Abiotic Stress Responses in Plants: Unraveling the Complexity of Genes and Networks to Survive - Abiotic Stress: Plant Responses and Applications in Agriculture - page 52

As a consequence of water loss and decreased cell volume, cell sap solute concentrations
increase and thereby cell osmotic potential decreases. As cell turgor also decreases, an
early effect common to these stresses is a sharp decrease in leaf expansion rate and over‐
all plant growth rate. Furthermore, an additional active decrease of the cell sap osmotic
potential is observed, as an attempt to keep cell hydration. In fact, at the metabolic level,
a common feature to these three stresses is the osmotic adjustment by synthesis of low-
molecular weight osmolytes (carbohydrates [24], betain [25] and proline [26]) that can
counteract cellular dehydration and turgor loss [27]. On the other hand, differences be‐
tween these stresses do also exist. While drought stress is mainly osmotic, ion toxicity,
namely Na + , is a distinctive feature of salinity stress. Cold stress, behinds physiological
drought, has an impact on the rate of most biochemical reactions, including photosyn‐
thetic carbon metabolism reactions, as enzyme activities are extremely temperature-de‐
pendent. Also water stress and salinity stress decrease photosynthesis, which create
conditions to increased photoinhibition, particularly under high irradiances.
2.3. Plant bioenergetics as a core to stress sensor
Despite the different physiological responses to early abiotic stress discussed previously,
a common point observed is the changes in the plant bioenergetic status. Such changes
may involve a decrease in the energy production and/or an increase in energy demand
to overcome the stress. The bioenergetics status is often considered as the chemical ener‐
gy provided by adenylate energy charge (AEC), as defined in [28], for which plants are
mainly dependent on photosynthesis.
The effect of abiotic stresses on photosynthesis can be perceptible: i) within the photochemical
reactions in the tylakoid membrane; ii) in the carbon reduction cycle in the stroma; iii) in the
carbohydrate use in the cytosol and; iv) on the CO 2 supply to the chloroplast dependent of
stomata, mesophyll and chloroplast conductance (reviewed by [29,30]). ATP and NADPH
resulting from photochemical reactions are used in all others processes except CO 2 supply to
the chloroplast in C3 plants, so any limitation in photosynthesis such as those imposed by
drought, can alter the plant bioenergetics status [31].
When the ATP and NADPH production by photochemical processes exceed the capacity for
utilization in CO 2 fixation, plants can use several processes to dissipate energy and avoid or
minimised photoinhibition (see 2.4). These processes include alternative electron sinks
dependent of O 2 such as the oxygenase reaction catalised by ribulose-1,5-bisphosphate
carboxylase/oxigenase (Rubisco, E. C. 4.1.1.39) which initiates photorespiration [32]. The light-
dependent O 2 uptake by photorespiration not only use ATP and reducing power from
photosynthetic electron transport system but also cause a loss of the CO 2 fixed by Calvin cycle.
Even in plants under no photoinhibitory conditions, photorespiration occur due to the capacity
of Rubisco to catalise the carboxylation and oxygenation of ribulose-1,5-bisphosphate,
depending on the CO 2 / O 2 ratio. At 25 ºC, photorespiration increases the cost of carbon (C)
fixation to 4.75 ATP and 3.5 NADPH per C fixed under atmospheric CO 2 and O 2 Concentrations,
which compares to 3 ATP and 2 NADPH per C fixed under no photorespiration conditions, e.
g. only 2% O 2 instead of atmospheric 21% O 2 [33]. In plants submitted to drought, a reduction