Physiological Responses of a Grapefruit Orchard to Irrigation with Desalinated Seawater.

Desalinated seawater (DSW) has emerged as a promising solution for irrigation in regions facing water scarcity. However, adopting DSW may impact the existing cultivation model, given the presence of potentially harmful elements, among other factors. A three-year experiment was carried out to assess the short-term effects of four irrigation waters-freshwater (FW), DSW, a mix 1:1 of FW and DSW (MW), and DSW with low boron (B) concentration (DSW-B)-on a 'Rio Red' grapefruit orchard. These irrigation waters exhibited varying levels of phytotoxic elements, some potentially harmful to citrus trees. Sodium (Na+) and chloride (Cl-) concentrations exceeded citrus thresholds in all treatments, except in DSW-B, whilst B exceeded toxicity levels in DSW and MW treatments. Leaf concentrations of Cl- and Na+ remained low in all treatments, whereas B approached toxic levels only in DSW and MW-irrigated trees. The rapid growth of the trees, preventing excessive accumulation through a dilution effect, protected the plants from significant impacts on nutrition and physiology, such as gas exchange and chlorophyll levels, due to phytotoxic elements accumulation. Minor reductions in photosynthesis in DSW-irrigated trees were attributed to high B in leaves, since Cl- and Na+ remained below toxic levels. The accelerated tree growth effectively prevented the substantial accumulation of phytotoxic elements, thereby limiting adverse effects on tree development and yield. When the maturation of trees reaches maximal growth, the potential accumulation of phytotoxic elements is expected to increase, potentially influencing tree behavior differently. Further study until the trees reach maturity is imperative for comprehensive understanding of the long-term effects of desalinated seawater irrigation.

Deficit Irrigation Applied to Lemon Trees Grafted on Two Rootstocks and Irrigated with Desalinated Seawater.

The use of desalinated seawater (DSW) for irrigation in semi-arid regions is taking hold. Citrus tolerance to ions that predominate in DSW and water stress depends on the rootstock. Deficit irrigation was applied to DSW-irrigated lemon trees and grafted on rootstocks with different tolerance (Citrus macrophylla (CM) and sour orange (SO)). Plants were irrigated with DSW or Control treatment (distilled water), and, 140 days later, irrigation treatments were started: full irrigation (FI) or DI (50% of the volume applied to FI). After 75 days, differences between CM and SO plants irrigated with DSW and under DI were found. The higher concentrations of Cl- and Na+ in CM and B in SO were the main causes of shoot growth reduction. The osmotic adjustment of CM plants was made possible by the accumulation of Na+, Cl-, and proline, but SO failed to adjust osmotically. In CM and SO plants, photosynthesis reduction was due to lower chlorophyll levels, but also to stomatal factors (CM plants) or alterations of the photochemical machinery (SO plants). Finally, unlike CM, SO had a good antioxidant system. In the future, knowing the different responses of CM and SO under these stressful conditions could be useful in citrus-growing areas.

Changes in Berry Tissues in Monastrell Grapevines Grafted on Different Rootstocks and Their Relationship with Berry and Wine Phenolic Content.

Monastrell grapevines grafted on the rootstocks 140Ru, 1103P, 41B, 110R, and 161-49C were subjected to regulated deficit irrigation (RDI) and partial root-zone irrigation (PRI). We analyzed the effects of the rootstock and irrigation method on the phenolic concentration in different berry tissues, its dilution/concentration due to the berry size, the anatomical and morphological traits of berries related to the phenolic compounds concentration, and the relationships of all these parameters with the final berry and wine phenolic content. The rootstock had an important effect on the accumulation of total phenolic compounds and anthocyanins in the skin (berries from 110R and 140Ru had the highest values). Moreover, the rootstock modified some anatomical and morphological characteristics that had a direct relationship with the final phenolic compounds concentration in the must. Large grapes and high must percentages (110R and 140Ru) produced a dilution effect, whereas small berries and a low must percentage increased the concentration (161-49C). For 110R, the small size of the cells of the epidermis and hypodermis in the grapes also could have contributed to the high phenolic compounds concentration in the skin. The percentage of cells in the skin with a uniform coloration was positively correlated with its total phenolic compounds and anthocyanins concentration and also with the phenolic quality of the wine. The PRI modified some specific morphological/anatomical skin/berry traits, and these may have contributed to important changes in the final concentration of phenolic compounds, depending on the rootstock. The better phenolic quality of the must and wines observed in some rootstocks under PRI could be due to smaller cells in the epidermis and hypodermis of the skin (161-49C), a higher percentage of cells with a uniform coloration in the hypodermis (110R), or a lower number of seeds per berry (161-49C). In contrast, the lower phenolic compounds concentration in the must of grapes observed in the most vigorous rootstocks under PRI could be due to a greater thickness of the epidermis (140Ru), greater cuticle thickness (41B), a higher number of seeds (140Ru), a lower skin/pulp ratio and percentage of skin (140Ru), a greater percentage of cells in the epidermis without coloration or with large inclusions, and a lower percentage of cells with a uniform coloration in the epidermis (140Ru). The final quality of the grape is related to some changes in histological and morphological aspects of the grape produced by the rootstock and irrigation strategy.

Mycorrhizal effectiveness in Citrus macrophylla at low phosphorus fertilization.

An experiment was conducted with seedlings of Citrus macrophylla Wester to study the effects of P nutrition on plants inoculated with a mixture (Rhizophagus irregularis and Funneliformis mosseae) of arbuscular mycorrhizal (AM) fungi. The treatments consisted of factorial combinations of two factors: mycorrhization (-AM: non-inoculated plants, and +AM: inoculated plants) and P nutrition (0, 0.1, 1, and 5 mM P). After the P treatments had been applied for 165 days, the AM fungi showed an important effect on plant growth and P uptake, but this effect depended on the P fertilization. In the absence of P fertilization, inoculation with the AM fungi had little impact on P nutrition and plant growth. However, when 0.1 or 1 mM P was supplied, inoculation had a clear beneficial effect on plant growth, since P nutrition was significantly improved, the maximum growth of the +AM plants occurring at 1 mM P. The supply of 5 mM P did not increase plant growth with regard to 1 mM P due to a lack of improvement in leaf P nutrition and photosynthesis. The higher demand of the AM fungi in the roots of the +AM plants for sucrose reduced the concentration of sucrose in the leaves of plants receiving 5 mM P, and of fructose and glucose in the roots of plants supplied with 0.1 or 1 mM P, relative to the -AM plants. The inoculated plants grown with 5 mM P had a decreased starch concentration in their roots, in order to supply the high sugar demand of the AM fungi. The C drain towards the AM fungi in the +AM plants may have been compensated by a higher photosynthetic rate and improved mineral nutrition. Inoculation improved plant P nutrition in the 0.1 and 1 mM P treatments but had a lesser effect at 5 mM P. The tissue levels of certain nutrients, such as Mg, improved with inoculation regardless of the P treatment, but those of other nutrients - such as Zn or Fe - increased more in the +AM plants with lower P supply. So, in general, the +AM C. macrophylla plants receiving the highest P supply did not show improved mineral nutrition relative to the -AM plants. Overall, the results indicate that when the availability of P to C. macrophylla plants is high, the lower benefits received by the plants from the C-for-P trade can convert a mutualistic relationship between the host plant and AM fungi into a parasitic one since colonization can persist even when the availability of P in the soil is high.

Influence of deficit irrigation timing on the fruit quality of grapefruit (Citrus paradisi Mac.).

The irrigation necessities for grapefruit production are very high. Due to the scarcity of water resources, growers use deficit irrigation (DI) - which could affect the fruit quality. Different DI strategies were studied: Control (irrigated at 100% ETc) and T1, T2 and T3 (50% ETc at phases I, II and III of fruit growth, respectively). Strategy T1 only delayed external maturation depending on the duration of the water stress. High water stress in T2 delayed fruit maturation, increased acidity and reduced the sugar concentration. Under T2, trees suffering moderate water stress showed increased flavonoid and phenolic contents but decreased lycopene levels. External maturation was delayed in T3 when severe stress occurred during the first part of phase III. Strategy T3 advanced internal ripening when moderate water stress occurred during the first 40 days of phase III, increasing sugar accumulation, promoted by the high acidity of the fruits. Moderate water stress also increased ß-carotene, flavonoids and phenolics levels.

Rapid estimation of nutritional elements on citrus leaves by near infrared reflectance spectroscopy.

Sufficient nutrient application is one of the most important factors in producing quality citrus fruits. One of the main guides in planning citrus fertilizer programs is by directly monitoring the plant nutrient content. However, this requires analysis of a large number of leaf samples using expensive and time-consuming chemical techniques. Over the last 5 years, it has been demonstrated that it is possible to quantitatively estimate certain nutritional elements in citrus leaves by using the spectral reflectance values, obtained by using near infrared reflectance spectroscopy (NIRS). This technique is rapid, non-destructive, cost-effective and environmentally friendly. Therefore, the estimation of macro and micronutrients in citrus leaves by this method would be beneficial in identifying the mineral status of the trees. However, to be used effectively NIRS must be evaluated against the standard techniques across different cultivars. In this study, NIRS spectral analysis, and subsequent nutrient estimations for N, K, Ca, Mg, B, Fe, Cu, Mn, and Zn concentration, were performed using 217 leaf samples from different citrus trees species. Partial least square regression and different pre-processing signal treatments were used to generate the best estimation against the current best practice techniques. It was verified a high proficiency in the estimation of N (Rv = 0.99) and Ca (Rv = 0.98) as well as achieving acceptable estimation for K, Mg, Fe, and Zn. However, no successful calibrations were obtained for the estimation of B, Cu, and Mn.

Alleviation of salt stress in citrus seedlings inoculated with arbuscular mycorrhizal fungi depends on the rootstock salt tolerance.

Seedlings of Cleopatra mandarin (Citrus reshni Hort. ex Tan.) and Alemow (Citrus macrophylla Wester) were inoculated with a mixture of AM fungi (Rhizophagus irregularis and Funneliformis mosseae) (+AM), or left non-inoculated (-AM). From forty-five days after fungal inoculation onwards, half of +AM or -AM plants were irrigated with nutrient solution containing 50 mM NaCl. Three months later, AM significantly increased plant growth in both Cleopatra mandarin and Alemow rootstocks. Plant growth was higher in salinized +AM plants than in non-salinized -AM plants, demonstrating that AM compensates the growth limitations imposed by salinity. Whereas AM-inoculated Cleopatra mandarin seedlings had a very good response under saline treatment, inoculation in Alemow did not alleviate the negative effect of salinity. The beneficial effect of mycorrhization is unrelated with protection against the uptake of Na or Cl and the effect of AM on these ions did not explain the different response of rootstocks. This response was related with the nutritional status since our findings confirm that AM fungi can alter host responses to salinity stress, improving more the P, K, Fe and Cu plant nutrition in Cleopatra mandarin than in Alemow plants. AM inoculation under saline treatments also increased root Mg concentration but it was higher in Cleopatra mandarin than in Alemow. This could explain why AM fungus did not completely recovered chlorophyll concentrations in Alemow and consequently it had lower photosynthesis rate than control plants. AM fungi play an essential role in citrus rootstock growth and biomass production although the intensity of this response depends on the rootstock salinity tolerance.

Physiological and growth changes in micropropagated Citrus macrophylla explants due to salinity.

Salinity is one of the major abiotic stresses affecting arable crops worldwide, and is the most stringent factor limiting plant distribution and productivity. In the present study, the possible use of in vitro culture to evaluate the growth and physiological responses to salt-induced stress in cultivated explants of Citrus macrophylla was analyzed. For this purpose, micropropagated adult explants were grown in proliferation and rooting media supplemented with different concentrations of NaCl. All growth parameters were decreased significantly by these NaCl treatments; this was accompanied by visible symptoms of salt injury in the proliferated shoots from 60mM NaCl and in the rooted shoots from 40mM NaCl. Malondialdehyde (MDA) increased with increasing salinity in proliferated shoots, indicating a rising degree of membrane damage. The concentration of total chlorophyll significantly decreased in the presence of NaCl, and this effect was more pronounced in the rooted explants. The Na(+) and Cl(-) concentrations in the explants increased significantly with the salinity level, but Cl(-) levels were higher in the proliferated explants than in the rooted explants. For osmotic adjustment, high concentrations of compatible solutes (proline and quaternary ammonium compounds-QAC) accumulated in salt-stressed plants in proliferation, but differences were not observed in rooted explants. In proliferation, proline and QAC were highly correlated with the sodium and chloride concentrations in the explants, indicating a possible role of these compounds in osmotic adjustment. The plant concentrations of NO(3)(-), K(+), Mg(2+), Ca(+) and Fe were also affected by the NaCl concentration of the medium. We suggest that the important deleterious effects in the in vitro explants of Citrus macrophylla grown at increasing NaCl concentrations were due mainly to toxic effects of saline ions, particularly Cl(-), at the cellular level.