In general, Fruit growth (in weight, volume and shape) is the result of cell division, cell enlargement and the formation of extracellular spaces.
In olives, it seems reasonable to add oil formation to this list.
The double sigmoid growth curve suggested for many drupes has often been considered applicable to the growth of olive fruit. Some authors, however, distinguish a greater number of growth stages and attribute each of them to different factors. According to some, the growth of olive fruit is rapid in the first phase, slower in the second (August, September) and rapid again in autumn in the third phase, with veraison. Decelerated growth, however, is not a necessary phase in the development of the olive fruit but is often linked to climatic factors, such as drought. With irrigation there is no deceleration of fruit growth and the fruit grows continuously until ripe.
It is therefore important to understand how agronomic practices, in particular fertilization, can affect the growth of olives, slowing down or accelerating fruit development.
The effect of nitrogen and potassium on olive growth and oil accumulation
The effect of summer soil and foliar applications of two doses of nitrogen and potassium on some characteristics of olive fruit and their seasonal variations were studied in a young, drip-irrigated olive grove.
The decline in the longitudinal (L)/transverse (D) axis ratio across the season demonstrates that the initial elongation period is followed by a period in which the transverse diameter of the fruit increases more rapidly. The transverse diameter (D) of the fruits increased by 46.83% between the first measurement (7 July) and the last (29 November). During the same period, the length of the longitudinal axis (L) increased by only 34.64%. Fruit and pulp dry weights increased continuously until the third sampling in early October, after which no significant changes occurred. A similar trend was found for most other measured characteristics, such as moisture and ash weight.
During July and October, the increase in fruit oil weight paralleled the increase in pulp weight. Between August and September, however, the increase in oil in the pulp was much faster. Subsequently, in October, the increases in pulp and oil were more limited, i.e. after the period of rapid increase in oil, a more or less stable maximum weight of oil per fruit was recorded.
The relative rates of change in fruit fresh weight (mg/g/day) were high in the periods 7 July-14 August and 14 August-5 October, but fell to values close to zero in October-November.
Both nitrogen and potassium inputs had a significant negative effect on pulp dry weight per fruit, which in November for nitrogen- and potassium-treated trees fell to 89% and 93%, respectively, compared to unfertilized controls. Foliar applications also caused a significant (10%) reduction in fruit pulp. Nitrogen spraying slightly reduced the percentage of oil on the pulp, while nitrogen fertilization on the soil had no effect.
The trees sprayed with potassium (KNO3) alone, however, had a slightly higher percentage of oil in the dry pulp.
Spraying with nitrogen (Urea) and potassium (KNO3) reduced oil production the most and in November the average percentage of oil in the pulp of the sprayed trees was equal to 88.39% of that of the control trees.
The decrease in fruit dry weight (pulp) following nitrogen application was explained by the greater number of fruits present as nitrogen was applied early and affected the number of fruits per tree.
Some responses of the olive tree to nitrogen during fruit development coincide with what has been reported in previous studies, suggesting that, at least in part, the accumulation of fruit dry matter occurs at the expense of nutrients taken from the leaves.
