plants and water

Question 1

drought

Answer 1

• threatens agricultural livelihoods and causes food insecurity
• e.g. Afghanistan 2018, caused large displacement, exacerbated migrant crises etc

Question 2

roots

Answer 2

• absorptive interface with soil
• lag on research on rooting behaviour, hormone signalling etc
• structure studied using CT scanning

Question 3

root structure

Answer 3

• Dicots, large fleshy primary tap root, branches of first-order lateral roots, may branch again into second-order lateral roots
• Monocots, primary seminal root branches into first and second order lateral roots, tend to be more branched, crown roots just under soil surface
• root system architecture adapted to environment, low nutrients leads to more branched lateral roots

Question 4

root hairs

Answer 4

• increases surface area for absorption
• elongations from single epidermal cell, vacuole pushes cell membrane and wall outwards
• interactions of hormones determines number (depends on environmental conditions), distribution etc
• relatively short lived, continuously regenerated
• specialised, thin permeable cell walls

Question 5

symbiotic root fungi, ectomycorrhizal

Answer 5

• 80% plant species
• mantle of fungal cells coat outer root surface, roots do not directly interact with soil
• enhances water and nutrient exchange
• increases protection
• Hartig net, fungi can penetrate through cell walls (not into cells through membrane)
• fungi extracts some carbon from plant
• increases surface area through mycelia

Question 6

root tissues

Answer 6

• cortex layer stores nutrients, water, carbohydrates
• vascular tissue bounded by endodermis
• phloem, mostly for transport of carbohydrate down to roots
• xylem

Question 7

endodermal layer

Answer 7

• specialised lignified cells
• Caspian strip waterproofed by suberin encloses cell wall
• water in apoplastic route (in channels within cell walls) forced into symplastic route (into cells)
• allows cellular control of water entering vasculature (removal of bacteria, viruses etc)

Question 8

plant mineral deficiencies

Answer 8

• plants need 16 or more nutrients (debated) taken up from soil
• leaf discolouration can diagnose deficiencies
• mobile nutrients include N, P, K, Mg
• able to move out of older leaves to younger plant parts when supplies are inadequate, older leaves are less efficient as they are at bottom of plant, apoptosis often triggered
• immobile nutrients include Mn, S, Cu, Fe, Ca, B
• plants struggle to move them elsewhere, deficiency symptoms first appear in new growth or young leaves and can be localised

Question 9

human mineral deficiencies

Answer 9

• all plant derived and therefore soil derived, problem with intensive agriculture depleting nutrients, can rotate crops/ supplement soil
• undernutrition (not receiving enough calories or correct balance of nutrients) accounts for 45% worldwide child death
• vitamin D deficiency a problem in UK
• vitamin A deficiency a problem in other countries, rice lacks vitamin A, can lead to disability and death (blindness)
• GMO ‘golden rice’ is fortified with vit A

Question 10

dicot stem xylem cells and tissues

Answer 10

• vascular bundles in circular arrangement around pith
• as the stem matures, secondary growth occurs through the vascular cambium
• produces additional xylem cells (secondary xylem) that increases stem diameter and transport capacity

Question 11

xylem

Answer 11

• cells dead at maturity
• undergoes programmed cell death (PCD)
• loses cytoplasm and connecting cell walls forming long tubes
• heavily lignified, rigid
• way lignin is deposited varies
• provides some mechanical strength (primary source in small plants)

Question 12

suction

Answer 12

• would not take H2O to tree tops
• maximum height of 10.6m = equilibrium point with a fixed vacuum taking into account strength of xylem
• after 10.6m gravity acts on water

Question 13

mechanism of journey of water to treetops

Answer 13

• still debated
• main theory is cohesion tension model
• from models of synthetic tree (microfluidic system formed in a synthetic hydrogel)
• passive transport of water by capillary action
• adhesion and cohesion creates upward movement until balanced by gravity
• evaporation of water via the stomata from mesophyll cells in leaves (transpiration) creates tension (negative pressure) in the xylem)
• if H2O evaporates at top of column upward flow occurs to equilibrium height

Question 14

cohesion

Answer 14

= attraction of like molecules
- water is polar
- 1 water molecules binds to 4 others (H bonds), strongly cohesive
- creates very strong surface tension

Question 15

adhesion

Answer 15

= attraction of unlike molecules
- between H2O molecules and xylem wall from charge interaction

Question 16

ABA and events in drought stressed plants

Answer 16

• drought stress simulated by witholding water
• stomata closure correlates with ABA levels (abscisic acid, plant hormone)
• modifying ABA response pathways could make plants more adaptable to drought as stomatal closure would happen more quickly, reducing water loss through transpiration

Question 17

cuticle layer

Answer 17

• complex cutin biopolymer excreted
• waterproofs leaf surface, hydrophobic
• decreases evapotranspiration from upper epidermis

Question 18

succulent leaves

Answer 18

• waxy leaves
• store water in fleshy leaves
• mucilaginous tissues within leaves can retain water through gel like properties
• reduces transpiration
• CAM photosynthesis

Question 19

minimising leaf surface e.g. pine needles

Answer 19

• narrow elongated shape reduce stomatal pores and area of epidermal surface, decreasing water loss through evapotranspiration
• thick waxy cuticle coating, barrier against evapotranspiration and reflects sunlight (reduced thermal energy)
• sunken stomata in grooves creates microenvironment that reduces evapotranspiration

Question 20

rolled leaves e.g. marram grass

Answer 20

• also has deep roots
• encloses internal space in rolled structure, les surface area exposed to direct sunlight and wind, creating microclimate

Question 21

lithops ‘living stones’

Answer 21

• native to southern Africa
• camouflage, colour mimics geology, protects against herbivory
• grow mostly underground with just the leaf pair exposed to limit evapotranspiration

Question 22

succulent stems e.g. cacti

Answer 22

• no leaves, stem photosynthesis decreases surface area and therefore evapotranspiration
• waxy cuticle
• sunken stomata into stem surface
• CAM photosynthesis, stomata open at night
• extensive shallow root system
• stem tissue adapted to store large amounts of water
• spines and hairs helps shade stem, collects dew and protects against herbivory