Interplay Between Environmental Signals and Endogenous Salicylic Acid Concentration - Salicylic Acid: Plant Growth and Development - page 64

higher by ca. 50 % at low light irradiance relative to dark (Genoud et al. 2002 ).
However, a further increase in light irradiance had no significant effect on
endogenous SA levels (Zeier et al. 2004 ). For sunflower (Helianthus annuus L.), a
plant that is adapted to grow best at full sun light, endogenous SA levels in
hypocotyl tissue showed consistent and appreciable (ca. 10-fold) increases as light
irradiance levels increased from very low to low and then to full sunlight (Kurepin
et al. 2010a ). These increases in endogenous SA levels of sunflower shoot tissue at
the increased light irradiance levels were associated with significant decreases in
hypocotyl elongation and biomass accumulation (Kurepin et al. 2010a ). Thus, the
difference in the endogenous SA response and its magnitude in response to
changes in light irradiance levels, vary between A. thaliana and sunflower. The
changes in growth and endogenous SA levels seen in the plant's response to light
irradiance level can also be ecotype-specific. The Stellaria longipes sun ecotype
grows in an alpine habitat characterized by very short plants and distant sur-
rounding vegetation. In contrast, the S. longipes shade ecotype grows in nearby
low elevation prairie grasslands, where the habitat is characterized by tall neigh-
bouring vegetation which causes canopy shading and/or shading by neighbours.
These two ecotypes grow just a few kilometres apart, but the habitat differences
caused by elevation make them an excellent model for light signaling research
(Emery et al. 1994 ). Plants of both ecotypes decrease their shoot growth in
response to increase in light irradiance levels and endogenous SA levels increase
coincidentally (Kurepin et al. 2012a ). This decrease in shoot growth in response to
an increase in light irradiance was proportionally similar for both sun and shade
ecotypes of S. longipes. However, the magnitude of changes in endogenous SA
levels varied between the two ecotypes. The sun ecotype plants showed less than a
2-fold increase in endogenous SA levels. Yet shade ecotype plants had more than a
3-fold increase in endogenous SA levels (Kurepin et al. 2012a ). Thus, the mag-
nitude of the SA concentration in response to increasing irradiance levels can vary
depending on species and ecotype. Even so, the high light irradiance-mediated
increase in endogenous SA levels may have ecological significance. For example,
it is a well-known fact that there is a higher susceptibility of younger trees to
pathogen attack in understory environments. This may be because the fungal
activity in understory environments is greater than in forest openings (Kitajima
and Augspurger 1989 ). Thus, when seeds of Betula papyrifera Marsh., (a species
of birch native to North America) were planted in a range of habitats differing in
light availability (from understory to open forest), the application of a fungicide
reduced losses in a habitat-dependent manner (O'Hanlon-Manners and Kotanen
2004 ). Thus, the fungicide was more effective in understory than in open habitats.
One possible explanation is that low light irradiance in the understory forest
habitat prevents the establishment of B. papyrifera plants because their ability to
modify endogenous SA levels in response to pathogen attack is depressed.
In both sunflower and S. longipes plants the high light irradiance-mediated
increase in endogenous SA levels was correlated with a decrease in shoot growth.
Simplistically, then, the role of SA could be classified as growth inhibitory for
light

irradiance-mediated responses. However, A. thaliana mutants with