124 S. Signorelli et al.
to environmental abiotic stress is one of the ways to improve the productivity of
legumes and aid in harnessing their potential nutritional value. Identification of
biochemical and physiological characters which contribute to improve the yield in
legumes under limiting conditions is a main objective of plant breeders for agri-
cultural and cattle-rearing regions. Thus, this chapter intends to provide an under-
standing of the mechanisms involved in the combined stress-tolerance responses in
legumes.
6.2 Environmental Stresses Induce Varied Plant
Responses
Plants are frequently subjected to stress—environmental condition that adversely
affects the growth, development and productivity thereof. Biotic stress can be im-
posed by organisms such as viruses, bacteria and fungi, while abiotic stress can be
due to an excess or deficit in some environmental factor. Among the environmental
conditions that cause damage are excess water, water deficit, soil salinity, extreme
temperatures, insufficient mineral nutrients in the soil and high- or low-light radia-
tion (Bohnert and Sheveleva 1998 ; Bray et al. 2000 ).
Resistance or susceptibility to stress depends on the species, genotype and stage
of development of the plant. Resistance mechanisms can be grouped into two cat-
egories—those that prevent exposure to stress and the other that results in tolerance.
Certain morphological features such as sunken stomata and deep roots are examples
of resistance mechanism that can prevent stress. However, other mechanisms of
resistance are achieved by acclimation, i.e. the maintenance of internal homeostasis
of the various organelles in response to changing environmental factors
(Bray et al. 2000 ).
Plants acclimate to manage the different types of stress triggering a wide range of
responses from the perception of stress at the cellular level, leading to the activation
of a very large number of genes. Key components of the stress response are the stim-
ulus itself, transducers, signal molecules, transcription regulators, responsive genes
that trigger morphological, biochemical and physiological adaptation involved in
this situation. In turn, the duration and severity with which stress is imposed deter-
mine how the plant will respond (Pastori and Foyer 2002 ; Bray et al. 2000 ).
Unlike resistance to biotic factors, resistance to water stress and other abiotic
factors, despite being clearly genetic, is not a result of the action of a specific gene
(Zhu et al. 1997 ). The ability of plants to withstand water stress is a multigenic trait
and biochemical pathways responsible for products or processes that improve the
overall strength can act additively, and also synergistically (Bohnert et al. 1995 ).
It is reported that several genes responsive to water stress not only perform their
functions protecting cells by producing metabolically important proteins under wa-
ter deficit but also in the regulation of genes involved in signal transduction in
response to stress. Thus, these gene products are classified into two groups: The
first group includes proteins that are involved in stress tolerance such as channel