Regulation of Photosynthetic Light Energy Capture
19
The example shown in Fig. 2.1 represents a leaf with a moderately high
photosynthetic capacity and in the absence of any other stress. The amount
of excess PFD, and hence the degree of center closure, obviously would be
greater in leaves having a lower photosynthetic capacity or in the presence
of environmental stresses that impose a further limitation on the photosynthetic rate. We wish to emphasize that these are predictions based on
the assumption that there were no regulation of energy interception or
dissipation.
2.3 Regulation of Light Interception
There are at least four different ways in which changes in light interception
can be achieved: changes in leaf orientation relative to the solar beam,
changes in leaf reflectance, rearrangements of the chloroplasts within the
leaf, and changes in the amount of chlorophyll per leaf area. Of these means
of regulating light interception, changes in leaf orientation are probably the
most powerful.
2.3.1 Changes in Leaf Orientation
Developmental. changes in leaf angle in response to the light environment
have long been recognized as a phenomenon common to many land plants.
In general, leaves developing in the shade have a predominantly horizontal
orientation and are arranged in a single layer, whereas those developing in
full sun often have steeper angles, especially under conditions of water stress
(Ehleringer 1988). A steeply angled leaf obviously intercepts considerably
less of the solar radiation during the midday hours when the radiation
load is maximal. This is especially important in water-stressed leaves when
stomatal closure leads to a severely reduced photosynthetic rate and prevents
the dissipation of the absorbed radiation as latent heat. A near-vertical leaf
orientation may therefore prevent both overexcitation of the photosynthetic
reaction centers and overheating the leaf, which may be lethal. A disadvantage is that such developmental changes in leaf angle are largely
irreversible when the stress is relieved.
Passive Leaf Movements. The passive wilting of many species such as sunflower and the leaf rolling in many grasses, such as Sorghum spp., are
common consequences of water stress in tissues without secondary cell
walls. Such wilting can be quite effective in reducing the energy load on the
plant, and may decrease water loss by as much as 50 to 70% (Begg 1980).
Since leaves without secondary wall thickenings are very sensitive to changes
19
The example shown in Fig. 2.1 represents a leaf with a moderately high
photosynthetic capacity and in the absence of any other stress. The amount
of excess PFD, and hence the degree of center closure, obviously would be
greater in leaves having a lower photosynthetic capacity or in the presence
of environmental stresses that impose a further limitation on the photosynthetic rate. We wish to emphasize that these are predictions based on
the assumption that there were no regulation of energy interception or
dissipation.
2.3 Regulation of Light Interception
There are at least four different ways in which changes in light interception
can be achieved: changes in leaf orientation relative to the solar beam,
changes in leaf reflectance, rearrangements of the chloroplasts within the
leaf, and changes in the amount of chlorophyll per leaf area. Of these means
of regulating light interception, changes in leaf orientation are probably the
most powerful.
2.3.1 Changes in Leaf Orientation
Developmental. changes in leaf angle in response to the light environment
have long been recognized as a phenomenon common to many land plants.
In general, leaves developing in the shade have a predominantly horizontal
orientation and are arranged in a single layer, whereas those developing in
full sun often have steeper angles, especially under conditions of water stress
(Ehleringer 1988). A steeply angled leaf obviously intercepts considerably
less of the solar radiation during the midday hours when the radiation
load is maximal. This is especially important in water-stressed leaves when
stomatal closure leads to a severely reduced photosynthetic rate and prevents
the dissipation of the absorbed radiation as latent heat. A near-vertical leaf
orientation may therefore prevent both overexcitation of the photosynthetic
reaction centers and overheating the leaf, which may be lethal. A disadvantage is that such developmental changes in leaf angle are largely
irreversible when the stress is relieved.
Passive Leaf Movements. The passive wilting of many species such as sunflower and the leaf rolling in many grasses, such as Sorghum spp., are
common consequences of water stress in tissues without secondary cell
walls. Such wilting can be quite effective in reducing the energy load on the
plant, and may decrease water loss by as much as 50 to 70% (Begg 1980).
Since leaves without secondary wall thickenings are very sensitive to changes