Regulation of 5-Aminolevulinic Acid (ALA) on Leaf Photosynthesis of Pear (Pyrus Pyrifolia Nakai)
|School||Nanjing Agricultural College|
|Keywords||ALA Pear Photosynthesis H2O2 Chemical thinning Fruit quality|
Photosynthesis, the most important biochemical reactions in the planet, is a process to synthesize energy-rich organic compounds from carbon dioxide (CO2) and water (H2O) and release oxygen (O2) by using solar energy in the green plants (including algae). Light is the driving force of the photosynthetic reaction, and light energy capture and transfer process will directly affect the photosynthetic efficiency of plant performance. However, too high of light intensity will hamper light energy conversion that is called photoinhibition. Only the photochemical reaction of photosynthesis and carbon assimilation in harmony, photosynthesis will achieve the best condition.5-Aminolevulinic acid (ALA) is the key biosynthetic precursor of porphyrin in all living organisms. In agricultural production, it has been suggested to be able used in a wide scale. Previous studies showed that exogenous ALA could increase plant photosynthesis in many herbaceous plants, but in woody fruit trees, there is no relative study reported. In this study, pear trees were used to study the characteristics of leaf photosynthesis and regulation of exogenous ALA or Alstrong, a fertilizer with ALA as the main active component on the leaf photosynthesis. The main results are as follows.1. It was suggested that the net photosynthetic rate of leaves in young pear trees(Pyrus pyrifolia Nakai cv’Zhongli 1’) was significantly higher than that of ’Huasu’ (Wang et al., Acta Hort Sin,2005,32:571-577), both of which are the precocious pear cultivars bred in China. In this work, the diurnal variations of photosynthesis of two cultivars were compared by a portable Ciras-2 photosynthetic system and a Plant Efficiency Analyzer (PEA). The results showed that the net photosynthetic rates (Pn) and carboxylation efficiency (CE) in’Zhongli 1’in adult stage trees were still greatly higher than that of ’Huasu’, which approved the former report. JIP-test analysis demonstrated that the performance index on an absorption basis (PIABS) and the PSII maximal photochemical efficiency (φPO) in’Zhongli 1’were also higher than that of’Huasu’. However, the amplitude of the K step (Wk), which reflects the inhibition level of oxygen evolving complex activity at the donor side of PSII reaction center, and the approximate initial slope of the fluorescence transient (Mo), which reflects the close level of PSII reaction center, were lower in’Zhongli 1’than that in’Huasu’. Additionally, the expression of the gene coding Rubisco small subunit of’Zhongli 1’leaves was higher than that of’Huasu’. Therefore, the higher leaf photosynthetic capacity of’Zhongli 1’was presumably related with greater conversion efficiency of light energy into biochemical energy during photosynthetic light reaction, and as well as the CO2 fixation during photosynthetic dark reaction.2. In this part of work, the concentration effect of 5-aminolevulinic acid (ALA) in "Alstrong" amino-acid leaf fertilizer was tested in 8 cultivars of pear (Pyrus pyrifolia Nakai) according to its promotion on leaf chlorophyll content and photosynthesis. The results showed that when ALA concentrations were in 0.05-5 mg·L-1, "Alstrong" could significantly improve the leaf SPAD, net photosynthetic rate (Pn) and the activity of both donor side and acceptance side of PSII reaction center at different levels dependent upon cultivars. Correlation analysis demonstrated that Pn of pear leaves was not closely related with leaf chlorophyll content (P>0.05), but with the photosynthetic performance index on absorption (PIABS), the activity of oxygen evolving complex (Wk), the maximum close rate for PSII reaction center (Mo), the probability that a trapped exciton moves an electron into the electron transport chain beyond QA-(Ψo), quantum yield of electron transport (φEo) and the proposition of energy dissipation through heat (φDo). These results suggested that the promotion by "Alstrong" amino-acid leaf fertilizer on Pn of pear leaves was related with the increment of photochemical energy conversion efficiency.3. The effects of 5-aminolevulinate (ALA), a key precursor in the biosynthesis of porphyrins such as chlorophyll and heme, on the net photosynthetic rate (Pn), the PSII maximal photochemical efficiency ((pPo), the Rubisco initial activity, hydrogen peroxide contents and the activity of antioxidant enzymes were investigated in pear (Pyrus pyrifolia Nakai cv. Akemizu) leaves. The results showed that ALA treatment could induce rise slightly in the H2O2 content, reduce the MDA content simultaneity, significantly improve the antioxidant enzyme (SOD, APX, CAT) activities and increasing the Pn,φPo, Rubisco initial activity of pear leaves. These data indicate that H2O2 may act as signaling molecules involved in regulating the activity of antioxidant enzymes, thereby increasing the photosynthetic capacity of pear leaves.4. Spraying with exogenous 5-aminolevulinic acid (ALA) in concentrations of 600, 900 and 1200 mg·L-1 at the late of full blooming could significantly thin flowers and young fruits of pear (Pyrus pyrifolia Nakai cv. Hosui), and then promote fruit weight and internal quality at harvest. Analysis with flowers treated by ALA showed that the superoxide anion production rate was greatly higher than that of the control, while the activities of superoxide dismutase (SOD) and catalase (CAT) were decreased, suggesting that oxidative stress was responsible for flower abortion in pear trees. Moreover, measurements of leaf gas exchange and chlorophyll fluorescence showed that although the ALA solution was applied at blooming, leaf photosynthesis was obviously promoted in a rather long time, at least 3 months, since the leaf net photosynthetic rate (Pn), apparent quantum yield (AQY), PSII photochemical efficiency ((pPo), and photosynthetic performance index (PIABS) in the treated trees were maintained higher than the control. These results implied that ALA application in suitable concentrations at early stage could not only chemically thin in order to decline manual thinning, but also promote leaf photosynthesis and fruit quality. This physiological function of ALA has not been reported before, which is of significance in fruit production.