4.3 Photosynthetic pigments content
Treating with redroot pigweed leachate led to leaf yellowing (chlorosis)
in majority of plants in this study. The cause of this chlorosis in
wheat plant is the decrease in chlorophyll content of leaves, a
phenomenon that has been noticed in the literature (Borella et
al., 2014, Dehghani et al., 2014, Singh and Sunaina, 2014).
Probably one of the reasons for the decrease in photosystem II
efficiency and photosynthesis fall down in wheat is the decrease in leaf
chlorophyll content. In cucumber species, the cause of leaf yellowing
was different from that in wheat. In the other hand, under allelopathic
stress condition, cucumber plants showed a significant increase in
carotenoid pigment content, which could be interpreted as a resistance
mechanism in this plant. There is no doubt that one of the detrimental
effects of amaranth allelopathy is the induction of oxidative stress in
plants (Bakhshayeshan-Agdam et al., 2019). The xanthophyll cycle
is one of the well-known mechanisms involved in resistance to oxidative
stress (Salehi-lisar and Bakhshayeshan-agdam, 2016) and an increase in
the concentration of components of this cycle can serve to the benefit
for plants under stress. After chlorophyll a, chlorophyll b is one of
the most important photosynthetic pigments. Besides, the ratio of
chlorophyll a to b is also determinant in plants and studies show a
decrease in this parameter in both plants. In addition to total
carotenoid content, beta-carotene and lutein concentrations as two
carotenoid pigments that play a key role in photosystem II antennae were
also investigated. Beta-carotene is a pigment that supplies the energy
needed to chlorophyll stimulation, reducing its content and function in
photosystem II antenna causes reducing of chlorophyll excitation and
decreasing of photosystem II function in electron transport (Telfer,
2002), -an event that was observed in wheat plant. Lutein was known as
the most important xanthophyll in plants which forms 60% of total
xanthophyll and 40% of total leaf carotenoid. This pigment acts as a
non-photochemical quenching of chlorophyll in reaction centers and
decline in its content is compensated by enhancing other carotenoids
involved in photosystem II antenna such as beta-carotene (Dall’Osto et
al., 2006), this occurrence was observed in cucumber plant.