Compressive/tensile stresses and lignified cells as resistance components in joints between cladodes of Opuntia laevis (Cactaceae): responses to applied stresses
Ls. Evans et al., Compressive/tensile stresses and lignified cells as resistance components in joints between cladodes of Opuntia laevis (Cactaceae): responses to applied stresses, ENVIR EXP B, 46(1), 2001, pp. 47-53
Cactaceae are a diverse group of plants with a wide variety of morphologies
and reproductive strategies. Many species have segmented stems in which te
rminal cladodes may be separated from main stem cladodes with varying amoun
ts of resistance. Previous results demonstrated that lignified xylem cells
in tensile portions of stem joints provide the main resistance to separatio
n of cladodes within cactus plants. The purpose of the present study was to
determine if stem joints of Oputia laevis would produce additional lignifi
ed xylem cells in response to additional externally applied stresses. Norma
l average stress levels, which accompany the addition of a new cladode, wer
e applied to 12 plants. In contrast, double the average stress levels were
applied to 13 other plants. After exposure to the two stress regimens for 6
months, the amount and location of lignified xylem cells in joint segments
were similar for both stress treatments. So, although the results support
the hypothesis that lignified xylem cells act as the main resistance to str
ess at joints of cladodes, doubling the normal amount of applied stress was
insufficient to alter the amount or location of lignified xylem cells in s
tem joints. These results indicate that normal amounts of lignified xylem c
ells can resist up to two times the normal amount of stress for 6 months wi
thout producing additional lignified xylem cells. (C) 2001 Elsevier Science
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