Allocation, within and between organs, and the dynamics of root length changes in two birch species

From Intra-plant to Regional Scale: June Temperatures and Regional Climates Directly and Indirectly Control Betula nana Growth in Arctic Alaska

Tundra shrubs reflect climate sensitivities in their growth-ring widths, yet tissue-specific shrub chronologies are poorly studied. Further, the relative importance of regional climate patterns that exert mesoscale precipitation and temperature influences on tundra shrub growth has been explored in only a few Arctic locations. Here, we investigate Betula nana growth-ring chronologies from adjacent dry heath and moist tussock tundra habitats in arctic Alaska in relation to local and regional climate. Mean shrub and five tissue-specific ring width chronologies were analyzed using serial sectioning of above- and below-ground shrub organs, resulting in 30 shrubs per site with 161 and 104 cross sections from dry and moist tundra, respectively. Betula nana growth-ring widths in both habitats were primarily related to June air temperature (1989–2014). The strongest relationships with air temperature were found for ‘Branch2’ chronologies (dry site: r = 0.78, June 16, DOY = 167; moist site: r = 0.75, June 9, DOY = 160). Additionally, below-ground chronologies (‘Root’ and ‘Root2’) from the moist site were positively correlated with daily mean air temperatures in the previous late-June (‘Root2’ chronology: r = 0.57, pDOY = 173). Most tissue-specific chronologies exhibited the strongest correlations with daily mean air temperature during the period between 8 and 20 June. Structural equation modeling indicated that shrub growth is indirectly linked to regional Arctic and Pacific Decadal Oscillation (AO and PDO) climate indices through their relation to summer sea ice extent and air temperature. Strong dependence of Betula nana growth on early growing season temperature indicates a highly coordinated allocation of resources to tissue growth, which might increase its competitive advantage over other shrub species under a rapidly changing Arctic climate.

Mean annual temperature influences local fine root proliferation and arbuscular mycorrhizal colonization in a tropical wet forest

Mean annual temperature (MAT) is an influential climate factor affecting the bioavailability of growth-limiting nutrients nitrogen (N) and phosphorus (P). In tropical montane wet forests, warmer MAT drives higher N bioavailability, while patterns of P availability are inconsistent across MAT. Two important nutrient acquisition strategies, fine root proliferation into bulk soil and root association with arbuscular mycorrhizal fungi, are dependent on C availability to the plant via primary production. The case study presented here tests whether variation in bulk soil N bioavailability across a tropical montane wet forest elevation gradient (5.2°C MAT range) influences (a) morphology fine root proliferation into soil patches with elevated N, P, and N+P relative to background soil and (b) arbuscular mycorrhizal fungal (AMF) colonization of fine roots in patches. We created a fully factorial fertilized root ingrowth core design (N, P, N+P, unfertilized control) representing soil patches with elevated N and P bioavailability relative to background bulk soil. Our results show that percent AMF colonization of roots increased with MAT (r 2 = .19, p = .004), but did not respond to fertilization treatments. Fine root length (FRL), a proxy for root foraging, increased with MAT in N+P-fertilized patches only (p = .02), while other fine root morphological parameters did not respond to the gradient or fertilized patches. We conclude that in N-rich, fine root elongation into areas with elevated N and P declines while AMF abundance increases with MAT. These results indicate a tradeoff between P acquisition strategies occurring with changing N bioavailability, which may be influenced by higher C availability with warmer MAT.

Root traits contributing to plant productivity under drought

Geneticists and breeders are positioned to breed plants with root traits that improve productivity under drought. However, a better understanding of root functional traits and how traits are related to whole plant strategies to increase crop productivity under different drought conditions is needed. Root traits associated with maintaining plant productivity under drought include small fine root diameters, long specific root length, and considerable root length density, especially at depths in soil with available water. In environments with late season water deficits, small xylem diameters in targeted seminal roots save soil water deep in the soil profile for use during crop maturation and result in improved yields. Capacity for deep root growth and large xylem diameters in deep roots may also improve root acquisition of water when ample water at depth is available. Xylem pit anatomy that makes xylem less "leaky" and prone to cavitation warrants further exploration holding promise that such traits may improve plant productivity in water-limited environments without negatively impacting yield under adequate water conditions. Rapid resumption of root growth following soil rewetting may improve plant productivity under episodic drought. Genetic control of many of these traits through breeding appears feasible. Several recent reviews have covered methods for screening root traits but an appreciation for the complexity of root systems (e.g., functional differences between fine and coarse roots) needs to be paired with these methods to successfully identify relevant traits for crop improvement. Screening of root traits at early stages in plant development can proxy traits at mature stages but verification is needed on a case by case basis that traits are linked to increased crop productivity under drought. Examples in lesquerella (Physaria) and rice (Oryza) show approaches to phenotyping of root traits and current understanding of root trait genetics for breeding.

Root traits contributing to plant productivity under drought

Geneticists and breeders are positioned to breed plants with root traits that improve productivity under drought. However, a better understanding of root functional traits and how traits are related to whole plant strategies to increase crop productivity under different drought conditions is needed. Root traits associated with maintaining plant productivity under drought include small fine root diameters, long specific root length, and considerable root length density, especially at depths in soil with available water. In environments with late season water deficits, small xylem diameters in targeted seminal roots save soil water deep in the soil profile for use during crop maturation and result in improved yields. Capacity for deep root growth and large xylem diameters in deep roots may also improve root acquisition of water when ample water at depth is available. Xylem pit anatomy that makes xylem less "leaky" and prone to cavitation warrants further exploration holding promise that such traits may improve plant productivity in water-limited environments without negatively impacting yield under adequate water conditions. Rapid resumption of root growth following soil rewetting may improve plant productivity under episodic drought. Genetic control of many of these traits through breeding appears feasible. Several recent reviews have covered methods for screening root traits but an appreciation for the complexity of root systems (e.g., functional differences between fine and coarse roots) needs to be paired with these methods to successfully identify relevant traits for crop improvement. Screening of root traits at early stages in plant development can proxy traits at mature stages but verification is needed on a case by case basis that traits are linked to increased crop productivity under drought. Examples in lesquerella (Physaria) and rice (Oryza) show approaches to phenotyping of root traits and current understanding of root trait genetics for breeding.

Plasticity in relative growth rate after a reduction in nitrogen availability is related to root morphological and physiological responses

BACKGROUND AND AIMS

To maximize growth and fitness a plant must adjust its phenotype by an amount and speed that matches changes in nitrogen availability. To determine how plastic ontogenetic changes in root physiological and morphological traits interact and whether or not these responses are likely to maximize growth, ontogenetic changes in relative growth rate (RGR, proportional rate of change of plant dry mass), unit root rate (URR, rate of change of plant dry mass per unit root length or area), specific root length (SRL, root length per dry root mass), specific root area (SRA, root area per dry root mass), and other root traits before and after a decrease in nitrogen supply, were studied in ten herbaceous species.

METHODS

Plants of each species were grown in hydroponic culture under controlled conditions in a control treatment where the supply of nitrogen remained constant at 1 mm, and in a stress treatment where the nitrogen supply was abruptly reduced from 1 to 0·01 mm during the growth period.

KEY RESULTS AND CONCLUSIONS

In the treatment series the number of bifurcations per root area and per root length, specific root area (SRA) and length (SRL), areal (URR(area)) and length-based (URR(mass)) unit root rate and RGR decreased, and root tissue density increased relative to the control. Species having greater plasticity in the percentage decrease in SRA at the end of the experiment also had smaller reductions in RGR; plasticity in SRA is therefore adaptive. In contrast, species which showed a greater reduction in URR(area) and in the number of bifurcations per root area and per root length, showed stronger reductions in RGR; plasticity in URR(area) and in the number of bifurcations per root area and per root length is therefore not adaptive. The plastic responses observed in SRA, SRL and in root tissue density constitute a set of plastic adjustments that would lead to resource conservation in response nutrient stress.

Patterns and relationships of plant traits, community structural attributes, and eco-hydrological functions during a subtropical secondary succession in central Yunnan, Southwest China

Human-induced changes in land use lead to major changes in plant community composition and structure which have strong effects on eco-hydrological processes and functions. We here tested the hypothesis that changes in traits of living plants have resulted in changes in structural attributes of the community that influenced eco-hydrological functions by altering eco-hydrological processes. This was done in the context of a subtropical secondary forest suc?cession following land abandonment in Central Yunnan (Southwest China). During the succession, species with high specific leaf area (SLA), high leaf nitrogen concentration (LNC), high specific root length (SRL), and low leaf dry matter content (LDMC) were progressively replaced by species with the opposite characteristics. The obtained results of correlation analyses were as follows: (1) Correlations were significant between community-aggregated SLA, LNC, and the leaf area index (LAI). Significant correlations were detected between LAI, canopy interception and stemflow, and surface runoff and soil erosion. (2) Significant correlations were also found between community-aggregated SLA, LNC, LDMC, and accumulated litter biomass. High accumulated litter biomass strongly increases the maximum water-retaining capac?ity of litter. However, significant correlations were not found between the maximum water-retaining capacity of litter and surface runoff and soil erosion. (3) Correlations were significant between community-aggregated SLA, LNC, and fine root biomass. Fine root biomass was not significantly related to the maximum water-retaining capacity of the soil, but was significantly related to surface runoff and soil erosion. These results suggest that canopy characteristics play a more important role in control of runoff and soil erosion at the studied site. It follows that plant functional traits are closely linked with canopy characteristics, which should be used as a standard for selecting species in restoration and revegetation for water and soil conservation.

Influence of nutrient supply and water vapour pressure on root architecture of Douglas-fir and western hemlock seedlings

Root growth responses of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] and western hemlock (Tsuga heterophylla Raf. Sarg.) seedlings to three nutrient concentrations and two shoot vapour pressure deficits were measured. Both species gained dry mass at high and medium nutrient treatments throughout the experiment, but not at low nutrition. Low nutrition gave highest ratios of projected leaf surface area to total root length in both species. Douglas-fir geometry differed from that of hemlock, with longer interior link lengths, particularly at the lowest nutrition. Douglas-fir showed greater numbers of exterior-interior links than hemlock. More links were observed at medium and high nutrition than at low nutrition for both species. Exterior-interior links increased over time for the two highest nutrient treatments. Significant topological differences were observed between species, the lowest and two highest nutrient treatments, and high and low vapour pressure deficits. Both species showed herring-bone root architecture at the lowest nutrition. This architectural configuration became more pronounced in hemlock seedlings grown under higher vapour pressure deficits. Faster-growing Douglas-fir had a dichotomous architecture at medium and high nutrition that was not influenced by increased vapour pressure deficits. Douglas-fir topology appears to be adapted to exploit soil nutrient patches while hemlock appears to rely on efficient exploitation of soil volume.

Dynamic phenotypic plasticity for root growth in Polygonum: a comparative study

Species differences in patterns of phenotypic plasticity may be an important aspect of adaptive diversity. Plasticity for functionally important root traits was studied in inbred field lineages of Polygonum persicaria and P. cespitosum (Polygonaceae). Replicate seedlings were grown in plexiglass rhizotrons under a range of constant and temporally variable moisture treatments. Plasticity was determined for final whole-plant biomass, root biomass allocation, and absolute and proportional root length. The dynamic aspect of root plasticity was examined by digitizing weekly tracings of the proportional deployment of each plant's root system to different vertical soil layers. Plants of both species expressed significant functionally adaptive phenotypic plasticity in the relative allocation, length, and vertical deployment of root systems in response to contrasting moisture conditions. Plasticity patterns in these closely related species were in general qualitatively similar, but for most traits differed in the magnitude and/or the timing of the plastic response. Dynamic changes in root deployment were more marked as well as faster in P. persicaria. Species differences in patterns of individual plasticity were generally consistent with the broader ecological distribution of P. persicaria in diverse as well as temporally variable moisture habitats.