Lenticels in the periderm allow for gas exchange o

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Lenticels: in the periderm allow for gas exchange o More width growth than height o Vascular cambium is a cylinder of meristematic cells one layer thick that develops from undifferentiated parenchyma cells o Secondary xylem accumulates as wood o Secondary phloem does not accumulate o Rings visible in wood o In temperate regions, the vascular cambium or perennials is dormant through the winter o Monocots cant make new vascular tissue, cant live in PA Regulation of Plant Growth I o Review of secondary growth Only eudicots have true secondary growth New xylem cells are much longer Air bubbles form in xylem Break up the water column
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Monocots can’t live in cold places because they can’t make new xylem come spring o Plant hormones Hormones control plant growth and development by affecting cell division, elongation, and differentiation Plant hormones are produced in very low concentration Signal transduction pathway Reception -> transduction-> response Hormone or environmental stimulus-> receptor-> relay proteins and second messengers-> activation of cellular responses Reception: internal (e.g. hormones) and external (e.g. light) signals are detected by receptors, proteins that change in response to specific stimuli Transduction: second messengers transfer and amplify signals from receptors to proteins that cause responses Response: a signal transduction pathway leads to regulation of one or more cellular activities o In most cases, these responses to stimulation involve increased activity of enzymes o This can occur by transcriptional regulation or post- translational modification of proteins De-etiolation (“Greening”) proteins Many enzymes that function in response during the greening of potatoes- produced or made functional- are directly involved in photosynthesis Others supply chemical precursors for chlorophyll production Other proteins may affect levels of various growth hormones Before exposure to light, potato is etiolated; after a week’s exposure to natural daylight the plant has undergone de- etiolation Light-> phytochrome activated by light ->cGMP/Ca2+ channel opened -> protein kinases activated-> transcription factors-> de-etiolation (greening) response proteins o Ca2+ levels can influence ion channels, cytockeletal proteins, and enzymes Hormones (plant growth regulators) are chemical signals that coordinate different parts of an organism and play a key role in signal transduction pathways; can act as a primary messenger between cells Major plant hormones: Gibberellins, Auxin, Cytokinins, Brassinosteroids, Abscisic Acid, Ethylene Gibberellins
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o Stimulate fruit growth, stimulate seed germination, control stem elongation o Apples drop if not many seeds are growing o Gibberellins send signal to aleurone (outside endosperm), aleurone secretes β -amylase and other enzymes, sugar and other nutrients are consumed Breweries o Thompson grapes Gibberellin-induced fruit growth o
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