When is breeding for drought tolerance optimal if drought is random?

* Increasing climatic unpredictability associated with characteristics of some species makes plant drought-tolerance an important drought-adaptation strategy. Using norm-of-reaction functions, or empirically determined functions that enable us to predict the state of a trait given the state of an environmental variable, allows modelling of plant performance when water availability varies randomly. * A mathematical model is proposed to evaluate drought-tolerance and growth strategies given a set of environmental parameters: the frequency of rainy days, the soil water-storage capacity, plant water use and plant growth rates. This model compares the performance of genotypes that differ in drought tolerance expressed as the ability to grow in drier soils, and assumes a general trade-off function between drought tolerance and maximum plant growth rate. * It is worth selecting plants with a greater degree of drought tolerance, expressed by the ability to grow in drier soils whenever the frequency of rains is smaller than the rate of soil water depletion. Otherwise, maximizing growth rate at the expense of drought tolerance is the best strategy. The nature of the trade-off between drought tolerance and plant growth rate also constrains the selection for optimal drought-adapted genotypes. * Breeders will have to consider these aspects of plant-environment interactions before establishing selection programs for drought adaptation.     

Assessment of changes in physiological and biochemical traits in four pistachio rootstocks under drought,salinity and drought + salinity stresses

In this study, 7-month-old UCB-1, Badami, Ghazvini and Kale-Ghouchi pistachio rootstocks were exposed to control, drought, salinity and drought + salinity environments for 60 d. Total chlorophyll and total carotenoid contents decreased in all cultivars under drought, salinity and drought + salinity stresses. Under drought and salinity stresses, alone or in combination, Na⁺ and Cl⁻ ions increased in all four pistachio rootstocks, while K⁺ ion decreased only in Ghazvini and Kaleh-Ghouchi cultivars. The enzyme activities of ascorbate peroxidase, polyphenol oxidase, catalase and guaiacol peroxidase increased in all cultivars when subjected to all three stresses with the exception of the ascorbate peroxidase activity in Kale-Ghouchi cultivar during drought stress. Oxidative stress parameters including electrolyte leakage, malondialdehyde, other aldehydes and hydrogen peroxide increased under all three stress conditions in all genotypes. The content of proline, total free amino acids and total soluble carbohydrates were enhanced under drought, salinity and drought + salinity stresses, whereas the protein content decreased in all pistachio rootstocks. In all evaluated traits, except for the K⁺ ion content and APX activity, the highest impacts was seen for drought + salinity > salinity > drought stresses, respectively. For the first time, we have proven that K⁺ ion content has a positive correlation with the ascorbate peroxidase, polyphenol oxidase, catalase and guaiacol peroxidase enzymes activities under drought + salinity stress. Finally, based on the bi-plot and cluster analyses, we have selected the UCB-1 > Badami > Ghazvini > Kale-Ghouchi cultivars as the most tolerant pistachio rootstocks under drought + salinity stress, respectively.     

How do riparian woody seedlings survive seasonal drought?

In semi-arid regions, a major population limitation for riparian trees is seedling desiccation during the dry season that follows annual spring floods. We investigated the stress response of first-year pioneer riparian seedlings to experimental water table declines (0, 1 and 3 cm day⁻¹), focusing on the three dominant cottonwood and willows (family Salicaceae) in California’s San Joaquin Basin. We analyzed growth and belowground allocation response to water stress, and used logistic regression to determine if these traits had an influence on individual survival. The models indicate that high root growth (>3 mm day⁻¹) and low shoot:root ratios (<1.5 g g⁻¹) strongly predicted survival, but there was no evidence that plants increased belowground allocation in response to drawdown. Leaf δ¹³C values shifted most for the best-surviving species (net change of +3.5 per mil from −30.0 ± 0.3 control values for Goodding’s willow, Salix gooddingii), implying an important role of increased water-use efficiency for surviving water stress. Both S. gooddingii and sandbar willow (S. exigua) reduced leaf size from controls, whereas Fremont cottonwood (Populus fremontii) sustained a 29% reduction in specific leaf area (from 13.4 to 9.6 m² kg⁻¹). The functional responses exhibited by Goodding’s willow, the more drought-tolerant species, may play a role in its greater relative abundance in dry regions such as the San Joaquin Basin. This study highlights the potential for a shift in riparian forest composition. Under a future drier climate regime or under reduced regulated river flows, our results suggest that willow establishment will be favored over cottonwood.     

Does a terminal drought tolerance QTL contribute to differences in ROS scavenging enzymes and photosynthetic pigments in pearl millet exposed to drought?

The control of reactive oxygen species (ROS) and the stability of photosynthetic pigments under stress conditions are hypothesized to contribute to drought tolerance. Here we studied how ascorbic peroxidase (APX), superoxide dismutase (SOD), catalase (CAT) isozyme activities and chlorophyll a, b (Chl a, b) and carotenoids (Car) contents responded to water stress and whether they related to presence of a terminal drought tolerance QTL in pearl millet. We used PRLT2/89-33 (QTL donor), H77/833-2 (sensitive), and near-isogenic lines (QTL-NILs) introgressed with the QTL in H77/833-2 background. Under water stress there was no significant change in the total APX activity; only the proportional APX5 activity increased, with higher band intensity in tolerant genotypes. There were no significant changes in total activities of CAT and SOD under water stress, with similar band intensities in all genotypes, and a new CAT isozyme was induced in all genotypes. The photosynthetic pigment content decreased under water stress, although not differently in any genotype. Under water stress, the activities of most APX, CAT and SOD isozymes were closely related to the total chlorophyll/carotenoids ratio. Overall, besides APX5, water stress did not lead to major changes in the profile of isoenzymes involved in ROS scavenging. Similarly, the pigment content under stress did not discriminate genotypes according to the presence/absence of the QTL. This absence of discrimination for the ROS scavenging enzymes and for the pigment content under stress suggests that these traits may not play a key role in terminal drought tolerance in pearl millet.     

Does plasmolysis increase the drought tolerance of plant cells?

Summary Iljin's experiments withRhoeo discolor, cabbage and red beet which seemed to demonstrate increased drought resistance of plasmolysed tissue have been repeated, but his results could not be confirmed. The tissue plasmolysed in sucrose solutions, died either during stepwise plasmolysis and deplasmolysis or else later on, during exposure to unsaturated air within one or two days, even at the highest humidities. Iljin's error was apparently produced by a wrong interpretation of his tests of viability: the plasma of his dead cells did not disintegrate and retained anthocyanin by tonoplast plasmolysis.Plasmolysis proved harmful to all three objects investigated.In view of these results and earlier criticism of Iljin's analogous experiments on frost resistance by others, all experimental evidence produced in support of Iljin's mechanical theory of drought resistance should be reexamined.     

Nitrogen fixation control under drought stress. Localized or systemic?

Legume-Rhizobium nitrogen fixation is dramatically affected under drought and other environmental constraints. However, it has yet to be established as to whether such regulation of nitrogen fixation is only exerted at the whole-plant level (e.g. by a systemic nitrogen feedback mechanism) or can also occur at a local nodule level. To address this question, nodulated pea (Pisum sativum) plants were grown in a split-root system, which allowed for half of the root system to be irrigated at field capacity, while the other half was water deprived, thus provoking changes in the nodule water potential. Nitrogen fixation only declined in the water-deprived, half-root system and this result was correlated with modifications in the activities of key nodule's enzymes such as sucrose synthase and isocitrate dehydrogenase and in nodular malate content. Furthermore, the decline in nodule water potential resulted in a cell redox imbalance. The results also indicate that systemic nitrogen feedback signaling was not operating in these water-stressed plants, since nitrogen fixation activity was maintained at control values in the watered half of the split-root plants. Thus, the use of a partially droughted split-root system provides evidence that nitrogen fixation activity under drought stress is mainly controlled at the local level rather than by a systemic nitrogen signal.     

How do herbivorous insects respond to drought stress in trees?

Increased frequency and severity of drought, as a result of climate change, is expected to drive critical changes in plant–insect interactions that may elevate rates of tree mortality. The mechanisms that link water stress in plants to insect performance are not well understood. Here, we build on previous reviews and develop a framework that incorporates the severity and longevity of drought and captures the plant physiological adjustments that follow moderate and severe drought. Using this framework, we investigate in greater depth how insect performance responds to increasing drought severity for: (i) different feeding guilds; (ii) flush feeders and senescence feeders; (iii) specialist and generalist insect herbivores; and (iv) temperate versus tropical forest communities. We outline how intermittent and moderate drought can result in increases of carbon‐based and nitrogen‐based chemical defences, whereas long and severe drought events can result in decreases in plant secondary defence compounds. We predict that different herbivore feeding guilds will show different but predictable responses to drought events, with most feeding guilds being negatively affected by water stress, with the exception of wood borers and bark beetles during severe drought and sap‐sucking insects and leaf miners during moderate and intermittent drought. Time of feeding and host specificity are important considerations. Some insects, regardless of feeding guild, prefer to feed on younger tissues from leaf flush, whereas others are adapted to feed on senescing tissues of severely stressed trees. We argue that moderate water stress could benefit specialist insect herbivores, while generalists might prefer severe drought conditions. Current evidence suggests that insect outbreaks are shorter and more spatially restricted in tropical than in temperate forests. We suggest that future research on the impact of drought on insect communities should include (i) assessing how drought‐induced changes in various plant traits, such as secondary compound concentrations and leaf water potential, affect herbivores; (ii) food web implications for other insects and those that feed on them; and (iii) interactions between the effects on insects of increasing drought and other forms of environmental change including rising temperatures and CO₂ levels. There is a need for larger, temperate and tropical forest‐scale drought experiments to look at herbivorous insect responses and their role in tree death.     

Phospholipase D δ knock-out mutants are tolerant to severe drought stress

Phospholipase D (PLD) is involved in different plant processes, ranging from responses to abiotic and biotic stress to plant development. Phospholipase Dδ (PLDδ) is activated in dehydration and salt stress, producing the lipid second messenger phosphatidic acid. In this work we show that pldδ Arabidopsis mutants were more tolerant to severe drought than wild-type plants. PLDδ has been shown to be required for ABA regulation of stomatal closure of isolated epidermal peels. However, there was no significant difference in stomatal conductance at the whole plant level between wild-type and pldδ mutants. Since PLD hydrolyses structural phospholipids, then we looked at membrane integrity. Ion leakage measurements showed that during dehydration of leaf discs pldδ mutant has less membrane degradation compared to the wild-type. We further analyzed the mutants and showed that pldδ have higher mRNA levels of RAB18 and RD29A compared to wild-type plants under normal growth conditions. Transient expression of AtPLDδ in Nicotiana benthamiana plants induced a wilting phenotype. These findings suggest that, in wt plants PLDδ disrupt membranes in severe drought stress and, in the absence of the protein (PLDδ knock-out) might drought-prime the plants, making them more tolerant to severe drought stress. The results are discussed in relation to PLDδ role in guard cell signaling and drought tolerance.     

Suburbs: dangers or drought refugia for freshwater turtle populations?

Urbanization is one of the most rapidly expanding forms of habitat alteration worldwide. Wildlife differs in their responses to urbanization depending upon species and site-specific factors. We used capture-mark-recapture to examine the abundance, population demographics, growth, and movements of the eastern long-necked turtle (Chelodina longicollis) in Australia over 1 year in a suburban environment and an adjacent nature reserve during drought. Contrary to expectations, sex ratios, injury incidence, and frequency of juvenile size classes did not differ between turtles in the suburbs and the nature reserve. Moreover, turtles in the suburbs were nearly 3 times more abundant, grew 5 times faster, and had populations comprised of more adults in the larger size classes than nature reserve populations. These findings, together with net movements from the nature reserves into the suburbs, suggest that suburban water bodies were the higher quality habitat, effectively buffering turtles from temporal fluctuations in environmental conditions during drought. However, reserve managers and urban planners need to recognize that suburban water bodies have the potential to attract turtles from nearby reserves during drought, and that even low levels of persistent mortality during these travels across reserve boundaries may have consequences for populations of long-lived vertebrates. © 2011 The Wildlife Society.     

How important is woody tissue photosynthesis in poplar during drought stress?

Key message

Woody tissue photosynthesis might play a key role in maintaining plant carbon economy and hydraulic function under unfavourable conditions such as drought stress.

Abstract

Within trees, a portion of respired CO₂ is assimilated by bark and woody tissue photosynthesis, but its physiological role remains unclear, in particular under unfavour able conditions like drought stress. We hypothesised that woody tissue photosynthesis will contribute to overall tree carbon gain both under sufficient water supply and during drought, and plays a role in maintaining the hydraulic function. We subjected half of the trees to a stem and branch light-exclusion treatment to prevent bark and woody tissue photosynthesis. Then, we measured leaf gas exchange and stem growth in Populus deltoides x nigra ‘Monviso’ trees both under well-watered and dry conditions. We additionally measured cavitation using acoustic emission in detached control and light-excluded branches to illustrate the role of woody tissue photosynthesis in xylem embolism repair. Under well-watered conditions, light exclusion resulted in reduced stem growth relative to control trees by 30 %. In response to drought, stem shrinkage of light-excluded trees was more pronounced as compared to control trees. During drought stress also maximum photosynthesis and transpiration rate tended to decrease more rapidly in light-excluded trees compared to control trees. Leaf fall in light-excluded branches together with the larger number of acoustic emissions in control branches indicates that in the latter more xylem vessels were still hydraulically functional under drought. Therefore, our study highlights that photosynthesis at branch and stem level might be a key factor in the resilience of trees to drought stress by maintaining both the plant carbon economy and hydraulic function.

     

Does the turgor loss point characterize drought response in dryland plants?

The water potential at turgor loss point (Ψ_tlp) has been suggested as a key functional trait for determining plant drought tolerance, because of its close relationship with stomatal closure. Ψ_tlp may indicate drought tolerance as plants, which maintain gas exchange at lower midday water potentials as soil water availability declines also have lower Ψ_tlp. We evaluated 17 species from seasonally dry habitats, representing a range of life‐forms, under well‐watered and drought conditions, to determine how Ψ_tlp relates to stomatal sensitivity (pre‐dawn water potential at stomatal closure: Ψg_s0) and drought strategy (degree of isohydry or anisohydry; ΔΨ_MD between well‐watered conditions and stomatal closure). Although Ψg_s0 was related to Ψ_tlp, Ψg_s0 was better related to drought strategy (ΔΨ_MD). Drought avoiders (isohydric) closed stomata at water potentials higher than their Ψ_tlp; whereas, drought tolerant (anisohydric) species maintained stomatal conductance at lower water potentials than their Ψ_tlp and were more dehydration tolerant. There was no significant relationship between Ψ_tlp and ΔΨ_MD. While Ψ_tlp has been related to biome water availability, we found that Ψ_tlp did not relate strongly to stomatal closure or drought strategy, for either drought avoiders or tolerators. We therefore suggest caution in using Ψ_tlp to predict vulnerability to drought.     

Integrated drought responses of black poplar: how important is phenotypic plasticity?

Climate change is expected to increase drought frequency and intensity which will threaten plant growth and survival. In such fluctuating environments, perennial plants respond with hydraulic and biomass adjustments, resulting in either tolerant or avoidant strategies. Plants' response to stress relies on their phenotypic plasticity. The goal of this study was to explore physiology of young Populus nigra in the context of a time‐limited and progressive water deficit in regard to their growth and stress response strategies. Fourteen French 1‐year‐old black poplar genotypes, geographically contrasted, were subjected to withholding water during 8 days until severe water stress. Water fluxes (i.e. leaf water potentials and stomatal conductance) were analyzed together with growth (i.e. radial and longitudinal branch growth, leaf senescence and leaf production). Phenotypic plasticity was calculated for each trait and response strategies to drought were deciphered for each genotype. Black poplar genotypes permanently were dealing with a continuum of adjusted water fluxes and growth between two extreme strategies, tolerance and avoidance. Branch growth, leaf number and leaf hydraulic potential traits had contrasted plasticities, allowing genotype characterization. The most tolerant genotype to water deficit, which maintained growth, had the lowest global phenotypic plasticity. Conversely, the most sensitive and avoidant genotype ceased growth until the season's end, had the highest plasticity level. All the remaining black poplar genotypes were close to avoidance with average levels of traits plasticity. These results underpinned the role of plasticity in black poplar response to drought and calls for its wider use into research on plants' responses to stress.     

Genetic dissection of drought tolerance in chickpea (Cicer arietinum L.)

Key message

Analysis of phenotypic data for 20 drought tolerance traits in 1–7 seasons at 1–5 locations together with genetic mapping data for two mapping populations provided 9 QTL clusters of which one present on CaLG04 has a high potential to enhance drought tolerance in chickpea improvement.

Abstract

Chickpea (Cicer arietinum L.) is the second most important grain legume cultivated by resource poor farmers in the arid and semi-arid regions of the world. Drought is one of the major constraints leading up to 50 % production losses in chickpea. In order to dissect the complex nature of drought tolerance and to use genomics tools for enhancing yield of chickpea under drought conditions, two mapping populations—ICCRIL03 (ICC 4958 × ICC 1882) and ICCRIL04 (ICC 283 × ICC 8261) segregating for drought tolerance-related root traits were phenotyped for a total of 20 drought component traits in 1–7 seasons at 1–5 locations in India. Individual genetic maps comprising 241 loci and 168 loci for ICCRIL03 and ICCRIL04, respectively, and a consensus genetic map comprising 352 loci were constructed (http://cmap.icrisat.ac.in/cmap/sm/cp/varshney/). Analysis of extensive genotypic and precise phenotypic data revealed 45 robust main-effect QTLs (M-QTLs) explaining up to 58.20 % phenotypic variation and 973 epistatic QTLs (E-QTLs) explaining up to 92.19 % phenotypic variation for several target traits. Nine QTL clusters containing QTLs for several drought tolerance traits have been identified that can be targeted for molecular breeding. Among these clusters, one cluster harboring 48 % robust M-QTLs for 12 traits and explaining about 58.20 % phenotypic variation present on CaLG04 has been referred as “QTL-hotspot”. This genomic region contains seven SSR markers (ICCM0249, NCPGR127, TAA170, NCPGR21, TR11, GA24 and STMS11). Introgression of this region into elite cultivars is expected to enhance drought tolerance in chickpea.     

Hydraulic limits preceding mortality in a piñon-juniper woodland under experimental drought

Drought‐related tree mortality occurs globally and may increase in the future, but we lack sufficient mechanistic understanding to accurately predict it. Here we present the first field assessment of the physiological mechanisms leading to mortality in an ecosystem‐scale rainfall manipulation of a piñon–juniper (Pinus edulis–Juniperus monosperma) woodland. We measured transpiration (E) and modelled the transpiration rate initiating hydraulic failure (E_crit). We predicted that isohydric piñon would experience mortality after prolonged periods of severely limited gas exchange as required to avoid hydraulic failure; anisohydric juniper would also avoid hydraulic failure, but sustain gas exchange due to its greater cavitation resistance. After 1 year of treatment, 67% of droughted mature piñon died with concomitant infestation by bark beetles (Ips confusus) and bluestain fungus (Ophiostoma spp.); no mortality occurred in juniper or in control piñon. As predicted, both species avoided hydraulic failure, but safety margins from E_crit were much smaller in piñon, especially droughted piñon, which also experienced chronically low hydraulic conductance. The defining characteristic of trees that died was a 7 month period of near‐zero gas exchange, versus 2 months for surviving piñon. Hydraulic limits to gas exchange, not hydraulic failure per se, promoted drought‐related mortality in piñon pine.     

Ultrastructural changes in wheat embryos during a “presowing drought hardening” treatment

Summary Changes in the ultrastructure of the shoot apex of wheat embryos were studied during a reputed drought hardening treatment consisting of 48 hours of hydration followed by dehydration to air dry weight and during a subsequent second hydration for 48 hours. In the initial dry state lipid droplets are closely packed against the cell walls and the mitochondria and plastids have little internal structure. During the first hydration the lipid droplets are reduced in number and disperse while the mitochondria and plastids assume a normal appearance. The nucleus does not change appreciably. Under the conditions used in this work dehydration to air dry weight is not lethal to the embryos although notable changes in cellular structure occur: 1. Massive condensation of the normally loosely organized chromatin, 2. some loss of internal mitochondrial structure, 3. specific association of short elements of rough ER with vacuoles and lipid droplets, and 4. formation of fibrillar elements in the cytoplasm. In some respects then, embryos dehydrated after a 48 hour soaking have a very different structure to that of embryos that have dried during maturation of the seed. Since these changes are, in the case of viable embryos, reversible upon rehydration they cannot be linked, as such, to any long term drought hardening effect of the treatment. It is suggested that some changes, particularly the condensation of chromatin, may indicate a protective response to water stress. The kind of information needed to sustain this suggestion is discussed.This project is supported by grants from the Rural Credits Development Fund and the Australian Research Grants Committee.