Comparing the impact of microplastics derived from a biodegradable and a conventional plastic mulch on plant performance.

Agricultural lands have been identified as plastic sinks. One source is plastic mulches, which are a source of micro- and nano-sized plastics in agricultural soils. Because of their persistence, there is now a push towards developing biodegradable plastics, which are designed to undergo (partial) breakdown after entering the environment. Yet, limited research has investigated the impacts of both conventional and biodegradable plastics on distinct plants. Moreover, comparisons among studies are difficult due to differences in experimental design. This study directly compares the effects of artificially weathered conventional polyethylene (PE) and starch-based biodegradable polybutylene adipate terephthalate (PBAT) on four food crops, including two monocots (barley, Hordeum vulgare, and wheat, Triticum aestivum L.) and two dicots (carrot, Daucus carota, and lettuce, Lactuca sativa L.). We investigated the effects of environmentally relevant low, medium, and high (0.01 %, 0.1 %, 1 % w/w) concentrations of PE and starch-PBAT blend on seed germination (acute toxicity), and subsequently on plant growth and chlorophyll through a pot-plant experiment (chronic toxicity). Germination of all species was not affected by both plastics. However, root length was reduced for lettuce and wheat seedlings. No other effects were recorded on monocots. We observed a reduction in shoot length and bud wet weight of carrot seedlings for the highest concentration of PE and starch-PBAT blend. Chronic exposure resulted in a significant decrease in shoot biomass of barley and lettuce. Additionally, a positive increase in the number of leaves of lettuce was observed for both plastics. Chlorophyll content was increased in lettuce when exposed to PE and starch-PBAT blend. Overall, adverse effects in dicots were more abundant than in monocots. Importantly, we found that the biodegradable plastic caused more commonly adverse effects on plants compared to conventional plastic, which was confirmed by a mini-review of studies directly comparing the impact of conventional and biodegradable microplastics.

Species-dependent responses of crop plants to polystyrene microplastics.

Only recently there has been a strong focus on the impacts of microplastics on terrestrial crop plants. This study aims to examine and compare the effects of microplastics on two monocotyledonous (barley, Hordeum vulgare and wheat, Triticum aestivum), and two dicotyledonous (carrot, Daucus carota and lettuce, Lactuca sativa) plant species through two complimentary experiments. First, we investigated the effects of low, medium, and high (103, 105, 107 particles per mL) concentrations of 500 nm polystyrene microplastics (PS-MPs) on seed germination and early development. We found species-dependent effects on the early development, with microplastics only significantly affecting lettuce and carrot. When acutely exposed during germination, PS-MPs significantly delayed the germination of lettuce by 24%, as well as promoted the shoot growth of carrot by 71% and decreased its biomass by 26%. No effect was recorded on monocot species. Secondly, we performed a chronic (21 d) hydroponic experiment on lettuce and wheat. We observed that PS-MPs significantly reduced the shoot growth of lettuce by up to 35% and increased its biomass by up to 64%, while no record was reported on wheat. In addition, stress level indicators and defence mechanisms were significantly up-regulated in both lettuce and wheat seedlings. Overall, this study shows that PS-MPs affect plant development: impacts were recorded on both germination and growth for dicots, and responses identified by biochemical markers of stress were increased in both lettuce and wheat. This highlights species-dependent effects as the four crops were grown under identical conditions to allow direct comparison. For future research, our study emphasizes the need to focus on crop specific effects, while also working towards knowledge of plastic-induced impacts at environmentally relevant conditions.

Early root growth and architecture of fast- and slow-growing Norway spruce (Picea abies) families differ-potential for functional adaptation.

The relationship between the growth rate of aboveground parts of trees and fine root development is largely unknown. We investigated the early root development of fast- and slow-growing Norway spruce (Picea abies (L.) H. Karst.) families at a developmental stage when the difference in size is not yet observed. Seedling root architecture data, describing root branching, were collected with the WinRHIZO™ image analysis system, and mixed models were used to determine possible differences between the two growth phenotypes. A new approach was used to investigate the spatial extent of root properties along the whole sample root from the base of 1-year-old seedlings to the most distal part of a root. The root architecture of seedlings representing fast-growing phenotypes showed ~30% higher numbers of root branches and tips, which resulted in larger root extensions and potentially a better ability to acquire nutrients. Seedlings of fast-growing phenotypes oriented and allocated root tips and biomass further away from the base of the seedling than those growing slowly, a possible advantage in nutrient-limited and heterogeneous boreal forest soils. We conclude that a higher long-term growth rate of the aboveground parts in Norway spruce may relate to greater allocation of resources to explorative roots that confers a competitive edge during early growth phases in forest ecosystems.

The community of needle endophytes reflects the current physiological state of Norway spruce.

This study investigated fungal endophytes in the needles of Norway spruce (Picea abies) cuttings in relation to host tree growth. We also determined the prevalence of endophytes in needles incubated for six months. The cuttings originated from clonal origins showing slow- and fast-growth in long-term field trials but the heritable differences in growth rate were not yet detected among the studied cutting. Endophytes were isolated from surface-sterilized needles with culture-free DNA techniques. No significant differences were observed between endophyte communities of slow- and fast-growing clonal origins. However, the endophyte community correlated with the current growth rate of cuttings suggesting that endophytes reflect short- rather than long-term performance of a host. The concentration of condensed tannins was similar in slow- and fast-growing clonal origins but it showed a negative relationship with endophyte species richness, implying that these secondary compounds may play an important role in spruce tolerance against fungal infections. More than a third of endophyte species were detected in both fresh and decomposing needles, indicating that many needle endophytes are facultative saprotrophs. Several potentially pathogenic fungal species were also found within the community of saprotrophic endophytes.

Profiling functions of ectomycorrhizal diversity and root structuring in seedlings of Norway spruce (Picea abies) with fast- and slow-growing phenotypes.

We studied the role of taxonomical and functional ectomycorrhizal (ECM) fungal diversity in root formation and nutrient uptake by Norway spruce (Picea abies) seedlings with fast- and slow-growing phenotypes. Seedlings were grown with an increasing ECM fungal diversity gradient from one to four species and sampled before aboveground growth differences between the two phenotypes were apparent. ECM fungal colonization patterns were determined and functional diversity was assayed via measurements of potential enzyme activities of eight exoenzymes probably involved in nutrient mobilization. Phenotypes did not vary in their receptiveness to different ECM fungal species. However, seedlings of slow-growing phenotypes had higher fine-root density and thus more condensed root systems than fast-growing seedlings, but the potential enzyme activities of ectomycorrhizas did not differ qualitatively or quantitatively. ECM species richness increased host nutrient acquisition potential by diversifying the exoenzyme palette. Needle nitrogen content correlated positively with high chitinase activity of ectomycorrhizas. Rather than fast- and slow-growing phenotypes exhibiting differing receptiveness to ECM fungi, our results suggest that distinctions in fine-root structuring and in the belowground growth strategy already apparent at early stages of seedling development may explain later growth differences between fast- and slow-growing families.

Endophyte communities vary in the needles of Norway spruce clones.

Endophytic fungi show no symptoms of their presence but can influence the performance and vitality of host trees. The potential use of endophytes to indicate vitality has been previously realized, but a standard protocol has yet to be developed due to an incomplete understanding of the factors that regulate endophyte communities. Using a culture-free molecular approach, we examined the extent to which host genotype influences the abundance, species richness, and community composition of endophytic fungi in Norway spruce needles. Briefly, total DNA was extracted from the surface-sterilized needles of 30 clones grown in a nursery field and the copy number of the fungal internal transcribed spacer (ITS) region of ribosomal DNA was estimated by quantitative PCR. Fungal species richness and community composition were determined by denaturing gradient gel electrophoresis and DNA sequencing. We found that community structure and ITS copy number varied among spruce clones, whereas species richness did not. Host traits interacting with endophyte communities included needle surface area and the location of cuttings in the experimental area. Although Lophodermium piceae is considered the dominant needle endophyte of Norway spruce, we detected this species in only 33% of samples. The most frequently observed fungus (66%) was the potentially pathogenic Phoma herbarum. Interestingly, ITS copy number of endophytic fungi correlated negatively with the richness of ectomycorrhizal fungi and thus potential interactions between fungal communities and their influence on the host tree are discussed. Our results suggest that in addition to environmental factors, endophyte communities of spruce needles are determined by host tree identity and needle surface area.