photosynthesis [25, 50, 51]. In the process of photosynthesis, the allelochemicals
mainly disturb the function of photosystem II [52, 53], followed by damage of D1
proteins[54]. It has been reported that the F v /F m and growth of the weeds [55] as well
as the number of active reaction center in the electron transport chain [56] are
decreased by the sorgoleone. In barnyard grass, the green pigments (i.e., chlorophyll
a and b, carotenoids) in leaves, the alpha-amylase activity in seeds, and the metabolism of photosynthetic process were significantly inhibited by a higher application
of essential oil from leaves of lemongrass [43]. In cucumber, application of root
extracts and root exudates of cucumber and derivatives of cinnamic acid and benzoic
acid reduced the net photosynthetic rate, transpiration, intercellular CO 2 concentration, and stomatal conductance [50].
Sorgoleone, an allelochemical found in sorghum, is inhibitor of PSII [57–59]
which interferes the binding of plastoquinone at the DI protein [59]. Some other
allelochemicals (e.g., 5-ethoxysorgoleone) found in sorghum have also been
reported to inhibit the activity of PSII [60]. Some allelochemicals produced by
cyanobacteria inhibit the electron transport after binding to PSII sites [61], e.g.,
fischerellin from the Fischerella muscicola [62]. The Myriophyllum spicatum (an
aquatic angiosperm) produce an allelochemical, i.e., tellimagrandin II, which disturbs the PSII mechanism by affecting the electron transport at non-heme iron [63].
2.6
Effect on Nutrient and Water Uptake
Nutrient and water uptake in the plant roots is also affected by the allelochemicals.
The functions of Na
+
/K
+
-ATPase is well known in the uptake and transportation
of ion at the plasma membrane which is reduced by the allelochemicals. In a study,
the allelochemicals (cinnamic acid and p-hydroxybenzoic acid) highly reduced the
root dehydrogenase capacity and ATPase activity and thus decreased the uptake
of potassium, nitrate, and phosphorus [64]. H
+ -ATPase action across the root cell
plasma lemma is decreased by sorgoleone and juglone, which influence the uptake
of water and solutes in the soybean, maize, and peas (Pisum sativum L.) [65, 66].
In maize seedlings, different allelochemicals such as p-coumaric acid, ferulic acid,
and trans-cinnamic acid have been reported to affect the uptake of nitrate and H
+
-
ATPase function in the plasma membrane [67]. The growth, uptake, and translocation of nutrients in radish (Raphanus raphanistrum L.) plants are affected by the
residues of sunflower (Helianthus annuus L.) [68].
In a study on wheat, Yuan et al. [69] correlated the allelochemical (e.g., 4-tertbutyl benzoic acid, ferulic acid, and benzaldehyde) application with the nitrogen
absorption. However, the correlation was more negative for ammonical form of
nitrogen than the nitrate form of nitrogen. Yu and Matsui [70] also reported reduction
in the uptake of nitrate, sulfate, potassium, calcium, magnesium, and iron due to
application of cinnamic acid and the root exudates of cucumber in cucumber
seedlings. It is interesting to note that the impact of allelochemicals on the ion
uptake is linked with the concentration and type of allelochemical. For example,
low concentration of an allelochemical (i.e., dibutyl phthalate) may enhance the
548
A. Nawaz et al.
mainly disturb the function of photosystem II [52, 53], followed by damage of D1
proteins[54]. It has been reported that the F v /F m and growth of the weeds [55] as well
as the number of active reaction center in the electron transport chain [56] are
decreased by the sorgoleone. In barnyard grass, the green pigments (i.e., chlorophyll
a and b, carotenoids) in leaves, the alpha-amylase activity in seeds, and the metabolism of photosynthetic process were significantly inhibited by a higher application
of essential oil from leaves of lemongrass [43]. In cucumber, application of root
extracts and root exudates of cucumber and derivatives of cinnamic acid and benzoic
acid reduced the net photosynthetic rate, transpiration, intercellular CO 2 concentration, and stomatal conductance [50].
Sorgoleone, an allelochemical found in sorghum, is inhibitor of PSII [57–59]
which interferes the binding of plastoquinone at the DI protein [59]. Some other
allelochemicals (e.g., 5-ethoxysorgoleone) found in sorghum have also been
reported to inhibit the activity of PSII [60]. Some allelochemicals produced by
cyanobacteria inhibit the electron transport after binding to PSII sites [61], e.g.,
fischerellin from the Fischerella muscicola [62]. The Myriophyllum spicatum (an
aquatic angiosperm) produce an allelochemical, i.e., tellimagrandin II, which disturbs the PSII mechanism by affecting the electron transport at non-heme iron [63].
2.6
Effect on Nutrient and Water Uptake
Nutrient and water uptake in the plant roots is also affected by the allelochemicals.
The functions of Na
+
/K
+
-ATPase is well known in the uptake and transportation
of ion at the plasma membrane which is reduced by the allelochemicals. In a study,
the allelochemicals (cinnamic acid and p-hydroxybenzoic acid) highly reduced the
root dehydrogenase capacity and ATPase activity and thus decreased the uptake
of potassium, nitrate, and phosphorus [64]. H
+ -ATPase action across the root cell
plasma lemma is decreased by sorgoleone and juglone, which influence the uptake
of water and solutes in the soybean, maize, and peas (Pisum sativum L.) [65, 66].
In maize seedlings, different allelochemicals such as p-coumaric acid, ferulic acid,
and trans-cinnamic acid have been reported to affect the uptake of nitrate and H
+
-
ATPase function in the plasma membrane [67]. The growth, uptake, and translocation of nutrients in radish (Raphanus raphanistrum L.) plants are affected by the
residues of sunflower (Helianthus annuus L.) [68].
In a study on wheat, Yuan et al. [69] correlated the allelochemical (e.g., 4-tertbutyl benzoic acid, ferulic acid, and benzaldehyde) application with the nitrogen
absorption. However, the correlation was more negative for ammonical form of
nitrogen than the nitrate form of nitrogen. Yu and Matsui [70] also reported reduction
in the uptake of nitrate, sulfate, potassium, calcium, magnesium, and iron due to
application of cinnamic acid and the root exudates of cucumber in cucumber
seedlings. It is interesting to note that the impact of allelochemicals on the ion
uptake is linked with the concentration and type of allelochemical. For example,
low concentration of an allelochemical (i.e., dibutyl phthalate) may enhance the
548
A. Nawaz et al.