(Borago officinalis L.) is an annual plant belongs to the family of Boraginaceae.
It is found in the North Africa, South America, the Mediterranean region,
Europe, and Asia Minor, and has industrial, pharmaceutical and forage uses 46.
The leaves, flowers and the oil extracted from the seeds are used as medicine.
It could be treated for different diseases such as arthritis, diabetes, heart
diseases, multiple sclerosis and eczema 5. The borage plant can grow up to 70
to 100 cm, and its stem and leaves are covered by coarse hairs. The stem is
erect with oval or lanceolate leaves that are rough and wrinkled. Each flower
of this plant can potentially produce four seeds. The borage seeds contain around 30% oil that is
rich in the gamma-linolenic acid.
growth is a dynamic process which is continuously shaped by environmental
conditions. Water stress is one of the most important abiotic stresses in the
arid and semi-arid regions that cause a wide variety of physiological and
biochemical changes that inhibit plant growth and development from germination
to productivity and disturb photosynthesis 40, 42. Drought stress in plants
occurs when the plant’s water intake is less than its loss. This may be related
with excessive water loss or less absorption or both 12.
response of plants to drought stress is complex and depends on the intensity
and duration of stress as well as the plant species and its stage of growth 45. Plants
may respond differently to drought stress: (i) drought escape by rapid
development which allows plants to finish their life cycle before severe water shortage,
(ii) drought avoidance by enhancing water uptake and reducing transpiration
rate (reduction of stomatal conductance and leaf area), (iii) drought tolerance
by maintaining tissue turgor during water stress via osmotic adjustment, and
(iv) plant survival under severe stress 63.
Water stress is considered as one of the abiotic stresses that
restrict plant growth and production, as well as physiological and biochemical
processes of plants Anjum et al. 2012. Plant response to water deficit is
complex and involves changes in the morphology, physiology and metabolism,
depending on factors such as plant species and variety, the dynamics, duration
and intensity of soil water depletion, environmental conditions, as well as
plant growth and phenological stages in which water deficit is occurred Lisar
et al. 2012. Plants can avoid drought by maximizing water uptake or minimizing
water loss Chaves et al. 2003 or accumulating some osmolytes to cope
with the stress Szira et al. 2008. Plant adaptation to water stress is the
result of different events, which lead to changes in tissue osmotic potential, antioxidant
defenses, growth rate and structure Duan et al. 2007.
of the responses of the plants to drought stress is change in photosynthetic
pigments. Chlorophylls and carotenoids absorb light energy and transfer it into
the photosynthetic apparatus of leaves. Therefore, determination of leaf
pigments content can provide a valuable tool to integrate and understand the
physiological and biochemical function of leaves 53. Chlorophyll plays a key
role in light trapping and converting it into chemical energy, so any decrease
in chlorophyll content may result in a reduction in photosynthesis. Chlorophylls
a and b are two main types of chlorophyll which work as photoreceptors in
photosynthesis 18. Drought stress could change chlorophyll and carotenoids
contents in plants 28. Severe water deficit disrupts the photosynthesis of
plants by changing chlorophyll content, affecting chlorophyll components and
damaging the photosynthetic activity 29. Reduction in chlorophyll content
during drought stress depends on the duration and severity of water limitation 65.
This reduction is mainly the consequence of damage to chloroplasts caused by
reactive oxygen species 56. There are also some reports which show an
enhanced accumulation of chlorophyll under drought stress 8, 57, 61.
carotenoids not only contribute in light absorbance, but also help plants to
resist drought stress 30. Carotenoids could protect the cell membranes from
light-dependent oxidative damage, and their role in scavenging reactive oxygen
species (ROS) has been well studied 59. They also play an important role in attracting
pollinators and in seed dispersal 62.
pigments present in the photosystems are believed to be damaged by
environmental stresses resulting in a reduced light-absorbing efficiency of
both photosystems (PSI and PSII) and hence a reduced photosynthetic capacity 66.
A number of earlier studies have shown that drought stress adversely affected
the functionality of both PSII and PSI, especially PSII. This led to decreased
electron transport through these two systems 67. It is well established that
PSII plays a key role in photosynthetic response to drought and unfavorable
environmental conditions 43. The light absorbed by chlorophyll molecules can
(i) drive photosynthesis (photochemistry); (ii) be re-emitted as heat; or (iii)
be reflected (fluorescence) 44. Despite the fact that the extent of chlorophyll
fluorescence does not comprise more than 1% to 2% of total light absorbed by
the chlorophyll, it gives a valuable insight into exploitation of the
excitation energy by PSII and other protein complexes of the thylakoid
membranes 51, particularly when plants are subjected to stressful conditions.
the day, the quality and quantity of photosynthetically active radiation (PAR) frequently
changes and plants try to keep a balance between the conversion of light energy
and protection of the photosynthetic apparatus from photo-inhibition or repair
of eventual damage 9. The measurements of reflectance or absorbance changes
of leaf chlorophyll content have been often applied to characterize the status
of the photosynthetic apparatus, but they are not showing enough information on
the photosynthetic activity 15. In contrast, the various methods based on
records of chlorophyll fluorescence were shown to be reliable and simple tools
for assessment of photosynthetic electron transport and related photosynthetic
processes 32. The dynamic changes in chlorophyll fluorescence are a direct
reflection of photosynthesis in crops 39. Analysis of chlorophyll
fluorescence and measurement of the maximum quantum yield of PSII
photochemistry (Fv/Fm) could be useful in determining
damage to light reaction systems in photosynthetic mechanisms under water
stress 48. Water stress influences Fv/Fm and decreases
the electron transport rate and the effective quantum yield of photosystem II
photochemistry. The Fv/Fm is a parameter which allows
detection of any damage to PSII and possible photoinhibition 1 and it is the
main chlorophyll fluorescence parameter which could be estimated easily. The
difference between F0 (chlorophyll fluorescence intensity measured
when all PSII reaction centers are assumed to be open) and Fm
(maximal chlorophyll fluorescence intensity measured when all photosystem II
reaction centers are closed) is the variable fluorescence (Fv) 14.
Drought stress in fact by affecting carbon replication negatively reduces
electron compliance and transport capacity and the system will reach Fm
state faster resulting in a reduction of variable fluorescence (Fv).
The maximum primary yield of photochemistry of PSII (Fv/F0)
provides an estimation of leaf photosynthetic capacity 34. Fv/F0
is an indicator of the size and the number of active photosynthetic reaction
Plants accumulate various organic and inorganic solutes in the
cytosol to diminish osmotic potential Rhodes and Samaras 1994. The process of
accumulation of such solutes under drought stress in the cell, in order to
maintain the cell volume and turgor against dehydration, is known as osmotic
adjustment Anjum et al. 2011. Compatible solutes (osmolytes) are
low-molecular-weight; highly soluble compounds that are usually nontoxic even
at high cytosolic concentrations and protect plants from stress through
different means such as contribution towards osmotic adjustment, detoxification
of reactive oxygen species, stabilization of membranes and native structures of
enzymes and proteins Farooq et al. 2009. Large numbers of compounds such as
proline and soluble sugars in plants play a key role in maintaining the osmotic
equilibrium and in the protection of membranes as well as macromolecules.
Proline is one of the
amino acids that is synthesized from glutamine in plants and appear most
commonly in response to stress. Proline acts as a free radical scavenger and
stabilizes cell membranes, which lead to the adjustment of cell metabolism and
growth under stressful conditions Verbruggen and Hermans 2008. In response to
stresses, the carbohydrate status of a leaf gets altered and this might serve
as a metabolic signal Chaves et al. 2003. It was stated that even sugar flux
may be a signal for regulation of plant metabolism Gibson 2005. Accumulation
of soluble sugars increases resistance to the drought in the plant Kameli and