1. Introduction
The method of petiole sap analysis is used for the determination of crop nutritional status since 1920 [
1,
2] and is carried out on fresh material, giving a semiquantitative evaluation of the extractable nutrients that are present in soluble inorganic forms in the plant just at the sampling moment [
3].
Xylem and phloem sap plus apoplastic solution are used to determine the nutrient concentration in a crop [
4], but in order to establish a crop a nutritional diagnostic is required for comparison with the sufficiency ranges of this crop [
5]. The sufficiency ranges are commonly used to interpret plant nutrient composition as well as nutrient deficiency, sufficiency, luxury consumption, or excess [
6].
There are several factors which can modify the nutrient sap concentration, such as fertilizer application, light intensity, phenological stage, and sampling position [
2,
7,
8,
9]. Regarding fertilizer application, Ikeda [
10] reported a significant decrease in sap concentration of NO
3−, H
2PO
4−, Ca
2+, and Mg
2+ in tomato grown in nutrient solution after 10 days without nutrient solution. Considering the light intensity, Leyva et al. [
11] found that in a tomato crop in the cycle of autumn–winter, when the days are shorter and the light intensity is lower than in summer months, there is a decrease in the synthesis of carboxylates and soluble carbohydrates used as osmolytes in plants resulting in a higher uptake of NO
3− to be used as osmolytes. Cadahía [
12] found a significant variation in the concentration of Cl
−, NO
3−-N, H
2PO
4−-P, SO
42−-S, Na
+, K
+, Ca
2+, and Mg
2+ in sap throughout the different phenological stages of a tomato crop. Garcia and Azuara [
13] reported a high variation between sampled leaves selected (young leaves, fully grown young leaves, mature leaves, and older leaves), reporting that the concentration of H
2PO
4−-P was higher in the young leaf because this ion is located in places with maximum metabolic activity. Similiarly, Llanderal et al. [
14] reported that the selection of sampled leaves (fully grown young leaves, mature leaves, and older leaves) did not modify the Ca
2+, Cl
−, SO
42—S, and Na
+ concentrations, whereas NO
3−-N, K
+, Mg
2+, and H
2PO
4−-P concentrations showed great variability due to the selection of the sample leaf, therefore it is necessary to be careful with the sample selection.
The wide variability of sufficiency ranges in tomato proposed by different researchers includes: NO
3−-N (700–1210 mg L
−1), K
+ (3500–5000 mg L
−1) [
2], PO
4H
2−-P (35–300 mg L
−1), Ca
2+ (280–1420 mg L
−1), Mg
2+ (190–2000 mg L
−1) [
12], Cl
− (750–4500 mg L
−1), and Na
+ (50–400 mg L
−1) [
15] which can be related to the different factors aforementioned. Therefore, the aim of this work is to evaluate different parameters in relation with the nutrient concentration in petiole sap in a tomato crop in a greenhouse cultivated in Mediterranean conditions and to propose an empirical model in order to determine the nutrient concentration in petiole sap.
4. Discussion
Concerning climatic parameters, the range of ETc obtained in our trial (1.2 to 2.4 mm day
−1) was within the ranges proposed by Fernández et al. [
25] and Baeza et al. [
26], who established a minimum value of 0.5 and a maximum value of 2.4 mm day
−1. The range of global radiation in our experiment was from 6.7 to 8.0 MJ m
−2 day
−1 and was inside in the range of 5 to 11 MJ m
−2 day
−1 reported by Baeza et al. [
26] for the production of tomato in Mediterranean greenhouses. The range of temperature during the experiment (10 to 17 °C) was in line with the findings reported by Baeza et al. [
26] who established a range from 4 to 25 °C inside the greenhouse in a tomato crop. Finally, the average relative humidity in our experiment (85% of RH) was inside the range established by Baudoin et al. [
27] (70%–90% of RH) for a tomato crop in the Mediterranean area.
As far as plant parameters are concerned, the values of LAI in our experiment ranged from 4.2 to 4.6. These values were inside the range (3.8 to 4.7) reported by different researchers such as Barraza et al. [
28] and Medrano et al. [
17]. Finally, the yield obtained in our experiment was lower compared to the production of this area with an average of 9.56 kg m
−2 [
29]. This yield reduction can be due to the low daily radiation inside the greenhouse, since Baudoin et al. [
27] recommended a minimum daily radiation in tomato of around 8.5 MJ m
−2 day
−1. Similiarly, Sandri et al. [
30] and Llanderal et al. [
31] reported the same results in a tomato greenhouse crop.
The mean values for pH, EC, and nutrient concentrations in soil solution in our experiment (pH (7.5–8.1) and EC (2.1–4.1 dS m
−1) values and concentration of H
2PO
4− (0.1–0.2 mmol L
−1), K
+ (3.1–7.6 mmol L
−1), Ca
2+ (2.5–5.9 mmol L
−1), Mg
2+ (2.5–11.3 mmol L
−1), Cl
− (1.3–13.2 mmol L
−1), and Na
+ (2.5–11.3 mmol L
−1) were inside the optimal ranges proposed by Lao et al. [
19]. On the other hand, the range of NO
3− concentration was higher than the optimal range proposed by Lao et al. [
19] (NO
3− 6.4–19 mmol L
−1). The higher NO
3− concentration found in our experiment can be related to an excessive nitrogen input which is well above the nutritional requirements of the crop.
The mean values of nutrient concentrations in petiole sap in our experiment were inside the optimal range proposed by Cadahía [
12] for H
2PO
4−-P (35–135 mg L
−1), K
+ (600–4590 mg L
−1), Ca
2+ (280–1420 mg L
−1), and Mg
2+ (190–200 mg L
−1) concentrations, and for Cl
− and Na
+, the values in our experiment were inside in the optimal range proposed by Urrestarazu [
15] (750–4500 and 50–400 mg L
−1, respectively). On the other hand, the range of NO
3−-N concentration was higher than the optimal range proposed by Cadahía [
12] (133–1000 mg L
−1). The high concentration of NO
3−-N in sap over our trial could be a result of the excess of NO
3− in the soil solution (SS) due to the high supply of nitrogen fertilizers, which is in agreement with Fontes and Ronchi [
32] who found a positive correlation between NO
3− concentration in soil solution and in petiole sap. Moreover, the high concentration of NO
3−-N in sap over our trial could be due to the low light intensity inside the greenhouses because under low-light conditions, there is a decrease in the activity of nitrate reductase as reported by Llanderal [
9].
The mean values of leaf nutrient concentration in our experiment were inside the optimal ranges for N (25–48), P (2.6–4.7), and K (16–31 mg g
−1 DW) proposed by Llanderal et al. [
33] and higher than the optimal values proposed by Casas and Casas [
34] for Cl (<5 mg g
−1) and Na (<1.8 mg g
−1 DW).
The variability of nutrient concentrations in sap are associated with several factors, such as climatic parameters, leaf area index, nutrient concentrations in soil solution, and the previous nutrient concentrations in petiole sap. All nutrient concentrations in the petiole sap showed a positive correlation with the crop ETc, and this can be related to the reduction of the water content in the plant as suggested by Hochmuth [
2]. No significant correlation between nutrient concentrations and temperature was found. The only significant correlation between RH and nutrient concentrations in sap was for Ca
2+. These results are in line with the findings reported by Armstrong and Kirkby [
35] who established that under high relative humidity conditions (95%), the nutrient uptake of calcium by mass flow was restricted. No correlation between NO
3−-N concentration and RH was found in our experiment and this could be due to the fact that NO
3−-N concentration is independent of the RH conditions as proposed by Erica et al. [
36].
No relationship between Cl
−, H
2PO
4−-P, Na
+, K
+, and Mg
2+ concentrations in sap and RH is due to the fact that ions such as Cl
−, H
2PO
4−-P, Na
+, K
+, and Mg
2+ are considered mobile in the phloem, so they can be deposited in plant organs or translocated [
14,
37].
The increase of nutrient concentration in sap could be related to the reduction of water content in the plant, since crop transpiration increases with increasing atmospheric vapor pressure deficit (VPD) [
38]. In plant parameters, LAI has a negative correlation with Cl
−, NO
3−-N, H
2PO
4−-P, K
+, and Na
+ concentration in sap which can be due to the dilution factor related to biomass increase proposed by Opstad [
39]. On the other hand, there was a positive correlation with Ca
2+ and Mg
2+ due to the accumulation of these ions over the crop cycle as reported by Llanderal [
9]. No correlation between nutrient concentrations in sap and soil solution is due to the fact that the concentrations in soil solution were in the optimal range as proposed by Lao et al. [
19]. Nevertheless, the physiological process such as nutrients uptake, bio-assimilation, and storage affect the nutrient concentrations in sap [
9].
The significant autocorrelation over the crop cycle of NO
3−−N, H
2PO
4−-P, K
+, Ca
2+, and Mg
2+ concentration relates to the capacity of plants to regulate nutrient concentrations in response to changes in environmental conditions [
40]. However, no significant autocorrelation of Cl
− and Na
+ over the experimental period occured due to the regulation developed by some wild tomato species through the accumulation of these ions into the vacuole for their osmotic regulation [
41], which in turn can be diluted for a growth and succulence mechanism as proposed by Cuartero et al. [
42].
Finally, comparing the different diagnostic methods established with the yield, our results report that the petiole sap method shows the best coefficient of correlation. The negative correlation in petiole sap (NO
3−-N, H
2PO
4—P, and K
+) and leaf analysis (N, P, and K) can be due to the dilution factor as a consequence of the increase of growth and yield in the plant [
31,
43]. Moreover, the negative correlation can be due to the translocation of these nutrients to the fruit [
44]. On the other hand, it is necessary to point out that the positive correlation between K
+ concentration in soil solution and yield is in line with the findings reported by Bugarín-Montoya et al. [
45] who proposed the same positive correlation in tomato crop.
5. Conclusions
The method of petiole sap shows the best coefficient of correlation with the yield, compared with the different diagnostic methods established: soil solution and leaf nutrient concentrations; therefore, nutrient sap concentration can be recommended as the most sensitive nutritional diagnosis methods related to the expected yield.
The highest problem of sap diagnosis methods is the wide range of nutrient concentrations related to optimal nutrient status. In this paper, a model is proposed to determine the nutrient concentrations in petiole sap in response to climatic parameters, nutrients in soil solution, and growth. The ETc, DPV, and LAI are the most significant variables that allow the development of these empirical prediction models regarding nutrient concentrations in petiole sap. It is important to highlight that the reduction of the water content in the plant increases the concentration of all the nutrients in petiole sap.
The high autocorrelation of nutrient concentrations in sap for one week suggests that a longer sampling period is needed in order to analyze the nutrient change in sap, therefore we recommend that the best option is a sampling period of 15 days. Nevertheless, it is necessary for other experiments to further support and confirm these findings due to the scarcity of previous knowledge.