Sodium interception by xylem parenchyma and chloride recirculation in phloem may augment exclusion in the salt tolerant Pistacia genus: context for salinity studies on tree crops

Physiological traits underlying sodicity tolerance in Jamun (Syzygium cumini L. Skeels) cultivars

Background

There is a lack of research on how sodicity stress affects tree growth and physiological relations in jamun (Syzygium cumini L. Skeels). An understanding of cultivar-specific morpho-physiological changes under sodic conditions might aid in the development of more sodicity-tolerant cultivars through genetic improvement, and help identify cultivars suitable for degraded sodic soils.

Methods

We assessed the effects of sodicity stress on tree growth, physiological relations, and ion uptake in four cultivars of jamun including CISH J-37 (J-37), CISH J-42 (J-42), Konkan Bahadoli (KB), and Goma Priyanka (GP).

Results

Jamun cultivars exhibited varying degrees of reduction in tree growth, leaf area, and gas exchange properties under sodic conditions. Elevated soil pH caused relatively larger declines in trunk cross sectional area (TCSA; >30%) and canopy volume (CV; >25%) in J-42 and KB. Reductions brought on by sodicity stress in leaf area were rather modest (<10%) across cultivars, suggesting that maintaining leaf area may be a key adaptive trait in jamun to cope with sodic conditions. In addition to displaying a notable increase in water use efficiency (WUE), cultivar J-37 also exhibited largely intact levels of relative chlorophyll and photosynthetic rate (P n) under sodic conditions. Despite a high intrinsic P n under control treatment, cultivar GP displayed a large drop in P n (37.16%) when exposed to sodicity stress. Comparatively greater increases in leaf phenolics in KB and GP seemed to be at the expense of growth and photosynthesis under sodic conditions. While superoxide dismutase (SOD) and catalase (CAT) activities showed notable spikes in both J-37 and GP, proline accumulation increased substantially only in cultivar J-37 in response to sodicity stress. Despite significant increases in leaf Na+ and Cl- concentrations, J-37 was found to be comparatively efficient in Na+ and Cl- exclusion from leaves when compared to other cultivars. Surprisingly, sodicity stress did not alter leaf K+, Ca2+ and Mg2+ levels noticeably across cultivars. Correlation analysis suggested that elevated leaf Cl- likely inhibited tree growth more than leaf Na+. Principal component analysis was reasonably efficient in discerning the shared and divergent responses to sodicity stress of the studied cultivars.

Conclusions

Membership function analysis revealed a reasonable resilience to sodicity stress only in cultivar J-37. Maintenance of photosynthesis, reduced uptake of Na+ and Cl- ions, increased and synergistic activities of SOD and CAT, and a higher leaf K+/Na+ ratio likely accounted for better performance of J-37 trees in sodic soils. Further investigations are needed to elucidate the molecular underpinnings of sodicity tolerance.

Similar and divergent responses to salinity stress of jamun (Syzygium cumini L. Skeels) genotypes

Background

Genetic variation for salt tolerance remains elusive in jamun (Syzygium cumini).

Methods

Effects of gradually increased salinity (2.0-12.0 dS/m) were examined in 20 monoembryonic and 28 polyembryonic genotypes of jamun. Six genotypes were additionally assessed for understanding salt-induced changes in gas exchange attributes and antioxidant enzymes.

Results

Salt-induced reductions in leaf, stem, root and plant dry mass (PDM) were relatively greater in mono- than in poly-embryonic types. Reductions in PDM relative to control implied more adverse impacts of salinity on genotypes CSJ-28, CSJ-31, CSJ-43 and CSJ-47 (mono) and CSJ-1, CSJ-24, CSJ-26 and CSJ-27 (poly). Comparably, some mono- (CSJ-5, CSJ-18) and poly-embryonic (CSJ-7, CSJ-8, CSJ-14, CSJ-19) genotypes exhibited least reductions in PDM following salt treatment. Most polyembryonic genotypes showed lower reductions in root than in shoot mass, indicating that they may be more adept at absorbing water and nutrients when exposed to salt. The majority of genotypes did not exhibit leaf tip burn and marginal scorch despite significant increases in Na+ and Cl-, suggesting that tissue tolerance existed for storing excess Na+ and Cl- in vacuoles. Jamun genotypes were likely more efficient in Cl- exclusion because leaf, stem and root Cl- levels were consistently lower than those of Na+ under salt treatment. Leaf K+ was particularly little affected in genotypes with high leaf Na+. Lack of discernible differences in leaf, stem and root Ca2+ and Mg2+ contents between control and salt treatments was likely due to their preferential uptake. Correlation analysis suggested that Na+ probably had a greater inhibitory effect on biomass in both mono- and poly-embryonic types. Discriminant analysis revealed that while stem and root Cl- probably accounted for shared responses, root Na+, leaf K+ and leaf Cl- explained divergent responses to salt stress of mono- and poly-embryonic types. Genotypes CSJ-18 and CSJ-19 seemed efficient in fending off oxidative damage caused by salt because of their stronger antioxidant defences.

Conclusions

Polyembryonic genotypes CSJ-7, CSJ-8, CSJ-14 and CSJ-19, which showed least reductions in biomass even after prolonged exposure to salinity stress, may be used as salt-tolerant rootstocks. The biochemical and molecular underpinnings of tissue tolerance to excess Na+ and Cl- as well as preferential uptake of K+, Ca2+, and Mg2+ need to be elucidated.

Contrasting allelic effects for pistachio salinity tolerance in juvenile and mature trees

Breeding perennial tree crops often requires prediction of mature performance from juvenile data. To assess the utility of juvenile screens to predict salinity tolerance of mature pistachio trees, we compared performance of 3-month ungrafted seedlings and 4-year-old grafted rootstocks under salinity stress. The QTL allele associated with higher salt exclusion from seedling leaves conferred lower growth in saline field conditions, suggesting that mapping QTL in seedlings may be easier than discerning the optimal allele for field performance.

It takes two: Reciprocal scion-rootstock relationships enable salt tolerance in 'Hass' avocado

Commercial avocado orchards typically consist of composite trees. Avocado is salt-sensitive, suffering from substantial growth and production depreciation when exposed to high sodium and chloride levels. Salt ions penetrate the roots and are subsequently transferred to the foliage. Hence, understanding distinct physiological responses of grafted avocado plant organs to salinity is of great interest. We compared the ion, metabolite and lipid profiles of leaves, roots and trunk drillings of mature 'Hass' scion grafted onto two different rootstocks during gradual exposure to salinity. We found that one rootstock, VC840, did not restrict the transport of irrigation solution components to the scion, leading to salt accumulation in the trunk and leaves. The other rootstock, VC152, functioned selectively, moderating the movement of toxic ions to the scion organs by accumulating them in the roots. The leaves of the scion grafted on the selective rootstock acquired the standard level of essential minerals without being exposed to excessive salt concentrations. However, this came with an energetic cost as the leaves transferred carbohydrates and storage lipids downward to the rootstock organs, which became a strong sink. We conclude that mutual scion-rootstock relationships enable marked tolerance to salt stress through selective ion transport and metabolic modifications.

Population genomics of apricots unravels domestication history and adaptive events

Among crop fruit trees, the apricot (Prunus armeniaca) provides an excellent model to study divergence and adaptation processes. Here, we obtain nearly 600 Armeniaca apricot genomes and four high-quality assemblies anchored on genetic maps. Chinese and European apricots form two differentiated gene pools with high genetic diversity, resulting from independent domestication events from distinct wild Central Asian populations, and with subsequent gene flow. A relatively low proportion of the genome is affected by selection. Different genomic regions show footprints of selection in European and Chinese cultivated apricots, despite convergent phenotypic traits, with predicted functions in both groups involved in the perennial life cycle, fruit quality and disease resistance. Selection footprints appear more abundant in European apricots, with a hotspot on chromosome 4, while admixture is more pervasive in Chinese cultivated apricots. Our study provides clues to the biology of selected traits and targets for fruit tree research and breeding.

Root vacuolar sequestration and suberization are prominent responses of Pistacia spp. rootstocks during salinity stress

Understanding the mechanisms of stress tolerance in diverse species is needed to enhance crop performance under conditions such as high salinity. Plant roots, in particular in grafted agricultural crops, can function as a boundary against external stresses in order to maintain plant fitness. However, limited information exists for salinity stress responses of woody species and their rootstocks. Pistachio (Pistacia spp.) is a tree nut crop with relatively high salinity tolerance as well as high genetic heterogeneity. In this study, we used a microscopy-based approach to investigate the cellular and structural responses to salinity stress in the roots of two pistachio rootstocks, Pistacia integerrima (PGI) and a hybrid, P. atlantica x P. integerrima (UCB1). We analyzed root sections via fluorescence microscopy across a developmental gradient, defined by xylem development, for sodium localization and for cellular barrier differentiation via suberin deposition. Our cumulative data suggest that the salinity response in pistachio rootstock species is associated with both vacuolar sodium ion (Na+) sequestration in the root cortex and increased suberin deposition at apoplastic barriers. Furthermore, both vacuolar sequestration and suberin deposition correlate with the root developmental gradient. We observed a higher rate of Na+ vacuolar sequestration and reduced salt-induced leaf damage in UCB1 when compared to P. integerrima. In addition, UCB1 displayed higher basal levels of suberization, in both the exodermis and endodermis, compared to P. integerrima. This difference was enhanced after salinity stress. These cellular characteristics are phenotypes that can be taken into account during screening for sodium-mediated salinity tolerance in woody plant species.

Ray fractions and carbohydrate dynamics of tree species along a 2750 m elevation gradient indicate climate response, not spatial storage limitation

Parenchyma cells in the xylem store nonstructural carbohydrates (NSC), providing reserves of energy that fuel woody perennials through periods of stress and/or limitations to photosynthesis. If the capacity for storage is subject to selection, then the fraction of wood occupied by living parenchyma should increase towards stressful environments. Ray parenchyma fraction (RPF) and seasonal NSC dynamics were quantified for 12 conifers and three oaks along a transect spanning warm dry foothills (500 m above sea level) to cold wet treeline (3250 m asl) in California's central Sierra Nevada. Mean RPF was lower for both conifer and oak species with warmer dryer ranges. RPF variability increased with elevation or in relation to associated climatic variables in conifers - treeline-dominant Pinus albicaulis had the lowest mean RPF measured (c. 3.7%), but the highest environmentally standardized variability index. Conifer RPF variability was explained by environment, increasing predominantly towards cooler wetter range edges. In oaks, NSC was explained by environment - values increasing for evergreen and decreasing for deciduous oaks with elevation. Lastly, all species surveyed appear to prioritize filling available RPF with sugar to achieve molarities that balance reasonable tensions over starch to maximize stored carbon. RPF responds to environment but is unlikely to spatially constrain NSC storage.