The Molecular Basis of ABA-Mediated Plant Response to Drought - Abiotic Stress: Plant Responses and Applications in Agriculture - page 116

Water stress reduces photosynthesis in the leaves of higher plants. It is linked with a de‐
creased diffusion of CO 2 from the atmosphere to the site of carboxylation [133-134]. Under‐
lying this process is the stomatal closure during short-term drought and photoinhibition
damage, and the inactivation of RuBisCO under long-term stress [135].
Stomatal closure is one of the first responses to drought conditions which might result in
cell dehydration or runaway xylem cavitation [136]. A good illustration of this process is
stomatal behavior in the midday, when either stomatal closure or decreased stomatal
conductance can be observed. Both responses are mediated by ABA synthesized in re‐
sponse to dehydration conditions [18]. When decreased stomatal conductance is com‐
bined with sustained high irradiance, leaves are subjected to excess energy relative to the
available CO 2 and the rate of reducing power can overcome the rate of its use in the Cal‐
vin cycle. These processes lead to the down-regulation of photosynthetic and even photo‐
inhibition. Plants have evolved mechanisms of defense to protect photosynthesis. Such
protection can be achieved by the regulated thermal dissipation that occurs in the light-
harvesting complexes [137].
Processes associated with the photosynthetic apparatus can be measured using chlorophyll
fluorescence. Experiments with chlorophyll fluorescence were first carried out by Kautsky
and Hirsch [138]. Since then, this technique has progressed quickly and chlorophyll fluores‐
cence can be easily measured using commercially available chlorophyll fluorimeters which
enable the measurements of the photochemical and non-photochemical processes involved
in the fluorescence quenching that occurs in the presence of light [139]. The Fv/Fm ratio rep‐
resenting the maximum quantum yield of the primary photochemical reaction of photosys‐
tem II (PSII) is the most often used parameter. Environmental stresses that affect PSII
efficiency lead to the characteristic decrease in the value of this parameter [140]. Fluores‐
cence kinetics of chlorophyll a, the 'OJIP/JIP-test' named after the basic steps of the transient
by which parameters quantifying PSII behavior are calculated (O is the fluorescence intensi‐
ty F0 (at 50 μs); J is the fluorescence intensities FJ (at 2 ms); I is FI (at 30 ms) and P is the
maximal fluorescence intensity, FP = FM) is an informative tool for studying the effects of
different environmental stresses on photosynthesis [141-142;10;143]. This analysis offers sim‐
ple equations to express the equilibrium between the inflow and outflow of the entire ener‐
gy flux within PSII; it also provides information about the fate of absorbed energy. Some of
the parameters calculated using the JIP-test are related to energy fluxes for light absorption
(ABS), the trapping of excitation energy (TR) and electron transport (ETR) per reaction cen‐
ter (RC) or per sample area called cross-section (CS). Their estimates are based on the analy‐
sis of several groups of measured and calculated parameters. Analyses performed using
these parameters are quick and the measurements are non-invasive [10].
In addition to the photosynthesis process, it was observed that the alteration of leaf an‐
gle caused by dehydration, towards smaller angles, would diminish intercepted radiation
and carbon assimilation, and also have an important protective role against excess solar
energy [144]. There is also a correlation between the rate of photosynthesis and the age
of the leaf. Younger leaves tend to be more resistant to drought than older ones. When a
severe reduction in the size of the leaf canopy occurs, as a result of shedding older