Light & Water

Question 1

Radiation that strikes on an object does three things

Answer 1

reflects off and back
absorption
transmission

Question 2

Greenhouse effect

Answer 2

Transmission of radiation through glass/atmosphere
Shorter wavelengths only
absorbed 
Objects in greenhouse heat up
Objects re-radiate as heat
long wavelengths only
Long waves cannot get back out through

Question 3

Light’s Characteristics

Answer 3

spectral quality (color or wavelength)
intensity (# of photons, amt. of energy)
duration (length of light period)
direction (angle of incidence)

Question 4

Spectral quality

Answer 4

380 violet
430 blue
500 green
560 yellow
600 orange
650 - 760 red

Question 5

Photosynthesis (pertaining to light)

Answer 5

Only red and blue light is absorbed by chlorophyll and other pigments
The absorbed part of the spectrum is called PAR
Photosynthetically Active Radiation

Question 6

Light Intensity (Quantity)

Answer 6

intensity is measured as micromoles/square meter/second
sunlight ranges between 1000-2000
interior light can range as low as 50

Question 7

Compensation point

Answer 7

at a certain light intensity, a plant captures as much energy through photosynthesis as it needs to survive
photosynthesis = respiration

at this point, the plant will neither die nor grow

below CP, a plant will die
above CP, a plant will grow

Question 8

Saturation Point

Answer 8

at a certain light intensity, a plant captures the maximum amount of energy through photosynthesis that it is able to do

At lower light intensities, a plant will respond to brighter light (grow faster)

Above the SP, brighter light has no effect on growth rate

Question 9

Compensation Point & Saturation Point (comparing shade and sun plants)

Answer 9

Sun-loving plant species tend to have a higher saturation point

Shade-loving species tend to have a lower compensation point

Question 10

Duration

Answer 10

plant growth rate and productivity is dependent on

• how much time the plant receives PAR above the Compensation Point
• how far above the CP it is

Question 11

Horticultural Practices with Light

Answer 11

spacing - light exposure
pruning - allow light penetration into tree or shrub canopy
eliminate weeds that compete for light
supplemental lighting

Question 12

Advances of supplemental lighting

Answer 12

increase intensity

increase duration

Question 13

Supplemental Lighting

Incandescent vs. Fluorescent

Answer 13

Supplemental
inefficient 
short life span
high in red portion
Fluorescent 
efficient 
expensive
high in blue

Question 14

HID

Answer 14

High Intensity Discharge lamps
-metal halide, sodium vapor 
-most efficient 
-spectral composition varies
metal halide - green/blue
sodium - red/orange

Question 15

LED

Answer 15

Light Emitting Diode lamps

• precise control of spectral composition, can change to match to plant needs, life cycle
• cool
• very efficient
• very long life span, low maintenance and disposal cost

Question 16

Other plant processes/ light

Answer 16

photoperiodism - LD vs. SD
seed dormancy - some seeds need light to germinate

branching & stem elongation
-both involve red vs. far red light

Question 17

Red / Far Red Light

Answer 17

much of red is absorbed

much of far red is transmitted

Question 18

Branching / stem elongation

Answer 18

sun - red:far red ration is balanced
shade - far red > red

red promotes short internodes, more branching in full sun
far red promotes less branching, longer internodes in the shade (“stretching” and more “open” plant for)

Question 19

What does water do in a plant

Answer 19

a solvent for chemical reactions
substrate for chemical reactions
carrier for mineral uptake
provides turgidity - maintains plant form and is a driving force for growth
provides cooling via evaporation 
reduces rapid temperature change

Question 20

Water in soil

Answer 20

water is found in spaces between soil particles

when spaces are full of only water (no air) the soil is saturated

water drains out of soil due to gravity

Question 21

The water soil holds after gravity is called

Answer 21

field capacity

larger pore capacity is filled with air, and the smaller spaces are filled with water

Question 22

Water removal from soil

Answer 22

evaporation
transpiration
through plant and incorporation into growth

Question 23

Factors influencing transpirational water loss by a plant

Answer 23

environmental

• temp
• humidity (higher humid, less water loss)
• wind
• light

plant properties

• leaf area, shape/form leaf
• stomata: density, size
• cuticle thickness and composition
• leaf surface: color, waxes, hairs, trichomes
• leaf exposure: aspect to sun, leaf curling

Question 24

When water is in short supply

Answer 24

at field capacity, water that a plant can use is called: available water

when all water is gone, there is still some water in soil, but it is held by the soil structure too tightly for plants to get; that stage is called the permanent wilting point

Question 25

At a time when transpirational water loss is greater than water absorption

Answer 25

stomata close
biochemical changes in plant cells
reduced growth and productivity 
leaf rolling and wilting
recovery if water is restored, or if severe enough, permanent wilting and plant death

Question 26

Supplemental water

types of irrigation

Answer 26

flood or furrow
supply canal
field canal
sprinkler 
pipe/risers/valve
nursery overhead irrigation 
water gun 
greenhouse auto-irrigation 

low volume
trickle, drip
capillary mat

Question 27

Issue: Water Use Efficiency, Nutrient Pollution, Recycling

Answer 27

Environmental concerns over wetlands preservation, pollution, etc. are changing horticulture so that more efficient water use, recapture and recycling of water are becoming more commonly used and eventually will be required by law

Question 28

Determining when to irrigate

Answer 28

aerial imaging - satellite/airplane
in soil devices - tensiometer, gypsum block, neutron probe, electrical conductivity meter
weather data (temp/precip./evap.)