Mineral Nutrition of Higher Plants, 2nd edition, H. Marschner. Academic Press, London (1995), 889, (ISBN 0-12-473543-6). Price: 29.95 Pound Sterling

Contents Summary 67 I. Introduction 68 II. Silicon transport in plants: to absorb or not to absorb 69 III. The role of silicon in plants: not just a matter of semantics 71 IV. Silicon and biotic stress: beyond mechanical barriers and defense priming 76 V. Silicon and abiotic stress: a proliferation of proposed mechanisms 78 VI. The apoplastic obstruction hypothesis: a working model 79 VII. Perspectives and conclusions 80 Acknowledgements 81 References 81 SUMMARY: Silicon (Si) is not classified as an essential plant nutrient, and yet numerous reports have shown its beneficial effects in a variety of species and environmental circumstances. This has created much confusion in the scientific community with respect to its biological roles. Here, we link molecular and phenotypic data to better classify Si transport, and critically summarize the current state of understanding of the roles of Si in higher plants. We argue that much of the empirical evidence, in particular that derived from recent functional genomics, is at odds with many of the mechanistic assertions surrounding Si's role. In essence, these data do not support reports that Si affects a wide range of molecular-genetic, biochemical and physiological processes. A major reinterpretation of Si's role is therefore needed, which is critical to guide future studies and inform agricultural practice. We propose a working model, which we term the 'apoplastic obstruction hypothesis', which attempts to unify the various observations on Si's beneficial influences on plant growth and yield. This model argues for a fundamental role of Si as an extracellular prophylactic agent against biotic and abiotic stresses (as opposed to an active cellular agent), with important cascading effects on plant form and function.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.15343

The controversies of silicon's role in plant biology.

Drought and salinity are the main abiotic stresses limiting crop yield and quality worldwide. Improving food production in drought- and salt-prone areas is the key to meet the increasing food demands in near future. It has been widely reported that silicon (Si), a second most abundant element in soil, could reduce drought and salt stress in plants. Here, we reviewed the emerging role of Si in enhancing drought and salt tolerance in plants and highlighted the mechanisms through which Si could alleviate both drought and salt stress in plants. Silicon application increased plant growth, biomass, photosynthetic pigments, straw and grain yield, and quality under either drought or salt stress. Under both salt and drought stress, the key mechanisms evoked are nutrient elements homeostasis, modification of gas exchange attributes, osmotic adjustment, regulating the synthesis of compatible solutes, stimulation of antioxidant enzymes, and gene expression in plants. In addition, Si application decreased Na+ uptake and translocation while increased K+ uptake and translocation under salt stress. However, these mechanisms vary with plant species, genotype, growth conditions, duration of stress imposed, and so on. This review article highlights the potential for improving plant resistance to drought and salt stress by Si application and provides a theoretical basis for application of Si in saline soils and arid and semiarid regions worldwide. This review article also highlights the future research needs about the role of Si under drought stress and in saline soils.

Mechanisms of silicon-mediated alleviation of drought and salt stress in plants: a review.

Light emitting diodes (LEDs) applied to microalgal production

Silicon (Si) can improve drought tolerance in plants, but the mechanism is still not fully understood. Previous research has been concentrating on Si’s role in leaf water maintenance in Si accumulators, while little information is available on its role in water uptake and in less Si-accumulating plants. Here, we investigated the effects of Si on root water uptake and its role in decreasing oxidative damage in relation to root hydraulic conductance in tomato (Solanum lycopersicum ‘Zhongza No.9’) under water stress. Tomato seedlings were subjected to water stress induced by 10% (w/v) polyethylene glycol-6000 in the absence or presence of 2.5 mM added silicate. The results showed that Si addition ameliorated the inhibition in tomato growth and photosynthesis, and improved water status under water stress. The root hydraulic conductance of tomato plants was decreased under water stress, and it was significantly increased by added Si. There was no significant contribution of osmotic adjustment in Si-enhanced root water uptake under water stress. The transcriptions of plasma membrane aquaporin genes were not obviously changed by Si under water stress. Water stress increased the production of reactive oxygen species and induced oxidative damage, while added Si reversed these. In addition, Si addition increased the activities of superoxide dismutase and catalase and the levels of ascorbic acid and glutathione in the roots under stress. It is concluded that Si enhances the water stress tolerance via enhancing root hydraulic conductance and water uptake in tomato plants. Si-mediated decrease in membrane oxidative damage may have contributed to the enhanced root hydraulic conductance.

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Silicon Enhances Water Stress Tolerance by Improving Root Hydraulic Conductance in Solanum lycopersicum L.

Chrysanthemum (Dendranthema grandiflorum Kitamura), a qualitative short-day plant, does not produce flowers when the night length is shorter than a cultivar-dependent critical value. Flowering is effectively inhibited when the required uninterrupted night period is broken by short periods of exposure to night interruption lights (NILs). The effects of the quality and positioning of the NIL on the morphogenesis, flowering, and photoperiodic gene expressions of chrysanthemum were investigated. Blue (NI-B), red (NI-R), far-red (NI-Fr), and white (NI-W) light-emitting diodes (LEDs) at an intensity of 10 μmol m−2 s−1 photosynthetic photon flux density (PPFD) were used to interrupt the dark period or plants grown in a closed walk-in growth chamber at night for a total of 4 h. The NILs were positioned to illuminate either the shoot tip, youngest mature leaf (vigorously growing third leaf from the shoot tip), or old leaf (third leaf from the stem base). The NI-R on the shoot tip and all NI-Fr treatments increased the plant extension growth, to a similar height observed in plants in long-day (LD) treatments. Extension growth was generally suppressed for plants in the NI-B, and plant height was the smallest when NI-B was shed onto the old leaf. Plants in all NIL treatments except NI-Fr showed a negative correlation between the plant height and the age of the leaf (or tissue) exposed to the NIL. Flowering was observed for plants in all treatments except for NI-R shed onto the shoot tip. The transcriptional factor PHYA was slightly expressed in plants which showed a flowering response. However, the CRY1, PHYB, and TFL was not expressed to levels reported previously, and the reason for low expression of these genes in these plants may be due to a combined effect of these genes with other flowering promoting and/or inhibiting genes. These results suggest that both the NIL quality and the age of the leaf (or tissue) exposed to the NIL affect the morphogenesis, flowering, and expression of transcriptional factors. Moreover, the previously known effects of the NIL quality on the morphogenesis as well as the vegetative to reproductive phase transition of plants were the most pronounced when the NIL irradiated the shoot tip, and the least pronounced when the NIL was shed onto the old leaf. Therefore, it seems that not only the appropriate quality, but also the appropriate positioning of the NIL, is important for optimizing flowering and plant extension growth.

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Both the Quality and Positioning of the Night Interruption Light are Important for Flowering and Plant Extension Growth

We investigated the effects of the quality of light used for interrupting the night period, termed the night interruption light (NIL), on morphogenesis, flowering, and the expression of photoperiodic genes in chrysanthemum (Dendranthema grandiflorum) cultivar ‘Gaya Yellow’, a qualitative short-day (SD) plant. Plants were raised in a closed-type plant factory under white (W) light-emitting diodes (LEDs) providing a light intensity of 180 μmol m−2 s−1 photosynthetic photon flux density, under a condition of long-day (LD, 16 h light/8 h dark), short-day (SD, 10 h light/14 h dark), or SD with a 4-h night interruption (NI) provided by 10 μmol m−2 s−1 PPF green (NI-G), blue (NI-B), red (NI-R), far-red (NI-Fr), or W (NI-W) LEDs. Plants grown in the LD condition were the tallest. The SD, NI-B, and NI-Fr conditions induced flowering. Phytochrome A (phyA) and cryptochrome 1 (cry1) were expressed at high levels in plants in NI-B, NI-Fr, and SD conditions. These results suggest that the NIL quality has significant implications on morphogenesis, flowering, and the expression of photoperiodic genes. Flowering was positively affected by the expression of phyA, cry1, and FLOWERING LOCUS T (FT), whereas it was negatively affected by the expression of phyB and anti-florigenic FT/TFL1 (AFT).

Night interruption light quality changes morphogenesis, flowering, and gene expression in Dendranthema grandiflorum

The current study has determined the effect of red and blue lights on the enhancement of growth, antioxidant property, phytochemical contents, and expression of proteins in Scrophularia kakudensis. In vitro-grown shoot tip explants of S. kakudensis were cultured on the plant growth regulator-free Murashige and Skoog (MS) medium and cultured under the conventional cool white fluorescent lamp (control), blue light-emitting diodes (LED) light, or red LED light. After 4 weeks, growth, stomatal ultrastructure, total phenols and flavonoids, activities of antioxidant enzymes, and protein expressions were determined. Interestingly, blue or red LED treatment increased the shoot length, shoot diameter, root length, and biomass on comparison with the control. In addition, the LED treatments enhanced the contents of phytochemicals in the extracts. The red LED treatment significantly elicited the accumulation of flavonoids in comparison with the control. In accordance with the secondary metabolites, the LED treatments modulated the activities of antioxidant enzymes. Moreover, the proteomic insights using two-dimensional gel electrophoresis system revealed the proteins involved in transcription and translation, carbohydrate mechanism, post-translational modification, and stress responses. Taken together, the incorporation of blue or red LED during in vitro propagation of S. kakudensis can be a beneficial way to increase the plant quality and medicinal values of S. kakudensis.

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Physiological and Proteomic Insights Into Red and Blue Light-Mediated Enhancement of in vitro Growth in Scrophularia kakudensis-A Potential Medicinal Plant.

We examined the effect of source, concentration, and application method of silicon (Si) on the growth, development, and photosynthetic capacity of Fragaria × ananassa ‘Maehyang’ and ‘Seolhyang’. We applied 0, 35, or 70 mg L−1 Si in a potassium silicate (K2SiO3), sodium silicate (Na2SiO3), or calcium silicate (CaSiO3) solution to plants via subirrigational supply or foliar application. Plant height of ‘Maehyang’ was highest with the 70 mg L−1 Si Na2SiO3 foliar application, but it was not significantly different among treatments in ‘Seolhyang’. Crown size was not significantly affected by source, concentration, or application method in both cultivars. Elemental concentrations in the shoot and root of ‘Maehyang’ were the highest in the 35 mg L−1 Si Na2SiO3 treatment for both application methods. Elemental concentrations in the shoot and root of ‘Seolhyang’ were the highest in the 70 mg L−1 Si K2SiO3 foliar application. Photosynthetic proteins abundantly increased in both cultivars with the 35 or 70 mg L−1 Si K2SiO3 treatment, for both application methods. Moreover, two important photosynthetic proteins, viz. PsaA and PsbA, were expressed and their expressions were higher with the 35 or 70 mg L−1 Si K2SiO3 treatment, for both application methods. These results suggested that 35 or 70 mg L−1 Si, supplied in the form of K2SiO3, promoted photosynthetic protein expressions the greatest, regardless of the application method, in both ‘Maehyang’ and ‘Seolhyang’.

Silicon application during vegetative propagation affects photosynthetic protein expression in strawberry

This study was conducted to examine the effect of light intensity and photoperiod of white LEDs as the artificial light source on the growth of leaf lettuce (Lactuca sativa L.) ‘Seonhong Jeokchukmyeon’ in a closed-type plant production system. Seedlings, transplanted at a density of 20 cm × 20 cm in a completely randomized design, were grown under white LEDs (FC Poibe Co. Ltd., Korea), at one of the 3 light intensities (100, 200, or 300 μmol · m−2 · s−1), and each with one of 3 photoperiods [12/12, 18/6, or 24/0 (Light/Dark)]. Plants were cultured for 22 days under the condition of 21 ± 2C, 60 ± 10% RH, and 400 ± 50 μmol · mol−1 CO2. The greatest leaf length and width, fresh and dry weights, and total anthocyanin content were obtained in the 24/0 photoperiod, regardless of the light intensity. Length of the longest root, fresh and dry weights, and number of leaves were greater in light intensity of 200 μmol · m−2 · s−1 than 100 or 300 μmol · m−2 · s−1. Chlorophyll value was the greatest in the photoperiod 12/12 than 18/6 or 24/0. The results obtained suggest that plant grew the best kept by light intensity at 200 or 300 μmol · m−2 · s−1, and photoperiod of 12/12 or 18/6. Additional key words : artificial light source, chlorophyll value, total anthocyanin content

https://www.koreascience.or.kr:443/article/JAKO201336448941860.pdf

Yoo Gyeong Park

Papers

Both the Quality and Positioning of the Night Interruption Light are Important for Flowering and Plant Extension Growth

Night interruption light quality changes morphogenesis, flowering, and gene expression in Dendranthema grandiflorum

Physiological and Proteomic Insights Into Red and Blue Light-Mediated Enhancement of in vitro Growth in Scrophularia kakudensis-A Potential Medicinal Plant.

Silicon application during vegetative propagation affects photosynthetic protein expression in strawberry

Growth of Lettuce in Closed-Type Plant Production System as Affected by Light Intensity and Photoperiod under Influence of White LED Light