 16S rRNA and ITS regions were sequenced to study root-zone bacterial and fungal communities, respectively, in invaded and non-invaded samples and analyzed using Quantitative Insights Into Microbial Ecology (QIIME). Though root-zone microbial community structure initially differed across locations, plant invasion shifted communities in similar ways. Indicator species analysis revealed that Operational Taxonomic Units (OTUs) closely related to Proteobacteria, Acidobacteria, Actinobacteria, and Ascomycota increased in abundance due to plant invasions. The Hyphomonadaceae family in the Rhodobacterales order and ammonia-oxidizing Nitrospirae phylum showed greater relative abundance in the invaded root-zone soils. Hyphomicrobiaceae, another bacterial family within the phyla Proteobacteria increased as a result of plant invasion, but the effect associated most strongly with root-zones of M. vimineum and R. davurica. Functional analysis using Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) showed bacteria responsible for nitrogen cycling in soil increased in relative abundance in association with plant invasion. In agreement with phylogenetic and functional analyses, greater turnover of ammonium and nitrate was associated with plant invasion. Overall, bacterial and fungal communities changed congruently across plant invaders, and support the hypothesis that nitrogen cycling bacteria and functions are important factors in plant invasions. Whether the changes in microbial communities are driven by direct plant microbial interactions or a result of plant-driven changes in soil properties remains to be determined.

A rootstock provides water conservation for a grafted commercial tomato (Solanum lycopersicum L.) line in response to mild-drought conditions: a focus on vegetative growth and photosynthetic parameters.

The development of water stress resistant lines of commercial tomato by breeding or genetic engineering is possible, but will take considerable time before commercial varieties are available for production. However, grafting commercial tomato lines on drought resistant rootstock may produce drought tolerant commercial tomato lines much more rapidly. Due to changing climates and the need for commercial production of vegetables in low quality fields there is an urgent need for stress tolerant commercial lines of vegetables such as tomato. In previous observations we identified a scion root stock combination ('BHN 602' scion grafted onto 'Jjak Kkung' rootstock hereafter identified as 602/Jjak) that had a qualitative drought-tolerance phenotype when compared to the non-grafted line. Based on this initial observation, we studied photosynthesis and vegetative above-ground growth during mild-drought for the 602/Jjak compared with another scion-rootstock combination ('BHN 602' scion grafted onto 'Cheong Gang' rootstock hereafter identified as 602/Cheong) and a non-grafted control. Overall above ground vegetative growth was significantly lower for 602/Jjak in comparison to the other plant lines. Moreover, water potential reduction in response to mild drought was significantly less for 602/Jjak, yet stomatal conductance of all plant-lines were equally inhibited by mild-drought. Light saturated photosynthesis of 602/Jjak was less affected by low water potential than the other two lines as was the % reduction in mesophyll conductance. Therefore, the Jjak Kkung rootstock caused aboveground growth reduction, water conservation and increased photosynthetic tolerance of mild drought. These data show that different rootstocks can change the photosynthetic responses to drought of a high yielding, commercial tomato line. Also, this rapid discovery of one scion-rootstock combination that provided mild-drought tolerance suggests that screening more scion-rootstock combination for stress tolerance may rapidly yield commercially viable, stress tolerant lines of tomato.

Effects of summer drought and winter freezing on stem hydraulic conductivity of Rhododendron species from contrasting climates.

We studied the limits to maximum water transport in three diffuse-porous evergreen shrubs exposed to frequent winter freeze-thaw events (Rhododendron maximum L. and R. catawbiense Michaux from the Appalachian Mountains) and to a severe summer drought (R. macrophyllum G. Don. from the Oregon Cascades). Percent loss of hydraulic conductivity (PLC), vulnerability curves to xylem embolism and freezing point temperatures of stems were measured over 2 years. Controlled freeze-thaw experiments were also conducted to determine the effect of thaw rate on PLC. During both years, native PLC was significantly higher in winter than in summer for R. macrophyllum. Seasonal changes in PLC were variable in both R. catawbiense and R. maximum. Only R. maximum plants growing in gaps or clearings showed higher PLC than understory plants. A rapid (2-4 day) natural recovery of high native PLC during the winter was observed in both R. maximum and R. macrophyllum. Compared with the bench-dehydration method, vulnerability curves based on the air-injection method consistently had less negative slopes and greater variation. Fifty percent PLC (PLC(50)) obtained from vulnerability curves based on the dehydration method occurred at -1.75, -2.42 and -2.96 MPa for R. catawbiense, R. maximum and R. macrophyllum, respectively. Among the study species, R. macrophyllum, which commonly experiences a summer drought, had the most negative water potential at PLC(50). In all species, stem freezing point temperatures were not consistently lower in winter than in summer. A single fast freeze-thaw event significantly increased PLC, and R. catawbiense had the highest PLC in response to freezing treatments. Recovery to control PLC values occurred if a low positive hydraulic pressure was maintained during thawing. Rhododendron macrophyllum plants, which commonly experience few freeze-thaw events, had large stem diameters, whereas plants of R. catawbiense, which had small stem diameters, suffered high embolism in response to a single freeze-thaw event. Both drought-induced and winter-induced embolism caused a significant reduction in hydraulic conductivity in all species during periods when drought or freeze-thaw events occurred in their native habitats. However, rapid recovery of PLC following freezing or drought maintained the species above their relatively low margins of safety for complete xylem dysfunction.

Stem photosynthesis in Psorothamnus spinosus (smoke tree) in the Sonoran desert of California.

Stem photosynthetic responses to environmental parameters were investigated with Psorothamnus spinosus in the Sonoran Desert of California. Light saturation of stem photosynthesis was equal to maximum midday summer irradance (1600-2000 µmol·m-2·s-1). The optimum temperature for stem photosynthesis was 39°C, and lower stem temperatures (27-35°C) caused significant decreases (up to 50%) in stem photosynthesis. Positive stem photosynthesis was maintained up to 51°C. Stem photosynthesis was relatively insensitive to increasing vpd up to 5 kPa; However, stem conductance decreased by 25% at a vpd of 5 kPa. At vpd greater than 5 kPa stem photosynthesis decreased relatively more than that of stem conductance causing a decrease in water use efficiency and an increase an intercellular carbon dioxide concentration. Maximum stem photosynthetic rates were low (6.2-10.6 µmol·m-2·s-1) on a stem surface area, but, stem photosynthetic rates of young shoots were substantially higher (19.5-33.3 µmol· m-2·s-1) on a projected area basis.

The influence of photoperiod on drought induction of dormancy in Lotus scoparius.

Many species of plants in Mediterranean climate regions have evolved deciduousness, causing reduced leaf area during the long summer drought characteristic of Mediterranean climates. This summer deciduous growth form has been considered a plant adaptation in Mediterranean regions allowing survival during periods of extreme water stress. Many studies have suggested the ecological importance of this growth form but few studies have examined the physiological stimulus for deciduousness.Previous data indicate that abscission in Lotus scoparius (a mediterranean California deciduous species) is influenced by both photoperiod and water stress in a complex manner. Here the physiological basis of long day enhanced leaf fall during water stress is investigated.Examination of water potential components indicate an osmotic adjustment with incresing water stress which enables the maintenance of turgor at lower water potentials. Osmotic adjustment in plants grown under long photoperiods was greater than that in short photoperiods. Therfore, long day enhanced abscission during water stress was not due to a greater susceptability to turgor loss during long days. Rather, long day treatment caused these plants to initiate dormancy (as indicated by soluble protein concentrations) during the onset of water stress. The dormant condition could not be released by subsequent release from water stress. Apparently, Lotus scoparius has evolved a photoperiodic control (presumably through growth regulators) over the initiation of dormancy during water stress. The adaptive significance of this photoperiodic control over the leaf abscission response to water stress relates to the variable climate of Mediterranean regions.

Summer water relations of the desert phreatophyte Prosopis glandulosa in the Sonoran Desert of southern California.

Prosopis is a genus of phreatophytic trees inhabiting hot deserts and semiarid grasslands of the world. Although desert trees are exposed to unusual environmental temperature and water stress, few investigations have evaluated their water relations. This is particularly true for Prosopis species growing in areas where a large portion of their water use comes from ground water.Water relations components for Prosopis glandulosa were studied at Harper's Well, near the Salton Sea, California during the summer months of 1980. Maximum temperatures (49° C), irradiance (2,000 µE/m2/sec), and vapor pressure deficit (5.3 kPa) were reached in July. During this time Prosopis glandulosa predawn xylem pressure potentials were below-3.0 MPa. Prosopis glandulosa at Harper's Well is able to maintain open stomata during high temperatures, high vapor pressure deficit and at low estimated turgor pressure. Leaf resistance measurements indicate that stomata are open primarily in the morning, but may reopen in the afternoon in trees with greater water resources. Osmotic potentials of juvenile shoots were higher (-1.0 to-2.5 MPa) than mature shoots (-3.5 MPa). Estimated turgor potential remained low (0.1-0.2 MPa) during the morning and early afternoon. Estimated turgor pressure increased from August to September as temperatures and vapor pressure deficit decreased. Leaf conductance was strongly associated with leaf vapor pressure deficit and estimated turgor potential but poorly associated with xylem pressure potential. Prosopis stomata seem to be uncoupled from tissue water potential until-4.8 MPa is reached.