PLANT PHYSIOLOGY , Vol 103, Issue 4 1097-1106, Copyright © 1993 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
Developmental Variability of Photooxidative Stress Tolerance in Paraquat-Resistant Conyza
Z. Amsellem, MAK. Jansen, ARJ. Driesenaar and J. Gressel
Departments of Plant Genetics (Z.A., M.A.K.J., J.G.) and Biochemistry (A.R.J.D.), The Weizmann Institute of Science, Rehovot 76100, Isarel
Paraquat-resistant hairy fleabane (Conyza bonariensis L. Cronq.) has been
extensively studied, with some contention. A single, dominant gene
pleiotropically controls levels of oxidant-detoxifying enzymes and
tolerance to many photooxidants, to photoinhibition, and possibly to other
stresses. The weed forms a rosette on humid short days and flowers in dry
long days and, thus, needs plasticity to photooxidant stresses. In a series
of four experiments over 20 months, the resistant and susceptible biotypes
were cultured in constant 10-h low-light short days at 25[deg]C. Resistance
was measured as recovery from paraquat. The concentration required to
achieve 50% inhibition of the resistant biotype was about 30 times that of
the susceptible one just after germination, increased to >300 times that
of the susceptibles at 10 weeks of growth, and then decreased to 20-fold,
remaining constant except for a brief increase while bolting. Resistance
increased when plants were induced to flower by long days. The levels of
plastid superoxide dismutase and of glutathione reductase were generally
highest in resistant plants compared to those of the susceptibles at the
times of highest paraquat resistance, but they were imperceptibly different
from the susceptible type at the times of lower paraquat resistance.
Photoinhibition tolerance measured as quantum yield of oxygen evolution at
ambient temperatures was highest when the relative amounts of enzymes were
highest in the resistant biotype. Resistance to photoinhibition was not
detected by chlorophyll a fluorescence. Enzyme levels, photoinhibition
tolerance, and paraquat resistance all increased during flowering in both
biotypes. Imperceptibly small increases in enzyme levels would be needed
for 20-fold resistance, based on the moderate enzyme increases correlated
with 300-fold resistance. Thus, it is feasible that either these enzymes
play a role in the first line of defense against photooxidants, or another,
yet unknown mechanism(s) facilitate(s) the lower level of resistance, or
the enzymes and unknown mechanisms act together.
