Genetic Variation in Drought-Tolerance Traits and Their Relationships to Growth in Pinus radiata D. Don Under Water Stress

The selection of drought-tolerant genotypes is globally recognized as an effective strategy to maintain the growth and survival of commercial tree species exposed to future drought periods. New genomic selection tools that reduce the time of progeny trials are required to substitute traditional tree breeding programs. We investigated the genetic variation of water stress tolerance in New Zealand-grown Pinus radiata D. Don using 622 commercially-used genotypes from 63 families. We used quantitative pedigree-based (Genomic Best Linear Unbiased Prediction or ABLUP) and genomic-based (Genomic Best Linear Unbiased Prediction or GBLUP) approaches to examine the heritability estimates associated with water stress tolerance in P. radiata. Tree seedling growth traits, foliar carbon isotope composition (δ13C), and dark-adapted chlorophyll fluorescence (Y) were monitored before, during and after 10 months of water stress. Height growth showed a constant and moderate heritability level, while the heritability estimate for diameter growth and δ13C decreased with water stress. In contrast, chlorophyll fluorescence exhibited low heritability after 5 and 10 months of water stress. The GBLUP approach provided less breeding value accuracy than ABLUP, however, the relative selection efficiency of GBLUP was greater compared with ABLUP selection techniques. Although there was no significant relationship directly between δ13C and Y, the genetic correlations were significant and stronger for GBLUP. The positive genetic correlations between δ13C and tree biomass traits under water stress indicated that intraspecific variation in δ13C was likely driven by differences in the genotype's photosynthetic capacity. The results show that foliar δ13C can predict P. radiata genotype tolerance to water stress using ABLUP and GBLUP approaches and that such approaches can provide a faster screening and selection of drought-tolerant genotypes for forestry breeding programs.

Evaluation of Responsivity to Drought Stress Using Infrared Thermography and Chlorophyll Fluorescence in Potted Clones of Cryptomeria japonica

As climate change progresses, the breeding of drought-tolerant forest trees is necessary. Breeding drought-tolerant trees requires screening for drought stress using a large number of individuals and a high-throughput phenotyping method. The aim of this study was therefore to establish high-throughput methods for evaluating the clonal stress responses to drought stress using infrared thermography and chlorophyll fluorescence methods in Cryptomeria japonica. The stomatal conductance index (Ig), maximum photochemical quantum yield of photosystem II (Fv/Fm), and axial growth of four plus-tree clones of C. japonica planted in pots were measured weekly for 85 days after irrigation was stopped. The phenotypic trait responsivity to drought stress was estimated by a nonlinear mixed model and by introducing the cumulative water index, which considers the past history of the soil water environment. These methods and procedures enabled us to evaluate the clonal stress responses in C. japonica and could be applied to large-scale clone materials to promote the breeding program for drought tolerance.

Assessment of changes in growth traits, oxidative stress parameters, and enzymatic and non-enzymatic antioxidant defense mechanisms in Lepidium draba plant under osmotic stress induced by polyethylene glycol

Lepidium draba is a weed with the medicinal properties which few researches have been done on it. In this study, some traits, related to the osmotic stress, in 14-day-old L. draba sprouts that were grown 9 days in the presence of various doses of polyethylene glycol 6000 (PEG 6000) including 0, 3, 6, 9, and 12%, with different osmotic potentials (- 0.04, - 0.12, - 0.23, - 0.34, and - 0.48 MPa, respectively) were investigated. Based on our results, germination percentage besides stem and root lengths decreased with increasing the concentrations of PEG. The contents of electrolyte leakage, malondialdehyde, other aldehydes, total protein, free amino acids, total soluble carbohydrate as well as free proline increased with increasing the concentrations of PEG. Also, for the first time, our results have proven that under osmotic stress, there is an adverse relationship between hydrogen peroxide content and the activity of catalase, peroxidase, ascorbate peroxidase, and guaiacol peroxidase enzymes, such that hydrogen peroxide content decreased with induction of PEG up to 6% and after that increased, while the activity of catalase, peroxidase, ascorbate peroxidase, and guaiacol peroxidase enzymes increased up to 6% PEG and after that decreased. The expression levels of catalase, peroxidase, ascorbate peroxidase, and guaiacol peroxidase genes showed the same pattern as was seen for these enzyme activities. According to the results of this study, it can be deduced that decreasing H₂O₂ content cannot be the main reason for other oxidative stress parameters to decrease. In this study, P5CS and P5CR gene expression levels increased with increasing levels of PEG up to 12% which was completely similar to free proline content. Based on our results, L. draba can be considered as a semi-tolerant plant to osmotic stress.

Variability and Plasticity in Cuticular Transpiration and Leaf Permeability Allow Differentiation of Eucalyptus Clones at an Early Age

Background and Objectives. Water stress is a major constraining factor of Eucalyptus plantations’ growth. Within a genetic improvement program, the selection of genotypes that improve drought resistance would help to improve productivity and to expand plantations. Leaf characteristics, among others, are important factors to consider when evaluating drought resistance evaluation, as well as the clone’s ability to modify leaf properties (e.g., stomatal density (d) and size, relative water content at the time of stomatal closure (RWCc), cuticular transpiration (Ec), specific leaf area (SLA)) according to growing conditions. Therefore, this study aimed at analyzing these properties in nursery plants of nine high-productivity Eucalyptus clones. Material and Methods: Five Eucalyptus globulus Labill. clones and four hybrids clones (Eucalyptus urophylla S.T. Blake × Eucalyptus grandis W. Hill ex Maiden, 12€; Eucalyptus urograndis × E. globulus, HE; Eucalyptus dunnii Maiden–E. grandis × E. globulus, HG; Eucalyptus saligna Sm. × Eucalyptus maidenii F. Muell., HI) were studied. Several parameters relating to the aforementioned leaf traits were evaluated for 2.5 years. Results: Significant differences in stomatal d and size, RWCc, Ec, and SLA among clones (p < 0.001) and according to the dates (p < 0.001) were obtained. Each clone varied seasonally the characteristics of its new developing leaves to acclimatize to the growth conditions. The pore opening surface potential (i.e., the stomatal d × size) did not affect transpiration rates with full open stomata, so the water transpired under these conditions might depend on other leaf factors. The clones HE, HG, and 12€ were the ones that differed the most from the drought resistant E. globulus control clone (C14). Those three clones showed lower leaf epidermis impermeability (HE, HG, 12€), higher SLA (12€, HG), and lower stomatal control under moderate water stress (HE, HG) not being, therefore, good candidates to be selected for drought resistance, at least for these measured traits. Conclusions: These parameters can be incorporated into genetic selection and breeding programs, especially Ec, SLA, RWCc, and stomatal control under moderate water stress.

Quantitative and comparative analysis of whole-plant performance for functional physiological traits phenotyping: New tools to support pre-breeding and plant stress physiology studies

Plants are autotrophic organisms in which there are linear relationships between the rate at which organic biomass is accumulated and many ambient parameters such as water, nutrients, CO₂ and solar radiation. These linear relationships are the result of good feedback regulation between a plants sensing of the environment and the optimization of its performance response. In this review, we suggest that continuous monitoring of the plant physiological profile in response to changing ambient conditions could be a useful new phenotyping tool, allowing the characterization and comparison of different levels of functional phenotypes and productivity. This functional physiological phenotyping (FPP) approach can be integrated into breeding programs, which are facing difficulties in selecting plants that perform well under abiotic stress. Moreover, high-throughput FPP will increase the efficiency of the selection of traits that are closely related to environmental interactions (such as plant water status, water-use efficiency, stomatal conductance, etc.) thanks to its high resolution and dynamic measurements. One of the important advantages of FPP is, its simplicity and effectiveness and compatibility with experimental methods that use load-cell lysimeters and ambient sensors. In the future, this platform could help with phenotyping of complex physiological traits, beneficial for yield gain to enhance functional breeding approaches and guide in crop modeling.

Responses of tree species to heat waves and extreme heat events

The number and intensity of heat waves has increased, and this trend is likely to continue throughout the 21st century. Often, heat waves are accompanied by drought conditions. It is projected that the global land area experiencing heat waves will double by 2020, and quadruple by 2040. Extreme heat events can impact a wide variety of tree functions. At the leaf level, photosynthesis is reduced, photooxidative stress increases, leaves abscise and the growth rate of remaining leaves decreases. In some species, stomatal conductance increases at high temperatures, which may be a mechanism for leaf cooling. At the whole plant level, heat stress can decrease growth and shift biomass allocation. When drought stress accompanies heat waves, the negative effects of heat stress are exacerbated and can lead to tree mortality. However, some species exhibit remarkable tolerance to thermal stress. Responses include changes that minimize stress on photosynthesis and reductions in dark respiration. Although there have been few studies to date, there is evidence of within-species genetic variation in thermal tolerance, which could be important to exploit in production forestry systems. Understanding the mechanisms of differing tree responses to extreme temperature events may be critically important for understanding how tree species will be affected by climate change.

Genetic architecture of carbon isotope composition and growth in Eucalyptus across multiple environments

In the context of climate change, the water-use efficiency (WUE) of highly productive tree varieties, such as eucalypts, has become a major issue for breeding programmes. This study set out to dissect the genetic architecture of carbon isotope composition (δ(13) C), a proxy of WUE, across several environments. A family of Eucalyptus urophylla × E. grandis was planted in three trials and phenotyped for δ(13) C and growth traits. High-resolution genetic maps enabled us to target genomic regions underlying δ(13) C quantitative trait loci (QTLs) on the E. grandis genome. Of the 15 QTLs identified for δ(13) C, nine were stable across the environments and three displayed significant QTL-by-environment interaction, suggesting medium to high genetic determinism for this trait. Only one colocalization was found between growth and δ(13) C. Gene ontology (GO) term enrichment analysis suggested candidate genes related to foliar δ(13) C, including two involved in the regulation of stomatal movements. This study provides the first report of the genetic architecture of δ(13) C and its relation to growth in Eucalyptus. The low correlations found between the two traits at phenotypic and genetic levels suggest the possibility of improving the WUE of Eucalyptus varieties without having an impact on breeding for growth.

Physiological and biochemical responses to severe drought stress of nine Eucalyptus globulus clones: a multivariate approach

Seasonal drought, typical of temperate and Mediterranean environments, creates problems in establishing plantations and affects development and yield, and it has been widely studied in numerous species. Forestry fast-growing species such as Eucalyptus spp. are an important resource in such environments, selected clones being generally used for production purposes in plantations in these areas. However, use of mono-specific plantations increases risk of plant loss due to abiotic stresses, making it essential to understand differences in an individual clone's physiological responses to drought stress. In order to study clonal differences in drought responses, nine Eucalyptus globulus (Labill.) clones (C14, C46, C97, C120, C222, C371, C405, C491 and C601) were gradually subjected to severe drought stress (<14% of field capacity). A total of 31 parameters, physiological (e.g., photosynthesis, gas exchange), biochemical (e.g., chlorophyll content) and hormonal (abscisic acid [ABA] content), were analysed by classic and multivariate techniques. Relationships between parameters were established, allowing related measurements to be grouped into functional units (pigment, growth, water and ABA). Differences in these units showed that there were two distinct groups of E. globulus clones on the basis of their different strategies when faced with drought stress. The C14 group (C14, C120, C405, C491 and C601) clones behave as water savers, maintaining high water content and showing high stomatal adjustment, and reducing their aerial growth to a great extent. The C46 group (C46, C97, C222 and C371) clones behave as water spenders, reducing their water content drastically and presenting osmotic adjustment. The latter maintains the highest growth rate under the conditions tested. The method presented here can be used to identify appropriate E. globulus clones for drought environments, facilitating the selection of material for production and repopulation environments.