Phenotypic characterization of drought responses in red clover (Trifolium pratense L.)

Introduction

Red clover (Trifolium pratense) is a protein-rich, short-lived perennial forage crop that can achieve high yields, but suffers increasingly from drought in different cultivation areas. Breeding for increased adaptation to drought is becoming essential, but at this stage it is unclear which traits breeders should target to phenotype responses to drought that allow them to identify the most promising red clover genotypes. In this study, we assessed how prolonged periods of drought affected plant growth in field conditions, and which traits could be used to distinguish better adapted plant material.

Methods

A diverse panel of 395 red clover accessions was evaluated during two growing seasons. We simulated 6-to-8-week drought periods during two consecutive summers, using mobile rain-out shelters, while an irrigated control field was established in an adjacent parcel. Plant growth was monitored throughout both growing seasons using multiple flights with a drone equipped with RGB and thermal sensors. At various observation moments throughout both growing seasons, we measured canopy cover (CC) and canopy height (CH). The crop water stress index (CWSI) was determined at two moments, during or shortly after the drought event.

Results

Manual and UAV-derived measurements for CH were well correlated, indicating that UAV-derived measurements can be reliably used in red clover. In both years, CC, CH and CWSI were affected by drought, with measurable growth reductions by the end of the drought periods, and during the recovery phase. We found that the end of the drought treatment and the recovery phase of approximately 20 days after drought were suitable periods to phenotype drought responses and to distinguish among genotypes.

Discussion

Multifactorial analysis of accession responses revealed interactions of the maturity type with drought responses, which suggests the presence of two independent strategies in red clover: 'drought tolerance' and 'drought recovery'. We further found that a large proportion of the accessions able to perform well under well-watered conditions were also the ones that were less affected by drought. The results of this investigation are interpreted in view of the development of breeding for adaptation to drought in red clover.

San Diego Thornmint (Acanthomintha ilicifolia) Populations Differ in Growth and Reproductive Responses to Differential Water Availability: Evidence from a Common Garden Experiment

The responses of rare plants to environmental stressors will determine their potential to adapt to a rapidly changing climate. We used a common garden approach to evaluate how six populations of the annual San Diego thornmint (Acanthomintha ilicifolia Lamiaceae; listed as endangered in the state of California and as threatened by the US Fish and Wildlife Service) from across the species range respond in terms of growth (biomass, height, and width) and reproduction (seed production, floral production, and next generation seed viability) to experimental differences in water availability. We found a significant irrigation-by-population interaction on the aboveground growth, wherein the differences in the magnitude and direction of treatment did not correlate directly with climate variables in natural populations. With respect to reproduction, the low-irrigation treatment produced more seeds per plant, more reproductive individuals, and a larger proportion of viable seed in most, but not all, populations. The seed production and the effect of irrigation on seed production correlated positively with rainfall at wild source populations. These results suggest that Acanthomintha ilicifolia responds to water limitation by creating more and higher-quality seed, and that plants locally adapted to a higher annual rainfall show a greater plasticity to differences in water availability than plants adapted to a lower annual rainfall, a finding that can inform the in situ demographic management and ex situ collection strategy for Acanthomintha ilicifolia and other rare California annuals.

ABA regulation of root growth during soil drying and recovery can involve auxin response

Abscisic acid (ABA) plays an important role in plant adaptation to water deficits, but its role in regulating root growth (primary root elongation and lateral root number) during different drought-phases remains unclear. Here, we exposed wild-type (WT) and ABA-deficient (not) tomato plants to three continuous drought-phases (moderate drying: day 0-21; severe drying: day 22-47 and re-watering: day 48-51). It was found that WT increased primary root growth during moderate drying; maintained more lateral roots, and greater primary root and total root length under severe drying; and produced more roots after re-watering. After RNA-Seq analysis, we found that the auxin-related genes in root showed different expression patterns between WT and not under drying or re-watering. Further, exogenous supply of IAA partially recovered the root growth of ABA-deficient not plants under three continuous drought-phases. Our results suggested that ABA regulation of tomato root growth during soil drying and recovery can involve auxin response.

Leaf water relations reflect canopy phenology rather than leaf life span in Sonoran Desert trees

Plants from arid environments display covarying traits to survive or resist drought. Plant drought resistance and ability to survive long periods of low soil water availability should involve leaf phenology coordination with leaf and stem functional traits related to water status. This study tested correlations between phenology and functional traits involved in plant water status regulation in 10 Sonoran Desert tree species with contrasting phenology. Species seasonal variation in plant water status was defined by calculating their relative positions along the iso/anisohydric regulation continuum based on their hydroscape areas (HA)-a metric derived from the relationship between predawn and midday water potentials-and stomatal and hydraulic traits. Additionally, functional traits associated with plant water status regulation, including lamina vessel hydraulic diameter (DHL), stem-specific density (SSD) and leaf mass per area (LMA) were quantified per species. To characterize leaf phenology, leaf longevity (LL) and canopy foliage duration (FD) were determined. Hydroscape area was strongly correlated with FD but not with leaf longevity (LL); HA was significantly associated with SSD and leaf hydraulic traits (DHL, LMA) but not with stem hydraulic traits (vulnerability index, relative conductivity); and FD was strongly correlated with LMA and SSD. Leaf physiological characteristics affected leaf phenology when it was described as canopy FD better than when described as LL. Stem and leaf structure and hydraulic functions were not only relevant for categorizing species along the iso/anisohydric continuum but also allowed identifying different strategies of desert trees within the 'fast-slow' plant economics spectrum. The results in this study pinpoint the set of evolutionary pressures that shape the Sonoran Desert Scrub physiognomy.

Bark water vapour conductance is associated with drought performance in tropical trees

Bark water vapour conductance (g_bark) is a rarely considered functional trait. However, for the few tree species measured to date, it appears high enough to create stem water deficits associated with mortality during droughts, when access to water is limited. I tested whether g_bark correlates with stem water deficit during drought conditions in two datasets of tropical trees: one of saplings in forest understories during an annual dry season and one of potted saplings in a shadehouse during extreme drought conditions. Among all 14 populations of eight species measured, g_bark varied more than 10-fold (0.86-12.98 mmol m^-2 s^-1). In the forest understories, g_bark was highly correlated with stem water deficit among four deciduous species, but not among evergreen species that likely maintained access to soil water. In the shadehouse, g_bark was positively correlated with stem water deficit and mortality among all six species. Overall, tree species with higher g_bark suffer higher stem water deficit when soil water is unavailable. Incorporating g_bark into soil-plant-atmosphere hydrodynamic models may improve projections of plant mortality under drought conditions.

Genome-Wide Analysis of the DYW Subgroup PPR Gene Family and Identification of GmPPR4 Responses to Drought Stress

Pentatricopeptide-repeat (PPR) proteins were identified as a type of nucleus coding protein that is composed of multiple tandem repeats. It has been reported that PPR genes play an important role in RNA editing, plant growth and development, and abiotic stresses in plants. However, the functions of PPR proteins remain largely unknown in soybean. In this study, 179 DYW subgroup PPR genes were identified in soybean genome (Glycine max Wm82.a2.v1). Chromosomal location analysis indicated that DYW subgroup PPR genes were mapped to all 20 chromosomes. Phylogenetic relationship analysis revealed that DYW subgroup PPR genes were categorized into three distinct Clusters (I to III). Gene structure analysis showed that most PPR genes were featured by a lack of intron. Gene duplication analysis demonstrated 30 PPR genes (15 pairs; ~35.7%) were segmentally duplicated among Cluster I PPR genes. Furthermore, we validated the mRNA expression of three genes that were highly up-regulated in soybean drought- and salt-induced transcriptome database and found that the expression levels of GmPPR4 were induced under salt and drought stresses. Under drought stress condition, GmPPR4-overexpressing (GmPPR4-OE) plants showed delayed leaf rolling; higher content of proline (Pro); and lower contents of H2O2, O2- and malondialdehyde (MDA) compared with the empty vector (EV)-control plants. GmPPR4-OE plants exhibited increased transcripts of several drought-inducible genes compared with EV-control plants. Our results provided a comprehensive analysis of the DYW subgroup PPR genes and an insight for improving the drought tolerance in soybean.

A heuristic classification of woody plants based on contrasting shade and drought strategies

Woody plants vary in their adaptations to drought and shade. For a better prediction of vegetation responses to drought and shade within dynamic global vegetation models, it is critical to group species into functional types with similar adaptations. One of the key challenges is that the adaptations are generally determined by a large number of plant traits that may not be available for a large number of species. In this study, we present two heuristic woody plant groups that were separated using cluster analysis in a three-dimensional trait-environment space based on three key metrics for each species: mean xylem embolism resistance, shade tolerance and habitat aridity. The two heuristic groups separate these species into tolerators and avoiders. The tolerators either rely on their high embolism resistance to tolerate drought in arid habitats (e.g., Juniperus and Prunus) or rely on high shade tolerance to withstand shaded conditions in wet habitats (e.g., Picea, Abies and Acer). In contrast, all avoiders have low embolism resistance and low shade tolerance. In arid habitats, avoiders tend to minimize catastrophic embolism (e.g., most Pinus species) while in wet habitats, they may survive despite low shade tolerance (e.g., Betula, Populus, Alnus and Salix). Because our approach links traits to the environmental conditions, we expect it could be a promising framework for predicting changes in species composition, and therefore ecosystem function, under changing environmental conditions.

WRKY transcription factors are involved in brassinosteroid signaling and mediate the crosstalk between plant growth and drought tolerance

Brassinosteroids (BRs) are critical for the plant growth and development. BRs signal through the plasma membrane localized receptor-like kinases to downstream transcription factors BES1/BZR1 to regulate the expression of thousands of genes for various BR responses. In addition to the role in plant growth and development, BRs have been implicated in responses to environmental stresses such as drought. However, the mechanism through which BRs regulate drought have just begun emerging. We have recently found that a group of WRKY transcription factors, WRKY46, WRKY54, WRKY70, which are well known for the function in abiotic and biotic stress, cooperates with BES1 to mediate BR-regulated drought response. The wrky46 wrky54 wrky70 triple mutants showed growth defect, likely due to impaired BR signaling as well as some reduction of endogenous BR level. WRKY46/54/70 cooperates with BES1 to regulate the expression of BR target genes to promote growth. We also found that WRKY46/54/70 negatively modulates drought tolerance by globally repressing drought-inducible gene expression. Thus, our result uncovers a new role for WRKY transcription factors in BR signaling and provides the molecular mechanism for BR-regulated plant growth and drought stress through WRKY46/54/70 and BES1 transcription factors.

Retention of stored water enables tropical tree saplings to survive extreme drought conditions

Trees generally maintain a small safety margin between the stem water potential (Ψstem) reached during seasonal droughts and the Ψstem associated with their mortality. This pattern may indicate that species face similar mortality risk during extreme droughts. However, if tree species vary in their ability to regulate Ψstem, then safety margins would poorly predict drought mortality. To explore variation among species in Ψstem regulation, I subjected potted saplings of six tropical tree species to extreme drought and compared their responses with well-watered plants and pretreatment reference plants. In the drought treatment, soil water potential reached <-10 MPa, yet three species, Bursera simaruba (L.) Sarg., Cavanillesia platanifolia (Bonpl.) Kunth and Cedrela odorata L. had 100% survival and maintained Ψstem near -1 MPa (i.e., desiccation-avoiding species). Three other species, Cojoba rufescens (Benth.) Britton and Rose, Genipa americana L. and Hymenaea courbaril L. had 50%, 0% and 25% survival, respectively, and survivors had Ψstem <-6 MPa (i.e., desiccation-susceptible species). The desiccation-avoiding species had lower relative water content (RWC) in all organs and tissues (root, stem, bark and xylem) in the drought treatment than in the reference plants (means 72.0-90.4% vs 86.9-97.9%), but the survivors of the desiccation-susceptible C. rufescens had much lower RWC in the drought treatment (44.5-72.1%). Among the reference plants, the desiccation-avoiding species had lower tissue density, leaf-mass fraction and lateral-root surface area (LRA) than the desiccation-susceptible species. Additionally, C. platanifolia and C. odorata had reduced LRA in the drought treatment, which may slow water loss into dry soil. Together, these results suggest that the ability to regulate Ψstem during extreme drought is associated with functional traits that favor retention of stored water and that safety margins during seasonal drought poorly predict survival during extreme drought.

Drought responses of flood-tolerant trees in Amazonian floodplains

BACKGROUND

Flood-tolerant tree species of the Amazonian floodplain forests are subjected to an annual dry period of variable severity imposed when low river-water levels coincide with minimal precipitation. Although the responses of these species to flooding have been examined extensively, their responses to drought, in terms of phenology, growth and physiology, have been neglected hitherto, although some information is found in publications that focus on flooding.

SCOPE

The present review examines the dry phase of the annual flooding cycle. It consolidates existing knowledge regarding responses to drought among adult trees and seedlings of many Amazonian floodplain species.

MAIN FINDINGS

Flood-tolerant species display variable physiological responses to dry periods and drought that indicate desiccation avoidance, such as reduced photosynthetic activity and reduced root respiration. However, tolerance and avoidance strategies for drought vary markedly among species. Drought can substantially decrease growth, biomass and photosynthetic activity among seedlings in field and laboratory studies. When compared with the responses to flooding, drought can impose higher seedling mortality and slower growth rates, especially among evergreen species. Results indicate that tolerance and avoidance strategies for drought vary markedly between species. Both seedling recruitment and photosynthetic activity are affected by drought,

CONCLUSIONS

For many species, the effects of drought can be as important as flooding for survival and growth, particularly at the seedling phase of establishment, ultimately influencing species composition. In the context of climate change and predicted decreases in precipitation in the Amazon Basin, the effects of drought on plant physiology and species distribution in tropical floodplain forest ecosystems should not be overlooked.

Does night-time transpiration contribute to anisohydric behaviour in a Vitis vinifera cultivar?

The hypothesis that vines of the Semillon wine grape variety show anisohydric behaviour was tested, i.e. that tissue hydration is unstable under fluctuating environmental conditions. Stomatal conductance and transpiration rates from leaves were measured during the day and at night. Leaf water potential (Psi(l)) in Semillon was negatively correlated to vapour pressure deficit (VPD) both predawn and during the day. Furthermore, Psi(l) fell to significantly lower values than in any of the nine other varieties examined. Night-time values of stomatal conductance (g(n)) and transpiration (E(n)) in Semillon were up to four times higher than in other varieties; plants enclosed in plastic bags overnight to reduce E(n) resulted in better plant-soil equilibration so that predawn Psi(l) in Semillon was the same as in Grenache. These data indicate that the hypothesis is supported, and that night-time transpiration contributes significantly to the low Psi(l) values in Semillon during warm, dry nights. The other contributing factor is daytime stomatal conductance (g(day)), which in Semillon leaves was higher than in other varieties, although the decline in g(day) with increasing VPD was greater in Semillon than in Shiraz or Grenache. The high values of g(day) were associated with high rates of transpiration (E(day)) by Semillon through a day when VPD reached 4.5 kPa. When compared to other varieties, Semillon was not unusual in terms of root length density, stomatal density, xylem sap abscisic acid, or leaf electrolyte leakage. Night-time and daytime water loss and insufficient stomatal regulation therefore account for the tendency to anisohydric behaviour shown by Semillon.