Agriculture Reference
In-Depth Information
variety of osmolytes such as sugars and sugar alcohols (polyols), proline,
tertiary and quaternary ammonium compounds, and tertiary sulphonium
compounds (Sairam and Tyagi, 2004). In tomato, fruit set failed due to
the disruption of sugar metabolism and proline transport during pollen
development under high temperature condition (Sato et al., 2006). Hexose
sensing in transgenic plants engineered to produce trehalose, fructans or
mannitol may be an important contributory factor to the stress-tolerant
phenotypes (Hare et al., 1998). Due to significant roles of osmolytes in
response to environmental stresses in plants, heat tolerance might be en-
hanced by increased accumulation of compatible solutes through tradi-
tional plant breeding, marker-assisted selection (MAS) or genetic engi-
neering approaches (Ashraf and Foolad, 2007).
10.4.4.3 PHOTOSYNTHESIS
Any constraint in photosynthesis can limit plant growth at high tempera-
tures. Alterations in various photosynthetic attributes under heat stress are
indicative of thermo tolerance of the plant as they show correlations with
growth. The primary sites of heat injury are photochemical reactions in
thylakoid lamellae and carbon metabolism in the stroma of chloroplast
(Wise et al., 2004). Chlorophyll fluorescence, the ratio of variable fluores-
cence to maximum fluorescence (
F
v
/F
m), and the base fluorescence (
F
0)
are physiological parameters that have direct correlation with heat toler-
ance (Yamada et al., 1996). In tomato, an increased chlorophyll
a
:
b
ratio
and a decreased chlorophyll: carotenoids ratio have been observed in the
tolerant genotypes under high temperatures, indicating that these changes
were related to heat tolerance of tomato (Camejo et al., 2005; Wahid and
Ghazanfar, 2006). Furthermore, under high temperatures, degradation of
chlorophyll
a
and
b
was more pronounced in developed compared to de-
veloping leaves (Karim et al., 1997, 1999). Being highly thermolabile, the
activity of PSII is greatly reduced or even partially stopped under high
temperatures (Bukhov et al., 1999; Camejo et al., 2005). Heckathorn et al.
(1998) reported that photo system II electron transport of tomato plants
was disturbed by stress of 42 °C for 6 h and exposure of continuous mild
heat stress (32-34 °C/22-26 °C day/night) may not have been high enough
to depress photosynthesis. Feller et al. (1998), however, reported that
