Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

High Temperature Stress
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Nonphotochemical quenching of PSII chlorophyll fluorescence is attributed
to three components: energy dependent quenching, qE, photoinhibitory quenching,
qI, and state transition quenching, qT. The effects of qI are not discussed in this
review. Energy dependent quenching, qE, results from the acidification of the lumen
and is closely associated with PSII and LHCII activities, the deepoxidation of violaxanthin
to zeaxanthin in the xanthophyll cycle, and in photoprotection. It is assumed that a
large portion of heat related qN is due to qE because of the increased pH gradient and
induction of cyclic electron transport during heat stress. Although, qT may play a
significant role as it was noted earlier that mild heat stress causes a state I to state II
transition.
An important protective mechanism for PSII is the de-epoxidation of
violaxanthin to zeaxanthin. Havaux and Tardy (1996) noted an increase in zeaxanthin
during a 2 hr heat stress at 35°C in potato leaves and a concurrent increase in the
thermal stability of PSII, indicating that this protection is also important at elevated
temperatures. Further, Havaux et al. (1996) noted a stabilization of the thylakoid membranes
to heat stress by increasing the amount of zeaxanthin in leaves with ascorbate.
A high ion gradient across the thylakoid membranes may also increase ionic
interactions at the lipid headgroups and thus stabilize the structure of the thylakoids.
Several authors noted a protective effect of light, which presumably forms a pH gradient
across the thylakoid membranes, on photosynthesis. Weis (1981a) found that the
518 nm electrochromic shift decay rate was less when chloroplasts were incubated in
the light rather than the dark. Havaux et al. (1991) found that PSII fluorescence was
stabilized when leaves where heated in the presence of light rather than the dark. The
induction of cyclic electron transport and the deactivation of rubisco may be adaptive
responses to high temperatures rather than maladaptive responses.
The dichotomy of observations showing an increased energy gradient across
the thylakoid membranes while the permeability of the thylakoids increase during heat
stress may be explained by a reduced ATP and NADPH requirement of the Calvin cycle
because of the deactivation of rubisco and/or an increase in cyclic electron transport
through PSI. An increased energy gradient and decreased lumen pH serve as protective
mechanisms for PSII and the thylakoid membranes. The qE quenching of qN is well
understood (Horton et al., 1996), and as mentioned earlier, it is also known that qN
increases with temperature.
Many authors have noted interaction effects of various variables with heat
stress, including thermoprotection from isoprene, solutes, and CO 2
. Isoprene (C 5
H 8
, 2-
methyl 1,3-butadiene) is produced in large quantities by plants and has been shown to
increase the thermal tolerance of photosynthesis (Sharkey and Yeh, 2001). By removing
endogenous isoprene using a nitrogen atmosphere and comparing this to leaves
producing isoprene or adding exogenous isoprene to leaves, studies have found a shift
to higher temperatures in leaves before heat damage occurs (Sharkey and Singsaas,
1995; Singsaas et al., 1997). Further, fosmidomycin-fed leaves showed greater tolerance