increases light interception by leaves, it also increases the heat load for these plants,
which could be disadvantageous, particularly if water is limited. In fact, some solar-
tracking species can change their leaf orientation from being perpendicular to the direc-
tion of light to parallel to the light, depending on water stress (Shackel and Hall 1979;
Forseth and Ehleringer 1980). Also, in many arid environments the leaves of plants that
do not solar track become more vertical with increasing light intensity, apparently as a
protective mechanism against photodamage from excess light (King 1997; Valladares and
Pugnaire 1999; Falster and Westoby 2003).
In young seedlings, the primary shoot generally grows upward in a seemingly straight-
forward integration of positive phototropism and negative gravitropism, and the primary
root grows downward in a similarly straightforward combination of positive gravitropism
and negative phototropism (Okada and Shimura 1992). However, the bulk of a mature
plant is made up of lateral organs—branches and leaves in the shoot and lateral and ad-
ventitious roots belowground—which grow at nonvertical orientations even in the ab-
sence of a phototropic stimulus. It is the growth of these lateral branches that gives plants
their characteristic form, and they have large effects on plant productivity.
Because these nonvertical orientations appear to be actively maintained via gravitropic
responses, Digby and Firn (1995) have termed the growth orientation of these organs to
be their gravitropic set-point angle, or GSA (see also Chapter 2 of this book). The GSA
of both shoots and roots can be altered by red light (Gaiser and Lomax 1993; Digby and
Firn 2002; Mullen and Hangarter 2003). Thus, the fine-tuning of organ positioning in
mature plants by directional light cues requires a more complex integration of growth re-
sponses, involving not only phototropism, gravitropism, and their interactions, but also
light-dependent changes in GSA. Because the GSA is a developmentally regulated vari-
able (Digby and Firn 1995), light regulation of GSA allows for differences in response
depending on the age of the specific organ. This may allow different parts of a mature
plant to tailor their growth in an appropriate manner for the specific environmental con-
ditions they encounter.
4.6 Conclusions and outlook
Tremendous progress in understanding the mechanisms of phototropism has been made
in recent years. Although phototropism has been intensively studied since the time of
Darwin’s classic experiments, it is only within the past decade that we have identified the
phototropins as the primary pigments involved in light perception in phototropism. It has
become increasingly obvious that the other two major groups of photoreceptors, the cryp-
tochromes and phytochromes, interact with phototropins and play both direct and indirect
roles in phototropic responses. Much of the current research focuses on a better under-
standing of the cellular and molecular events downstream from the primary photorecep-
tors. There is increased recognition that the interaction between and among the primary
photoreceptors is important in phototropism and other important light-regulated develop-
mental processes (Mas et al. 2000; Folta and Spalding 2001; Whippo and Hangarter
2003; Lariguet and Fankhauser 2004; Kumar and Kiss 2007).
One recent approach we have used to better understand the relationship between pho-