PLANT SCIENCE

1
PLANT SCIENCE
2
Longitudinal Section through a leaf Structure
Function for Photosynthesis and Transpiration
3
Factors affecting Transpiration

• The rate of water loss from plant stems and
  leaves varies depending on internal and external
  conditions
• Main internal condition?
• Caused by?

4
Scanning electron micrographs of epidermis
w/guard cells stoma(ta)
5

• The roles of ion channels in ABA signaling 
  Figure 1. A model for roles of ion channels in
  ABA signaling.

6

• ABA triggers cytosolic calcium (Ca2cyt)
  increases (McAinsh et al., 1990 Fig. 1, left
  panel). Ca2cyt elevations activate two
  different types of anion channels
  Slow-activating sustained (S-type Schroeder and
  Hagiwara, 1989) and rapid transient (R-type
  Hedrich et al., 1990) anion channels. Both
  mediate anion release from guard cells, causing
  depolarization (Fig. 1, left panel). This change
  in membrane potential deactivates
  inward-rectifying K (Kin) channels and
  activates outward-rectifying K (Kout) channels
  (Schroeder et al., 1987), resulting in K efflux
  from guard cells (Fig. 1, left panel). In
  addition, ABA causes an alkalization of the guard
  cell cytosol (Blatt and Armstrong, 1993) which
  directly enhances Kout channel activity (Blatt
  and Armstrong, 1993 Ilan et al., 1994 Miedema
  and Assmann, 1996) and down-regulates the
  transient R-type anion channels (Schulz-Lessdorf
  et al., 1996). The sustained efflux of both
  anions and K from guard cells via anion and
  Kout channels contributes to loss of guard cell
  turgor, which leads to stomatal closing (Fig.
  1).       As vacuoles can take up over 90 of
  the guard cells volume, over 90 of the ions
  exported from the cell during stomatal closing
  must first be transported from vacuoles into the
  cytosol (MacRobbie, 1998 MacRobbie, 1995).
  Ca2cyt elevation activates vacuolar K (VK)
  channels proposed to provide a pathway for
  Ca2-induced K release from the vacuole (Ward
  and Schroeder, 1994). At resting Ca2cyt, K
  efflux from guard cell vacuoles can be mediated
  by fast vacuolar (FV) channels, allowing for
  versatile vacuolar K efflux pathways (Allen and
  Sanders, 1996). The pathways for anion release
  from vacuoles remain elusive.       Stomatal
  opening is driven by plasma membrane
  proton-extruding H-ATPases. H-ATPases can drive
  K uptake via Kin channels (Fig. 1, right panel
  Kwak et al., 2001). Cytosolic Ca2 elevations in
  guard cells down-regulate both Kin channels
  (Schroeder and Hagiwara, 1989) and plasma
  membrane H-ATPases (Kinoshita et al., 1995),
  providing a mechanistic basis for ABA and Ca2
  inhibition of K uptake during stomatal opening
  (Fig. 1, right panel).

7
Abiotic factors affecting transpiration rates

• Light intensity
• increases rates
• Darkness stomatal closure and therefore
  decreases transpiration rates

8
Temperature

• Heat is required for evaporation of water from
  surface of spongy mesophyll cells as temp
  increases transpiration does also
• Higher temps increases osmosis rates within the
  spongy mesophyll cells

9
Humidity

• Leaf air spaces are almost always nearly
  saturated
• Therefore the lower the humidity outside the leaf
  the greater the concentration gradient and the
  greater the rate of transpiration and vice versa.

10
Wind

• Wind disrupts any potential build-up of water
  vapor near and around the stomata
• Wind therefore ensures a gradient for
  transpiration to continue .
• No wind lower rates

11
Specific example Transpiration in Xerophytes

• Plants that are adapted to grow in very dry
  environments
• Cereus giganteus saguaro cactus
• Locale - Desert southwest
• Adaptations include

12

• Vertical stems
• Very thick waxy cuticle
• Spines instead of leaves
• CAM carbon fixation physiology

13
CAM revisited