When S. ×utahensis plants dehydrated as a result of decreasing substrate volumetric water contents, plants closed their stomata to reduce transpiration and stomatal conductance as a drought acclimation to maintain plant water status and prevent water losses and further dehydration . Although CO2 uptake is limited when stomata are closed and stomatal conductance reduced , plants had a lower proportion of visibly wilted leaves in this study or better aesthetic quality under drought conditions in other reports. These plants were considered drought tolerant in ornamental plant evaluations in semiarid regions in Australia and the United States . Shepherdia ×utahensis reduced its midday stomatal conductance at lower water availability and can be considered as a low water-use landscape plant. Plants have the capacity of regulating stomatal conductance that is related to their habitat aridity. Kjelgren et al. reported that plants native to arid regions, such as Dianella revoluta ‘Breeze’ and Ptilotus nobilis , showed greater reduction of stomatal conductance compared with those from humid areas. Because of restricted transpiration, plants with acclimation capability may reduce leaf size to enhance convective heat loss to mitigate heat stress that causes high leaf-to-air VPD and leaf wilting . The fact that leaf-to-air VPD increased when substrate volumetric water content decreased is likely a direct consequence of increased leaf temperature because leaf vapor pressure is estimated by leaf temperature.
To avoid heat stress, french flower bucket leaf energyis balanced primarily using sensible heat loss under drought . The efficacy of sensible heat loss relates to boundary layer resistance, which is positively correlated to leaf width . Under drought conditions, cell division and leaf expansion are limited , and smaller leaves are beneficial for dissipating heat through convection and conduction to maintain leaf temperature close to air temperature . In this study, S. ×utahensis produced smaller leaves under water stress and leaf size of plants grown at the substrate volumetric water content of 0.05m3 ·m−3 was 51% smaller than those at 0.40m3 ·m−3 . This result is in line with previous studies that consistently reported reductions in leaf size under water stress for drought-tolerant ornamental plants . For instance, Zollinger et al. suggested that small leaves allow Lavandula angustifolia and Penstemon ×mexicali ‘Red Rocks’ to reduce water loss when irrigation intervals were increased from 1week to 4weeks. Toscano et al. also found that leaf size of Viburnum tinus ‘Lucidum’ decreased by 19% to acclimate to drought stress. Shepherdia ×utahensis decreased total leaf area under water stress as a result of reductions in leaf number and size . However, plants with decreased total leaf area have fewer stomata and less light interception, which controls transpiration and leaf temperature, respectively . Reduced total leaf area has been reported as a means of avoiding drought stress in ornamental plants such as Lavandula angustifolia, Pittosporum tobira , and Viburnum tinus ‘Lucidum’ . The root growth of S. ×utahensis was enhanced at low substrate volumetric water content, while shoot growth was inhibited, resulting in a higher root-to-shoot ratio , which helps plants to obtain water more efficiently. Rosa hybrida ‘Ferdy’ and Populus cathayana have been observed to increase root growth to maintain water status under water stress .
Drought-tolerant plants native to the western United States also produce small leaves and deep roots to reduce water demand and loss and increase water uptake . In this study, as substrate volumetric water content decreased, leaves of S. ×utahensis curled as stem water potential became more negative. At the substrate volumetric water content of 0.05m3 ·m−3 , the leaf curling index was 0.17, suggesting that the light interception area was 83% that of flattened leaves. Similarly, Dianella revoluta ‘Breeze’ and Ctenanthe setosa have been shown to minimize sunlight exposure through leaf curling under water deficit . Although light-harvesting efficiency is reduced, leaf curling limits water loss from transpiration and protects plants from overheating to sustain photosystem functions and other biochemical/physiological processes . In addition, as the rooting substrate became drier in this study, specific leaf area decreased, indicating that leaves became thicker , which prevented leaves from overheating. Plants may decrease specific leaf area to acclimate to water stress as reported in Ptilotus nobilis . The trichome density of S. ×utahensis in this study was affected by substrate water availability and plant water status . Water-stressed S. ×utahensis produced densely packed trichomes, resulting in a silvery appearance, while well-watered plants had fewer trichomes to cover epidermal cells and exhibited a greener color . Trichomes promote leaf reflectance , which helps balance energy and reduce heat stress . Positive effects of trichomes on leaf reflectance of visible light have been reported on Verbascum thapsus and Salix commutata . However, because trichomes are broad-spectrum reflectors , the reflectance of PAR, blue, green, and red light is proportional to the trichome density . When substrate volumetric water content decreased, the reflectance of green light did not increase as much as blue light and red light due to the chlorophyll in the epidermal cells . Increased leaf reflectance has been shown to sacrifice the efficacy of light-harvesting pigments and reduce the net assimilation rate when plants are grown in drier conditions. Previous research also suggested that trichomes improved the reflectance of near infrared light . However, in this study, denser trichomes produced in drier substrate did not affect near infrared light reflectance of S. ×utahensis . Slaton et al. reported similar results that near-infrared light reflectance was not affected by increased trichome density in 48 species. More studies are needed to evaluate the effects of trichomes on near-infrared reflectance. Increased trichome density has smaller effects on decreasing gas exchange coed with the effects on leaf reflectance . However, densely packed trichomes covering the stomata of S. ×utahensis may increase resistance to transpiration and reduce water loss . Leaf trichomes also increase leaf roughness and increase the laminar boundary layer to restrict air movement across leaf surfaces to reduce transpiration . Eriogonum corymbosum and S. rotundifolia produce leaf trichomes for better protection from wind and to maintain water status . Densely packed trichomes add an atmospheric boundary layer that imposes additional resistance to water vapor diffusion . However, CO2 influx is also limited by the boundary layer resistance, decreasing the net assimilation rate . Although trichome-induced boundary layer resistance has a smaller effect on transpiration than stomatal conductance , it still provides an advantage for desert plants to survive in dry and hot conditions. The genetic regulation of trichome density of Caragana korshinskii has been reported by Ning et al. . However, it is unclear how xeric plants change their trichome density to acclimate to drought conditions. A negative correlation between leaf trichome density and leaf size or epidermal cell size occurred in this study , which suggests that cell expansion maycontrol trichome density. Low trichome coverage fraction, which was related to greater space between trichomes, showed when epidermal cell density decreased, indicating cell expansion may coordinate trichome density. Ascensão and Pais reported the number of trichomes is determined during leaf lifespan, and leaf cell differentiation does not affect trichome number. Similar results showed in our research that plants had similar total numbers of trichomes per leaf at different substrate volumetric water contents. This may indicate that S. ×utahensis develops trichomes independent of leaf development. In fact, trichomes develop at the early stage of leaf development and often earlier than stomatal development . For instance, trichomes of Inula viscosa are fully developed and reach mature size when leaves are 2mm long; however, a mature leaf is 6–8cm long .
Ocimum basilicum forms trichomes at an early stage of leaf development and trichomes then grow independently . In the same study, trichomes covered young leaves but became more widely spaced when leaf cells started to expand . In our study, the total number of epidermal cells per leaf was similar on plants at different substrate volumetric water contents, bucket flower which indicates cell differentiation might have minor effects on regulating trichome density. In contrast, cell expansion might be the main factor for regulating trichome density because leaf size, epidermal cell size, and the space among trichomes changed along with substrate volumetric water contents and correlated significantly with trichome density of S. ×utahensis . Ehleringer found a negative correlation between leaf size and trichome density of Encelia farinosa, but cell size was not determined. Cell enlargement at high soil moisture levels amplified leaf size and the space among trichomes, reducing the trichome density on the S. ×utahensis leaves in this study. The relationships between trichome density, epidermal cell size and density, and leaf reflectance might indicate changes in cell size predominantly controls trichome density to modify leaf reflectance. Modifying leaf reflectance via the change in cell size helps rapidly acclimate to environmental change without compromising whole leaf function . Cell-expansion driven leaf anatomic change has been widely reported on adjusting stomatal density . For instance, Murphy et al. observed that cell expansion was the predominant factor for coordinating vein and stomata density of eight angiosperm species under sun and shade. Stomatal density decreases and the size of guard cells increases when leaf water potential increases , suggesting cell expansion not only enlarges the distance between epidermal appendages but also increases their size. Environmental factors also promote leaf trichome density. Such factors include increased leaf-to-air VPD and drought , all of which negatively affect plant water status. For instance, high leaf-to-air VPD may increase water loss via transpiration, leading to plant dehydration. Leaf trichome density of Cucumis sativus increased when air humidity decreased from 90 to 20% at 28°C, causing leaf-to-air VPD to increase from 0.4 to 3.0kPa . Shibuya et al. did not investigate cell or leaf expansion of C. sativus, but increased leaf-to-air VPD may promote trichome density because rising leaf-to-air VPDs reduces cell size , making space between trichomes smaller. In this study, higher leaf-to-air VPD and smaller leaves were observed when S. ×utahensis plants grew at the lower θtand the smaller epidermal cell size resulted in greater trichome density. Therefore, because increased leaf-to-air VPD and drought led to a reduction in cell enlargement and denser trichomes in S. ×utahensis, leaf trichome density was regulated using turgor-pressure-driven cell expansion to acclimate to drought conditions.Modern plant trade disturbs historical ecological relationships and creates opportunities for the development of novel pathogenic interactions , often with correlated genetic changes . However, pathogens must be adapted to the environment of the novel host before they meet, or they will not be able to survive and reproduce . That does not mean pathogens necessarily pre-adapted to the exact same host, but either could have adapted to a similar host earlier and retained that adaptation until encountering a novel host. Convergent evolution in diverse pathogen populations can allow for divergent strains to have the ability to infect the same hosts. Three potential mechanisms of genetic change that can accompany host shifts are nucleotide changes leading to different alleles in the core genome of a pathogen , whole gene gain and loss in the pan-genome, leading to unique sets of genes in individual strains, or regulatory/epigenetic changes. Due to the recent increase in whole genome sequencing of plant pathogens, we can now more effectively use phylogenetic analyses to investigate their genetic associations to both novel and historical host plants . Understanding the phylogenetic relationships between specific host and pathogens should allow the development of preemptive plans to protect natural ecosystems as well as agriculture from the emergence of novel pathogens. Xylella fastidiosa is an insect-transmitted, xylem-limited bacterial plant pathogen found across the Americas, and as of recently, globally. X. fastidiosa is considered to be a generalist pathogen, because, as a species; it reportedly infects at least 563 species belonging to 82 botanical families . The lack of host specificity that X. fastidiosa exhibits as a species contrasts with increased plant host specificity in smaller clades and strains . It is still debated whether a pathogen like X. fastidiosa should be considered a generalist species that “leaps” between phylogenetically distant hosts or, alternatively, a crawler at shallower clades . The difference is biological as there are unique implications for either evolutionary path. X. fastidiosa could be repeatedly evolving specialization or it could have biological and genetic traits as a species that make particular hosts of disparate plant taxa suitable. From an applied perspective, there have been recent calls from government agencies for increased focus on understanding the host range of X. fastidiosa. This is because the pathogen has been deemed likely to spread and to be of extremely high risk to crops of agricultural value , 2015. Xylella fastidiosa causes disease in a range of high value crops, including Pierce’s disease of grapevines, citrus variegated chlorosis disease in sweet oranges, almond leaf scorch, leaf scorch of coffee, olive quick decline syndrome , spanning North and South America, Europe, the Middle East, and Taiwan .