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Jonathan M. Frantz, Sushant Khandekar and Scott Leisner

The use of copper (Cu) in agriculture is widespread as a pesticide, and it is present in high concentrations in certain types of manures. As the use of Cu continues and manure management is incorporated into sustainable systems, the likelihood of Cu toxicity increases. Supplemental silicon has been used to successfully counteract potential micronutrient toxicity. There is currently considerable debate regarding the value of including silicon (Si) as a nutrient in fertility programs and as such, it is not part of a typical management practice in floriculture crop production in the United States. We investigated the potential for Si to ameliorate the effects of Cu toxicity in both a Si-accumulating [zinnia (Zinnia elegans)] and a Si-non-accumulating [snapdragon (Antirrhinum majus)] species. Using visible stress indicators and dry weight analysis, it initially appeared that Si was a significant benefit to only zinnia under Cu toxicity. Enzymatic assays and elemental analysis of leaves, stems, and roots revealed that both species responded to supplemental Si, showing evidence of reduced stress and nutrient concentrations more similar to healthy, control plants than plants exposed to Cu toxicity. Although there appear to be differences in the extent of Si-mediated amelioration of Cu toxicity between these two plants, both responded to supplemental Si. This adds to the growing body of evidence that all plants likely have Si-mediated responses to stress, and its inclusion into fertility programs should be more broadly considered than current practices.

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Jie Li, Scott M. Leisner and Jonathan Frantz

Copper (Cu) is an essential micronutrient for plants and is the a.i. in pesticides for some pathogens and algae. Elevated doses of Cu can cause toxicity in plants. While silicon (Si) is reported to alleviate the toxicity of some heavy metals, its role in reducing the symptoms induced by excess Cu is unclear. Therefore, the role of Si in plant response to Cu stress was investigated in arabidopsis [Arabidopsis thaliana (L.) Heyn.]. Based on plant symptoms (a reduction of leaf chlorosis as well as increased shoot and root biomass) and a reduction of phenylalanine ammonia lyase [PAL (EC 4.3.1.5), a stress-induced enzyme] activity in the shoot, Si was found to alleviate copper stress. Real-time reverse transcriptase-polymerase chain reaction analyses indicated that the RNA levels of two arabidopsis copper transporter genes, copper transporter 1 (COPT1) and heavy metal ATPase subunit 5 (HMA5) were induced by high levels of Cu, but were significantly decreased when Si levels were also elevated. Taken together, our findings indicate that Si addition can improve the resistance of arabidopsis to Cu stress, and this improvement operates on multiple levels, ranging from physiological changes to alterations of gene expression.

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Jonathan Frantz, Dharmalingam Pitchay, David Tay, Jennifer Ehrenberger, John Gray, Scott Heckathorn and Scott Leisner

Zonal geranium (Pelargonium ×hortorum) and scented geranium (Pelargonium sp.) together are among the top-selling floriculture plants in the United States today, with several hundred cultivars and species available each year. With such diversity in appearances, growth habits, and developmental traits, we hypothesize a correspondingly wide range of nutritional uptake and partitioning characteristics. Mature leaves from 55 cultivars or breeding lines of zonal geranium and 60 species of Pelargonium sp. were sampled from paired plants twice throughout the year from the Ornamental Plant Germplasm Center. Their tissue was analyzed for essential macro- and micronutrients using ICP-OES. Generally, macronutrients varied by a factor of 3–5, whereas micronutrients varied by up to 100-fold. The micronutrient boron was a notable exception with variation across the samples of only about a factor of 5. With this information, attempts will be made to correlate tissue nutrient concentrations with genetic source (cultivars and breeding lines) and environmental conditions from the origin of the different Pelargonium species from South Africa. This work illustrates the challenges in associating specific sufficiency or deficiency values for nutrient concentrations in tissue of plants based on only a few cultivars, species, or locations.