Chapter 5 - Light (PPT) Flashcards
Light
- energy source for photosynthesis
- often determines plant morphology
- often a trigger for fall leaf color, abscission, and flowering
Electromagnetic radiation
- both a wave
- and a pulse of energy (photons or quanta)
Light quality (wavelength λ)
- measured in nm (10^-9m)
- 380 to 770 nm
- the shorter the wavelength, the higher the energy
- plants don’t absorb green light as well as blue or red light
Light quantity (intensity)
- how bright the light is
- best measured as photosynthetic photon flux (ppf)
Light duration (photoperiod)
- how long the day is vs. the night
- Affects photosynthesis (Food production) AND plant development (Trigger)
Photosynthesis
12H2O + 6CO2 –> C6H12O6 + 6O2 + 6H2O
- A plant that gets no light dies quickly
- A plant that gets too little dies slowly
Chloroplasts
- Organelles that contain chlorophyll
- Membranes inside are called thylakoids
- Stacks of thylakoids are called grana
- Colorless areas in between are stroma
Chloroplast Pigments
- Chlorophyll
- Chlorophyll a (two types)
- Cholorphyll b
- Absorb primarily in red and blue wavelengths
- Carotenoids
- Xanthophylls
Photosynthesis Process
- Pigments are excited by light energy
- Chlorophyll b and carotenoids pass energy to chlorophyll a (accessory pigments)
- Makes energy for the dark reactions (CO2 fixation)
Dark Reaction Process (CO2 fixation)
-CO2 enters leaves and is fixed to another compound
-Fixation pathways:
-C3 - three-carbon molecule (most hort plants)
-C4 - four-carbon is also used (corn, bermudagrass)
(C3 and C4 processes are spatially separated)
-CAM - uses both pathways within the same cell (moss rose, pineapple)
C3 vs C4 vs CAM
-Under cool temps, C4 and CAM pathways have no advantage
- Under warm temps, C3 pathway struggles:
- Photorespiration - oxygen is fixed rather than CO2 (creates toxins, wastes energy to break those toxins down)
- C4 pathway uses an enzyme with greater affinity for CO2 (most efficient)
- CAM plants open stomates at night and fix CO2 then close stomates during the day and do photosynthesis while preventing water loss
- ONLY AN ADVANTAGE UNDER WARM CONDITIONS:
- C4 and CAM = warm season plants
- C3 = cool season plants
Spacing & Orientation
- makes best use of light
- increases air circulation (help prevent and control disease)
- Increase plant quality (size, shape, color, fullness, shelf life)
Optimal Spacing
- space costs money - especially in GH
- space is often wasted when plants are young (orchards, veggie gardens)
- intercropping or thinning might be used to avoid wasting space
- balance
- space far enough to maximize yield w/o wasting space (losing profit)
- use proper methods to avoid poor germination
- get rid of weeds (mulches, herbicides, cultivation, mowing)
- GH and hydroponics may allow for re-spacing efficiencies, BUT labor is expensive
- in nurseries, intermix rows of upright plants with rows of spreading plants
- in gardens consider multiple rows rather than single rows
- in landscapes, consider the final size and plant height!
Orientation Issues
-Row orientation can make a difference, but other logistical factors may trump
- GH orientation is also commonly debated:
- above 40 degrees N, ridge should be East-West
- Below 40 degrees N, ridge should be North-South
- Gutter-connected should be North-South
- BUT, wind also has an effect
