Plant Growth

Growth in Animals Animals grow throughout the whole organism many regions & tissues at different rates

Growth in Plants Specific regions of growth: meristems stem cells: perpetually embryonic tissue regenerate new cells apical shoot meristem growth in length primary growth apical root meristem lateral meristem growth in girth secondary growth

Apical meristems shoot root

Root structure & growth protecting the meristem

protecting the meristem Shoot growth Apical bud & primary growth of shoot region of stem growth axillary buds “waiting in the wings” protecting the meristem Young leaf primordium Apical meristem Older leaf primordium Lateral bud primordium Vascular tissue

Growth in woody plants Woody plants grow in height from tip Primary xylem Growth in woody plants Woody plants grow in height from tip primary growth apical meristem Woody plants grow in diameter from sides secondary growth lateral meristems vascular cambium makes 2° phloem & 2° xylem cork cambium makes bark Primary phloem Epidermis Lateral meristems Secondary xylem Primary phloem Primary xylem Secondary phloem Annual growth layers Bark

Secondary growth Secondary growth growth in diameter thickens & strengthens older part of tree cork cambium makes bark growing ring around tree vascular cambium makes xylem & phloem

Why are early & late growth different? Vascular cambium Phloem produced to the outside Xylem produced to the inside bark phloem cork cambium xylem late vascular cambium early last year’s xylem

Woody stem How old is this tree? cork cambium vascular cambium late early 3 2 1 xylem phloem bark

Tree trunk anatomy tree girdling What does girdling do to a tree? Aaaargh! Murderer! Arborcide! Tree trunk anatomy tree girdling What does girdling do to a tree?

Where will the carving be in 50 years?

Plant hormones auxin gibberellins abscisic acid ethylene and more…

Auxin (IAA) Effects controls cell division & differentiation phototropism growth towards light asymmetrical distribution of auxin cells on darker side elongate faster than cells on brighter side apical dominance

Gibberellins Family of hormones Effects over 100 different gibberellins identified Effects stem elongation fruit growth seed germination plump grapes in grocery stores have been treated with gibberellin hormones while on the vine

Abscisic acid (ABA) Effects slows growth seed dormancy high concentrations of abscisic acid germination only after ABA is inactivated or leeched out survival value: seed will germinate only under optimal conditions light, temperature, moisture

One bad apple spoils the whole bunch… Ethylene Hormone gas released by plant cells Effects fruit ripening leaf drop like in Autumn apoptosis One bad apple spoils the whole bunch…

Fruit ripening Adaptation Mechanism hard, tart fruit protects developing seed from herbivores ripe, sweet, soft fruit attracts animals to disperse seed Mechanism triggers ripening process breakdown of cell wall softening conversion of starch to sugar sweetening positive feedback system ethylene triggers ripening ripening stimulates more ethylene production

Apoptosis in plants Many events in plants involve apoptosis What is the evolutionary advantage of loss of leaves in autumn? Many events in plants involve apoptosis response to hormones ethylene auxin death of annual plant after flowering senescence differentiation of xylem vessels loss of cytoplasm shedding of autumn leaves The loss of leaves each autumn is an adaptation that keeps deciduous trees from desiccating during winter when the roots cannot absorb water from the frozen ground. Before leaves abscise, many essential elements are salvaged from the dying leaves and are stored in stem parenchyma cells. These nutrients are recycled back to developing leaves the following spring. Fall color is a combination of new red pigments made during autumn and yellow and orange carotenoids that were already present in the leaf but are rendered visible by the breakdown of the dark green chlorophyll in autumn. Photo: Abscission of a maple leaf. Abscission is controlled by a change in the balance of ethylene and auxin. The abscission layer can be seen here as a vertical band at the base of the petiole. After the leaf falls, a protective layer of cork becomes the leaf scar that helps prevent pathogens from invading the plant (LM).

Responses are critical for plant success Tropisms—growth of plant organs toward or away from stimuli Phototrophism Gravitropism Thigmotrophism Other Stimuli Turgor movements Environmental stress

Phototropism Biological clocks control circadian rhythms in plants—cycle with a frequency of about 24 hours Set due to environmental signals Most cued by light-dark cycle resulting from earth’s rotation

Photoperiodism synchronizes plant response Plants detect the time of year by the photoperiod Photoperiodism controls flowering Critical night length If daytime is broken by brief peroids of darkness—plants flower (summer) If night is interrupted by short exposure to light—plants do not flower (winter)

Phytochromes Photoreceptors in many plant responses to light and photoperiod—help to measure length of darkness in a photoperiod Phytochromes alternate between two photoreversible forms: Pr—red absorbing Pfr—far red absorbing

Phytochromes entrain biological clock Pfr gradually reverts to Pr when light not present Decrease in Pfr = night Increase in Pfr = day

Gravitropism Orientation of a plant in response to gravity Roots display positive gravitropism—curve down Shoots display negative gravitropism—curve upward

Thigmotropism Directional growth in response to touch

Turgor movements Rapid, reversible plant responses Rapid leave movements—travel from the leaf that was stimulated to adjacent leaves along the stem Sleep movements—lowering of leaves to a vertical position in evening and raising of leaves to a horizontal position Due to daily changes in turgor pressure

Responses to Environmental Stress Water Deficit Oxygen Deprivation Salt Stress Heat Stress Cold Stress

Water Deficit Leaves and roots have control systems to cope with water deficits Leaves—reduce transpiration Roots—reduce growth

Oxygen Deprivation Water logged soil lacks air spaces that provide oxygen Some plants form air tubes that extend from roots to the surface

Salt stress Excess salt in the soil can cause a water deficit in the plant and have toxic effects on the plant Plants produce compatible solutes that keep the water potential of cells more negative than the soil solution.

Heat Stress Transpiration helps plants respond to excessive heat—which can denature the enzymes and destroy the metabolism of the plant. Plants can produce heat shock proteins when exposed to excessive temperatures—back up to transpiration

Cold Stress Cold causes a change in the fluidity of cell membranes Subfreezing are the most severe form of cold stress because ice crystals begin to form in the plant Plants respond by altering the lipid composition of their membranes

Defense against herbivores Plants defend themselves both physically and chemically: Physical—thorns and spines Chemical—distasteful or toxic compounds Recruit predatory animals to help defend

Defense against pathogens The hypersensitive response (HR) contains the infection by: Photoalexins—antimicrobial compounds released from wounded cells Activation of genes encoding pathogen resistant (PR) proteins –act as signals to adjacent cells Lignin synthesis and cross-linking of cell walls—actions aimed at isolating the infection.

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