6 Combined Abiotic Stress in Legumes 127
functions, such as being a free radical scavenger, a cell redox balancer, a cytosolic
pH buffer and a stabilizer for subcellular structures, especially during osmotic and
salt stresses (Szabados and Savouré 2010 ).
During drought establishment, plants exhibit a decrease in stomatal conductance
with the consequent decrease in CO 2 assimilation. Stomatal closure has been con-
sidered as the main reason for the inhibition of photosynthesis under drought. How-
ever, it was demonstrated that limiting stomatal water losses is not so important
to maintain photosynthetic activity. For example, it has been observed in leaves
of various species, reductions in photosynthesis occur without apparent effects on
stomatal conductance (Teskey et al. 1986 ; Hutmacher and Krieg 1983 ), suggest-
ing that factors independent of stomatal behaviour impact photosynthesis in plants
subjected to drought.
The use of split root system has helped in gaining knowledge about the impact
of drought on the process of nodulation in legumes (Larrianzar et al. 2014 ). Nod-
ule number is mainly regulated at the systemic level through a signal which is
produced by nodule/root tissue, translocated to the shoot and transmitted back
to the root system. This process involves shoot Leu-rich repeat receptor-like
kinases. In contrast, local and systemic mechanisms regulate nitrogenase activ-
ity in nodules (Esfahani et al. 2014 ). Under drought and heavy metal stress, the
regulation is mostly local, whereas the application of exogenous nitrogen seems
to exert a regulation of nitrogen fixation both at the local and systemic levels
(Marino et al. 2007 ).
6.2.2 Response of Plants to Heat Stress
High temperature at early sowing resulted in poor crop establishment due to fail-
ure of seed germination, emergence and reduced vigour (Khalaffalla 1985 ; Weaich
et al. 1996 ). In such situations, avoidance mechanisms, such as transpiration, leaf
rolling, hairiness or wax layers, may play a role in dissipating the heat load. How-
ever, in general, transpiration is the most important heat-dissipating system through
latent heat loss (Kramer 1983 ).
Plants exposed to high temperatures, at least 5 °C above their optimal growing
conditions, exhibit cellular and metabolic responses required for the plants to
survive under this condition (Guy 1999 ). These effects include changes in the
organization of organelles, cytoskeletal reorganization and membrane functions,
accompanied by a decrease in the synthesis of some proteins and overexpression
of HSPs, the production of phytohormones such as abscisic acid (ABA) and
antioxidants and other protective molecules (Bita and Gerats 2013 ; Maestri et al.
2002 ; Bray et al. 2000 ). Under heat stress, about 5 % of plant transcripts (∼ 1500
genes) are up regulated, twofold or more (Rizhsky et al. 2004 ; Larkindale and
Vierling 2008 ; Finka et al. 2011 ). A significant fraction of these transcripts encode
heat-induced chaperones. For example, 88 out of 1780 in Arabidopsis thaliana, and
117 out of 1509 in wheat, are associated with HSP-based protection mechanism
(Liu et al. 2008 ; Ginzberg et al. 2009 ; Bokszczanin and Fragkostefanakis 2013 ).