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

110
T.D. Sharkey and S.M. Schrader
1997). The authors of this study concluded that stomatal closure resulted in less
evaporative cooling in the canopy and so an increase in canopy temperature relative to
a crop grown in today’s level of CO 2
and at the same air temperature. Thus, energy
balance considerations can cause elevated CO 2
to reduce grain yield. Because rice is a
major crop and is grown in many environments near its high temperature limit, the effect
of high temperature stress on reproductive yield is a pressing problem deserving of
substantial effort, if major crop failures are to be avoided as global temperatures rise.
3.2. Seedling Establishment
Heat can limit plant growth at the seedling stage, because the temperature near the soil
can be very high as a result of a boundary layer of air near the soil surface (Campbell
and Norman, 1998). Soil temperature can exceed 50°C when the sun is bright. Metabolism
in seedlings of many species can respond to heat through the induction of hsps
(Vierling, 1991). Many of the studies of hsp-derived thermotolerance use assays of
seedling establishment (Hong and Vierling, 2000; Queitsch et al., 2000; Hong et al.,
2003), which is appropriate, even though hsps have effects well beyond their effect on
this phase of the plant’s development.
3.3. Photosynthesis
Even in the absence of any injury, photosynthesis of C 3
plants would be expected to
decline as temperature increases because photorespiration increases with temperature
faster than does photosynthesis (Schuster and Monson, 1990). However, it is also well
known that heat directly damages the photosynthetic apparatus, with photosystem II
often considered a key weak link (Santarius, 1975; Santarius and Müller, 1979; Berry and
Björkman, 1980) but only above 45°C (Terzaghi et al., 1989; Thompson et al., 1989;
Gombos et al., 1994; Çajánek et al., 1998). One effect of high temperature is destruction
of the oxygen-evolving complex with the loss of a 33 kDa extrinsic protein and Mn 2+
(Enami et al., 1994b). However, while reversible reductions in PSII-dependent electron
transport can be seen at less than 40°C, irreversible effects and loss of the 33 kDa
protein occur at >42°C (Yamane et al., 1998). Thus, damage to PSII cannot explain
widely observed, heat-induced depression in photosynthesis seen at temperatures
between 35°C and 40°C. This chapter will focus primarily, but not exclusively, on the
effects of moderately high temperature (35°C to 40°C) on photosynthesis.
3.3.1. The Role of Stromal Proteins
There are some studies linking heat shock proteins and photosynthetic capacity
(Heckathorn et al., 1998; Downs et al., 1999; Heckathorn et al., 2002; Barua et al., 2003).
The chloroplast-localized small hsp appears to be correlated with temperature range of