ABSTRACT
Leaf detachment is a common signal that triggers both the differentiation of dormant epiphyllous buds as well as the onset of foliar senescence in Kalanchoe pinnata Lam. (Pers.). The present study looked for any probable correlations among selected attributes of foliar senescence, e.g. soluble proteins, chlorophylls a and b (Chl(a+b)), and membrane stability index (MSI), and the antioxidative status, e.g. phenolics, ferric reducing ability in plasma equivalence (FRAP(eq)), and membrane protection index (MPI), during epiphyllous bud differentiation. The experimental system comprised 0.75-cm leaf discs, with or without a dormant epiphyllous bud, cultured in vitro and exposed for ten days to continuous light or dark. A steady depletion of soluble proteins and Chl(a+b), and lowering of MSI in the leaf discs were observed, the decline being relatively faster and of higher magnitude in discs exposed to dark rather than to light. The pigment loss in discs with differentiating epiphyllous buds was greater and faster than in those lacking buds, a somewhat reverse situation was observed in case of soluble proteins. Simultaneously, a time-dependent decrease in the level of phenolics was also observed. Their content was found to be lower in discs exposed to dark as compared to light, pointing to a relationship with a higher rate of senescence-related degradative processes in the dark. The change in the content of Chl(a+b) was found to be significantly correlated with the variation in the level of phenolics. The average FRAP(eq) after ten days was one half that of the initial level, which could be correlated with the decreasing levels of phenolics (intra-correlation) and maximally correlated with variations in Chl(a+b) and protein contents (inter-correlation). Aqueous alcohol foliar extracts significantly (p < 0.05) protected membranes against peroxidative stress, although the pattern was not found to be in line with that of the phenolics content or FRAP(eq). The diminishing Chl(a+b) content was found to be maximally correlated with alterations in the membrane protection.
Subject(s)
Antioxidants/chemistry , Kalanchoe/chemistry , Plant Leaves/chemistry , Kalanchoe/growth & developmentABSTRACT
Leaves of Kalanchoe pinnata have crenate margins with each notch bearing a dormant bud competent to develop into a healthy plantlet. Leaf detachment is a common signal for inducing two contrastingly different leaf-based processes, i.e. epiphyllous bud development into plantlet and foliar senescence. To investigate differentiation of bud and its correlation, if any, with foliar senescence, thidiazuron (TDZ), having cytokinin activity and ethrel (ETH), an ethylene releasing compound, were employed. The experimental system was comprised of marginal leaf discs, each harbouring an epiphyllous bud. Most of the growth characteristics of plantlet developing from the epiphyllous bud were significantly inhibited by TDZ but promoted by ETH. The two regulators modulated senescence in a manner different for leaf discs and plantlet leaves. Thus, TDZ caused a complete retention whereas ETH a complete loss of chlorophyll in the leaf discs. In contrast, the former resulted in a complete depletion of chlorophyll from the plantlet leaves producing an albino effect, while the latter reduced it by 50% only. In combined dispensation of the two regulators, the effect of TDZ was expressed in majority of responses studied. The results presented in this investigation clearly show that the foliar processes of epiphyllous bud differentiation and senescence are interlinked as TDZ that delayed senescence inhibited epiphyllous bud differentiation and ETH that hastened senescence promoted it. A working hypothesis to interpret responsiveness of the disc-bud composite on lines of a source-sink duo, has been proposed.
