in plants under stress conditions [42, 73, 74]. Ascorbic acid is also the major
nonenzyme antioxidant in the apoplast [75], where it also plays a key role in the
perception of stressful environmental stimuli and stress signaling [76, 77]. Plant
tolerance to environmental stressors can be enhanced by the exogenous use of useful
molecules such as proline, amino acids, humic acid, and other antioxidants [78]. The
physiological responses of herbaceous plants to the exogenous AsA have been
extensively studied [79–82]. However, the effects of exogenous AsA applications
on fruit tree species subject to water stress have been poorly studied, and there are
currently no studies on the impact of exogenous AsA on water-stressed deciduous
fruit trees and their responses after wetting. Water stress can inhibit the growth of
young fruit trees and reduce the growth, yield, and quality of fruits of mature
trees [83].
Ascorbic acid is the richest plant antioxidant [84] and is important for the
photoprotection and regulation of photosynthesis by stomatal or nonstomatal
factors [85, 86]. Foliar application of AsA in young peach trees can be a useful
practice to overcome short periods of water scarcity. With regard to gas exchange,
exogenous uses of AsA to young water-stressed peach trees significantly increased
the assimilation of CO 2 in both varieties (Scarletprince and CaroTiger) to the
control levels in a restorative watering step. Biosynthesis of AsA occurs on the
internal mitochondrial membrane by the oxidation of L-galacto-1,4-lactone
(L-GalL). The exogenous application of L-Gal, which is a precursor to ascorbate
synthesis, increases CO 2 assimilation, photosynthetic electron transport velocity,
and ultraviolet conduction [87]. Also, AsA plays a role in photosynthesis and
donates electrons to photosystems I and II when the primary electron donor system
is damaged [88]. Application of ascorbate results in increased photosynthesis,
growth rate, and chlorophyll concentration in wheat plants under water stress
compared to untreated plants [81]. This is of the utmost importance to alleviate
the negative effects of water stress on the reduction of photosynthesis in young
trees in commercial orchards experiencing a period of water stress (especially in
areas where the current practice is to start irrigation after the second year); in
young container-grown and field-grown trees in nurseries not only in drought
periods but also when field trees are excavated and very fine roots are destroyed
causing temporary water stress on trees while the roots are not recovering.
Accumulation of osmolytes such as proline in water-stress plants can contribute
to lower osmotic potential after wetting and allow water to move into cells [89].
In addition to ascorbate, other biologically active molecules also have a positive
effect on a number of plants. Recently, melatonin has been shown to have a
regulating effect on ripening and preventing disease. For example, pre-melatonintreated grape berries exhibit a higher endogenous accumulation of melatonin, which
not only increases grain size and weight but also enhances the synchronized grain
maturation. The application of melatonin after harvesting effectively delays aging
and maintains the quality of the peaches stored at ambient temperature. Exogenous
melatonin pretreatment improves anthocyanin accumulation by regulating gene
expression and increases antiradical activity in cabbage sprouts. Melatonin reduces
injuries caused by low temperatures in peach fruits by increasing the protective
394
L. Koleva-Valkova and A. Harizanova
nonenzyme antioxidant in the apoplast [75], where it also plays a key role in the
perception of stressful environmental stimuli and stress signaling [76, 77]. Plant
tolerance to environmental stressors can be enhanced by the exogenous use of useful
molecules such as proline, amino acids, humic acid, and other antioxidants [78]. The
physiological responses of herbaceous plants to the exogenous AsA have been
extensively studied [79–82]. However, the effects of exogenous AsA applications
on fruit tree species subject to water stress have been poorly studied, and there are
currently no studies on the impact of exogenous AsA on water-stressed deciduous
fruit trees and their responses after wetting. Water stress can inhibit the growth of
young fruit trees and reduce the growth, yield, and quality of fruits of mature
trees [83].
Ascorbic acid is the richest plant antioxidant [84] and is important for the
photoprotection and regulation of photosynthesis by stomatal or nonstomatal
factors [85, 86]. Foliar application of AsA in young peach trees can be a useful
practice to overcome short periods of water scarcity. With regard to gas exchange,
exogenous uses of AsA to young water-stressed peach trees significantly increased
the assimilation of CO 2 in both varieties (Scarletprince and CaroTiger) to the
control levels in a restorative watering step. Biosynthesis of AsA occurs on the
internal mitochondrial membrane by the oxidation of L-galacto-1,4-lactone
(L-GalL). The exogenous application of L-Gal, which is a precursor to ascorbate
synthesis, increases CO 2 assimilation, photosynthetic electron transport velocity,
and ultraviolet conduction [87]. Also, AsA plays a role in photosynthesis and
donates electrons to photosystems I and II when the primary electron donor system
is damaged [88]. Application of ascorbate results in increased photosynthesis,
growth rate, and chlorophyll concentration in wheat plants under water stress
compared to untreated plants [81]. This is of the utmost importance to alleviate
the negative effects of water stress on the reduction of photosynthesis in young
trees in commercial orchards experiencing a period of water stress (especially in
areas where the current practice is to start irrigation after the second year); in
young container-grown and field-grown trees in nurseries not only in drought
periods but also when field trees are excavated and very fine roots are destroyed
causing temporary water stress on trees while the roots are not recovering.
Accumulation of osmolytes such as proline in water-stress plants can contribute
to lower osmotic potential after wetting and allow water to move into cells [89].
In addition to ascorbate, other biologically active molecules also have a positive
effect on a number of plants. Recently, melatonin has been shown to have a
regulating effect on ripening and preventing disease. For example, pre-melatonintreated grape berries exhibit a higher endogenous accumulation of melatonin, which
not only increases grain size and weight but also enhances the synchronized grain
maturation. The application of melatonin after harvesting effectively delays aging
and maintains the quality of the peaches stored at ambient temperature. Exogenous
melatonin pretreatment improves anthocyanin accumulation by regulating gene
expression and increases antiradical activity in cabbage sprouts. Melatonin reduces
injuries caused by low temperatures in peach fruits by increasing the protective
394
L. Koleva-Valkova and A. Harizanova