Exam 3 Indoor Crop Production

Temperature → Energy balance

¤Energy received by plants includes:
Absorbed radiant energy from lamps
Absorbed infrared radiation from surroundings

¤Energy leaving plants includes:
Energy lost through emitting infrared radiation

Temperature → Radiation

¤Leaves have low absorption in near infrared range (700 – 1,500 nm), most reflected or transmitted

¤Leaves have high absorption in far infrared range (1,500 – 30,000 nm), contributes significantly to thermal energy load P

Primary sources of radiant energy:
_Lamps
-Reflectors

Temperature → Radiation (Sources in production)

¤HPS lamps have surface temperature of over 100 oC and emit large quantities of far infrared radiation

-Results in increased leaf temperature under these sources regardless of air temperature

-As comparison, LEDs have surface temperature of ~30 oC and fluorescent lamps of ~40 oC

Temperature → Energy balance (in Production)

¤Energy received by plants includes:
-Absorbed radiant energy from lamps
-Absorbed infrared radiation from surroundings

¤Energy leaving plants includes:
-Energy lost through emitting infrared radiation
-Heat convection
-Heat conduction
-Heat loss through evaporation

Temperature → Heat conduction

Heat transferred via conduction from leaf cells to air molecules (energy flows from high energy molecules to low energy molecules) -Limited without convective movement due to low thermal conductivity of air
-Rate of flow depends on temperature differentia

Temperature → Heat convection

Heat is transferred via convection when air moves across the plant. Two types of convection:

Free (natural) → heat transferred from leaves causes air to warm, expand, and decrease in density. Buoyant warm air moves upward away from plant

Forced → caused by wind or fans. Speeds of more than 0.5 m·s–1 are required for gas exchange, so 0.5 – 1.0 m·s–1 is common target

T/F?
Plant growth and development rates are NOT temperature dependent.

FALSE:

Plant growth and development rates ARE temperature dependent

-If temperatures are within the linear range, biomass and developmental stage can be well correlated with cumulative temperature

Temperature → Basis of: “Growing Degree Days” or “Heat Unit”

(Growing Degree Days =
(maximum temp. + minimum temp.)/2)-Base Temp.

**base temp is crop specific→

-Field crop production
-Outdoor temperature may be above/below optimum

T/F? For indoor production, consideration of these thresholds isn’t necessary

EXAMPLE: 
leafy crop that is harvestable after 40 d when grown under average temperature of 20 oC
-Cumulative temperature = 800 degree days

what is average temperature was 22 oC instead??

¤If average temperature was 22 oC instead, crop would be harvestable after 36 d (800/22 = 36.4)

Humidity (VPD)

Measure of water vapor in the air

¤Air can hold more water vapor at higher temperatures
-relative humidity is temperature-dependent

Humidity (VPD) → Relative humidity

¨ Relative humidity
– water vapor content of air based on the maximum amount of water the air can hold for a given temperature and pressure

¤Often expressed as percentage of water vapor content to the maximum at a given temperature

¤Thus, if air temperature decreases with no change in water vapor content…what happens to relative humidity? → 
IT INCREASES

Plants add water vapor to the air through _________?

Plants add water vapor to the air through transpiration

E→ transpiration
V_in→ Water in leaf
V_air→ Water vapor concentration in air
R_s→ Stomatal resistance (how closed are stomates)
R_b→ Boundary Layer resistance (thickness of still air surrounding leaf)

¤This must be dealt with for indoor vertical production
¤Air-conditioning can take care of this as water vapor condenses on cooling coils or dehumidifiers must be used
¤Condensation can be recycled to conserve water

Transpiration rate is affected by:

¤Water vapor concentration in air
¤Leaf temperature
¤Boundary layer and stomatal resistances

Vapor pressure deficit (VPD)

the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated at the same air temperature and is expressed in units of pressure (kPa)

¤Ideal range for indoor production is 0.8 – 0.95 kPa

When VPD is low

When VPD is low, transpiration will be inhibited and can lead to condensation on leaves and surfaces

¤Ideal range for indoor production is 0.8 – 0.95 kPa

When VPD is high

When VPD is high, plant must draw more water from roots to prevent wilting (can result in closed stomata)

¤Ideal range for indoor production is 0.8 – 0.95 kPa

Psychometric Chart

Graphical representation of the relationship between: Water vapor, Temperature, Energy

Absolute humidity

Actual water vapor content of the air (g/kg air)

Psychometric Chart → Saturation curve:

air at a specific temperature can contain a maximum amount of water vapor (g/kg)

¤If the air is cooled beyond the saturation point, what happens?