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Institut für Sonderkulturen und Produktionsphysiologie

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Browsing Institut für Sonderkulturen und Produktionsphysiologie by Subject "Exotherm"

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    Mechanisms of frost adaptation and freeze damage in grapevine buds
    (2002) Badulescu Valle, Radu Virgil; Blaich, Rolf
    Mechanisms of frost hardening in compound (latent) buds of the grapevine cultivar ?Bacchus? were tested with different methods during three winters. The investigated parameters were LTE/HTE (low temperature exotherm/high temperature exotherm), water content, starch, sugar- and anions combination and bud histology. Water content from wood and buds was determined regularly every 2 weeks from March 1998 until Mai 2000. The lowest water content in wood and buds (about 40 %) was found between November and February. In general shoot sections and buds from the apical shoot area contained less water than in the basal area. Sugars and anions were analyzed with HPLC. The highest concentrations of soluble sugars were found in basal buds of the shoot, the lowest concentration in buds of the apical shoot area. Sucrose was the predominant soluble sugar, it was accompanied by glucose, fructose, sucrose, raffinose, and also stacchyose which was hitherto not described for grapevine buds. The concentration of soluble sugars increased during autumn and reached its maximum (around 150 mg/g dry matter) in November/December until the beginning of January then it decreased again to around 30 mg/g at bud burst. The predominant anion was sulphate while chloride could be detected only in traces. The anions reached their maximum at the beginning of January and in mid April. To evaluate the exotherm measuring method, model experiments were carried out with water drops (1µl) on filter paper and with small plant parts (leaf, stems, flower parts). Both the plant parts and the destilled water on the cellulose fiber freeze mainly between ?8 and ?15°C (an influence of the low osmotic value of the plant sap could not be found). After the first freezing the specimen were thawed and freezing repeated. The freezing points of the first and the second freezing cycle were significantly correlated. This shows that freezing does not occur at random, but is determined by ice nucleation sites characteristic for each sample. These sites even survive the physical destruction of the cells by the ice cristals. Further model experiments were carried out to get indications on possible barriers to ice cristal growth in plant tissue. Exotherm analysis was used to determine the freezing point of grapevine buds which is accompanied by a transient temperature rise called exotherm. The grapevine buds show 2 or more exotherms, one or two HTEs (high temperature exotherms) between ? 5 °C and ?10°C and the LTE (low temperature exotherm, sometimes more than one ) between ?10°C and ?25°C depending on the frost adaption of the buds. The HTEs are assumed to indicate the freezing of surface water or apoplastic water in the subtending tissue (bud pad), whereas the LTE (or LTEs) seem to be caused by freezing of the primary (and secondary) buds (shoot primordiy of the compound bud). The temperature minimum of the LTEs (down to ? 25 °C) is reached in January/February and is not influenced by humidity which, however, changes the THE values occuring usually around ? 10 ° and ? 4 °C, which are influenced by water in the bud scales. The LTEs of the buds in the lower area of the shoot were higher as compared to the buds in the middle and upper area of the shoot. The LTE analysis clearly shows the frost adaptation of the latent buds which usually reaches a maximum by the end of January but a clear relation to the changing air temperatures could not be established. Histological and cytological analyses were used to test for frost damage in bud parts and for changes during the cold adaptation. A modified staining method was developed to differentiate the cells. During automn and winter the buds contained a lot of starch grains which dissolved at bud burst. A permeability barrier between bud pad and shoot primordia could not be found, however it could be directly shown, that a HTE causes no cell damage in the buds, while after the appearence of the LTE(s) a disintegration of protoplasts in primary and secondary buds could be found. This is a direct evidence that LTEs indicates the death of the eyes in the complex grapevine bud. If after the appearance of the HTE the buds were held one day at this temperature before further cooling, no LTEs would appear. This and similar observations during the frost storage of grapevine cuttings is discussed in terms of the (harmless) ice formation in the bud base at moderate minus temperatures which would result in a freeze drying effect due to the lower water potential of the bud pad (in comparison to the non frozen eyes) and a further increase of the frost resistance of the growing points. If frost adapted grapevine shoots from the field were kept at 20°C deacclimation occurred after about 10 days. Accidentally wetted buds showed exotherms above ?4°C. In these buds and the watering water ice nucleating bacteria (Pseudomonas fluorescens) could be found.