A plasma oscillation is a collective oscillation of charge density and its restoring electric field. In a three-dimensional bulk charged superfluid, long-range Coulomb forces give the longitudinal collective mode a nonzero long-wave plasma frequency. Different geometry and electromagnetic boundary conditions can change that limit.

With immobile ions, negligible collisions and electron density $n_e$, the plasma frequency is ${\omega }_p^2=n_e{e}^2/(m_e{ϵ}_0)$. Combining inertial electron current with vacuum displacement current gives ${\omega }^2={\omega }_p^2+c^2{k}^2$. Transverse fields are evanescent below ${\omega }_p$ and propagate above it.

A guard cell is one of the specialized epidermal biological cells bordering a stoma. Changes in solute content alter water uptake and turgor pressure, changing the size of the pore. This couples transpiration and entry of carbon dioxide for photosynthesis.

Calcium-dependent signaling and regulation of anion channels participate in closing responses. Experiments imposing different calcium patterns tested how intracellular dynamics influence stomatal movement, showing why a single concentration measurement may miss information carried by its timing.^[ref]

Photoperiodism is dependence of a biological response on the daily timing of light and darkness. In plants, flowering can depend on whether illumination coincides with an internally timed sensitive phase, as described by the external coincidence model.

Equal total hours of light need not produce equal flowering responses if the light occurs at different phases of the circadian clock. Light-dependent regulation of CONSTANS provides a molecular example connecting the timing of illumination to FLOWERING LOCUS T activity.^[ref]

A cryptochrome is a member of a light-sensitive protein family involved in biological timing and development. In plants, blue-light perception by cryptochromes contributes to responses including growth regulation and entrainment of the circadian clock.

A light input that resets the circadian clock can change the timing of a later response even after illumination ends. This distinguishes a clock-mediated effect from a response that simply follows the current light level; photoperiodism can depend on both.

NRT1.1, also called CHL1 or NPF6.3, is an Arabidopsis thaliana nitrate transporter with a nutrient-sensing role. Its nitrate-dependent effects on auxin transport connect external nitrogen availability to lateral-root development.

Changing transport or regulation can therefore alter root architecture as well as the amount of nutrient entering a biological cell. Experiments comparing nitrate conditions and auxin responses tested this connection, rather than inferring it solely from a growth correlation.^[ref]

A phototropin is a blue-light-responsive receptor with kinase activity that regulates responses including phototropism, chloroplast relocation and stomatal opening. Light perception changes intracellular regulation, which in turn alters movement or growth.

Mutants defective in the chloroplast high-light avoidance response identified a phototropin homologue required for that response. Comparing movement under strong and weak light helps separate avoidance from the different response of positioning chloroplasts to capture light.^[ref]

A flavonoid is a member of a broad family of plant metabolites sharing related carbon-ring structures. Members contribute to functions including pigmentation and chemical signaling; membership in the family does not assign one universal biological role.

In a legume-rhizobial interaction, selected root-exuded flavonoids act as signals that promote bacterial symbiotic responses. This is a useful example of signal transduction between different organisms, rather than simply a nutrient moving from one partner to another.

A host gene contributing to reduced disease; not all resistance genes encode NLR receptors.

A pattern recognition receptor detects characteristic molecular patterns associated with microbes or damaged tissue. In plant immunity, surface receptors connect extracellular recognition to intracellular signal transduction and defence.

Recognition of a widely shared pattern can provide broad surveillance, but the presence of that pattern does not by itself measure whether the interacting organism will cause disease. Pathogens can also interfere with signaling, which helps explain the additional surveillance provided by intracellular NLRs.

A nucleotide-binding leucine-rich repeat receptor, or NLR, is an intracellular immune-receptor protein characterized by a nucleotide-binding region and leucine-rich repeats. In plants, NLRs can detect pathogen-associated molecules or their effects and initiate defensive signaling.

An NLR can work as a sensor, a signaling helper or part of a paired NLR system. Different individuals can carry different receptor inventories, so one reference genome does not fully describe the species-wide pan-NLRome.^[ref]

A host resistance and pathogen avirulence specificity relationship; it is not the complete architecture of all plant disease resistance.

A deleterious hybrid phenotype that can arise from incompatible immune-gene interactions and inappropriate defence activation.

EDS1, ENHANCED Disease SUSCEPTIBILITY 1, is a plant immunity signaling component that forms complexes with partner proteins, including PAD4. Such interactions connect particular immune-recognition pathways to changes in defence activity.

The pathway is not present unchanged in every plant. Comparative genomic work identified convergent losses of EDS1, PAD4 and associated components, providing evidence for coevolution of an interacting molecular system.^[ref]

PAD4, PHYTOALEXIN DEFICIENT 4, is a plant immunity signaling component that interacts with EDS1. Their coupled activity helps connect immune recognition to downstream responses; PAD4 is not itself the generic name of a pathogen-recognition receptor.

Comparative analyses found repeated joint losses of the EDS1-PAD4 pathway and associated genes in several plant lineages. This makes pathway composition an evolutionary question: absence of one component can be informative about the others.^[ref]

Allocation and regulatory conflicts between growth and defence; the sign and magnitude of the relationship depend on environment and genotype.

Plant epigenetics investigates changes in gene activity associated with regulatory states such as DNA methylation and chromatin organization, including states that persist through mitosis or reproduction. Stable expression differences need not be caused by a changed DNA sequence, but independence from sequence variation must be tested.

The Karma epiallele connects this distinction to agriculture: reduced methylation of an intronic retrotransposon was associated with altered splicing and the low-yield mantled form of tissue-cultured oil palm. Somaclonal variation can thus involve a regulatory change as well as a sequence mutation.^[ref]

Persistence is a measurable property. Comparisons of Arabidopsis thaliana lines descended from a common ancestor found many changes at individual methylated positions but greater stability across larger methylated regions. Calling every transient response an epimutation would obscure this difference.^[ref]

Plant breeding changes the inherited characteristics of cultivated plants by choosing parents, generating variation and selecting descendants. Crosses followed by repeated selection can combine disease resistance with useful yield or quality traits. Genetic recombination determines which combinations can be recovered, while linkage drag can preserve undesirable neighbouring material.

Evaluation across environments matters because a selected genotype need not retain the same advantage under drought, different soils or a different pathogen. Experiments manipulating chromosomal crossovers investigate a mechanism that limits how readily useful parental variants can be separated and combined.^[ref]

The wheat Ph1 locus regulates pairing and genetic recombination during meiosis. In a genome containing related chromosome sets, appropriate pairing helps maintain regular inheritance while limiting exchange between related but nonhomologous chromosomes.

Experiments distinguish effects on chromosome synapsis from effects on the later formation of chromosomal crossovers. That distinction matters for plant breeding: increasing useful exchange need not be equivalent to removing every constraint on pairing.^[ref]