Plant Biology - Growth of Plants - Quick Guides

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Study GuidePlant Biology–Growth of Plants1. Responsive Growth Movements: TropismsPlants may not move from place to place like animals, but theydo respond to their environmentbygrowing in specific directions. These directional growth responses are calledtropisms.•When a plant growstowarda stimulus, it shows apositive tropism.•When it growsaway froma stimulus, it shows anegative tropism.Different types of stimuli—such as light, gravity, or touch—cause different kinds of tropic movements.1.1Phototropism (Response to Light)Phototropismis the growth response of plants tolight coming from one direction.•Shootsusually grow toward light, so they arepositively phototropic.•Rootsgrow away from light, making themnegatively phototropic.How does this happen?Experiments withoat seedlingshave helped scientists understand this process. A plant hormonecalledauxin (IAA)plays a key role.•Auxin moves to theshaded sideof the shoot.•Cells on the shaded sideelongate morethan those on the lighted side.•This uneven growth causes the shoot tobend toward the light.Ingreen stems, the process is slightly different. Here, anIAA inhibitorprevents cells on the lightedside from elongating, while cells on the shaded side keep growing. This difference again causes thestem to bend toward the light.Role of light colorNot all light affects growth equally.Blue light, especially wavelengthsbelow 500 nm, is the mosteffective in triggering phototropic responses.

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Study Guide1.2Gravitropism (Response to Gravity)Gravitropismis a plant’s growth response togravity.•Shootsgrow upward, away from gravity, so they arenegatively gravitropic.•Rootsgrow downward, toward gravity, making thempositively gravitropic.How do plants sense gravity?The exact mechanism isstill not fully understood. Scientists have proposed many ideas, but noneexplains everything. Over millions of years, plants likely developedmultiple waysto respond togravity.Certain substances are known to be involved in this response:•Auxin (IAA)•Calcium ions (Ca²⁺)•Possiblyabscisic acid (ABA)A long-standing theory suggests thatstarch grainsinside the root cap move downward when a rootis placed horizontally. This movement may help the plant sense gravity and trigger growth changes,but this ideahas not yet been conclusively proven.1.3Thigmotropism (Response to Touch)Thigmotropismis the growth response of a plant or plant part totouch or contact with a solidobject.A common example is seen inclimbing plants:•Theirtendrils wrap around supportslike sticks or wires.•Cells on the side touching the supportshorten, while cells on the opposite sideelongate.•This uneven growth causes the tendril to curl around the object.Plant hormones such asauxin (IAA)andethyleneare believed to play an important role in thisresponse.

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Study GuideSummaryTropisms help plantsadapt and surviveby growing in directions that improve their access to light,water, support, and stability. Even though plants appear still, their growth responses show howactively they interact with their environment.2. Other Plant MovementsEven though plants are fixed in one place by their roots, they are far from motionless. Many plantparts—such as leaves, flowers, and traps—can move when they receive signals from theirenvironment. These movements help plants survive and respond to changing conditions.What are nastic movements?These plant movements are callednastic movements. Unlike tropic movements, they arenotdirected toward or away from a stimulus. Instead, the movement happens no matter where thestimulus comes from.Types of nastic movements•Touch-based movementsare calledthigmonasticorseismonasticmovements.•A classic example is the rapid closing of traps in insect-eating plants like the Venus flytrap.•Daily “sleep” movementsof leaves are callednyctinastic movements.•These occur because of regular cycles of light and darkness, such as leaves folding at nightand opening during the day.How do these movements happen?Most nastic movements occur due tochanges in turgor pressure. After a plant senses a stimulus,water moves in or out of specialparenchyma cells, causing the cells to swell or shrink. This changein pressure leads to visible movement of plant parts.2.1Thigmomorphogenesis: Growth Changes Caused by TouchPlants constantly experience mechanical forces in nature. Wind, rain, animals brushing past, andeven repeated shaking all affect how a plant grows.What is thigmomorphogenesis?Thigmomorphogenesisis the growth response of plants to repeated mechanical disturbances liketouching or shaking.

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Study GuideHow does the plant change?When plants are regularly disturbed:•They growshorterin height•Their stems becomethicker•They developstronger supportive tissuesThese changes make the plant sturdier and better able to survive harsh conditions.What happens inside the plant?Mechanical stress activates certaingenesinside plant cells. One important gene producescalmodulin, a protein that binds calcium and plays a key role in regulating growth.This process is similar to other growth responses that involve calmodulin.The plant hormoneethylene, which helps regulate growth, also plays an important role in thisresponse.2.2Solar Tracking (Heliotropism)Some plants can move their leaves or flowers to follow the sun across the sky during the day. Thisbehavior helps them absorb maximum sunlight for photosynthesis.How solar tracking worksA well-known example is thesunflower:•In themorning, the flowers faceeast•Atmidday, they face upward toward the sun•In theevening, they turnwesttoward the setting sunWhat is this movement called?This movement is known asheliotropism.However, it isnot a true tropic movementbecause it doesnot involve growth.What causes the movement?Like many nastic movements, solar tracking happens due tochanges in turgor pressureinparenchyma cells. Water movement within these cells allows the plant parts to bend and follow thesun smoothly throughout the day.

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Study GuideKey TakeawayPlants may seem still, but they are highly responsive organisms. Through nastic movements, growthchanges, and solar tracking, plants constantly adjust to their environment—using clever internalmechanisms rather than muscles or nerves.If you’d like, I can also:•Turn this intoexam-ready notes•Adddiagrams with explanations•CreateAP-style questions•Or simplify it even more for quick revision3. Circadian Rhythms3.1What Are Circadian Rhythms?Many plants show activities that repeat on a regular24-hour cycle. For example, some flowersalways open in the late afternoon, while others open at night or during the day.This repeating pattern of growth or activity that happens about once every 24 hours is called acircadian rhythm.Examples You Can Easily NoticeCircadian rhythms control many important processes in plants, such as:•Sleep movements of leaves, where leaves fold or droop at night and reopen during the day•Opening and closing of flowers, especially night-blooming and day-blooming plants•Cell divisionin root tips•Production of proteins and hormonesThese activities happen on a daily schedule, even when conditions seem unchanged.

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Study Guide3.2Controlled from the InsideCircadian rhythms areendogenous, which means they are controlled frominside the plant. Plantshave an internal timing system known as thebiological clock.This clock allows plants to “know” when to carry out certain activities during the day or night.3.3How the Environment HelpsEven though circadian rhythms are internally controlled, theenvironment helps keep the clockaccurate. Light and darkness act like signals that adjust the plant’s internal clock so it stays in syncwith the outside world.This process of resetting the biological clock is calledentrainment.3.4Why Entrainment MattersEntrainment ensures that plant activities, such as flowering, stay aligned with:•Day and night cycles•Seasonal changes in day lengthBecause of this, plants can respond correctly in different seasons bymeasuring changes indaylight.3.5Role of PhytochromePlants use a special light-sensitive pigment calledphytochrometo detect light changes.Phytochrome helps the biological clock track day length and plays a key role in controlling circadianrhythms, especially those related to flowering.SummaryCircadian rhythms help plants stay on a daily schedule. An internal biological clock controls theserhythms, the environment keeps the clock adjusted, and phytochrome helps plants sense light andseasonal changes.

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Study Guide4. PhotoperiodismPhotoperiodismis how plants respond to changes in day length and night length over a 24-hourcycle. In simple terms, plants use light and darkness to tell what season it is. This helps them timeimportant events likeflowering, so they bloom at the best possible time of year.4.1Phytochrome: The Plant Light SensorPlants detect light using a special pigment calledphytochrome. This pigment exists in two forms:•Pr–absorbsred light•Pfr–absorbsfar-red lightThese two forms can change into each other:•In daylight,Pr changes to Pfr, andPfr can change back to Pr.•By the end of the day, plant cells usually reach a balance of about60% Pfr and 40% Pr.At night, something important happens:•Pfr slowly converts back to Pr or breaks down•Pr stays stable in the dark4.2Why Night Length MattersPfr is the active formof phytochrome. It acts like a switch that turns on plant responses such asfloweringandseed germination.•When enoughPfris present, certain plant responses begin.•BecausePfr breaks down at night, thelength of the night(not the day) is what reallycontrols these responses.Here’s how it works:•Short nights (long days):More Pfr remains → favors processes that need a lot of Pfr•Long nights (short days):More Pfr turns into Pr → favors responses that need only a smallamount of PfrEven thoughPris made from amino acids, it isinactiveuntil it converts toPfr.

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