6 Combined Abiotic Stress in Legumes 125
proteins involved in the movement of water across membranes, enzymes necessary
for the biosynthesis of osmolytes, proteases and macromolecules that can protect
membranes, among others. The second group includes factors involved in the regu-
lation of signal transduction and gene expression, such as protein kinases, transcrip-
tion factors and 14-3-3 proteins, among others (Bray 1997 ; Shinozaki and Yamagu-
chi-Shinozaki 1997 ).
Higher temperatures primarily affect photosynthesis, in particular CO 2 as-
similation because Rubisco activation is inhibited. Plants exposed to excessive
temperatures have specific metabolic cellular response characterized by low
protein synthesis, and induction of the synthesis of heat shock proteins (HSPs).
In addition to altering the pattern of gene expression, the high temperature can
damage cellular structures such as organelles and cytoskeleton (Bray et al. 2000 ;
Tang et al. 2007 ).
Water stress and high temperatures interact strongly with each other and have
opposite effects on photosynthesis. For example, in response to high temperature,
plants open their stomata to cool their leaves by transpiration, but if there is also
water deficit condition, plants would not be able to open the stomata and hence leaf
temperature will increase (Rizhsky et al. 2002 ). While both types of stress have
been extensively studied individually, few studies (Lu and Zhang 1999 ; Rizhsky
et al. 2002 ; Rizhsky et al. 2004 ) focused on impacts of combined heat and water
stress—a common situation prevailing under field conditions. It is possible that
combination of these stress factors can alter the metabolism of the plant differently,
compared to when a single stress is imposed (Xu and Zhou 2006 ).
6.2.1 Plants Response to Water Stress
Water deficit is one of the most widespread environmental factor stresses that occurs
when the transpiration rate exceeds the absorption of water from the root system.
Water deficit at the cellular level may result in an increase of solute concentration,
changes in cell volume, disruption of water potential gradient, turgor loss, loss of
membrane integrity and protein denaturation. The ability of the plant to respond to
water deficit and survive depends on mechanisms that involve the integration of
cellular responses throughout the plant (Bray et al. 2000 ).
Water deficit is a common plant environmental stress that dramatically limits
growth and development. Water stress can trigger a significant decrease in crop
productivity and quality, especially evident in grain and forage legumes. Lotus
japonicus is a well-established model legume closely related to forage legumes
such as Lotus corniculatus, Lotus tenuis and Lotus uliginosus (Choi et al. 2004 ;
Díaz et al. 2005a). Alfalfa is a legume species with great plasticity that can suc-
ceed in semiarid, subhumid and humid regions and for that reason is called the
“queen of forage legumes”. However, it requires well-aerated and deep soils and
is morphologically and physiologically adapted to withstand prolonged water defi-
ciencies. In marked contrast to their drought-tolerant nature, these plants are very
sensitive to a lack of oxygen that is common in flooding soils.