Changes in photosynthetic capacity and polypeptide patterns during natural senescence and rejuvenation of Cucurbita pepo L. (zucchini) cotyledons

Abstract

Natural senescence of Cucurbita pepo (zucchini) cotyledons was accompanied by a gradual degradation of reserve proteins (globulins) and an intensive decrease in the content of both large subunit (LSU) and small subunit (SSU) of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). The net photosynthetic rate, the primary photochemical activity of PSII, estimated by the variable fluorescence (F v )/maximal fluorescence (F m ) ratio (F v /F m ) and the actual quantum yield of PSII electron transport in the light-adapted state (UPSII) also progressively decreased during natural senescence. In contrast, the fraction of the absorbed light energy, which is not used for photochemistry (LNU) increased with progression of senescence. The decline in the photosynthetic rate started earlier in ontogenesis compared with the down-regulation of the functional activity of PSII, thus suggesting the existence of protective mechanisms which maintain higher efficiency of the photochemical electron transport reactions of photosynthesis compared with the dark reactions of the Calvin cycle during earlier stages of natural senescence. Decapitation of the epicotyl above the senescing cotyledons resulted in full recovery of the polypeptide profile in the rejuvenated cotyledons. In addition, the photosynthetic rate increased reaching values that exceeded those measured in juvenile cotyledons. The photochemical efficiency of PSII also gradually recovered, although it did not reach the maximum values measured in the presenescent cotyledons. KeywordsCotyledons of Cucurbita pepo L. (zucchini) Á Natural senescence Á Photosynthesis Á Polypeptide pattern Á Rejuvenation Abbreviations PSIIPhotosystem II UPSIIThe actual quantum yield of PSII electron transport in the light-adapted state LSU

Different effects of dark treatment on pigment composition and photosystem I and II activities in intact cotyledons and primary leaves of Cucurbita pepo (zucchini)

“…As we aimed to investigate simultaneously the effect of darkness on both cotyledons and leaves, we used 14-day-old plants. Our recent results showed that the amount of reserve globulins in the cotyledons at that growth stage was significantly reduced (Ananieva et al 2008a), thus indicating that the mixotrophic metabolism typical for the early stages of cotyledon development had been changed to fully autotrophic characteristic for the true leaves (Kutik et al 1998). Thus, although cotyledons and leaves were not of the same age, they were characterized with similar physiological status.…”

“…The differences found in total chlorophyll content as well as in the life span between cotyledons and primary leaves can be interpreted in terms of the specific functions of cotyledons, being reserve organs during the early stages of germination which later function as photosynthesizing organs (La Rocca et al 1996;Kutik et al 1998;Ananieva et al 2008a). The timecourse changes in total chlorophyll content in zucchini cotyledons were in accordance with the progressive decline in the photosynthetic rate reported recently (Ananieva et al 2008a), thus ensuring their longer life span and slower senescence as compared to primary leaves. Moreover, the time of cotyledon death coincided with the formation of the 9th-10th differentiated leaves and the beginning of flowering, thus confirming the correlative control of their senescence (Lim et al 2007).…”

“…In senescing Cucumis sativus cotyledons an age-dependent migration of the LHCII complex and its association with PSI leading possibly to destacking of grana structure was found (Prakash et al 2003). Our recent results on natural senescence of Cucurbita pepo (zucchini) cotyledons have shown that the decline in the net photosynthetic rate estimated by CO 2 uptake started earlier in ontogenesis compared with the downregulation of PSII electron transport (Ananieva et al 2008a). Thus, during earlier stages of natural senescence, the existence of protective mechanisms maintaining higher efficiency of the photochemical electron transport reactions compared with the dark reactions of the Calvin cycle could be suggested.…”

Nitric oxide induces cotyledon senescence involving co-operation of the NES1/MAD1 and EIN2-associated ORE1 signalling pathways in Arabidopsis

“…Previous studies have focused on characterizing age-dependent cotyledon senescence in different plant species, such as soybean, cucumber, cotton, and pumpkin (Bick and Strehler, 1971; Kanazawa et al , 2000; Xie et al , 2008; Ananieva et al , 2008 b ) and light irradiation accelerated cotyledon senescence (Watanabe et al , 1994; Wilhelmová et al , 2004). In this study, the phenotype of nitric oxide-induced cotyledon senescence was investigated and the nes1 mutant was characterized as having early cotyledon senescence which was induced by nitric oxide.…”

Effect of ontogeny, heterophylly and leaf position on the gas exchange of the hop plant

“…The hop plant grows up to 8 m in height, resulting in the presence of leaves of different age on the plant. The leaves in the lower parts achieved photosynthetic maturity earlier than leaves in the upper parts of the hop plants, reaching senescence earlier, which results in a lower rate of gas exchange (Ananieva et al 2008). In the lower parts, the offshoot leaves were formed at a time when the hop plant had not yet reached its final height and the bine leaves were still unformed.…”