Hydraulic lift and tolerance to salinity of semiarid species: consequences for species interactions

Whole plant response of Pongamia pinnata to drought stress tolerance revealed by morpho-physiological, biochemical and transcriptome analysis

BACKGROUND

Pongamia is considered an important biofuel species worldwide. Drought stress in the early growth stages of Pongamia influences negatively on the germination and seedling development. Due to lack of cultivar stability under drought stress conditions, establishment of successful plantation in drought hit areas becomes a major problem. To address this issue, drought stress response of four Pongamia genotypes was studied at morphological, physio-chemical and transcriptome levels.

METHODS AND RESULTS

Drought stress was levied by limiting water for 15 days on three months old seedlings of four genotypes. A significant effect of water stress was observed on the traits considered. The genotype NRCP25 exhibited superior morpho-physiological, biochemical drought responses. Also, the genotype had higher root length, photosynthetic pigments, higher antioxidant enzymes and solute accumulation compared to other genotypes. In addition, transcript profiling of selected drought responsive candidate genes such as trehalose phosphate synthase 1 (TPS1), abscisic acid responsive elements-binding protein 2 (ABF2-2), heat shock protein 17 (HSP 17 kDa), tonoplast intrinsic protein 1 (TIP 1-2), zinc finger homeodomain protein 2 (ZFP 2), and xyloglucan endotransglucolase 13 (XET 13) showed only up-regulation in NRCP25. Further, the transcriptome responses are in line with key physio-chemical responses exhibited by NRCP25 for drought tolerance.

CONCLUSIONS

As of now, there are no systematic studies on Pongamia drought stress tolerance; therefore this study offers a comprehensive understanding of whole plant drought stress responsiveness of Pongamia. Moreover, the results support important putative trait indices with potential candidate genes for drought tolerance improvement of Pongamia.

Past, present, and future geographic range of the relict Mediterranean and Macaronesian Juniperus phoenicea complex

AIM

The aim of this study is to model the past, current, and future distribution of J. phoenicea s.s., J. turbinata, and J. canariensis, based on bioclimatic variables using a maximum entropy model (Maxent) in the Mediterranean and Macaronesian regions.

LOCATION

Mediterranean and Macaronesian.

TAXON

Cupressaceae, Juniperus.

METHODS

Data on the occurrence of the J. phoenicea complex were obtained from the Global Biodiversity Information Facility (GBIF.org), the literature, herbaria, and the authors' field notes. Bioclimatic variables were obtained from the WorldClim database and Paleoclim. The climate data related to species localities were used for predictions of niches by implementation of Maxent, and the model was evaluated with ENMeval.

RESULTS

The potential niches of Juniperus phoenicea during the Last Interglacial period (LIG), Last Glacial Maximum climate (LGM), and Mid-Holocene (MH) covered 30%, 10%, and almost 100%, respectively, of the current potential niche. Climate warming may reduce potential niches by 30% in RCP2.6 and by 90% in RCP8.5. The potential niches of Juniperus turbinata had a broad circum-Mediterranean and Canarian distribution during the LIG and the MH; its distribution extended during the LGM when it was found in more areas than at present. The predicted warming in scenarios RCP2.6 and RCP8.5 could reduce the current potential niche by 30% and 50%, respectively. The model did not find suitable niches for J. canariensis during the LIG and the LGM, but during the MH its potential niche was 30% larger than at present. The climate warming scenario RCP2.6 indicates a reduction in the potential niche by 30%, while RCP8.5 so indicates a reduction of almost 60%.

MAIN CONCLUSIONS

This research can provide information for increasing the protection of the juniper forest and for counteracting the phenomenon of local extinctions caused by anthropic pressure and climate changes.

Is Variation in Conspecific Negative Density Dependence Driving Tree Diversity Patterns at Large Scales?

Half a century ago, Janzen and Connell hypothesized that the high tree species diversity in tropical forests is maintained by specialized natural enemies. Along with other mechanisms, these can cause conspecific negative density dependence (CNDD) and thus maintain species diversity. Numerous studies have measured proxies of CNDD worldwide, but doubt about its relative importance remains. We find ample evidence for CNDD in local populations, but methodological limitations make it difficult to assess if CNDD scales up to control community diversity and thereby local and global biodiversity patterns. A combination of more robust statistical methods, new study designs, and eco-evolutionary models are needed to provide a more definite evaluation of the importance of CNDD for geographic variation in plant species diversity.

Drought Tolerance and Competition in Eastern Redcedar (Juniperus virginiana) Encroachment of the Oak-Dominated Cross Timbers

On the dry, western edge of the eastern deciduous forest of the USA (Cross Timbers), the drought-tolerant, evergreen eastern redcedar (Juniperus virginiana) is encroaching into post oak- (Quercus stellata) dominated woodlands. The overall goal of this study was to examine whether the drought tolerance strategies of eastern redcedar provide it a competitive advantage over post oak and whether this is a key attribute facilitating its successful establishment in the Cross Timbers. Specifically, we assessed xylem water potential and leaf gas exchange of these two species growing in single-species stands and in a mixed-species stand. We found that both species exhibit a similar degree of isohydry and close their stomates to the same extent in response to declining xylem water potentials. Both species had similar relative reductions in gas exchange in response to drought, despite differences in xylem anatomy. However, post oak had leaf-level gas exchange rates approximately 5× greater than eastern redcedar during periods of high moisture availability. Therefore, it did not appear that eastern redcedar encroachment into an oak-dominated forest is facilitated by growing season differences in carbon gain, although evergreen eastern redcedar can conduct gas exchange year-round when conditions are favorable while post oak is deciduous. We found that volumetric soil water content (0-45 cm) was lower in the pure eastern redcedar stand than the mixed-species or pure post oak stand which may indicate that eastern redcedar may experience favorable soil moisture conditions when encroaching into open oak woodlands. Moreover, water potentials in eastern redcedar tended to be more negative in pure stands compared to the mixed stand. Our results suggest the two species may be using water from different depths, reducing competition. Overall, our findings indicate that eastern redcedar encroachment into formerly oak-dominated Cross Timbers forests likely will continue under moderate drought, in the absence of fire, with consequences for water budgets, carbon cycling, grazing forage, wildlife habitat, and wildfire risk.

What Do Plants Leave after Summer on the Ground?—The Effect of Afforested Plants in Arid Environments

The implementation of afforestation programs in arid environments in northern China had modified the natural vegetation patterns. This increases the evaporation flux; however, the influence of these new covers on the soil water conditions is poorly understood. This work aims to describe the effect of Willow bushes (Salix psammophila C. Wang and Chang Y. Yang) and Willow trees (Salix matsudana Koidz.) on the soil water conditions after the summer. Two experimental plots located in the Hailiutu catchment (Shaanxi province, northwest China), and covered with plants of each species, were monitored during Autumn in 2010. The monitoring included the soil moisture, fine root distribution and transpiration fluxes that provided information about water availability, access and use by the plants. Meanwhile, the monitoring of stable water isotopes collected from precipitation, soil water, groundwater and xylem water linked the water paths. The presence of Willow trees and Willow bushes reduce the effect of soil evaporation after summer, increasing the soil moisture respect to bare soil conditions. Also, the presence of soil water with stable water isotope signatures close to groundwater reflect the hydraulic lift process. This is an indication of soil water redistribution carried out by both plant species.

Seasonal and daily variations in primary and secondary metabolism of three maquis shrubs unveil different adaptive responses to Mediterranean climate

Maquis species play a central role in the maintenance of coastal ecosystems thanks to anatomical, physiological and biochemical features evolved to cope with severe stress conditions. Because the seasonal and daily dynamics of physiological and biochemical traits of maquis species are not fully addressed, we performed a field study on three coexisting Mediterranean shrubs (Pistacia lentiscus L. and Phillyrea latifolia L., evergreen schlerophylls, and Cistus incanus L., semi-deciduous) aiming at detecting the main adaptive differences, on a seasonal and daily basis, in primary and secondary metabolism along with the principal climatic determinants. These species differed in their physiological and biochemical responses especially on a seasonal level. In P. latifolia, a great investment in antioxidant phenylpropanoids contributed to maintain high photosynthetic rates throughout the whole growing season. In C. incanus, high carotenoid content associated with chlorophyll (Chl) regulation alleviated oxidative damage during the hot and dry summers and help recover photosynthesis in autumn. In P. lentiscus, high abscisic acid levels allowed a strict control of stomata, while fine Chla/Chlb regulation concurred to avoid photoinhibition in summer. Temperature resulted the most important climatic factor controlling the physiological and biochemical status of these coexisting shrubs and, thus, in determining plant performances in this Mediterranean coastal habitat.

Contrasting water sources and water-use efficiency in coexisting desert plants in two saline-sodic soils in northwest China

Soil degradation resulting from various types of salinity is a major environmental problem, especially in arid and semiarid regions. Exploring the water-related physiological traits of halophytes is useful for understanding the mechanisms of salt tolerance. This knowledge could be used to rehabilitate degraded arid lands. To investigate whether different types of salinity influence the water sources and water-use efficiency of desert plants (Karelinia caspia, Tamarix hohenackeri, Nitraria sibirica, Phragmites australis, Alhagi sparsifolia, Suaeda microphylla, Kalidium foliatum) in natural environments, we measured leaf gas exchange, leaf carbon and xylem oxygen isotope composition and soil oxygen isotope composition at neutral saline-sodic site (NSS) and alkaline saline-sodic site (ASS) in northwest China. The studied plants had different xylem water oxygen isotope compositions (δ¹⁸ O) and foliar carbon isotope compositions (δ¹³ C), indicating that desert plants coexist through differentiation in water use patterns. Compared to that at the NSS site, the stem water in K. caspia, A. sparsifolia and S. microphylla was depleted in ¹⁸ O at the ASS site, which indicates that plants can switch to obtain water from deeper soil layers when suffering environmental stress from both salinity and alkalinisation. Alhagi sparsifolia had higher δ¹³ C at the ASS site than at the NSS site, while K. caspia and S. microphylla had lower δ¹³ C, which may have resulted from interspecific differences in plant alkali and salt tolerance ability. Our results suggest that under severe salinity and alkalinity, plants may exploit deeper soil water to avoid ion toxicity resulting from high concentrations of soluble salts in the superficial soil layer. In managed lands, it is vital to select and cultivate different salt-tolerant or alkali-tolerant plant species in light of local conditions.

The effect of consumer pressure and abiotic stress on positive plant interactions are mediated by extreme climatic events

Environmental extremes resulting from a changing climate can have profound implications for plant interactions in desert communities. Positive interactions can buffer plant communities from abiotic stress and consumer pressure caused by climatic extremes, but limited research has explored this empirically. We tested the hypothesis that the mechanism of shrub facilitation on an annual plant community can change with precipitation extremes in deserts. During years of extreme drought and above-average rainfall in a desert, we measured plant interactions and biomass while manipulating a soil moisture gradient and reducing consumer pressure. Shrubs facilitated the annual plant community at all levels of soil moisture through reductions in microclimatic stress in both years and herbivore protection in the wet year only. Shrub facilitation and the high rainfall year contributed to the dominance of a competitive annual species in the plant community. Precipitation patterns in deserts determine the magnitude and type of facilitation mechanisms. Moreover, shrub facilitation mediates the interspecific competition within the associated annual community between years with different rainfall amounts. Examining multiple drivers during extreme climate events is a challenging area of research, but it is a necessary consideration given forecasts predicting that these events will increase in frequency and magnitude.

Hydraulic redistribution: limitations for plants in saline soils

Hydraulic redistribution (HR), the movement of water from wet to dry patches in the soil via roots, occurs in different ecosystems and plant species. By extension of the principle that HR is driven by gradients in soil water potential, HR has been proposed to occur for plants in saline soils. Despite the inherent spatial patchiness and salinity gradients in these soils, the lack of direct evidence of HR in response to osmotic gradients prompted us to ask the question: are there physical or physiological constraints to HR for plants in saline environments? We propose that build-up of ions in the root xylem sap and in the leaf apoplast, with the latter resulting in a large predawn disequilibrium of water potential in shoots compared with roots and soil, would both impede HR. We present a conceptual model that illustrates how processes in root systems in heterogeneous salinity with water potential gradients, even if equal to those in non-saline soils, will experience a dampened magnitude of water potential gradients in the soil-plant continuum, minimizing or preventing HR. Finally, we provide an outlook for understanding the relevance of HR for plants in saline environments by addressing key research questions on plant salinity tolerance.

Phytochemical, ethnomedicinal uses and pharmacological profile of genus Pistacia

Pistacia genus belong to family Anacardiaceae and it is versatile in that its member species have food (P. vera), medicinal (P. lentiscus) and ornamental (P. chinensis) values. Various species of this genus have folkloric uses with credible mention in diverse pharmacopeia. As a trove of phenolic compounds, terpenoids, monoterpenes, flavonoids, alkaloids, saponins, fatty acids, and sterols, this genus has garnered pharmaceutical attention in recent times. With adequate clinical studies, this genus might be exploited for therapy of a multitude of inflammatory diseases, as promised by preliminary studies. In this regard, the ethnomedicinal, phytochemistry, biological potencies, risks, and scopes of Pistacia genus have been reviewed here.

Linking Populus euphratica hydraulic redistribution to diversity assembly in the arid desert zone of Xinjiang, China

The hydraulic redistribution (HR) of deep-rooted plants significantly improves the survival of shallow-rooted shrubs and herbs in arid deserts, which subsequently maintain species diversity. This study was conducted in the Ebinur desert located in the western margin of the Gurbantonggut Desert. Isotope tracing, community investigation and comparison analysis were employed to validate the HR of Populus euphratica and to explore its effects on species richness and abundance. The results showed that, P. euphratica has HR. Shrubs and herbs that grew under the P. euphratica canopy (under community: UC) showed better growth than the ones growing outside (Outside community: OC), exhibiting significantly higher species richness and abundance in UC than OC (p<0.05) along the plant growing season. Species richness and abundance were significantly logarithmically correlated with the P. euphratica crown area in UC (R² = 0.51 and 0.84, p<0.001). In conclusion, P. euphratica HR significantly ameliorates the water conditions of the shallow soil, which then influences the diversity assembly in arid desert communities.

Water release through plant roots: new insights into its consequences at the plant and ecosystem level

Hydraulic redistribution (HR) is the passive movement of water between different soil parts via plant root systems, driven by water potential gradients in the soil-plant interface. New data suggest that HR is a heterogeneous and patchy process. In this review we examine the main biophysical and environmental factors controlling HR and its main implications at the plant, community and ecosystem levels. Experimental evidence and the use of novel modelling approaches suggest that HR may have important implications at the community scale, affecting net primary productivity as well as water and vegetation dynamics. Globally, HR may influence hydrological and biogeochemical cycles and, ultimately, climate.