Unit 7: Chapter 36: Transport In Plants Chapter 37: Plant Nutrition Chapter 38: Plant Reproduction

UNIT 7 Chapter 36: Transport in Plants Chapter 37: Plant Nutrition Chapter 38: Plant Reproduction

Types of Transport  Transport of water and minerals occurs on three levels:  In/out of individual cells  Short distance  Long distance

 Differences in water potential drive transport of water in plant cells  = s + p

 Pure water, s = 0  Addition of solutes decreases s  Water moves from areas of high water potential to areas of low water potential

 Water potential impacts uptake and loss of water  Flaccid cell  Cell loses water  Cell will eventually plasmolyze

 Turgid cell  Cell gains/maintains water

Turgid plant

Absorption of Water & Minerals  Root hairs and mycorrhizae increase surface area and enhance absorption  Water and minerals transported = xylem sap

 Ascent of xylem sap depends mainly on transpiration  Xylem sap not “pushed” by pressure in roots, but “pulled” up

 Force created by transpiration and cohesion/adhesion of water molecules extends down shoot, into roots and even into soil

 Water potential gradient drives water up  Air in the xylem, cavitation, breaks the chain

Control of Transpiration  A plant can transpire more than its weight in water every day  Flow in xylem can reach 75cm/min

 Guard cells control the size of the stomata and balance the plant’s water needs and loss

 Transpiration-to-photosynthesis is amount of water loss per gram of CO2 fixed  Many plants ~600:1 – 600g of water lost per 1g of CO2 fixed  C4 plants (ex. corn) ~300:1

 Transpiration also results in evaporative cooling, which can cool leaves 10-15°C  Prevents denaturation of enzymes and disruption of metabolism

 Each stoma is flanked by a pair of guard cells suspended by other epidermal cells over an air space  Water into guard cells = turgid  stoma open  Water out of guard cells = flaccid  stoma closed

 K+ plays an important role in osmosis in guard cells  Presence of K+ ions lowers s and water flows in or out based on where s is lower

 There are three cues that initiate the opening of stomata in the morning  1. Blue-light receptors promote active uptake of K+ into guard cells

 Photosynthesis provides ATP  2. Depletion of CO2 as photosynthesis begins  3. Internal “clock” within guard cells that cycles on a 24-hour basis – circadian rhythm

Xerophytes  Xerophytes are plants that are well adapted to arid (very dry) environments  CAM plants are xerophytes (family Crassulaceae)

 A number of anatomical adaptations exist to reduce water loss  Concentration of stomata on lower surface of leaf  Presence of trichomes  Placement of stomata within “crypts”

Phloem Transport  Phloem sap is typically moved from sugar sources to sugar sinks  Sources: sugar being produced by photosynthesis, esp. in mature leaves  Sinks: growing parts of the plant, fruits  Sinks usually receive sugar from the sources closest to them  Can be in any direction – even against pull of gravity

 Pressure flow is the primary force behind the translocation of phloem sap

 Phloem sap flows from source to sink at about 1m/hr  Flow is fastest near sources  Sugar concentration is highest

 Water is recycled because of differences in 

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Nitrogen Requirements • 80% of the atmosphere is nitrogen (N2), but plants can still suffer deficiencies • Plants can only use nitrogen in certain forms • Ammonium (NH4+) or nitrate (NO3-) ions

• Bacteria in the soil metabolize unusable forms of nitrogen • Nitrogen fixation makes nitrogen available

• Nitrogen fixing bacteria convert N2 into NH4+ and ammonifying bacteria convert decomposing organic material into NH4+ • Nitrifying bacteria convert NH4+ into NO3-

• ALL (eukaryotic) life on earth depends on nitrogen fixation

Parasitic & Carnivorous Plants • Some plants supplement or replace their photosynthesis by taking advantage of other plants • ex. Indian pipe

• Epiphytes are autotrophic plants, but they simply live on other plants • Not truly parasitic • ex. some mosses and ferns

• Carnivorous plants supplement their nutrition by digesting animals • Typically found in areas with poor soil conditions • Use photosynthesis for carbohydrates, but get some nitrogen and minerals from animals • Modified leaves trap animals and secrete enzymes END

Flower Structure: Review

Pollination

• Pollination is the attachment of pollen to a flower’s stigma • Pollen released is carried by wind or animals • Pollen grain produces a pollen tube • Grows through style, into ovary and discharges sperm

• Zygote gives rise to an embryo • Ovule develops into a seed, ovary develops into a fruit containing seed(s)

Plant Reproduction Terminology

• Plant biologists distinguish between complete flowers, those having all four organs, and incomplete flowers, those lacking one or more of the whorls • A bisexual flower (“perfect flower”) is equipped with both stamens and carpals • A unisexual flower is missing either stamens or carpels

• A monoecious plant has male and female flowers on the same individual plant • ex. corn

• A dioecious species has male flowers and female flowers on separate plants • ex. date palms

Prevention of Self-Fertilization

• Some flowers self-fertilize, but most angiosperms have mechanisms that make this difficult • Barriers prevent self-fertilization to maintain genetic variety

• In some species stamens and carpels mature at different times • May be arranged so that it is unlikely that an animal pollinator could transfer pollen from the anthers to the stigma of the same flower

• Most common anti-selfing mechanism is self-incompatibility • Ability to reject its own pollen • Biochemical block prevents fertilization

• Self-incompatibility systems are analogous to the immune response of animals • Difference is that the animal immune system rejects non-self, self-incompatibility in plants is a rejection of self

• Based on genes for self-incompatibility, called S-genes, with as many as 50 different alleles in a single population • If a pollen grain and the carpel’s stigma have matching alleles at the S-locus, pollen grain fails to initiate or complete the pollen tube

Double Fertilization

• Pollen grain lands on stigma, absorbs moisture and begins producing a pollen tube • Directed by a chemical attractant, the pollen tube enters the ovary through the micropyle and discharges two sperm within the embryo sac

• Both sperm fuse with nuclei in the embryo sac • One sperm fertilizes the egg to form the zygote • Other sperm combines with the two polar nuclei to form a triploid nucleus in the central cell • Gives rise to the endosperm, a food-storing tissue of the seed

Fig. 38.9

• After double fertilization, the ovule develops into a seed, and the ovary develops into a fruit enclosing the seed(s) • As the embryo develops, the seed stockpiles proteins, oils, and starch

Fruit Development

• As the seeds are developing from ovules, the ovary of the flower is developing into a fruit • Pollination triggers hormonal changes that cause the ovary to begin its transformation into a fruit • If a flower has not been pollinated, fruit usually do not develop, and the entire flower withers and falls away

• The wall of the ovary becomes the pericarp, the thickened wall of the fruit • In some angiosperms, other floral parts contribute to what we call a fruit

Seeds & Seed Germination

• As a seed matures, it dehydrates and enters a dormancy phase • Extremely low metabolic rate, suspension of growth and development

• Conditions required to break dormancy • Some seeds germinate as soon as they are in a suitable environment • Remains dormant until some specific environmental cue causes them to break dormancy

• Seeds of many desert plant germinate only after a substantial rainfall, ensuring enough water • Where natural fires are common, many seeds require intense heat to break dormancy • Other seeds require a chemical attack or physical abrasion • ex. through an animal’s digestive tract before they can germinate

• Germination of seeds depends on imbibition, the uptake of water due to the low water potential of the dry seed • Expanding seed ruptures its seed coat and triggers metabolic changes in the embryo that enable it to resume growth

• Enzymes begin digesting the storage materials of endosperm and the nutrients are transferred to the growing regions of the embryo

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