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Kiwi ( Actinidia deliciosa ) calli were grown for two successive 45-day subcultures on agarised medium in order to evaluate the metabolic changes occurring during the growth of the callus. At different times (10, 35 and 45 days of each subculture), increase in fresh weight, oxygen uptake rate, levels of inorganic cations (K + , Ca 2+ , Mg 2+ and Na + ) and of some metabolites (sucrose, glucose-6-phosphate, malic acid, amino acids and adenosine phosphates) were measured. After an adaptive growth phase (0–6 days) and a phase of high growth (6–12 days), the callus growth rate decreased sharply and then remained essentially constant up to the 45th day. The parameter listed above changed with time following a similar pattern in the two analysed subcultures. In the first 10 days, most of them increased (in particular, oxygen uptake rate and the energetic status of the cells); the levels of Na + decreased. After the first 10 days, the levels of glucose-6-phosphate (glu-6-P) and of the adenylate pool and the availability of the high-energy phosphate bonds of ATP and ADP decreased and the Na + levels began to increase; in this period, the rate of oxygen uptake increased, but this corresponded to neither increased availability of the high-energy phosphate bonds of ATP and ADP nor increased growth rate. In the last period (35–45 days) the main metabolic parameters dropped, with a large increase in Na + levels. Transfer onto new medium restored, after the adaptive period, the maximum growth and the levels of the parameters listed above. The data show that during the subculture periods of Actinidia deliciosa calli important metabolic changes occur. They are probably linked to a different status of suffering (for reduction of nutrient availability, accumulation of catabolic products, and/or variations in oxygen and CO 2 diffusion) with the activation of metabolic mechanisms of adaptation and repair. These metabolic changes appear not severe enough to inhibit the growth of the calli. The results obtained with Actinidia deliciosa calli are discussed in relation to protocols of selection and micropropagation. |
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