Abiotic Stress Responses in Plants: Unraveling the Complexity of Genes and Networks to Survive - Abiotic Stress: Plant Responses and Applications in Agriculture - page 53

of photosynthesis and photorespiration is observed as a result of the lower CO 2 and O 2
availability in the chloroplast. However, in this situation, the photorespiratory pathway is less
decreased than photosynthesis, as firstly suggested Lawlor and co-workers [34, 35]. In fact,
despite the much higher affinity of Rubisco for CO 2 than O 2 , the CO 2 concentration is almost at
the sub saturating level in C3 plants. Thus any decrease in stomatal conductance or in the gases
solubility limits the carboxylase activity while the oxigenase activity is unaffected or less
affected [36, 37]. In C4 plants, the higher CO2 concentration at the Rubisco level allows a lower
decrease in the photosynthesis / photorespiration ratio under water deficit [38] than the one
observed in C3 plants, despite the C4 pathway having per se specific energy costs. The less
efficient light use for CO 2 fixation caused by photorespiration lowers the quantum yields of
photosynthesis in C3 plants under drought [39] or high temperature but this was not observed
in C4 plants [40]. Since photorespiration is the major cause of a lower bioenergetic balance in
photosynthetic tissues of C3 plants, increasing plant growth by overcoming the limitation of
photosynthesis imposed by Rubisco is still an important target of research and plant improve‐
ment [41-46].
In C3 and C4 plants under water deficit, the photosynthetic rate decreases with the leaf relative
water content and water potential [47-52]. This decrease is frequently correlated to the
impairment of photochemical processes in C3 plants [53, 54], including inhibition of ATP
synthesis [55-56]. It is still unclear if photosynthesis is primarily limited by water deficit
through the restriction of CO 2 supply to metabolism (stomatal limitation) [47] or by the
impairment of other processes which decrease the potential rate of photosynthesis (non-
stomatal limitation). Nevertheless research efforts on these subjects are relevant to improve
plants responses to stress [56].
Biochemical modeling of leaf photosynthesis in C3 and C4 plants [57-61] can provide useful
insights into the evaluation of stomatal and non-stomatal limitations of photosynthesis, as
previously shown in drought stressed Medicago truncatula plants [52] and Paspalum dilatatum
plants under water deficit [38], elevated CO2 [62] and dark chilling [63]. Photosynthesis light
curves allow the determination of the relative contribution of respiration, photosynthesis and
photorespiration to the light energy dissipation [64]. Additionally, they are an expeditious
method to screen plants with improved resistance to water deficit, as also shown with
M.truncatula transgenic lines [39].
The role of plant mitochondria in the bioenergetic balance is complex and involves cytocrome
c oxidase but also several other processes such as alternative dehydrogenases and alternative
oxidase that are independent of the adenylate control [65]. An increase in leaf respiratory
energy demand to overcome the drought stress via respiration was referred in leaves in few
studies [66-69]. More often, in drought plants, no change or a decrease in respiration is
observed in leaves but the variations were always minor comparing to photosynthesis, despite
the interdependence of the two processes through photorespiration [70]. However, at the
whole-plant level, the contribution of respiration to the plant bioenergetics status is relevant
because respiration can account for a release of 30-70% of the C fixed daily in well-watered
plants, whereas in drought plants the proportion of C lost increases, mainly due to the decrease
observed on photosynthesis [69-73].