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.