Abscisic acid substantially inhibits senescence of cucumber plants (Cucumis sativus) grown under low nitrogen conditions
Introduction
The availability of nitrogen (N) in natural soils frequently limits plant growth and development. Nitrogen-deficient areas have been found throughout the world, e.g., the soil N content is decreased in both dry land (Fan and Zhang, 2000) and tropical regions (Batjes, 1996). The production of plant biomass depends on the level of N in the soil. The artificial application of N to plants can result in a large increase in plant biomass (Taylor et al., 2005). Photosynthesis is also affected by the concentration of N in leaves (Sinclair and Horie, 1989). Plants grow slowly and have leaves that are pale green under deficient-N conditions. It is also the case that N moves into younger tissues from older ones (Mae and Ohira, 1981), so the structures of chloroplasts are substantially degraded for the senescence by nitrogen deficiency. Leaf senescence plays an important role in the plant's N economy (Feller and Fischer, 1994), and N deficiency limits plant growth and chlorophyll accumulation (Walker et al., 2001).
Plant hormones regulate the growth and development of plants under various environmental conditions related to changes in their qualities and quantities. There are numerous reports on the close relationships between leaf senescence and endogenous levels of abscisic acid (ABA). ABA content increases during senescence (Gepstein and Thimann, 1980, Dong et al., 2008) or N deficiency (Zdunek and Lips, 2001, Schraut et al., 2005). Although there are also numerous reports concerning the relationship between the level of ABA and chlorophyll degradation in leaves, as described above, we propose a new role for ABA in the accumulation of chlorophyll in cucumber (Cucumis sativus) leaves grown under low-N conditions. We recently found that the endogenous levels of ABA increased in cucumber plants grown under low-N conditions, and exogenously applied ABA resulted in the inhibition of the senescence of leaves in these plants. In this study, we report the effects of ABA on the biosynthesis and degradation of chlorophyll, and senescence in cucumber plants grown under low-N conditions in the presence of ABA.
Section snippets
Plant material
After being dipped in 70% ethanol for 30 s, cucumber (Cucumis sativus L. cv. Hokusin) seeds were sterilized in a solution of 10% bleach (Kao, Japan) for 10 min, rinsed with sterile water three times, and placed on 1% agar medium without nutrients for 8–10 d. Seedlings were transferred to Technopots (SUMIRON, Japan) with low-N (deficient-N) medium containing 1% agar medium, consisting of 2.5 mM K-PO4 buffer (pH5.5), 2 mM MgSO4, 50 μM Fe-EDTA, 70 μM H3BO3, 14 μM MnCl2, 0.5 μM CuSO4, 1 μM ZnSO4, 0.2 μM Na2MoO
Chlorophyll accumulation in cucumber plants in the presence or absence of ABA
Fig. 1 shows cucumber plants grown in the presence or absence of ABA under sufficient-N or low-N conditions at 30 d after ABA treatment. There was little effect on the leaf color between the control and ABA-applied cucumber plants under sufficient-N conditions. The cucumber plants grown in the ABA-free medium turned yellow under low-N conditions, while the plants grown in the medium containing ABA remained green in a dose-dependent manner even under same conditions (Fig. 1). The levels of
Discussion
Several reports show that the endogenous levels of ABA in plants increased during senescence (Ray et al., 1983, Kumar et al., 2008). When ABA was applied to detached leaves, chlorophyll degradation was greatly promoted (Gepstein and Thimann, 1980, Annamalainathan et al., 1996, Ferrante et al., 2004); however, ABA is less effective with regard to plant senescence and chlorophyll degradation in intact plants. Chlorophyll contents were slightly different between the control and ABA-applied plants
Acknowledgement
This study was supported in part by the Joint Research Grant made by the Arid Land Research Center, Tottori University.
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