16.2 PowerPoint 2 Plant responses 5.1.5 16.2 Plant responses to abiotic stress Links here are Photosynthesis (chapter 17) 9.5 Plant adaptations to very dry and very watery conditions When environmental conditions change, plants have to cope or die Abiotic stress? Changes in daylength, cold, heat, water availability, winds, salinity…. Plant responses can be physical: thick cuticles, hairy leaves, sunken stomata or can be physiological adaptations

Why is it so important for plants to be able to respond to their surroundings? 2 marks They are rooted to the ground so cannot move their bodies (1); Therefore very important that they are sensitive so they can grow in the right direction and make the best of the circumstances where they have germinated (1). Leaf loss in deciduous plants as a response to abiotic stress:

Leaf loss in deciduous plants as a response to abiotic stress: Plants in temperate climates experience great environmental changes in a year Daylight hours Temperature changes Water availability Seasonal changes have an impact on the amount of photosynthesis possible Balance lost between the amount of glucose required for respiration to maintain leaves is greater than the amount of glucose produced by photosynthesis. Winds So most deciduous trees in a temperate climate lose all of their leaves in winter and are dormant WHITEBOARDS: What are the survival advantages of this?

Leaf Abscission/fall Abscission (from the Latin ab, meaning away, and scindere, meaning to cut) is the shedding of various parts of an organism, such as a plant dropping a leaf, fruit, flower, or seed Trees in temperate countries shed their leaves in autumn. Survival advantage Reduces water loss through leaf surfaces Avoids frost damage Avoid fungal infections through damp, cold leaf surfaces Plants have limited photosynthesis in winter

Stages in leaf abscission Falling light levels results in low auxin levels Drop in auxin causes an increase in ethene production This causes the cells in abscission zone to be more sensitive to ethene Gene switching occurs to stimulate new enzyme production (now required proteins) More ethene produced, inhibits auxin production This increases production of enzyme cellulase, which digests the cell walls in outer layer of abscission zone (separation layer) Vascular bundles are sealed off and a fatty protective layer produced to prevent entry of pathogens Other cells are stimulated to retain water and swell putting a strain on outer layer Other abiotic factors play a part, winds, rain Separates the petiole from the stem A neat, waterproof scar is left

Daylength sensitivity Plants are also sensitive to the LACK of light available: photoperidism Many different plant responses are affected by the photoperiod: Breaking dormancy of leaf buds so they open Timing of flowering in a plant Formation of tubers for overwintering This sensitivity is due to:

Phytochromes: Plant photoreceptors Like animals plants detect the quantity, direction and wavelength of light using photoreceptors and respond to the changes in light conditions All messages in plants are chemicals Responses are slower Phytochromes: Plant photoreceptors Exist in 2 forms: Pr Pfr Each absorbs light at a different wavelength, and the ratio of both changes depending on the levels of light

Phytochromes A wide range of responses are regulated by phytochromes such as Seed germination Stem elongation Leaf expansion Chlorophyll formation Flowering In most plants P FR is physiologically active and stimulates the activities

Two isomers are photoreversible: Phytochromes: PR : absorbs red (R; 660 nm) light P FR : absorbs far red (FR; 730 nm) light Two isomers are photoreversible: Synthesised in Pr form Redlight Pr P fr Far red light P fr Pr sunlight contains more RED light SO SUNLIGHT: Pr P fr and P fr Pr Pr P fr DOMINATES SO P fr accumulates in light DARKNESS: P fr Pr slowly! P fr biologically/metabolically active Pr more stable

Produce a flow diagram to explain the process of abscission 8-10 marks Falling light levels = decreased concentration of auxin So leaves produce hormone ethene Ethene initiates gene switching in abscission zone at base of leaf stalk Gene switching causes production of new enzymes (cellulase) New enzymes digest and weaken cell walls in outer layer of abscission zone (separation layer) Vascular bundles sealed off, fatty material deposited in cells on stem side of separation layer Layer forms protective scar when leaf falls, preventing pathogen entry Cells in separation zone respond to hormonal cues by retaining water and swelling putting more strain on outer layer Further abiotic factors finish process (wind etc) Strain is too much and leaf separates from plant leaving neat waterproof scar

Abiotic factor: decrease in temperature Preventing freezing Abiotic factor: decrease in temperature If cells freeze membranes are disrupted and they will die Many plants have developed a mechanism for survival: Cytoplasm and sap in vacuoles contain solutes which lower the freezing point Some plants produce sugars/polysaccharides/amino acids which act as an antifreeze to prevent the cytoplasm from freezing Gene expression triggered due to reduction in day length and temperature fall and ABA produced, triggers gene switching so plants make chemicals (sugars,proteins) which lower freezing point of cytoplasm.

1. Explain how plant hormones are involved in protecting the plant cells from damage in freezing conditions 5 marks Chemicals such as abscisic acid (1); trigger gene switching (1); so plants make chemicals such as sugars or proteins (1); which lower the freezing point of the cytoplasm (1); or protect the cells against damage by ice crystals if they do freeze (1).

Stomatal control: ABA Abiotic stress causes release of ABA (abscisic acid) which leads to stomatal opening/closure Leaf cells produce ABA when under stress which closes stomata Scientists also believe that water availability at roots provides a warning As soil water levels fall, ABA produced in roots is transported to leaves Binds to receptors on plasma membranes of guard cells Changes ionic concentration in guard cells, reduces water potential so reducing turgor and so the guard cells close the stomata and loss of water by transpiration reduced.

Quick thinking: whiteboards Why do many trees in temperate climates lose all of their leaves in winter? 6 marks Exam question Plant responses to environmental changes …13 marks

The amount of photosynthesis that can take place decreases as day length is reduced and temperatures fall (1); so the amount of glucose produced by photosynthesis falls (1); the amount of glucose needed for respiration to maintain leaves through the winter (1); produce chemicals to prevent freezing damage increases (1); it becomes more efficient to lose the leaves and become dormant until the days lengthen and temperatures increase again (1).