Abstract
ABSTRACTIn our study, the effects of water stress on photosynthesis and photosynthetic electron transport chain (PETC) were studied in several ways, including monitoring the change of gas exchange parameters, modulated chlorophyll fluorescence, rapid fluorescence induction kinetics, reactive oxygen species (ROS), antioxidant enzyme activities and D1 protein levels in apple leaves. Our results show that when leaf water potential (ψw) is above –1.5 MPa, the stomatal limitation should be the main reason for a drop of photosynthesis. In this period, photosynthetic rate (PN), stomatal conductance (Gs), transpiration rate (E) and intercellular CO2 concentration (Ci) all showed a strong positive correlation with ψw. Modulated chlorophyll fluorescence parameters related to photosynthetic biochemistry activity including maximum photochemical efficiency (Fv/Fm), actual photochemical efficiency of PSII (ΦPSII), photochemical quenching coefficient (qP) and coefficient of photochemical fluorescence quenching assuming interconnected PSII antennae (qL) also showed a strong positive correlation as ψw gradually decreased. On the other hand, in this period, Stern-Volmer type non-photochemical quenching coefficient (NPQ) and quantum yield of light-induced non-photochemical fluorescence quenching [Y(NPQ)] kept going up, which shows an attempt to dissipate excess energy to avoid damage to plants. When ψw was below –1.5 MPa, PN continued to decrease linearly, while Ci increased and a ‘V’ model presents the correlation between Ci and ψw by polynomial regression. This implies that, in this period, the drop in photosynthesis activity might be caused by non-stomatal limitation. Fv/Fm, ΦPSII, qP and qL in apple leaves treated with water stress were much lower than in control, while NPQ and Y(NPQ) started to go down. This demonstrates that excess energy might exceed the tolerance ability of apple leaves. Consistent with changes of these parameters, excess energy led to an increase in the production of ROS including H2O2 and O2•−. Although the activities of antioxidant enzymes like catalase (CAT), superoxide dismutase (SOD) and peroxidase (POD) increased dramatically and ascorbate peroxidase (APX) decreased in apple leaves with drought stress, it was still not sufficient to scavenge ROS. Consequently, the accumulation of ROS triggered a reduction of net D1 protein content, a core protein in the PSII reaction center. As D1 is responsible for the photosynthetic electron transport from plastoquinone A (QA) to plastoquinone B (QB), the capacity of PETC between QA and QB was considerably downregulated. The decline of photosynthesis and activity of PETC may result in the shortage of adenosine triphosphate (ATP) and limitation the regeneration of RuBP (Jmax), a key enzyme in CO2 assimilation. These are all non-stomatal factors and together contributed to decreased CO2 assimilation under severe water stress.
Highlights
Water availability is an important factor affecting plant growth and yield in arid and semi-arid regions, where plants are often subjected to periods of drought (Chaves et al, 2003)
The excess energy has the potential to trigger an increase in the production of reactive oxygen species (ROS) including O2- and H2O2, which has been proven to hinder the synthesis of PSII core D1 (Murata and Takahashi, 2008)
As some studies indicated before, the decrease in photosynthesis is usually caused by stomatal limitation under mild to moderate drought condition when both of Gs and Ci decline while non-stomatal limitation is the main reason for the decrease in photosynthesis when Ci increases and Gs reaches a minimum inflection point (Pérez-López et al, 2012; Zhou et al, 2013)
Summary
Water availability is an important factor affecting plant growth and yield in arid and semi-arid regions, where plants are often subjected to periods of drought (Chaves et al, 2003). Many metabolic processes including photosynthesis are negatively affected. Previous studies have illustrated the decrease in photosynthesis of leaves is usually caused by stomatal limitation under mild to moderate drought condition and nonstomatal limitation under severe drought condition (Degl'Innocenti et al, 2009; Misson et al, 2010). Such decrease in photosynthesis leads to plants absorbing more light energy than can be consumed by photosynthetic carbon fixation. Consistent with the inhibition of D1 synthesis, the activity of photosynthetic electron transport chain (PETC) down-regulates
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