Abstract
Fluctuating light is a typical light condition in nature and can cause selective photodamage to photosystem I (PSI). The sensitivity of PSI to fluctuating light is influenced by the amplitude of low/high light intensity. Tobacco mature leaves are tended to be horizontal to maximize the light absorption and photosynthesis, but young leaves are usually vertical to diminish the light absorption. Therefore, we tested the hypothesis that such regulation of the leaf angle in young leaves might protect PSI against photoinhibition under fluctuating light. We found that, upon a sudden increase in illumination, PSI was over-reduced in extreme young leaves but was oxidized in mature leaves. After fluctuating light treatment, such PSI over-reduction aggravated PSI photoinhibition in young leaves. Furthermore, the leaf angle was tightly correlated to the extent of PSI photoinhibition induced by fluctuating light. Therefore, vertical young leaves are more susceptible to PSI photoinhibition than horizontal mature leaves when exposed to the same fluctuating light. In young leaves, the vertical leaf angle decreased the light absorption and thus lowered the amplitude of low/high light intensity. Therefore, the regulation of the leaf angle was found for the first time as an important strategy used by young leaves to protect PSI against photoinhibition under fluctuating light. To our knowledge, we show here new insight into the photoprotection for PSI under fluctuating light in nature.
Highlights
Y(NA) reflected the photosystem I (PSI) redox state that influenced the risk of PSI photoinhibition, and the performances of Y(ND) and Y(NA) indicated that PSI was insusceptible to constant high light
The response of photosystem II (PSII) to fluctuating light (FL) does not differ between vertical and horizontal leaves (Figure 4). These results indicate that vertical young leaves are more sensitive to PSI photoinhibition when exposed to natural FL conditions
Many previous studies have documented the molecular mechanisms of photoprotection for PSI against FL conditions
Summary
Upon a sudden increase in irradiance, the rapid increased electron flow from photosystem II (PSII) is accompanied by the relatively slower kinetics of diffusional conductance and CO2 fixation [3,4], leading to the transient PSI over-reduction [5,6,7,8]. Under such conditions, the electron donation from PSI to O2 increases, generating reactive oxygen species (ROS) within PSI [9]. The decrease in PSI activity suppresses CO2 fixation and impairs plant growth [16,18,19]
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