Abstract Induced defenses occur predominately in young,
developing plant tissues that rely upon carbohydrate im-
port to support their growth and development. To test the
hypothesis that the induced production of carbon-based
defenses is dependent upon photoassimilate import, we
examined the response of developing leaves of hybrid
poplar (Populus deltoides × P. nigra) saplings to wound-
ing by gypsy moth caterpillars (Lymantria dispar L.) and
exogenous jasmonic acid (JA). Growth rates, condensed
tannin contents and acid invertase activities were mea-
sured for individual leaves and the translocation of
13
C-labeled resources between orthostichous source-sink
pairs was quantified. Results showed a substantial in-
crease in the activity of cell wall invertase in sink leaves
wounded by gypsy moth caterpillars and treated with JA.
JA-induced sink leaves also imported 3–4 times as much
13
C-labeled carbon from orthostichous source leaves rel-
ative to controls and allocated a significant portion of
this imported
13
C to condensed tannin biosynthesis. Re-
duced carbohydrate flow to these leaves, caused by
source leaf removal, resulted in reduced condensed tan-
nin levels and the emergence of a growth–defense trade-
off. These results indicate that (1) induced sink strength
is elicited by insect wounding and JA application in hy-
brid poplar foliage, (2) imported resources are allocated
to the production of carbon-based defenses, and (3) the
level of induced defense in leaves can be constrained by
the ability of leaves to import carbohydrates from source
tissues. Together, these results suggest that within-cano-
py variations in induced resistance may arise in part be-
cause of uneven distribution of resources to induced fo-
liage.
Keywords Tannin biosynthesis · Sink strength · Populus ·
Plant defense
Introduction
Responsiveness of plant tissues to biotic and abiotic
stimuli tends to wane with age. This trend has been
observed for responses to herbivores (Wolfson and
Murdock 1990; Hartley and Lawton 1991; Karban and
Baldwin 1997; Kearsely and Whitham 1997; McAuslane
et al. 1997; Wait et al. 1998; Agrawal et al. 1999;
Cipollini and Bergelson 2000), to pathogens (Braga
1986; Coleman et al. 1987; Meier et al. 1993; Chen et al.
2000) and abiotic stresses (Coleman et al. 1987; Wait et
al. 1998). Similarly, production of morphological defens-
es such as thorns, spines, and hairs (e.g., Baur et al.
1991) and elective abscission in response to insects or
disease (Williams and Whitham 1986; Preszler and Price
1993; Vloutoglou and Kalogerakis 2000) can be induced
only in young, growing tissues. This produces plants that
are extremely heterogeneous spatially and temporally,
which is thought to complicate foraging by herbivores
and perhaps slow their adaptation to plant chemistry
(Whitham and Slobodchikoff 1982; Schultz 1983; Karban
and Baldwin 1997; Honkanen and Haukioja 1998;
Orians et al. 2000). Youthful induction could be adap-
tive, comprising preferential defense of more valuable
tissues (Krischik and Denno 1983; Bazzaz 1984), or may
simply reflect physiological (Mooney and Gulmon 1982)
or genetic (Whitham and Slobodchikoff 1982) con-
straints.
It is not clear how young leaves develop induced re-
sponses since they do not generally possess the required
resources to support them. Developing leaves typically
have few endogenous reserves (Kozlowski and Pallardy
1997) and often have lower photosynthetic rates com-
pared with mature leaves (Larson and Gordon 1969;
Larson et al. 1972; Hanson et al. 1988a, b). Until they
become photosynthetically competent, young leaves are
physiological sinks and must import materials from ex-
T.M. Arnold (
✉
) · J.C. Schultz
Pesticide Research Laboratory, Pennsylvania State University,
University Park, PA 16802, USA
e-mail: arnoldt@cofc.edu
Present address:
T.M. Arnold, Department of Biology, College of Charleston,
SC 29401, USA
Oecologia (2002) 130:585–593
DOI 10.1007/s00442-001-0839-7
PLANT ANIMAL INTERACTIONS
Thomas M. Arnold · Jack C. Schultz
Induced sink strength as a prerequisite for induced tannin biosynthesis
in developing leaves of
Populus
Received: 2 April 2001 / Accepted: 1 October 2001 / Published online: 14 November 2001
© Springer-Verlag 2001