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Chapter 35 Roots and mineral nutrition
Objectives:
1. Understand root anatomy
2. Understand how water and minerals move
from the soil to the xylem of root systems
3. Understand secondary growth in roots
4. Learn about soil and its importance
Primary Roots
Fibrous roots
Taproot
• Tissues
– epidermis
– ground tissues (cortex, pith)
– vascular tissues (xylem and phloem)
Adventitious roots: arise from the stem!
Root apical
meristem
(area of cell
division)
• Root cap
– protective layer that covers the root tip
– covers delicate root apical meristem
– may orient root so that it grows
downward
Root
cap
Root
cap
250
µm
Fig. 35-2a, p. 750
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• Epidermis
water
– protects the root
Root
hairs
Soil
air
• Root hairs
– short-lived extensions of epidermal
cells
– help absorb water and dissolved
minerals
Soil
particles
Epidermis
Root
hairs
Fig. 35-2b, p. 750
(meristematic)
• Cortex
– parenchyma cells, often store starch
• Endodermis
– innermost layer of cortex
– regulates movement of water and
minerals into root xylem
Stele (Vascular Cylinder)
• Pericycle
– gives rise to lateral roots, lateral meristems
• Xylem
– conducts water and dissolved minerals
• Phloem
– conducts dissolved sugar
Xylem and Phloem
• Herbaceous eudicot root
– xylem forms solid core in center of root
• Monocot root
– center of pith surrounded by ring of alternating
xylem and phloem bundles
– no vascular cambium
– no secondary growth
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Epidermis
(meristematic)
Cortex
Stele
250
µm
Eudicot
root
Fig. 35-3a, p. 751
Cortex cells
Monocot Root
Endoderm
is cell
Pericycle
cell
Phloem
cell
Xylem
vessel
elements
Stele of eudicot
root
25
µm
Fig. 35-3b, p. 751
Inner laryer
of cortex
Fig. 34-5
ATP and ion transport
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Secondary Vascular Tissues
Prop roots
Adventitious roots
pneumataphores
Haustoria
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Soil:
Symbiotic relationships: a close
association between two unrelated
organisms
Weathering (parent rock)
•Inorganic
particles
• Mycorrhizae (roots associated with
fungi)
•organisms
• Rhizobia (nitrogen fixing bacteria) - Nodules (house bacteria)
- plants get nitrogen from bacteria
- bacteria turn atmospheric N to NH3
Negative charges
K+ Mg+ bind to negs.
Contractile roots (bulbs)
Loam: 20%, 40%, 40%
Too much sand: lose water/minerals
Too much clay: lack air water
Cation exchange
16 elements are needed for plant growth
30-60% pore spaces
- water
- minerals
- gases
• Macronutrients (9) + Micronutrients (7)
•
•
•
•
Carbon
Oxygen
Hydrogen
Nitrogen
Soil water or
atmosphere
Leaching
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Humans and soil
• Farming can deplete minerals
- mineral removal
- adding minerals (N, P, K+)
- Salinization
• Soil erosion:
4.4 billion tons/year
1/3 of U.S. top soil is gone!!!
Salinization !!
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