Water loss prevention plays a greater role than ROS scavenging in dehydration tolerance of Kalanchoe tubiflora epiphyllous buds

Different plants and plant organs have various strategies to cope with drought stress.Kalanchoe tubifloraplants have a strong ability to prevent water loss and to scavenge reactive oxygen species. The epiphyllous bud of the plant serves as a reproduction unit and is able to generate plantlets even under conditions of extreme drought. The aim of this project was to study the antioxidant-stress response and osmoregulation ofK. tubifloraepiphyllous buds to dehydration and to compare the drought-tolerance mechanisms of the plant body and the epiphyllous buds. With the progression of bud dehydration, relative water content, seedling survival rate and dry weight per seedling decreased, and electrolyte leakage, hydrogen peroxide and malondialdehyde content and superoxide anion production rate increased. The activities of the antioxidant enzymes superoxide dismutase, catalase, ascorbate peroxidase, dehydroascorbate reductase and glutathione reductase decreased under dehydration. The content of proline, soluble sugars and soluble protein increased in dehydrated buds proportionally with the extent of water loss. These data indicate that the drought-tolerance mechanism of theK. tubiflorabud was different from that of its mother plant. The buds invest more energy to prevent water loss during drought stress, and their antioxidant defense weakens.

Correlations among Attributes of Senescence and Antioxidative Status of Leaf Discs during Epiphyllous Bud Differentiation in Kalanchoe pinnata Lam. (Pers.)

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 (Chla+b), and membrane stability index (MSI), and the antioxidative status, e.g. phenolics, ferric reducing ability in plasma equivalence (FRAPeq), 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 Chla+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 Chla+b was found to be significantly correlated with the variation in the level of phenolics. The average FRAPeq 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 Chla+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 FRAPeq. The diminishing Chla+b content was found to be maximally correlated with alterations in the membrane protection.

Leaves and epiphyllous shoots in Chisocheton (Meliaceae): a continuum of woody leaf and stem axes

The pinnately compound leaves of Chisocheton tenuis, a small tree from Papua New Guinea, exhibit indeterminate growth and periodically produce new pinnae from a leaf tip bud. Inflorescences and vegetative shoots arise from epiphyllous buds on the adaxial surface of the rachis between the pinna pairs. Axillary buds occur on the stem but are always vegetative. The structure and ontogeny of leaves, axillary buds, and epiphyllous buds are documented with sections and scanning electron microscopy. Although epiphyllous inflorescences are described from herbarium specimens of C. tenuis, only vegetative shoots were collected as epiphyllous outgrowths. These epiphyllous shoots formed woody stem and rachis axes similar to the stem and rachis on the original shoot. Epiphyllous buds first appear on a leaf primordium without evidence of an ontogenetic displacement from an earlier axillary site. Later, epiphyllous buds and pinnae arise in an acropetal order close to the meristem at the tip of the leaf. Epiphyllous inflorescences in Chisocheton pohlianus are described from herbarium material. Leaf and bud structure and ontogeny in a related species, Chisocheton montanus, which lacks epiphyllous buds and has axillary inflorescences, are similar to C. tenuis except that no meristems occur on the rachis. Possible morphological interpretations for these examples of unusual organography are presented. Epiphyllous buds and leaf tip meristems are examples of heterotopy. Key words: buds, epiphylly, Chisocheton, heterotopy, homology, leaves, Meliaceae, meristems.

The branching of Trichomanes proliferum (Hymenophyllaceae)

The general morphology, anatomy, and meristem histology of Trichomanes proliferum were studied in order to explain the morphogenesis of this fern. As opposed to Bierhorst's conclusions, T. proliferum was found to be a typical fern with normal tetrahedral shoot apical cells and lenticular leaf apical cells. The leaf is a lateral production of the shoot apical meristem. This species is similar morphologically to other species in the genus Trichomanes: the shoot apical meristems on the creeping stolons produce "lateral systems," composed of a leaf and a bud, which are extraaxillary, as in other Trichomanes species with a creeping filiform stolon. The unique morphology of this fern is due to two supplementary branching systems: a lateral branching of the stolon, which is leafless at the fork, and an epiphyllous budding, which results in the formation of additional leaves by a different process of development. Key words: fern, branching, morphogenesis, histogenesis, epiphyllous buds.