Comparison Of Photosynthetic Characteristics And Photoprotection Capacity In Flag Leaves Of Xiaoyan 54 And 8602, And Their Hybrids, Xiaoyan 41 And Xiaoyan 81

Photosyntheis and its active period are important factors determing wheat production. In wheat production, it is hard to control the external conditions that affect photosynthesis. However, it is feasible to enhance photosynthesis and extend its active period by perfecting the photosynthetic characteristics and photoprotection capacity of wheat leaves through cultivating and breeding.In this study, gas exchange, chlorophyll fluorescence, 820nm transmission, pigments, antioxidant enzymes, D1 protein and PsaA protein were assayed to compare photosynthetic characteristics and capacity of photoprotection of flag leaves of Xiaoyan 54 (XY54) and 8602, and their hybrids, Xiaoyan 41 (XY41) and Xiaoyan 81 (XY81) after anthesis. In this way, we expect to clear the differences in photosynthetic characteristics and photoprotection capacity between the parients and their hybrids, and then provide some valuable information for wheat breeding. The main results are as follows:(1) At the period of anthesis, there were some differences among these four cultivars (lines) in the content of chlorophylls. The pigment contents were higher in the hybrids, especially XY81, than their parents. Photosynthetic rate (Pn) and carboxylation efficiency (CE) were the highest in 8602 whereas stomatal conductance (Gs) was lower in XY41 and CE was the lowest in XY54. These four cultivars (lines) had similar intercellular CO2 concentration (Ci), ratio of variable F_K to the amplitude F_J–F_O (Wk), the maximum quantum yield of PSII (F_v/F_m), efficiency that an electron moves further than QA (Ψo), efficiency with which an electron from the intersystem electron carriers moves to reduce end electron acceptors at the PSI acceptor side (δRo), relative variable transmission at 820nm (ΔI/Io), and actually photochemical efficiency of PSII (ΦPSII). After anthesis, decreases in the pigment contents, Pn, CE, Gs, Fv/Fm,δRo andΔI/Io, and increases in Wk were faster in XY54 in contrast with other three cultivars (lines). No difference was detected in Ci,Ψo, orΦPSII among these four cultivars (lines). The change of above parameters were similar in 8602 and the hybrids during the whole period after anthesis. With similar chlorophyll contents, Pn was higher in XY54 in contrast with other three cultivars (lines). At late filling period, the four cultivars (lines) exhibited identically diurnal change in Pn and photosynthetic light reactions. The above results indicate that, compared with XY 54, the relatively slower decrease in photosynthetic rate in XY41 and XY81 was not correlated with their chlorophyll contents, stomatal conductance and photosynthetic light reaction capacities, but with their different dark reaction capacities. XY41 and XY81 may inherit the high dark reaction capacity from 8602, one of their parents.(2) At the period of anthesis, the content of carotenoids in XY81 and XY41 was similar as that in XY54, but higher than that in 8602. No differences were found among the four cultivars (lines) in terms of ratio ofΦPSII/ΦCO₂, non-photochemical quenching (NPQ), the content of D1 protein and PsaA protein, and the activity of catalase (CAT) or ascorbate peroxidase (APX). However, the activity of superoxide dismutase (SOD) was higher in XY54 compared with other three cultivars (lines). After anthesis, the content of carotenoids in XY81 and XY41 decreased slower than that in XY54, but identically with that in 8602. NPQ increased similarly in the four cultivars (generations). The activity of SOD increased whereas that of APX decreased faster in XY54 in contrast with those three generations. CAT showed a similar activity in the four cultivars (lines) after anthesis. At the late stage after anthesis, compared with those three cultivars (lines), XY54 had a faster increase in ratio ofΦPSII/ΦCO₂ and more pronounced degradation of D1 protein and PsaA protein. The above results suggest that the two hybrids might inherbit the higher carotenoids contents of XY54, but their degradation of carotenoids was correlated with the genetic characteristics from 8602. At the late stage after anthesis, heat dissipation depedent on photoinhibition was more predominant in XY54. XY54 might enhance its Mehler reaction to consume more absorbed light energy. During Mehler reaction, more produced superoxide anion was converted to hydrogen peroxide by SOD in XY54. Nevertheless, such accumulated hydrogen peroxide was not scavenged timely by APX. As a result, the photodamage became more severe in XY54.