Water relations and keeping-quality of cut Gerbera flowers

Abstract

The aim of the present investigation is to study the internal water relations,of ageing Gerbera inflorescences and their consequence on keepingquality of cut inflorescences. As in all parts of this paper, the term "flower" will be used to describe an inflorescence with its supporting stem.A great problem during vase-life of cut Gerbera flowers is ',stem break", a sudden bending of the stem. As described in part 1, this phenomenon was caused by a water shortage in the flower. The water-stress was a result of a decline of the absorption rate, due to an increase of the resistance to the water flow between the vase and the petals. In roses (which show a similar phenomenon) a water deficit in the flower-neck occurs because of competition between the various organs when the water supply is limiting (Zieslin et al., 1978). Also in the Gerbera flower there seems to be a competition for the available water, between flower head and stem.The increase in flow resistance causing stem break, was a result of microbial activity in the vase water. "Stem-plugging" by bacteria can be considerable already after 2 days for many flower species (Aarts, 1957). Silver-ions can extend keeping-quality of cut carnation flowers by their anti-ethylene effect (Halevy and Kofranek, 1977; Veen and Van de Geijn, 1978). However, the prevention of stem break by silver nitrate in the vase water is related to its bactericidal effect, as the mobility of silver supplemented as silver nitrate, is very low in flower stew (Veen and Van de Geijn, 1978; Nowak, 1979). Moreover, a pretreatment (1-24 h) of Gerbera stems with silver nitrate does not counteract the detrimental effect of etephon (Nowak, 1979). Mayak et al. (1977) demonstrated, that a pretreatment of the carnation stem with silver nitrate reduced the microbial population of the vase solution by the release of silver from the impregnated stem. They found that another important beneficial effect of such a pretreatment of the stem base is to decrease the toxic effect of metabolites produced by bacteria. So, the use of silver nitrate as a short pretreatment immediately after cutting will have advantages above other bactericides.There are 2 different pathways for water uptake by a Gerbera stem: a direct one through the xylem vessels at the cut surface and an indirect one through the cavity in the stem. Only the direct water uptake is strongly inhibited by bacterial activity in the vase water. Stem break can be prevented therefore without the use of chemicals by cutting the stem through the cavity in its center. The beneficial effect of this treatment could be improved by making a small hole in the stem as an air outlet from thecavity, together with a high water level in the vase in order to promote the rise of water inside the cavity.Stem stiffness consists of the strength from turgor of the cells and that of the structural elements. Gerbera cultivars with structurally strong stew do not show the phenomenon of stem break when a water deficit develops (De Jong, 1978). Breeding for flowers with structurally strong stems only will prevent stem break, however, not the water stress caused by microbial activity in the vase water. It is worthwhile therefore to select flowers not only with a structural strong stem, but also with a hollow one all the year round from an early stage of development.When Gerbera flowers were placed in water with silver nitrate, there was. still a gradual increase in the resistance for water flow through the stem ("physiological plugging") causing a decrease of water potential of the petals (part II). This decrease of petal water potential, however, was not accompanied by stem break. Possible explanations for this apparent discrepancy are discussed in detail in part II. The calculated resistance of the flower stem was obtained without induced pressure differences between 2 sides of a cut stem piece, so the results could not be due to artifacts caused by artificial pressures as suggested by Carpenter and Rasmussen (1973) for roses. The physiological plugging could be prevented by a constant low pH of the vase water.Even when the stem resistance for water flow remained constant at a low pH, water absorption of the cv.'Wageningen Rood' became lower than transpiration after 5 days, resulting in a decrease of flower fresh weight and petal water content (as a percentage of dry weight and as relative water content). The water potential of the petals, however,' remained steady, which seems a rather conflicting result. It should be realised, that the pressure chamber method used for estimating petal water potential, actually measures the non-osmotic component of the xylem water potential. As there ate no semi-permeable membranes between petal xylem elements and vase water, it is an accurate value for calculating stem flow resistance, but not for the actual water potential of the petal cells. This problem is discussed in more detail later on.Ageing petals of cut Gerbera flowers without stem plugging (part Ill) showed that the water content (W.C.) as a percentage of dry weight was correlated with ion leakage (I.L.) and with petal dry weight (D.W.) as given by the formula: W.C. = a + b(I.L.) - c(I.L.) 2 - d(D.W.). The increase in W.C. during the first days of vase life of cut flowers was due to a decrease of dry weight, while the sudden decrease in W.C. after some days of vase life was correlated with an increase in I.L., indicating a change in the semi- permeability of the membranes. Flowers ageing on the plant did not show the sharp decrease of W.C., whereas also the increase of I.L. was absent. The date at which I.L. of cut flowers increased depended on the cultivar and was affected by temperature and cytokinin treatments. The influence of temperature on the onset of the decrease of W.C. and increase of I.L. showed the importance for keeping-quality of a low temperature during storage and transport of the flowers.In part IV is demonstrated that the internal water relations of ageing petal-tissue were influenced to a large extent when flowers were separated from the plant. Sap osmotic potential (ψ osm ) of petals of cut flowers cv. 'Wageningen Rood' increased the first 6 days of vase life, followed by a decrease. Pressure potential (ψ press ) decreased during the entire vase period. When flowers were left on the plant, ψ osm was steady during the first 6 days and increased thereafter, whereas ψ press was steady until day 6 and then decreased. This different behaviour of the various components of water potential was due to the increase of ion leakage of petal cells of ageing cut flowers, whereas ion leakage remained constant when flowers were ageing on the plant. An increase of ion leakage of petal cells will decrease the osmotic potential of the xylem fluid of the petals. This change in xylem osmotic potential will not influence the potential difference between vase water and petal cells and thus absorption rate of vase water. However, it will decrease the water potential of the petal cells and therefore water content, osmotic potential and pressure Potential. The increase of ion concentration of the xylem fluid of the petals will cause a water shortage in the petal cells, even when these cells still act as good osmometers.Comparison of the results with literature data are difficult because of different measuring techniques and experimental circumstances. Mayak et al. (1974) using an isopiestic method, found a decline of petal water potential after 6 days with roses. However, they demonstrated an increase in stem water flow resistance when the flowers aged. Osmotic values of ageing carnation petals as given by Mayak et al. (1978) are conflicting with that given by Acock and Nichols (1979).Changes in ion leakage from the petal cells dominate the petal water relations of cut Gerbera flowers as discussed already previously. To obtain a better understanding of factors involved in keeping-quality of cut flowers, it will be important to know more about the triggering processes inducing the changes in ion leakage. Some experiments in this aspect are described in parts V and VI.It is known for many plant 'species that root-synthesized cytokinins are transported to the shoots, while ageing of leaves is hastend by excising and retarded by exogenous cytokinins. Moreover, ageing of cut flowers can be retarded by application of cytokinins. Therefore, experiments were done to investigate if differences in ion leakage between Gerbera petals of flowers ageing in a vase and on the plant could be ascribed to differences in cytokinin activities (part V). Cytokinin activities in petal-extracts of 3 cultivars, differing in their keeping-quality, were also compared. Activities decreased when the flowers aged (except on day 8 of the experiment). However, there were no differences between flowers ageing in a vase or on the plant. With the 3 cultivars used, there was no correlation between cytokinin content of petals at day of harvest and their keeping-quality. The results suggest strongly that no correlation exists between cytokinin activities of Gerbera petal cells and changes in their ion leakage.From the data given in part VI, it is concluded that changes in pressure potential of Gerbera petal cells can induce changes in the leakage of ions from the cells. The positive results of "pulsing" flowers, either with sugar (Kohl and Rundle, 1972; Mayak et al., 1973; Nichols, 1974; Sacalis and Chin, 1976) or mineral salts (Halevy, 1976), could also be a result of enhancing the pressure potential of the petal cells. Carnation flowers grown under dry conditions kept longer than those grown under moist irrigation regime (Hanan and Jasper, 1969; Mayak and Kofranek, 1976). It is likely, that the "dry grown" flowers have a higher pressure potential at the same water potential, than the "moist grown" flowers, as was found with leaves of plants grown in culture solutions with different osmotic potential (Jarvis and Jarvis, 1963).For the 3 Gerbera cultivars used, there was a correlation between their keeping-quality and their pressure potential at day of harvest.When a decrease of pressure potential initiates an increase of ion leakage, and subsequently causes a decrease of water content, the process of ageing will accelerate itself, once it has began.There seems to be a discrepancy between the conclusion that pressure potential of petal cells influences ion leakage of the cells and the data in Fig. 4 of part III where induced changes of water content did not influence ion leakage. The changes in water content in part III, however, were induced within 24 h, while the results in part VI were obtained when pressure potential was influenced during some days.The dominant influence of ion leakage on water relations, as demonstrated in parts III and IV, and the results of part VI suggest strongly that keeping pressure potential of flower petals above a certain level will be very important for a good keeping-quality of cut flowers. It might be possible that pressure potential at day of harvest, can be a selection criterion for potential keeping-quality of cut flowers.<p/