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

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CHAPTER 2
WATER STRESS
A. YOKOTA, K. TAKAHARA AND K. AKASHI
Graduate School of Biological Sciences, Nara Institute of Science and Technology,
Nara 630-0101, Japan
(Email: yokota@bs.naist.jp)
Key words: Abscisic acid, compatible solutes, desiccation, drought,
photosynthesis, rubisco
1. INTRODUCTION
Plants use ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) to fix CO 2
during
photosynthesis. RuBisCO also reacts with O 2
, lowering productivity through inevitable
photorespiration and increasing the CO 2
compensation point of C 3
plants to 50 to
70 bar. The Km value for CO 2
of RuBisCO is 10 to 15 µM and CO 2
activation is necessary
for activity (Roy and Andrews, 2000). Diffusion barriers to CO 2
in the stomata, plasma
membranes, cytosolic fluid and chloroplast envelopes lower CO 2
concentrations around
RuBisCO to approximately 7 µM during active photosynthesis (von Caemmerer and
Evans, 1991; Noctor et al., 2002), even with the aid of aquaporins for quick diffusion at
plasma membranes (Terashima and Ono, 2002; Uehlein et al., 2003). Consequently, less
than 20% of all RuBisCO catalytic sites actually participate in photosynthetic CO 2
fixation in chloroplasts (McCurry et al., 1981). Nevertheless, plants fix a total of 200
Gtons of CO 2
every year by investing a large amount of nitrogen in RuBisCO synthesis
and by maximizing the density of stomata per leaf unit area and the size of stomatal
apertures (Terashima et al., 2005).
These properties of RuBisCO are the most critical factors influencing the physiology
of plants under water-stressed conditions (Whitney and Andrews, 2001). The
amount of water transpired from leaves through stomata is 500 to 1000 times more than
the amount of CO 2
absorbed on a molar basis (Larcher, 1995). Consequently, plants
need an enormous amount of water for growth. The water use efficiency of C 3
plants is
1.3 to 2 g of dry matter production per kg of water used and this is 2-fold higher in C 4
plants. This indicates the importance of water as a determinant of plant productivity in
the field; for example, in the US drought is the most serious environmental stress
affecting agricultural production (Table 1) (Boyer, 1982).
K.V. Madhava Rao, A.S. Raghavendra and K. Janardhan Reddy (eds.),
Physiology and Molecular Biology of Stress Tolerance in Plants, 15–39.
© 2006 Springer. Printed in the Netherlands.
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