Alejandra Zúñiga-Feest

Phosphorus-mobilization ecosystem engineering: the roles of cluster roots and carboxylate exudation in young P-limited ecosystems

BACKGROUND Carboxylate-releasing cluster roots of Proteaceae play a key role in acquiring phosphorus (P) from ancient nutrient-impoverished soils in Australia. However, cluster roots are also found in Proteaceae on young, P-rich soils in Chile where they allow P acquisition from soils that strongly sorb P. SCOPE Unlike Proteaceae in Australia that tend to proficiently remobilize P from senescent leaves, Chilean Proteaceae produce leaf litter rich in P. Consequently, they may act as ecosystem engineers, providing P for plants without specialized roots to access sorbed P. We propose a similar ecosystem-engineering role for species that release large amounts of carboxylates in other relatively young, strongly P-sorbing substrates, e.g. young acidic volcanic deposits and calcareous dunes. Many of these species also fix atmospheric nitrogen and release nutrient-rich litter, but their role as ecosystem engineers is commonly ascribed only to their diazotrophic nature. CONCLUSIONS We propose that the P-mobilizing capacity of Proteaceae on young soils, which contain an abundance of P, but where P is poorly available, in combination with inefficient nutrient remobilization from senescing leaves allows these species to function as ecosystem engineers. We suggest that diazotrophic species that colonize young soils with strong P-sorption potential should be considered for their positive effect on P availability, as well as their widely accepted role in nitrogen fixation. Their P-mobilizing activity possibly also enhances their nitrogen-fixing capacity. These diazotrophic species may therefore facilitate the establishment and growth of species with less-efficient P-uptake strategies on more-developed soils with low P availability through similar mechanisms. We argue that the significance of cluster roots and high carboxylate exudation in the development of young ecosystems is probably far more important than has been envisaged thus far.

Ecotypic Differentiation in Morphology and Cold Resistance in Populations of Colobanthus quitensis (Caryophyllaceae) from the Andes of Central Chile and the Maritime Antarctic

Abstract Colobanthus quitensis is one of only two vascular plant species that occur in the Antarctic islands. We evaluated morphological and physiological traits and survival after freezing in individuals of C. quitensis collected in two populations: one from the Andes of central Chile and the other from the maritime Antarctic. We addressed whether these populations are ecotypes in terms of shoot morphology and physiology, and cold resistance. We also evaluated genetic differentiation at the internal transcribed spacer (ITS) regions. Under controlled growing conditions, Andean plants had longer and narrower leaves and longer peduncles. However, shoot biomass was similar in both populations. There was a 1.17% sequence divergence of ITS regions between populations. Survival to freezing at temperatures from 0° to −16°C was not different in nonacclimated plants. After cold-acclimation at 4°C, freezing tolerance was greater in Antarctic plants. Both Andean and Antarctic plants increased soluble sugars with increasing acclimation time, but it did not differ between populations. Shoot water content decreased with acclimation time in both populations, but Antarctic plants maintained slightly higher water contents after cold exposure. Shoot growth did not differ during acclimation. The studied populations of C. quitensis represent different morphological and cold resistant ecotypes despite their genetic similarity. However, the mechanistic basis of the higher survival to freezing of Antarctic plants was not elucidated.

Light regulation of sucrose-phosphate synthase activity in the freezing-tolerant grass Deschampsia antarctica

Deschampsia antarctica, a freezing-tolerant grass that has colonized the Maritime Antarctic, has an unusually high content of sucrose (Suc) in leaves, reaching up to 36% of dry weight. Suc accumulation has often been linked with increased activity of sucrose phosphate synthase (SPS; EC: 2.4.1.1.14). SPS, a key enzyme in sucrose biosynthesis, is controlled by an intricate hierarchy of regulatory mechanisms including allosteric modulators, reversible covalent modification in response to illumination, and transcriptional regulation. We hypothesized that during long day conditions in the Antarctic summer D. antarctica can maintain high SPS activity longer by indirect light regulation, thereby leading to a high sucrose accumulation in the leaves. The objectives of this study were to investigate a possible indirect light regulation of SPS activity and the effect of cold and day length on transcriptional and protein level of SPS in D. antarctica. Although SPS activity did not display an endogenous rhythm of activity in continuous light, activation of SPS at the end of the dark period was observed in D. antarctica. This activation of SPS is possibly controlled by covalent modification, because it was inhibited by okadaic acid while the SPS protein level did not significantly change. The highest SPS activity increase was observed after 21 days of cold-acclimation under long day conditions. This increased activity was not related to an increase in SPS gene expression or protein content. High SPS activity in cold long days leading to hyper accumulation of Suc appears to be among the features that permit D. antarctica to survive in the harsh Antarctic conditions.

Divergent functioning of Proteaceae species: the South American Embothrium coccineum displays a combination of adaptive traits to survive in high‐phosphorus soils

Summary 1. Proteaceae species in south-western Australia thrive on phosphorus-impoverished soils, employing a phosphorus-mining strategy involving carboxylate-releasing cluster roots. Some develop symptoms of phosphorus toxicity at slightly elevated soil phosphorus concentrations, due to their low capacity to down-regulate phosphorus uptake. In contrast, Proteaceae species in Chile, e.g. Embothrium coccineum J.R. Forst. & G. Forst., occur on volcanic soils, which contain high levels of total phosphorus, but phosphorus availability is low. 2. We hypothesised that the functioning of cluster roots of E. coccineum differs from that of south-western Australian Proteaceae species, in accordance with the difference in soil phosphorus status. With more phosphorus to be gained from the soil with high levels of total phosphorus, we expect less investment in biomass and more release of carboxylates. Furthermore, we hypothesised that E. coccineum regulates its phosphorus-uptake capacity, avoiding phosphorus toxicity when grown at elevated phosphorus levels. To test these hypotheses, E. coccineum seedlings were grown at a range of phosphorus supplies in nutrient solution. 3. We show that E. coccineum allocated at least five times less biomass to cluster roots that released at least nine times more carboxylates per unit cluster root weight compared with south-western Australian species (e.g. Banksia, Hakea). The highest phosphorus supply caused a growth inhibition and high leaf phosphorus concentration, without symptoms of phosphorus toxicity. We accept our hypotheses on the functioning of cluster roots and the high capacity to reduce the net phosphorus uptake in plants grown at a high-phosphorus supply. 4. This novel combination of traits indicates divergent functioning of Proteaceae species from southern South America, exposed to frequent phosphorus input due to volcanic activity, in contrast with the functioning of south-western Australian Proteaceae species that thrive on severely phosphorus-impoverished soils. These traits could explain the functioning of E. coccineum on soils that are rich in total phosphorus, but with a low concentration of available phosphorus.

Sugars and enzyme activity in the grass Deschampsia antarctica

Deschampsia antarctica is a freezing-tolerant plant and the only native Poaceae that grows in the Maritime Antarctic. During the long days of the growing season this plant accumulates sucrose (Suc) in the leaves to 36% of the dry weight. The mechanism that leads to this high accumulation is unknown. The effect of day length and low temperature on sucrose phosphate synthase (SPS) (EC: 2.4.1.14) activity and sugar accumulation was studied in D. antarctica and compared with other Poaceae. Three different day lengths: short (SD) (8/16 h), medium (MD) (16/8 h) and long (LD) (21/3 h); and two temperatures: 4°C (cold-acclimated) and 15°C (non-acclimated) were tested. The highest contents in total soluble sugars (TSS) and Suc were reached in crowns and leaves, respectively, in cold-acclimated plants under LD. TSS and Suc contents and SPS activity with cold acclimation were higher in D. antarctica than in other agricultural (wheat, oat and barley) and non-agricultural (D. caespitosa and D. beringensis) Poaceae species. Suc/TSS ratio was higher in all Deschampsia species than in agricultural Poaceae species. SPS activity and sucrose content in leaves were positively correlated only in LD cold acclimated plants. This result shows that SPS activity is responsive to day length in D. antarctica.

The effect of phosphorus on growth and cluster-root formation in the Chilean Proteaceae: Embothrium coccineum (R. et J. Forst.).

One of the main factors that favours the formation of cluster roots is a low supply of phosphorus (P). The soils of southern Chile are mainly formed from volcanic ash, characterized by low levels of available P. Embothrium coccineum, a Chilean Proteaceae species produces cluster roots (CR). The factors that control CR formation in Chilean Proteaceae have not been extensively studied. The objective of this work was to assess the effects of P on the growth and cluster-root formation of E. coccineum. Plants were produced from seeds collected at two different locations: Valdivia and Pichicolo both at 39ºS. They were cultured under similar greenhouse conditions, from June to September, watered twice a week using: distilled water (W), full strength Hoagland’s nutrient solution (H) or Hoagland without P (H-P). At the end of the experiment, height, total dry biomass, number of cluster roots (CR) per plant, CR /total root weight, were measured. Also acid exudation of CR was assayed using bromocresol purple on sterile agar plates. Treatments significantly affected growth and proportion of CR, the highest growth was observed with H. Under all treatments plants produced a similar number of CR. However, the proportion of CR biomass was higher with W and H-P than with H. Plants under W exhibited the lowest growth and low shoot/root ratio. Acid exudation of CR was not detectable in our experiment. These results are discussed comparing CR formation in low P conditions on Lupinus albus and other Proteaceae species, and the possible role of CR formation in E. coccineum considering its wide geographical distribution.

Low temperature regulates sucrose-phosphate synthase activity in Colobanthus quitensis (Kunth) Bartl. by decreasing its sensitivity to Pi and increased activation by glucose-6-phosphate

Colobanthus quitensis (Kunth) Bartl. is widely distributed from Mexico to the Antarctic. C. quitensis is a freezing resistant species that accumulates sucrose in response to cold. We tested the hypothesis that low temperature modifies the kinetic properties of C. quitensis sucrose phosphate synthase (SPS) to increase its activity and ability to synthesize sucrose during cold acclimation. Cold acclimation caused a fourfold increment in sucrose concentration and a 100% increase in SPS activity, without changes in the level of SPS protein. Cold acclimation did not affect the optimal temperature and pH for SPS activity. However, it caused a tenfold increase in the inhibition constant (Ki) for inorganic phosphate (Pi) calculated as a function of fructose-6-phosphate (Fruc-6-P). SPS from cold acclimated plants also exhibited a higher reduction of its Michaelis constant (Km) for glucose-6-phosphate (Gluc-6-P) with respect to non-acclimated plants. We suggest that the increase in C. quitensis SPS Ki for Pi and the increase in activation by Gluc-6-P in response to cold keep SPS activated, leading to high sucrose accumulation. This may be an important adaptation that allows efficient accumulation of sucrose during the harsh Antarctic summer.

Soil nitrogen, and not phosphorus, promotes cluster-root formation in a South American Proteaceae, Embothrium coccineum

PREMISE OF THE STUDY Cluster roots are a characteristic root adaptation of Proteaceae species. In South African and Australian species, cluster roots promote phosphorus (P) acquisition from poor soils. In a South American Proteaceae species, where cluster roots have been scarcely studied and their function is unknown, we tested whether cluster-root formation is stimulated by low soil nutrition, in particular low P-availability. METHODS Small and large seedlings (< 6- and > 6-months old, respectively) of Embothrium coccineum and soil were collected across four different sites in Patagonia (Chile). We determined cluster-root number and relative mass, and leaf Pi concentration per mass (Pimass) and per area (Piarea) for each seedling, and tested relationships with Olsen-P (OP), sorbed-P (sP) and total nitrogen (N) using generalized linear mixed-effects models and model selection to assess the relative strength of soil and plant drivers. KEY RESULTS Best-fit models showed a negative logarithmic relationship between cluster-root number and soil nitrogen (N), and between cluster-root relative mass and both leaf Piarea and soil N, and a positive logarithmic relationship between cluster-root number and leaf Piarea. Cluster-root relative mass was higher in small than in large seedlings. CONCLUSIONS Contrary to that found in South African and Australian Proteaceae, cluster roots of E. coccineum do not appear to be driven by soil P, but rather by soil N and leaf Piarea. We suggest that cluster roots are a constitutive and functional trait that allows plants to prevail in poor N soils.

Responses of two températe evergreen Nothofagus species to sudden and gradual waterlogging: relationships with distribution patterns

Los efectos del anegamiento gradual sobre las especies arboreas han sido poco estudiados. Nothofagus nitida y N. dombeyi son especies siempreverdes templadas diferencialmente distribuidas sobre gradientes de humedad del suelo; solo la primera es comun en sitios de drenaje pobre. Comparamos la tasa de crecimiento relativo (RGRH) y la perdida foliar de plantulas de dos a tres anos sujetas experimentalmente durante dos meses a drenaje normal (humedad del suelo en capacidad de campo), anegamiento repentino y anegamiento gradual, para determinar que regimen de anegamiento predice mejor diferencias interespecificas en tolerancia, las cuales son sugeridas desde las distribuciones naturales. RGRH resulto similar entre las especies pero difirio entre tratamientos (controles > anegamiento gradual = anegamiento repentino). El anegamiento repentino causo una masiva perdida foliar en ambas especies, pero el anegamiento gradual causo mayor perdida en N. dombeyi que en N. nitida indicando cierto grado de aclimatacion por parte de esta ultima. Regresiones lineares indicaron que RGRH fue negativamente afectado por la perdida foliar en ambas especies pero mas en N. dombeyi, sugiriendo que otro mecanismo causo el decrecimiento de RGRH en N. nitida. Para evaluar el efecto del anegamiento sobre el desempeno en el largo plazo, se realizaron trasplantes reciprocos en la cordillera de la Costa entre un sitio de drenaje limitado dominado por N. nitida y sin N. dombeyi, y un sitio sin limitaciones de drenaje dominado por N. dombeyi y sin N. nitida. Luego de dos temporadas de crecimiento, N. dombeyi presento menores area foliar especifica (SLA) y RGRH en el sitio de drenaje limitado que en su sitio original. La sobrevivencia en el sitio de drenaje pobre fue 100 % para N. nitida y 73,5 % para N. dombeyi. Reducidos crecimiento y sobrevivencia de N. dombeyi asociados a los efectos negativos de masiva perdida foliar y reducida SLA podrian excluir a esta esp

Physiological and morphological responses to permanent and intermittent waterlogging in seedlings of four evergreen trees of temperate swamp forests

Waterlogging decreases a plants metabolism, stomatal conductance (gs) and photosynthetic rate (A); however, some evergreen species show acclimation to waterlogging. By studying both the physiological and morphological responses to waterlogging, the objective of this study was to assess the acclimation capacity of four swamp forest species that reside in different microhabitats. We proposed that species (Luma apiculata [D.C.] Burret. and Drimys winteri J.R. et G. Forster.) abundant in seasonally and intermittently waterlogged areas (SIWA) would have a higher acclimation capacity than species abundant in the inner swamp (Blepharocalyx cruckshanksii [H et A.] Mied. and Myrceugenia exsucca [D.C.] Berg.) where permanent waterlogging occurs (PWA); it was expected that the species from SIWA would maintain leaf expansion and gas exchange rates during intermittent waterlogging treatments. Conversely, we expected that PWA species would have higher constitutive waterlogging tolerance, and this would be reflected in the formation of lenticels and adventitious roots. Over the course of 2 months, we subjected seedlings to different waterlogging treatments: (i) permanent (sudden, SW), (ii) intermittent (gradual) or (iii) control (field capacity, C). Survival after waterlogging was high (≥80%) for all species and treatments, and only the growth rate of D. winteri subjected to SW was affected. Drimys winteri plants had low, but constant A and g during both waterlogging treatments. Conversely, L. apiculata had the highest A and g values, and g increased significantly during the first several days of waterlogging. In general, seedlings of all species subjected to waterlogging produced more adventitious roots and fully expanded leaves and had higher specific leaf area (SLA) and stomatal density (StD) than seedlings in the C treatment. From the results gathered here, we partially accept our hypothesis as all species showed high tolerance to waterlogging, maintained growth, and had increased A or g during different time points of waterlogging. Differences in leaf (SLA) and stomata functioning (gs, StD) plasticity likely allows plants to maintain positive carbon gains when waterlogging occurs. The species-specific differences found here were not entirely related to microhabitat distribution.