How Can Plants Choose the Most Promising Organs? - Communication in Plants: Neuronal Aspects of Plant Life - page 56

4.2.2
Stress Increases Competition
The two-shoot pea plants were stressed by removing parts of the reserve
seed tissues (the cotyledons, Fig. 4.2b). This treatment, in addition to the
obvious reduction in shoot growth, also increased shoot inequality (Sachs
and Novoplansky 1997). When the larger shoot was removed the weaker
one always survived and grew to regenerate a new plant (Fig. 4.1f). This
suggests that the weaker shoot is responding not only to stress but also to
an enhanced competition with the stronger shoot. Such results were not
limited to cases where there was a reduction of reserve storage materials.
Thestressofweaklightandlimitedmineralnutritionhadthesameeffects
(Fig. 4.2c,d).
4.2.3
Unequal Light Conditions
The two shoots could be subjected to different conditions, simulating en-
vironmental heterogeneity at the level of an individual plant (Novoplansky
et al. 1989). Reducing the light available to one of the two shoots strongly
inhibited its growth and increased shoot inequality (Fig. 4.3e). When light
was excluded from one shoot, it died within 2 weeks in 80% of cases
(Fig. 4.3f). Here, again, the response depended on the presence of the shoot
that remained in the light. When the more fortunate shoot was removed,
the darkened shoot elongated rapidly, forming small, yellow leaves. This
is the common form of seedlings whose entire shoot system is in dark
conditions, part of the etiolation syndrome.
4.2.4
TheRateofShootDevelopmentandLeafRemoval
The inequality of the two shoots tended to increase with time, and relations
were not reversed unless the individual shoots were subjected to additional
treatments. This suggests that relative shoot size had an influence on the
subsequent development. Size is a parameter that can be readily manipu-
lated, simulating natural herbivory. When the entire top part of one shoot
wasremovedtheshootcouldcontinuedevelopmentonlybymeansofone
or more of the buds subtended by its remaining leaves. Such development
occurred readily, but only when the competing intact shoot was removed
(Fig. 4.1g). A bud is thus at a pronounced disadvantage relative to a grow-
ing shoot apex. This is, of course, the bud inhibition mentioned before, the