FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

important findings from local as well as international literature about the relationship between specific nutrients and grape quality, some of which having been mentioned in Chapter 5 already, are summarised below for the sake of completeness. N ITROGEN Nitrogen has a positive effect on growth, but maximum fruit yield (fertility) is obtained at a lower N nutrient level than that at which maximum vegetative growth is obtained. However, when there is no vegetative growth during the period following fruit set, the reaction to berry enlargement treatments (e.g. GA 3 or cytokinin) will not be according to expectations. Grapevines with a low N nutrient level also display smaller berry size and lower titratable acidity, especially lactic acid (Choné et al ., 2006). Overly vigorous growth, induced by an over-supply of nitrogen, may lead to “bloated” berries, that will have a dilution effect on colourants and aroma components, thus naturally being detrimental to produce good quality grapes. When N availability is high (especially after veraison) cells tend to be larger, which has a negative effect on storage ability and disease resistance. An indirect result is that larger leaves and longer shoots lead to dense canopies (Figure 26), thus helping to create favourable conditions for diseases (specifically Botrytis), as well as reduced sunlight penetration, with negative consequences for colour development specifically, but also for general grape quality. Carbohydrates that are obtained through photosynthesis are then used for the manufacture of protoplasm and are consequently not available for accumulation of, e.g. sugar, which will lead to delayed ripening. In situations where nitrogen is over-supplied it has also been found that the skins of the grapes are thinner, sugar accumulation is sluggish and acidity levels decline (Keller et al ., 1998). It has thus also been found (Ruiz et al ., 2004) that there is a correlation between the amino acid N levels of grape skins and the occurrence soft berries for Thompson Seedless. This correlation has furthermore also been found between the amino acid N and putrescine. The latter is associated with dying/dead tissues, and consequently the appearance of soft berries. This means that the over-supply of N leads to the occurrence of soft berries. Low carbohydrate reserves and large cells can also contribute towards the frost sensitivity of grapevines. Furthermore, excessive applications can lead to water berry (pallo negro) and a general decline in carbohydrate contents of plant tissues. Too much nitrogen in relation to potassium during the spring period can lead to leaf edge yellowing and necrosis (Figure 27), while it will also cause false symptoms of K deficiency.

96 | THE RELATIONSHIP BETWEEN FERTILISATION, THE NUTRITIONAL STATUS OF A VINEYARD AND GRAPE QUALITY