Variations limitation can cause oxidative stress [16] which plays

Variations in
physiological traits of borage were mainly associated with changes in water
availability or differences in environmental conditions between two years
rather than with seed priming (Tables 3 and 4). Numerous physiological and
metabolic responses occurring during drought help plants to overcome osmotic stress
injuries. Therefore, the ability to maintain the functionality of the
photosynthetic machinery under drought stress is of major importance. Enhancing
chlorophyll content with declining water supply (Fig. 1; Table 5) may be associated
with a decrease in leaf area 17, and an increase in leaf thickness and chloroplast
density in stressed plants. This could be a defensive response to reduce the
harmful effects of drought stress 19. Increasing chlorophyll content under
stress conditions could help leaves to cope with environmental stress and to
compensate for any damage that could affect the integrity and function of the
photosynthetic system.

Despite
the non-significant effect of year on chl a (Table 5) and chl b (Fig. 1B)
contents, greater increment of chlorophyll a content in the second year (12.1%)
resulted in higher chl a/chl b ratio in this year, compared with the first year
(Fig. 2A). This increment may be related to aggravation of drought effects in
second year due to relatively higher temperature (Table 1). The reduction in
chlorophyll a/b ratio under water stress (Fig. 2B) is the result of larger increase
in chlorophyll b than chlorophyll a in drought condition. Ashraf and Mehmood 6
also found a decrease in Chl a/b ratio in three out of four Brassica species
under water-deficit condition.

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Water
limitation can cause oxidative stress 16 which plays a critical role in destruction
of the cell membrane and chloroplasts and also degradation of photosynthetic
system 55, leading to a reduction in photosynthesis. Plants are able to
protect cell structures against active radicals produced under stress through
production of antioxidant compounds such as carotenoids 13. Carotenoids are
necessary for photo-protection of photosynthesis and they play an important
role as a precursor in signaling during the plant development under abiotic and
biotic stresses. Plant species that have a higher content of carotenoids under oxidative
stress show effective defense and better tolerance against drought stress 20.
The results of our work revealed that irrigation intervals up to 120 mm
evaporation is not created changes in leaf carotenoid content, but further
dehydration acted as an incentive to produce more carotenoids in borage leaves (Fig.
3).

Chlorophyll fluorescence is useful to
study the effects of environmental stresses on plants since photosynthesis is
often reduced in plants experiencing adverse conditions such as water deficit,
temperature stress, nutrient deficiency, polluting agents, and pathogen attack 4.
In general, the increase of F0 (Fig. 4A) and decrease of Fm
(Table 6) under drought stress concomitantly led to an increase in F0/Fm
and a decrease in Fv/F0 and Fv/Fm
in both years (Table 6). The low Fm, Fv/F0 and
Fv/Fm, and high F0/Fm in 2013
compared with 2012 (Table 6) was due to high temperatures during reproductive
stages of plants (Table 2). Initial fluorescence (F0) is
fluorescence level when plastoquinone electron acceptor pool (QA) is fully
oxidized 34. F0 is influenced by any environmental stress, which
alters the pigment of PSII 50.
Any
decrease in F0 due to stress reflects damage to the regulatory
processes external to the reaction centers of the PSII (P680) 27. The
F0 value may enhance if the PSII reaction center is
compromised or if the transfer of excitation energy from the antenna to the
reacĀ­tion centers is impaired 52. Thus, the increase in F0
observed in plants may be associated with damage to the photosynthetic
apparatus 60. The fluorescence rises from F0 when the primary
acceptor QA is oxidized to maximal fluorescence (Fm) when all
reaction centers are closed 49. The reduction of Fm due to drought
stress could be related with increased non-photochemical dissipation as heat or
with decreased activity of the water-splitting enzyme complex 38.

The
higher F0/Fm under water stress (Table 6) indicates that
the initial rate of reduction of the plastoquinone A (PQA) was higher than the
rate of plastoquinone B (PQB) and the activity of photosystem I (PSI) 38,
when the plants were subjected to a longer irrigation interval. Rohacek 51
suggested the increase relation F0/Fm as stress indicator.
Increment of F0/Fm under stress has been
documented for mango cultivars 38, Brassica species 31 and pepper
cultivars 64. Decreasing Fv/F0 in borage as a result of
water stress (Table 6) could be associated with a disruption of photosynthesis in
donor part of the PSII. Fv/F0 ratio strongly negatively
correlated with the chlorophyll index under water stress 2. Reduction in Fv/F0
under stressful conditions has also been reported by other authors 2, 35, 41,
47.

It was reported that the suppression in
photosynthetic rate can occur due to a number of biotic and abiotic factors,
which can considerably alter fluorescence emission kinetic characteristics of
plants 11. The Fv/Fm ratio can be used to detect damages
to photosystem II and possible photo-inhibition 1. Decreasing FV/F0
and FV/Fm under drought stress (Table 6) revealed that
the PSII may be damaged in different degrees due to water limitation, and the
primary reaction of photosynthesis may be inhibited 34. Drought stress
through adverse effects on the CO2 entry, reduces the capacity of
reception and electron transport. As a result, the fluorescence rapidly
maximized (Fm) leading to a reduction in variable fluorescence (Fv).
Indeed, limitation of CO2 absorption due to stomatal closure under
drought stress disturbs the balance between photochemical activity of
photosystem II and the photosynthesis electron need and damages the centers of
photosystem II 37. Reduction in Fv/Fm under stress suggests
that the total amount of light energy transformed in PSII reaction center was
decreased. Thus, the changes observed in photochemical activity of PSII can
contribute to the limitations of photosynthesis activity under water deficit 58.
Baker and Bowyer 10 indicated that alterations of PSII activity under water
stress are related to photo-inhibition rather than to a direct damage to PSII. Drought
stress causes considerable damage to the oxygen evolving center (OEC) coupled
with PSII 54 as well as degradation of D1 polypeptide leading to
the inactivation of the PSII reaction center 26, 67. These changes could
generate reactive oxygen species (ROS), which ultimately lead to photo-inhibition
and oxidative damage 3, 24.

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