Φυτά με πράσινους βλαστούς: συγκριτική ανατομική και φυσιολογική μελέτη

Abstract

Despite its significant contribution to the net carbon gain of plants and itsdistinct functional properties, stem photosynthesis has not yet received adequatescientific attention. For this reason, a combination of anatomical and physiologicalmethods was used to characterize the photosynthetic machinery of the green petiolesand pedicels of the monocotyledonous geophyte Zantedeschia aethiopica and thegreen stems of the dicotyledonous semi-woody species Dianthus caryophyllus, incomparison to the corresponding leaves. Both the green petioles/pedicels of Z.aethiopica and the green stems of D. caryophyllus possess all the anatomicalprerequisites of an actively photosynthesizing organ i.e. considerable number ofstomata with typical underlying substomatal chambers, chlorenchyma cells which aresimilar to the leaf palisade chlorenchyma cells and considerable amount of bothintercellular spaces and palisade free cell walls. Yet, the palisade cells of Z.aethiopica petioles/pedicels show a peculiar arrangement with their long axis parallelto the longitudinal organ axis. Furthermore, petiole/pedicel photosyntheticcharacteristics resemble those of leaves under adversity i.e. reduced Rubiscoactivity/content, high photorespiration rates and significant cyclic electron flowaround PSI. It is concluded that these are innate attributes of petiole/pedicelphotosynthesis serving particular functions like the increased nitrogen fixing activityof the species, the qualitative adjustment of the petiole/pedicel amino acid content, theactive decarboxylation of C4-organic acids and the rapid induction of nonphotochemicalquenching.Stem photosynthesis in D. caryophyllus was more efficient than leafphotosynthesis, as a result of the greater rates of stem C3 cycle and a possible organspecificvariation of the specificity factor of Rubisco. In general, D. caryophyllusstems display a photosynthetic pattern of optimal carbon assimilation in the expenseof photoprotection. It could be hypothesized that this kind of adaptation could be dueto the vertical orientation of stems, which results in lower incident light intensities invivo and may include the use of C4-organic acids coming up with the transpirationstream as an additional carbon source.