Regulation of Photosynthetic Light Energy Capture
23
are kept essentially normal to the solar beam, thereby maximizing light
interception. However, when the plants are gradually subjected to water
stress, the leaves begin instead to move paraheliotropically. The onset of
such movements usually coincides with stomatal closure. During soil drying
cycles, paraheliotropic movements first become apparent for short periods
just after solar noon but the leaves are diaheliotropic for the rest of the
day. The duration of the paraheliotropic periods increases as the drought
intensifies until the leaves are paraheliotropic for most of the day. This
mode of response is illustrated in Fig. 2.4 in which the diurnal changes in
cos(i) of well-watered plants of Townsville stylo (Stylosanthes humilis) are
compared with those of a plant in the field at the beginning of the dry season
when the minimum leaf water potential had fallen to -2.6MPa. During
midday, the radiation intercepted by the leaves of the stressed plant was
only about 20% of that intercepted by those of the unstressed plant.
These light evasion responses can be induced in the absence of water
stress by direct exposure of the pulvini to high irradiance (Koller and Shak
1990), indicating that the primary elicitor of the paraheliotropic response is
high irradiance, and that water deficit and high temperature act by lowering
the threshold for that response.
2.3.2 Changes in Leaf Reflectance
Many species, mostly from arid climates, are capable of regulating light
interception in the longer term by changing leaf reflectance. This is achieved
by varying the degree of pubescence, i.e., air-filled hairs or trichomes, and
in some species through a deposit of salt crystals on the leaf surface. A
striking example is Encelia farinosa, a native of the low interior California
deserts. Leaves that develop during the months with moderate temperatures
and relatively favorable water relations are only weakly pubescent and the
leaf hairs are alive and water-filled. Leaf reflectance is low and hence the
absorptance is high (Fig. 2.5, upper curve). As drought and temperature
increase, the hairs increase in both density and length and gradually become
filled with air, resulting in a strongly increased reflectance (Ehleringer and
Bjorkman 1978). The development of pubescence is strongly correlated with
declining precipitation, increasing heat load and, especially, midday water
potential (Ehleringer 1982). Under extreme conditions, such as those found
in Death Valley, California, the heavily pubescent leaves appear nearly
white and absorb as little as 29% of the photosynthetically active radiation
(Fig. 2.5, lower curve). Because the leaf hairs lose their water, the absorptance of near-infrared radiation is also strongly reduced, resulting in a
further reduction of the heat load.
Responses similar to those in E. farinosa were also found in the South
American desert species E. canescens. In both western North America and
23
are kept essentially normal to the solar beam, thereby maximizing light
interception. However, when the plants are gradually subjected to water
stress, the leaves begin instead to move paraheliotropically. The onset of
such movements usually coincides with stomatal closure. During soil drying
cycles, paraheliotropic movements first become apparent for short periods
just after solar noon but the leaves are diaheliotropic for the rest of the
day. The duration of the paraheliotropic periods increases as the drought
intensifies until the leaves are paraheliotropic for most of the day. This
mode of response is illustrated in Fig. 2.4 in which the diurnal changes in
cos(i) of well-watered plants of Townsville stylo (Stylosanthes humilis) are
compared with those of a plant in the field at the beginning of the dry season
when the minimum leaf water potential had fallen to -2.6MPa. During
midday, the radiation intercepted by the leaves of the stressed plant was
only about 20% of that intercepted by those of the unstressed plant.
These light evasion responses can be induced in the absence of water
stress by direct exposure of the pulvini to high irradiance (Koller and Shak
1990), indicating that the primary elicitor of the paraheliotropic response is
high irradiance, and that water deficit and high temperature act by lowering
the threshold for that response.
2.3.2 Changes in Leaf Reflectance
Many species, mostly from arid climates, are capable of regulating light
interception in the longer term by changing leaf reflectance. This is achieved
by varying the degree of pubescence, i.e., air-filled hairs or trichomes, and
in some species through a deposit of salt crystals on the leaf surface. A
striking example is Encelia farinosa, a native of the low interior California
deserts. Leaves that develop during the months with moderate temperatures
and relatively favorable water relations are only weakly pubescent and the
leaf hairs are alive and water-filled. Leaf reflectance is low and hence the
absorptance is high (Fig. 2.5, upper curve). As drought and temperature
increase, the hairs increase in both density and length and gradually become
filled with air, resulting in a strongly increased reflectance (Ehleringer and
Bjorkman 1978). The development of pubescence is strongly correlated with
declining precipitation, increasing heat load and, especially, midday water
potential (Ehleringer 1982). Under extreme conditions, such as those found
in Death Valley, California, the heavily pubescent leaves appear nearly
white and absorb as little as 29% of the photosynthetically active radiation
(Fig. 2.5, lower curve). Because the leaf hairs lose their water, the absorptance of near-infrared radiation is also strongly reduced, resulting in a
further reduction of the heat load.
Responses similar to those in E. farinosa were also found in the South
American desert species E. canescens. In both western North America and