Differential response to copper stress in the reproductive resources and allocation of metallophyte Kummerowia stipulacea

Long period exposure to serious cadmium pollution benefits an invasive plant (Alternanthera philoxeroides) competing with its native congener (Alternanthera sessilis)

Many aggressive plants possess high tolerance to heavy metals, but little is known about their invasiveness at heavy metal polluted sites. We performed a greenhouse experiment to examine the impacts of Cd (0, 10, 30, 60, and 100 mg kg^-1) and inter-specific competition on the reproductive capability of an invasive plant, Alternanthera philoxeroides, and its native congener, Alternanthera sessilis. We also examined the population dynamics of both native and invasive species in a simulated field experiment. Compared with A. philoxeroides, native A. sessilis was a stronger competitor as measured by vegetative growth, sexual reproduction, and dominance status in a mixed culture. However, A. philoxeroides showed great plasticity in root mass ratio that was positively affected by inter-specific competition and high Cd levels. Such high root allocation might allow for delayed growth of A. philoxeroides rhizomes as the relative cover of A. philoxeroides to A. sessilis in the field experiment gradually increased and > 1 after nine months culture, especially at high Cd treatment. Our results suggest that the invasiveness of A. philoxeroides is highly context- and time-dependent. In severely polluted environments, clonal propagation of A. sessilis is likely inhibited by the synergistic negative effects of inter-specific competition and heavy metal pollution, and a long time co-existence of these two competing species would facilitate the colonization of invasive plant.

Effects of manganese stress on phenology and biomass allocation in Xanthium strumarium from metalliferous and non-metalliferous sites

Xanthium strumarium is an annual pseudometallophyte. To reveal the mechanisms of this species to adapt to metallicolous environmental conditions, phenological traits and biomass allocation of metallicolous and non-metallicolous populations of X. strumarium under six Mn²⁺ concentrations by pot culture experiments were performed. The results showed that both time to bolting and time to fruit setting in the metallicolous population were earlier than those in the non-metallicolous population. The number of flowers, fruits, seeds and 1000-seed weight in the metallicolous population were higher than those in the non-metallicolous population under Mn stress. Reproductive allocation and harvest index in the metallicolous population were higher than those in the non-metallicolous population. Furthermore, all the Mn concentrations in leaves, stems, roots, and fruits of the metallicolous population were higher than the counterparts of non-metallicolous population. These results suggested that metallicolous population had higher tolerance to Mn stress than non-metallicolous population, the earlier flowering and fruiting, and the enhancement in reproductive allocation may contribute to plant tolerance to Mn toxicity for X. strumarium.

Divergent biology of facultative heavy metal plants

Among heavy metal plants (the metallophytes), facultative species can live both in soils contaminated by an excess of heavy metals and in non-affected sites. In contrast, obligate metallophytes are restricted to polluted areas. Metallophytes offer a fascinating biology, due to the fact that species have developed different strategies to cope with the adverse conditions of heavy metal soils. The literature distinguishes between hyperaccumulating, accumulating, tolerant and excluding metallophytes, but the borderline between these categories is blurred. Due to the fact that heavy metal soils are dry, nutrient limited and are not uniform but have a patchy distribution in many instances, drought-tolerant or low nutrient demanding species are often regarded as metallophytes in the literature. In only a few cases, the concentrations of heavy metals in soils are so toxic that only a few specifically adapted plants, the genuine metallophytes, can cope with these adverse soil conditions. Current molecular biological studies focus on the genetically amenable and hyperaccumulating Arabidopsis halleri and Noccaea (Thlaspi) caerulescens of the Brassicaceae. Armeria maritima ssp. halleri utilizes glands for the excretion of heavy metals and is, therefore, a heavy metal excluder. The two endemic zinc violets of Western Europe, Viola lutea ssp. calaminaria of the Aachen-Liège area and Viola lutea ssp. westfalica of the Pb-Cu-ditch of Blankenrode, Eastern Westphalia, as well as Viola tricolor ecotypes of Eastern Europe, keep their cells free of excess heavy metals by arbuscular mycorrhizal fungi which bind heavy metals. The Caryophyllaceae, Silene vulgaris f. humilis and Minuartia verna, apparently discard leaves when overloaded with heavy metals. All Central European metallophytes have close relatives that grow in areas outside of heavy metal soils, mainly in the Alps, and have, therefore, been considered as relicts of the glacial epoch in the past. However, the current literature favours the idea that hyperaccumulation of heavy metals serves plants as deterrent against attack by feeding animals (termed elemental defense hypothesis). The capability to hyperaccumulate heavy metals in A. halleri and N. caerulescens is achieved by duplications and alterations of the cis-regulatory properties of genes coding for heavy metal transporting/excreting proteins. Several metallophytes have developed ecotypes with a varying content of such heavy metal transporters as an adaption to the specific toxicity of a heavy metal site.

Copper-induced alteration in sucrose partitioning and its relationship to the root growth of two Elsholtzia haichowensis Sun populations

Hydroponic culture was used to comparatively investigate the copper (Cu)-induced alteration to sucrose metabolism and biomass allocation in two Elsholtzia haichowensis Sun populations with one from a Cu-contaminated site (CS) and the other from a non-contaminated site (NCS). Experimental results revealed that biomass allocation preferred roots over shoots in CS population, and shoots over roots in NCS population under Cu exposure. The difference in biomass allocation was correlated with the difference in sucrose partitioning between the two populations. Cu treatment (45 μM) significantly decreased leaf sucrose content and increased root sucrose content in CS population as a result of the increased activities of leaf sucrose synthesis enzymes (sucrose phosphate synthetase and sucrose synthase) and root sucrose cleavage enzyme (vacuolar invertase), which led to increased sucrose transport from leaves to roots. In contrast, higher Cu treatment increased sucrose content in leaves and decreased sucrose content in roots in NCS population as a result of the decreased activities of root sucrose cleavage enzymes (vacuolar and cell wall invertases) that led to less sucrose transport from leaves to roots. These results provide important insights into carbon resource partitioning and biomass allocation strategies in metallophytes and are beneficial for the implementation of phytoremediation techniques.

Interactive Effects of Arbuscular Mycorrhizal Fungi and Copper Stress on Flowering Phenology and Reproduction of Elsholtzia splendens

Plant responses to heavy metal contamination may depend on the presence of arbuscular mycorrhizal fungi (AMF). Elsholtzia splendens is an indicator species for the presence of copper (Cu) mines because both its flowering phenology and reproduction are tolerant to heavy metals. To test whether effects of Cu on the flowering phenology and reproduction of E. splendens depend on the presence of AMF, we conducted a factorial experiment with two Cu treatments (with or without Cu addition) crossed with two AMF treatments (with or without AMF inoculation). Without AMF, Cu addition significantly delayed the onset dates, ending dates and peak dates of flowering and decreased flowering duration. However, AMF inoculation reversed the effects of Cu stress, with recovered flowering onset and ending dates and increased the flowering duration. Cu addition significantly decreased inflorescence width and number, inflorescence biomass, vegetative biomass and total seed number, but significantly increased 1000-seed weight. AMF inoculation significantly increased vegetative biomass. Two-way ANOVA results showed that the interactive effects between Cu addition and AMF inoculation were significant on the inflorescence number, vegetative biomass and total seed number. These results indicate that AMF can alleviate the Cu stress on the flowering phenology and reproduction of E. splendens.