Use of the Diagnosis and Recommendation Integrated System (DRIS) for Determining the Nutritional Balance of Durian Cultivated in the Vietnamese Mekong Delta

: Durian is one of the most valuable and expensive fruits in Vietnam and Southeast Asia. Leaf nutritional disorders are considered to be causes of reductions in fruit yield and quality. However, studies on the nutritional balance in durian leaf are limited. In this study, we used the DRIS method for leaf limitation nutrient diagnosis at the stage 2 months before durian flowering. Our objectives were to (i) establish DRIS norms for the macronutrients (N, P, K, Ca, Mg, and S) in durian leaf and (ii) determine nutrient value ranges that are insufficient or excessive in durian production. A total of 180 leaf samples were collected and examined from 90 durian orchards cultivated on alluvial soils in the Vietnamese Mekong Delta. The results indicate that DRIS establishment for durian was highly reliable due to the significant positive correlation (r > 0.5) among nutrient indices. The limiting nutrients in durian leaf at the investigation stage were S, Mg, Ca, and P, where S and Mg were the most deficient. The optimal ranges of nutrients in durian leaf were determined and recommended in this study. Further studies are necessary to validate the efficiency of DRIS using nutrient omission trials under durian cultivation.


Introduction
Durian (Durio zibethinus Murr.) is a fruit originating from Borneo, Southeast Asia (SA) [1].It is widely cultivated in the tropical regions of SA, such as Indonesia, Thailand, Vietnam, Malaysia, and the Philippines [2].It is known as the 'king of fruits' due to its unique taste and nutritional value [3].Therefore, it is one of the most expensive and highvalue fruits for both domestic markets and export [4].In the Vietnamese Mekong Delta (VMD), the area of durian cultivation was 30,000 ha in 2020, and this is estimated to increase rapidly in the future.This is a significant concern for local authorities due to farmers lacking experience in durian growing.Durian cultivation has numerous requirements (farming technique, nutrient management, etc.) because these factors are the most effective in determining fruit productivity and quality [5].Farmers who cultivate durian in the VMD usually undertake nutrient management based on their experience.They apply fertilizers in excess of the actual mass requirement of the plants, harming the growth and development of durian.In addition, nutrient disorders and imbalances in durian leaf cause increasing fruit physiological disorders, resulting in a reduction in flesh quality and, therefore, decreasing the value and price of durian [6].Thus, leaf nutrient balancing is considered the best strategy for sustainable and effective durian cultivation.
Macronutrients, including nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S), are considered the main elements that significantly affect durian fruit yield and quality (smell and taste) [6].N plays many vital roles in growth, flowering, fruit setting and development, and physiological processes in the flesh of durian [6].N also contributes primarily to the photosynthesis and respiration of the plant [7].P directly affects energy storage and transfer, and enzyme activation [8].P is necessary for nutrient uptake, flower initiation, and fruit growth [9].K aids in the synthesis of proteins and carbohydrates; it also improves the size, taste, and color of fruits [10].Ca is essential for cell walls, cell division, and creating new roots; it also affects carbohydrate translocation and N absorption [10].Mg plays an important role in chlorophyll structure, regulating the uptake of other essential elements and carbohydrate translocation [11].S acts in the biosynthesis of sulfur-containing amino acids and various secondary metabolites [12].Therefore, deficiencies or excesses of these elements adversely affect durian cultivation and production.In many countries, fertilizer recommendations and nutritional status diagnosis in fruit plants are determined based on leaf analysis [13][14][15] because foliar analysis reveals the actual uptake of the plant from fertilization and soil.This is considered a measurement method for the evaluation of the current nutritional status of crops, which might be replaced by soil analysis [16].
Leaf diagnosis is a complex method because it is affected by foliar age and other interactions, which directly influence nutrient uptake and distribution [14].Various approaches are utilized to interpret the nutritional status of plants using leaf analysis, such as the identification of deficiency symptoms, determining the ratio of nutrients, or using the diagnosis and recommendation integrated system (DRIS) [17,18].DRIS was introduced and developed by Beaufils (1957Beaufils ( , 1971Beaufils ( , and 1973) as a solution for tackling the difficulties inherent in diagnostic methods utilizing leaves [19].Currently, DRIS is an interesting method because it helps in the identification of nutrient imbalances, insufficiencies, or excesses and ranks them in order of importance to determine remedial steps [14,17,20].DRIS has been successfully used for the evaluation of nutritional imbalances in fruit plants, such as mango [21], guava [22], bananas [23], and pineapple [17].According to Villamil-Carvajal et al. [24], DRIS norms must be developed for each crop.This is because the accuracy of DRIS norms is affected by the specific cultivar, climate, soil, plant nutrient management, etc. Presently, research concerning the application of DRIS in assessing leaf nutritional status for durian remains lacking and unclear.Therefore, to fill this knowledge gap, we undertook a study on 90 farms that cultivate durian in the VMD.The objectives of this study were to (i) establish DRIS norms for the macronutrients (N, P, K, Ca, Mg, and S) of durian leaf at the stage 2 months before durian flowering and to (ii) determine the ranges of optimal nutrients for durian leaf.

Study Description, Soil, and Climate
To carry out the study, we collected durian leaves from 90 orchards in Can Tho city and Hau Giang province (Figure 1).These locations occupy the largest area of durian cultivation, which is considered to have high potential for durian production in the VMD region [25].The average annual precipitation and air temperature in the period 2020-2022 were 1818 mm and 27.6 • C, respectively [26].Our previous studies [26,27] indicated that soils are classified as alluvial soils, with high fertility at the soil depth of 0-20 cm, rich soil organic matter (25-30 g kg −1 ) and total N content (1.0-1.5 g kg −1 ), and optimal soil bulk density (~1.0 g cm −3 ); data were shown on a dry weight soil.The pH value of soils was 4.5-5.0,which was extracted in distilled water with a ratio of soil/distilled water (1:2.5).According to Amran et al. [28], who reported that durian thrives well in soil pH ranges from 5.5 to 6.5.Therefore, soil pH of the study locations was lower than that in optimal threshold of approximately 1.0 unit.5.5 to 6.5.Therefore, soil pH of the study locations was lower than that in optimal threshold of approximately 1.0 unit.

Leaf Sampling and Analysis
In this study, the 'Ri 6' durian cultivar was investigated because it is popular and comprises approximately 70% of the durian varieties cultivated in the VMD [29].Before collecting leaf samples, we surveyed the fruit yield of durian in 2022 at 200 orchards.To establish DRIS norms, data should be divided into two groups, including high-yielding and low-yielding [30].The high-yielding group was calculated using the mean of the fruit yield population + (0.5 × SD) [30].We successfully separated two fruit yield groups: low yield (<17.6 Mg ha −1 ) and high yield (≥17.6 Mg ha −1 ).Next, we selected 90 durian orchards for leaf collection, with 45 low and 45 high fruit yields.Two leaf samples were taken from each orchard.A total of 180 leaf samples were collected in this research.
To be eligible for sampling, each orchard area had to be greater than 3000 m 2 , and the durian plant had to be at least 6 years old.In addition, farming techniques such as irrigation, cultivation, planting density, flower induction, and insect and disease management were similar among orchards.We conducted leaf sampling in August 2023, 2 months

Leaf Sampling and Analysis
In this study, the 'Ri 6' durian cultivar was investigated because it is popular and comprises approximately 70% of the durian varieties cultivated in the VMD [29].Before collecting leaf samples, we surveyed the fruit yield of durian in 2022 at 200 orchards.To establish DRIS norms, data should be divided into two groups, including high-yielding and low-yielding [30].The high-yielding group was calculated using the mean of the fruit yield population + (0.5 × SD) [30].We successfully separated two fruit yield groups: low yield (<17.6 Mg ha −1 ) and high yield (≥17.6 Mg ha −1 ).Next, we selected 90 durian orchards for leaf collection, with 45 low and 45 high fruit yields.Two leaf samples were taken from each orchard.A total of 180 leaf samples were collected in this research.
To be eligible for sampling, each orchard area had to be greater than 3000 m 2 , and the durian plant had to be at least 6 years old.In addition, farming techniques such as irrigation, cultivation, planting density, flower induction, and insect and disease management were similar among orchards.We conducted leaf sampling in August 2023, 2 months before durian flowering.According to Morales et al. [30], 2 months before the flowering stage is considered important due to plants requiring high amounts of nutrients for sprouting, flowering, and fruit setting; therefore, this stage is suitable for leaf collection.
Leaf samples collected from the plant showed no symptoms of nutritional deficiency and pest infestations.Leaves were picked from the 5th and 6th positions from the bud according to the method of Suarta et al. [31].Each leaf sample included 40 leaves collected from 5 durian trees.After collection, durian leaves were rinsed with distilled water 4 times to remove dust and insects.Subsequently, they were put in an oven for drying at 65 • C for 4 days and then ground and stored in vials to analyze mineral nutrients.
The contents of mineral nutrients (N, P, K, Ca, Mg, and S) in the leaves were analyzed based on the method of Houba et al. [32].Firstly, leaf samples were converted to inorganic forms using a mixture of 12 g of salicylic acid, 36 mL of water, and 200 mL of 96% H 2 SO 4 .Secondly, a solution of hydrogen peroxide (H 2 O 2 , 30%) was added during the leaf digestion process.Finally, N was determined using the Kjeldahl method; P was analyzed via a UV spectrophotometer (UV-1800, Shimadzu, Tokyo, Japan); K, Ca, and Mg were measured via atomic absorption spectrophotometry (iCE 3500, Thermo Scientific, Waltham, MA, USA) at wavelengths of 766, 422, and 285 nm, respectively; and S was determined using the colorimetric method.

DRIS Indices
DRIS indices are the values calculated after DRIS norms are established.According to Khuong et al. [17], DRIS indices should be determined based on the following equation: where IX1 is the DRIS index for the X1 nutrient.f X1 X2 is the function calculated for the X1 and X2 nutrient ratio; ). X1/X2 is the nutrient ratio for diagnosis and x1/x2 is the value of nutrient ratio that is determined from DRIS norms.CV is the coefficient of variation, and Y is the number of functions used for the calculation of the total nutrients.

Establishing Durian Leaf Nutrient Optimum Ranges
In this study, nutrient optimum ranges of leaves were established using the average nutrient concentration of the high-productivity group [30].The value ranges of durian leaf nutrients were calculated by taking into account the standard deviation (SD).Leaf nutrient classifications of durian were in accordance with the procedure of Morales et al. [30], including excessive (> 4  3 SD), high ( 2 3 < SD < 4 3 ), optimum (− 2 3 < SD < 2 3 ), low (− 4 3 < SD < − 2 3 ), and deficient (<− 4  3 SD).In our study, a total of five classes for durian foliar diagnosis were suggested.

Data Analysis
We used Microsoft Excel (ver.16) and IBM SPSS Statistics (ver.20) software for data analysis.Student's t-test was used to compare averages of nutrient concentration between low-and high-productivity groups.Pearson's correlation was applied to evaluate the relationships between nutritional variables.Data were interpreted on a dry-weight basis.

The Concentration of Leaf Nutrients under Low and High Durian Fruit Yields
Table 1 shows the nutrient concentrations for low and high durian fruit productivity.There was a significant difference in nutrients between the low-and high-productivity groups.The mean of the N concentration in the high-productivity group was higher by 3.10 g kg −1 than that in the low-productivity group.Similarly, the average contents of P, K, Ca, Mg, and S of the high-productivity group were 0.93, 4.10, 3.90, 1.68, and 3.00 g kg −1 higher than those in the low-productivity group, respectively.CV in each nutrient population ranged from 20.6% to 50% in both groups.Figure 2 shows the relationship in respect of nutrient concentration between highand low-yielding groups.There was a strong positive (r = 0.78 ***) relationship between P and K nutrients in the high-yielding group (Figure 2a).Similarly, Ca was positively correlated with Mg (r = 0.56 ***), and Mg was also positively correlated with S (r = 0.52 ***).Negative correlations were observed between the nutrient concentrations in the highproductivity group, such as the following: P and Ca (r = −0.47***), P and Mg (r = −0.61***), P and S (r = −0.45***), K and Ca (r = −0.61***), K and Mg (r = −0.72 ***), and K and S (r = −0.47***).However, there was no significant correlation among nutrients in the low-yielding group (Figure 2b).

DRIS Norms for Durian.
Table 2 shows the values of the mean, SD, CV, and σ 2 of nutrient ratios in two durian fruit productivity groups (low and high).After the calculation of the nutrient ratios in leaves, 15 nutrient ratios were selected from the DRIS norms to determine DRIS indices.These were

DRIS Norms for Durian
Table 2 shows the values of the mean, SD, CV, and σ 2 of nutrient ratios in two durian fruit productivity groups (low and high).After the calculation of the nutrient ratios in leaves, 15 nutrient ratios were selected from the DRIS norms to determine DRIS indices.These were N/P, K/N, N/Ca, N/Mg, N/S, K/P, Ca/P, P/Mg, P/S, K/Ca, K/Mg, K/S, Ca/Mg, Ca/S, and S/Mg.

DRIS Indices of Durian Leaf
The nutrient indices for durian were successfully established from the DRIS norms (Table 3).Among the five nutrients investigated, we observed that the N and K concentrations in durian leaves were quite high.In particular, the K index was slightly abundant (0.76), and the N nutrient index was excessive (2.17).Meanwhile, the concentrations of P, Ca, Mg, and S in durian leaf were deficient.The means of those nutrient indices (P, Ca, Mg, and S) were −0.71, −1.09, −2.36, and −3.71, respectively.S and Mg contents were extremely deficient compared with those of Ca and P. The limited nutrients in durian leaf at the stage 2 months before flowering followed the pattern S > Mg > Ca > P.

Discussion
We observed that there was a significant difference in nutrient concentrations between low and high fruit productivity (Table 1).In this study, we collected leaves from orchards that had similar farming techniques (see Section 2.2).However, fertilization management and use were different in the two productivity groups.The orchards of the high-yielding group received more than approximately 50% N-P-K chemical fertilizers than the lowyielding group.This is considered one reason for the concentrations of nutrients in the high-productivity group being higher than those in the low-productivity group.The results of our study are in agreement with Morales et al. [30], Khuong et al. [17], and Xuan et al. [20].They reported that nutrient concentrations are higher in high-yielding groups compared with low-yielding groups.A previous study indicated that differences in the amount of fertilizer applied significantly affect leaf nutritional contents [35].
Imbalanced fertilizer application also affected leaf nutrients.We realized that farmers growing durian often do not use or apply fertilizers containing elements such as Ca, Mg, and S in the long term.This leads to a decrease in or depletion of these nutrients in the soil and, thus, a deficiency in the leaves.In contrast, N and P are applied in large amounts by farmers because they believe that these nutrients will increase fruit yield quality.Various studies have indicated that the use of imbalanced fertilizers leads to decreased soil fertility and nutrition insufficiency in leaves, resulting in reductions in plant growth and productivity [36,37].According to Ong et al. [38], 10 tons of durian fruit removed approximately 4 P-3 Ca-5 Mg (kg).Unfortunately, the authors did not reveal the data in respect of S in their study.In this study, the average durian fruit yield was 16 tons year −1 ; therefore, around 6.5, 5.0, and 5.0 kg (P, Ca, and Mg, respectively) were used by fruit each year.Hence, farmers should return these elements via fertilizer (granular or foliar) application to offset the rate of nutrient loss from fruits.Other factors affecting the concentration of nutrients in leaves are low soil pH and high iron and aluminum contents, resulting in a decrease in the concentrations of exchangeable Ca and Mg in the soil.A previous study [39] reported that durian orchards in the VMD had low soil pH (pH < 5) and exchangeable cations (Ca 2+ and Mg 2+ ).Moreover, most durian trees are cultivated in raised beds in which the surface soil is always higher by 0.5 m compared with the water in the canal.This reduces the exchangeable cations because these cations runoff due to rainfall and irrigation [26].Although growers applied a certain amount of P for their gardens, P is a limited nutrient in durian leaf.P use efficiency decreased due to P being precipitated by Al and Fe under low pH conditions, resulting in increasing insoluble hydroxide compounds that are unavailable for plant uptake [26].Therefore, soil pH regulation is necessary for improving the availability of P in soil.
The CV values of DRIS norms were low, with ranges of 16.8-80.9%and 31.2-65.3% in the high-and low-productivity groups, respectively (Table 2).Walworth and Sumner [19] indicated that DRIS norms are highly reliable when the CV is low (<100%) and σ 2 L /σ 2 H is high.This is because a high CV could lead to large deviations in results between nutrient ratios.Our results are consistent with those of Llanderal et al. [40], who successfully determined the limiting nutrients of pepper using DRIS norms, which had a low CV and high σ 2 L /σ 2 H . Another study showed that DRIS norms established from higher σ 2 L /σ 2 H and lower CV values in nutrient ratios were more reliable for calculating the DRIS index [17].
Horticulturae 2024, 10, 561 9 of 11 A similar study also concluded that DRIS norms were highly accurate and efficient when the nutrient ratios had a low CV and high σ 2 L /σ 2 H [41,42]. Therefore, the DRIS norms in our study were reliable for DRIS index calculation due to the high σ 2 L /σ 2 H and low CV of the nutrient ratios.
From the results of the DRIS norms in Table 2, we selected 15 nutrient ratios, which were used to calculate the DRIS indices (Table 3).The results indicate that there was a strong positive correlation (r > 0.5) among nutrient indices (Figure 3).Therefore, these indices have high reliability and could be used widely for the nutritional diagnosis of durian leaf cultivated on the VMD.In addition, we observed that durian leaves had a shortage of P, Ca, Mg, and S (DRIS index < 0).Meanwhile, there was excessive N and K (DRIS index > 0) in durian leaf.According to Beverly [43], the nutrient index is excessive when the value is higher than zero.Further, nutrients are insufficient if the value of the nutrient index is smaller than zero, and nutritional balance is confirmed if the nutrient index is zero [44,45].

Conclusions
DRIS indices were successfully established for the nutritional diagnosis (N, P, K, Ca, Mg, and S) of durian and had high reliability.The concentrations of P, Ca, Mg, and S in leaves were deficient, while the contents of N and K were excessive.Therefore, to ensure sustainable durian development, we recommend that farmers should apply complex chemical fertilizers that contain P, Ca, Mg, and S to replenish their orchards.In addition, enhancing and regulating soil pH should be necessary to increase soil available nutrients (P, Ca, and Mg) for durian.Based on the results of our previous studies [27,47], the applications of lime, rice husk biochar, or compost were not only increasing soil pH but also supplying nutrients for the soil.The use of soil conservation practices (leguminous cover cropping and rice straw mulching) is also considered the best strategy for improving soil pH and avoiding the loss of nutrients or exchangeable cations on the surface of raised soil beds.

Figure 1 .
Figure 1.Map of location for collecting durian leaf samples.

Figure 1 .
Figure 1.Map of location for collecting durian leaf samples.

Figure 2 .
Figure 2. The relationship between durian leaf nutrient concentrations in the high (a) and low (b) productivity groups.* and *** indicate p < 0.05 and p < 0.001, respectively.

Figure 2 .
Figure 2. The relationship between durian leaf nutrient concentrations in the high (a) and low (b) productivity groups.* and *** indicate p < 0.05 and p < 0.001, respectively.

Figure 3
Figure 3 presents the relationship between the nutrient indices (N, P, K, Ca, Mg, and S) established from DRIS norms.A strong positive correlation was observed between DRIS indices for durian leaf.In particular, there was a positive correlation between IN and IP (r = 0.69), IN and IK (r = 0.55), IN and ICa (r = 0.60), IN and IMg (r = 0.48), and IN and IS (r = 0.54).IP correlated positively with IK, ICa, IMg, and IS, with r = 0.80, 0.89, 0.59, and 0.77, respectively.Similarly, IK correlated positively with ICa (r = 0.65), IMg (r = 0.68), and IS (r = 0.59).There was a strong positive correlation between ICa and IMg, ICa and IS, and IMg and IS (r = 0.68, 0.79, and 0.59, respectively).Horticulturae 2024, 10, x FOR PEER REVIEW 8 of 12

Table 4 .
Durian leaf nutrient optimum ranges for the stage 2 months before flowering.

Table 4 .
Durian leaf nutrient optimum ranges for the stage 2 months before flowering.