1. Introduction
Persimmon (
Diospyros kaki Thunb.) is a typical fruit crop of the Mediterranean region, but in Italy it is considered a minor species, being cultivated on an area of less than 2900 ha [
1]. The intensive and specialized persimmon cultivation is located mainly in Emilia-Romagna region, where this species finds optimal environmental conditions. Persimmon fertilization management is often based on the knowledge acquired from other major fruit crops such as peach [
2], apple [
3] and pear [
4], while little is known on its nutrient needs. The widespread use of vigorous rootstocks requires careful management of water and nutrients in order to control tree vegetative growth and fruiting. If nutrients supply exceed plants needs it results in an excessive vigour that is known to delay the end of the unproductive juvenile period in young orchards and increases fruit drop in mature ones [
5,
6]. Furthermore, the improper use of fertilizers increases the risk of water pollution [
7,
8] with negative impact on surface and ground water quality. An adequate management of fertilization requires the knowledge of soil fertility as well as tree needs in each specific phenological phases, in order to synchronise nutrient supply with plant demand [
9]. During the vegetative season part of the nutrients up-taken by roots is allocated to fruits and removed from the orchard at harvest; on the other hand, nutrients partitioned to vegetative organs (shoots and leaves) can be translocated within the tree or return back to the ground through abscissed leaves and pruned wood. The development of a correct fertilization plan should, thus, take into consideration the distribution of nutrients within plant organ, their mobility and availability during the vegetative season. Fertilization should aim at maintaining inputs (fertilizers, crop residues, irrigation water and rain falls) equal to outputs, that include quantities removed with fruits and stored in the perennial organs (skeleton and roots), and lost through erosion, runoff, immobilization and volatilisation.
An effective way to evaluate tree nutritional status is leaf analysis, particularly if done in the summer, that is able to evidence lack or excess of nutrients by comparing results of analysis with appropriate indices obtained in the same area of cultivation. However, currently, there are not leaf indexes for Italian persimmon and usually those from Australia [
10,
11], Spain [
12] and Turkey [
13] are used.
The aim of the present investigation was to (1) define summer leaf indexes for Italian cultivation and (2) assess the nutrient requirements of the two main varieties of persimmon grown in Emilia-Romagna Region by estimating the quantities of nutrients removed and recycled.
2. Materials and Methods
2.1. Characteristics of the Orchards Investigated
The study was carried out in 2018–2019 and involved seven farms specialized in persimmon (
Diospyros kaki Thunb.) cultivation, located in the provinces of Modena, Ravenna and Forlì-Cesena (between 44°, 38′ and 44°, 08′ N and between 10°, 55′ and 12°, 14′ E), in the Central and South-Eastern part of Emilia-Romagna region (Po valley, Italy). Ten mature orchards located in the seven farms were selected for similar features, including: (1) cultivars: Kaki Tipo, a local cultivar, and Rojo Brillante, a Spanish variety [
14] introduced in Italy about 20 years ago, that are the two most used varieties in Emilia-Romagna region; (2) rootstock: both varieties were grafted on seedlings; (3) tree spacings: 4.3–4.5 m between rows and 3.5–4.5 m between trees along the row (average planting density of 614 and 616 trees ha
−1 for Kaki Tipo and Rojo Brillante, respectively); (4) soil type: mainly loam and silt-loam; (5) soil management: use of herbicide on tree row, and alleys covered with spontaneous grass mowed regularly during the season; (6) pest control, irrigation and fertilization managed according to the Integrated Crop Management guideline of the Emilia-Romagna region [
15]. In detail, Kaki Tipo orchards were fertilized, yearly, with an average of 53 kg ha
−1, 20 kg ha
−1 and 63 kg ha
−1 of nitrogen (N), phosphorus (P) and potassium (K), respectively; while Rojo Brillante received 71 kg N ha
−1, 17 kg P ha
−1 and 63 kg K ha
−1 of N, P and K.
2.2. Plant Organ Sampling and Analysis
In each of five orchards per cultivar, four plots of five trees each were selected and used for sample collection. At the beginning of August, leaf samples were taken from the middle part of shoots. Leaf chlorophyll was measured by SPAD 502 (Minolta Co., Ramsey, NJ, USA) and leaf area was measured by a portable area meter (Li-3000, LiCor Inc., Lincoln, NE, USA). Leaves were then washed, oven-dried and milled, and analysed for the concentration of N, P, K, calcium (Ca), magnesium (Mg), sulphur (S), boron (B), copper (Cu), iron (Fe), manganese (Mn) and zinc (Zn). Nitrogen was determined by the Kjeldahl method, while other macro and micronutrients were determined by spectrometry emission (ICP-OES, Ametek Spectro, Arcos, Kleve, Germany), after mineralization in an Ethos TC microwave lab station (Milestone, Bergamo, Italy). At fruit harvest, plant yield was recorded and a representative sample of fruits was collected to measure dry matter and fruit mineral composition as previously described. In autumn, the naturally abscised leaves and the wood removed with pruning in one tree per plot were collected, weighed, oven-dried, milled and analysed for mineral composition as described above; in addition, leaf area was measured as described for summer leaves. To collect senescent leaves, the area below trees was delimited and the fallen leaves were collected several times until complete defoliation. The biomass and mineral composition of perennial organs (roots and skeleton) were recorded in winter on four trees from one orchard of both Rojo Brillante (12 years old) and Kaki Tipo (19 years old). Each tree was harvested and divided in roots, trunk and branches; a sub-sample of this organs was oven-dried, milled and analysed for macro and microelement concentration as previously described.
The mineral content of each organ was calculated by multiplying its nutrient concentration by its dry weight (DW). Leaf nutrient content was also referred to leaf specific weight calculated by dividing DW for leaf area. The knowledge of the nutrient content in tree organs allowed us to calculate the recycled and removed fractions. In detail, recycled nutrients are those that at the end of the vegetative season return to the soil and were calculated as the sum of nutrients content in abscised leaves and pruning wood. Removed nutrients were calculated as the sum content in root, skeleton and fruits at harvest. Total orchard demand was calculated as the sum the recycled and removed fraction multiplied by the number of trees per hectare. Finally, to estimate the annual contribution in terms of biomass and of removed nutrients by perennial organs, all the data were divided by the age of plants.
2.3. Statistical Analysis
Data of biomass, nutrients concentration and nutrients content for each cultivar are reported as means ± standard deviation (SD) of 20 samples for leaves (summer and abscised, fruits at harvest and pruned wood) and of 4 samples for skeleton and roots. Leaf nutrients content was analysed with the software R according to a discriminant canonical analysis (DCA). Pearson correlation coefficient was employed to estimate the linear relationship between leaf nutrient concentration and SPAD measurements.
4. Discussion
The annual biomass produced by persimmon tree (leaves, fruits and pruning wood) was almost 20 kg per tree, equal to 12 t ha
−1, a value similar to that reported for peach trees [
2]. Pruned wood and abscised leaves contributed each for 25–30% of the total tree biomass, with values of pruned wood correlated to cultivar vigour that is known to be higher in Kaki Tipo than Rojo Brillante. The contribution of persimmon pruned wood and leaves to total plant biomass was higher than that reported for apple [
16] and pear [
17] grafted on dwarfing rootstocks, but similar to peach grafted on the vigorous hybrid GF677 [
2]. About 50% of the annual biomass was partitioned to fruits, similarly to what previously observed for peach [
2]. This result confirms that Emilia-Romagna has the optimal pedo- climate conditions for persimmon intensive cultivation, with yields higher than the Italian average [
1].
The intensity of the green color of the leaves depends on several factors such as soil fertility, especially N availability, the age of the leaf and the genotype (cultivar, rootstock and their combination). In our study difference were observed between Kaki Tipo and Rojo Brillante (lower for Rojo Brillante than Kaki Tipo), despite similar orchard management; moreover, for both cultivars, our SPAD values were higher than those reported for Fuyu cultivar (45-50 SPAD unit) [
18]. This variability is related to the genotype, as reported for other crops such as apple [
19] and grapevine [
20]. The relationship between summer values of leaf chlorophyll and N concentration was significant and the coefficient of correlation was similar (r = 0.78–0.91) to that reported in Fuyu cultivar [
18,
21]. Moreover, we observed a positive correlation between SPAD and leaf S (r = 0.35, *), B (r = 0.42; **), Cu (r = 0.48, **) and Mn (r = 0.53, ***). Consequently, summer leaf chlorophyll could be a useful tool to predict nutrient deficiencies, but without precise information on nutrient concentration [
19].
The mineral composition of the leaves in summer showed a nutritional status similar for the two persimmon varieties; Ca, N and K were in line with values reported for several deciduous fruit crops [
22]. In Kaki Tipo, summer leaf nutrient concentrations were similar to those reported in Italy [
23], Spain [
12] and Turkey [
13]. In other studies, conducted in Australia [
24] and New Zealand [
10], K leaf values were significantly higher than those found in the present study. Manganese is the most representative micronutrient in persimmon leaf, as reported in the literature [
10,
12,
23]. For a correct interpretation of the analytical results it is important to compare data with indices available in the same area of diagnosis. This study, therefore, represents a first attempt to define preliminary indices for mineral diagnosis of summer leaves in both Kaki Tipo and Rojo Brillante cultivated in Emilia Romagna region, as already done for other fruit species.
The discriminant canonical analysis of leaf mineral composition in summer and winter, confirm, regardless of genotype, the mobility of N, P, K and S that, at the end of the season translocated to storage organs (roots, stem and branches), as described for deciduous trees [
25,
26], but also for persimmon [
10,
27]. Other nutrients such as Ca and microelements are not involved in this process and are accumulated in leaf until abscission [
10]. Magnesium was translocated in small quantities and our results contrast with those reported in a study on Rojo Brillante carried out in Spain, which showed an increase of Mg concentration until leaf abscission [
27]. In the present study, it was estimated that 50% of N and P, and 67% of K of summer leaves were translocated before leaf abscission, regardless of the cultivar [
28], while the residual fractions were potentially recycled in the orchard through the decomposition and mineralization processes [
29]. The contribution of recycled fraction through the fallen leaves of persimmon reached 70–80% of the total content in the tree for Ca, Mg, B and Mn, while it was about 36% for N, 20% for P and 20–25% for K.
The mineral content of the one-year-old shoots removed by winter pruning showed values comparable to those reported for persimmon Fuyu [
30], except for Ca, that was almost double in our study, while K level was higher than that reported in the literature for the pruned wood of peach [
31] and pear [
32]. This means that the soils of the investigation are rich in Ca and K. Like abscissed leaves, also pruned wood represents an important source of nutrients that are recycled in the orchard; however, its decomposition process is longer than leaves because the presence of lignin [
33]. The return of mineral nutrients to the soil through the pruning wood of persimmon accounts for 45% of the total plant N and K, 65% of the total P, and about the 25% for Ca and Mg, in the case of the Kaki Tipo cultivar.
Fruits at harvest accounted for the highest fraction of nutrients removed from the orchard, representing the major net fraction of the crop. In the present study this fraction was estimated to be about 32%, 50%, 62% and 31% of the total annual requirement of N, P, K and S, respectively; Mg, B, Cu and Zn were removed by fruits for the 10–15%, while the fraction of Ca, Fe and Mn were minimal and therefore their removal by fruits negligible. As for peach [
31] and in general for most fruit tree species [
34], K was the most important mineral nutrient in persimmon fruit and its amount was about twice that of N and almost twenty times higher than Ca, confirming the results reported in other studies [
11,
12,
30,
35]. The remaining nutrients removed annually were stored in the roots and in the skeleton). In our conditions, these reserves, even if represent smaller quantities (4–12, 0.2–1.1, 1.4–4 kg ha
−1 of N, P and K, respectively), are essential for plant metabolism since they support new tissue development in spring [
25].
The sum of the nutrients removed and recycled annually provides the total uptake of minerals during the season and the quantified values are in line with those recommended in Australia [
24].
5. Conclusions
In the conditions of our investigation, the two main varieties of persimmon industry cultivated in Emilia Romagna region showed a similar annual nutrients demand, estimated for macronutrients in about 90 kg N ha−1, 10 kg P ha−1, 80–90 kg K ha−1, 130–160 kg Ca ha−1, 25 kg Mg ha−1, while for micronutrients the requirement reaches 1000–2000 g ha−1 only for Fe and Mn. Except for K, more than half of yearly plant request of nutrients return to the soil at the end of season and can be reused with positive effect on the nutrient use efficiency. The recycled fraction mentioned above, therefore, must be considered, along with both soil fertility and tree nutritional status, in the definition of the fertilization plan, in order to minimize the external chemical inputs. Finally, this study proposes references indices for summer leaf analysis of Kaki Tipo and Rojo Brillante in Emilia Romagna region, with the aim of proving a correct interpretation of data in the same area of cultivation.