Ana Quiñones

Flooding affects uptake and distribution of carbon and nitrogen in citrus seedlings

Soil flooding has been widely reported to affect large areas of the world. In this work, we investigated the effect of waterlogging on citrus carbon and nitrogen pools and partitioning. Influence on their uptake and translocation was also studied through ¹⁵N and ¹³C labeling to provide insight into the physiological mechanisms underlying the responses. The data indicated that flooding severely reduced photosynthetic activity and affected growth and biomass partitioning. Total nitrogen content and concentration in the plant also progressively decreased throughout the course of the experiment. After 36 days of treatment, nitrogen content of flooded plants had decreased more than 2.3-fold compared to control seedlings, and reductions in nitrogen concentration ranged from 21 to 55% (in roots and leaves, respectively). Specific absorption rate and transport were also affected, leading to important changes in the distribution of this element inside the plant. Additionally, experiments involving labeled nitrogen revealed that ¹⁵N uptake rate and accumulation were drastically decreased at the end of the experiment (93% and 54%, respectively). ¹³CO₂ assimilation into the plant was strongly reduced by flooding, with δ¹³C reductions ranging from 22 to 37% in leaves and roots, respectively. After 36 days, the relative distribution of absorbed ¹³C was also altered. Thus, ¹³C recovery in flooded leaves increased compared to controls, whereas roots exhibited the opposite pattern. Interestingly, when carbohydrate partitioning was examined, the data revealed that sucrose concentration was augmented significantly in roots (37-56%), whereas starch was reduced. In leaves, a marked increase in sucrose was detected from the first sampling onwards (36-66%), and the same patter was observed for starch. Taken together, these results indicate that flooding altered carbon and nitrogen pools and partitioning in citrus. On one hand, reduced nitrogen concentration appears to be a consequence of impaired uptake and transport. On the other hand, the observed changes in carbohydrate distribution suggest that translocation from leaves to roots was reduced, leading to significant starch accumulation in leaves and further decreases in roots.

Nitrogen remobilization response to current supply in young citrus trees

Internal nitrogen (N) storage and remobilization processes support seasonal growth (flowering/fructification and subsequent leaf development) in particular in early spring, when soil temperatures are unfavourable for adequate N uptake. Storage nitrogen mobilization in young citrus trees was studied under two contrasting N supplies; high N (HN) and low N dose (LN) in the critical period of flowering and fruit set. 15N labelling technique was used to distinguish N derived from internal remobilization from that taken up by the roots. Regardless N supply, the greatest N remobilization took place from the beginning of the vegetative activity until flowering. Low N availability significantly increased (+14%) N retranslocation at the end of June drop agreeing with the hypothesis that reserve mobilization depends on soil N availability during flowering and fruit set. At the end of fruit drop, N remobilization contributed up to 70% and 61% of total N of young organs for LN and HN, respectively. Remobilized N was mainly recovered in abscised organs of both HN and LN trees and to a lesser extent in new flush leaves; however a greater percentage partitioned to abscised organs of LN as a consequence of the greater remobilization rate and the increased fruit abscission. Old leaves of LN remobilized significantly higher N, while woody organs and root system did not show differences between HN and LN supplied trees. The results presented in this paper demonstrate that the amount of N remobilized by young citrus plants depends on external N availability. Thus, low N application rates in early stages (flowering and fruit set) lead to higher translocation of N stored during the previous cycle to developing new organs.

Impact of fertilizer-water management on nitrogen use efficiency and potential nitrate leaching in citrus trees

In citrus production, the combination of adequate irrigation and nitrogen (N) management is considered a key factor in improving N uptake efficiency, so as to minimize nitrate ( ) leaching below the root zone and maintain optimal crop yield. The response of eight-year-old citrus trees grown in lysimeters to two combinations of fertilizer and water management techniques – split N application in drip irrigation (DI-F) vs. a single broadcast application with flood irrigation (FI-B) – was compared by means of the use of a 15N-labeled fertilizer (ammonium nitrate). Soil sampling 54, 116, 175 and 238 days after the beginning of the experiment and destructive harvest of trees at the end of the growing cycle allowed the determination of the fate of the labeled fertilizer supplied. The 15N recovered in the different compartments of the plant-soil system and fertilizer nitrogen uptake efficiency (FNUE, 15N taken up by the plant per 15N supplied) was also quantified. Irrigation-fertilizer management practices had no effect on either total tree biomass or in fruit yield; however, a significantly greater development of coarse roots in the DI-F treatment was recorded. Fertilizer-N uptake was 25% higher in trees in the DI-F than in the FI-B treatment; fine roots and fruits of DI-F trees accumulated 2.1- and 1.4-fold higher fertilizer-N, respectively, compared to the corresponding organs of FI-B trees. At the end of the experiment, -15N concentrations in the 0–20 cm and 60–90 cm layers of DI-F trees were higher than those in FI-B trees, whereas no differences were recorded for -15N concentrations of both treatments. Potential nitrate leaching (PNL) 54 days after the beginning of the experiment represented 8% of fertilizer-15N delivered in FI-B, whereas in DI-F it was almost negligible. In DI-F trees, PNL become noticeable on day 175, highlighting the effect of this irrigation method in reducing and slowing down the movement through the soil profile. At the end of the experiment PNL represented 14% and 5% of fertilizer supplied for FI-B and DI-F, respectively. FNUE was significantly higher (12%) with the DI-F treatment when compared to FI-B treated trees. Comparison of the two water-fertilizer management practices studied in the present work allowed us to conclude that the DI-F treatment increased FNUE while diminishing nitrate leaching, when compared to the FI-B treatment.

Flooding Impairs Fe Uptake and Distribution in Citrus Due to the Strong Down-Regulation of Genes Involved in Strategy I Responses to Fe Deficiency in Roots

This work determines the ffects of long-term anoxia conditions—21 days—on Strategy I responses to iron (Fe) deficiency in Citrus and its impact on Fe uptake and distribution. The study was carried out in Citrus aurantium L. seedlings grown under flooding conditions (S) and in both the presence (+Fe) and absence of Fe (-Fe) in nutritive solution. The results revealed a strong down-regulation (more than 65%) of genes HA1 and FRO2 coding for enzymes proton-ATPase and Ferric-Chelate Reductase (FC-R), respectively, in –FeS plants when compared with –Fe ones. H+-extrusion and FC-R activity analyses confirmed the genetic results, indicating that flooding stress markedly repressed acidification and reduction responses to Fe deficiency (3.1- and 2.0-fold, respectively). Waterlogging reduced by half Fe concentration in +FeS roots, which led to 30% up-regulation of Fe transporter IRT1, although this effect was unable to improve Fe absorption. Consequently, flooding inhibited 57Fe uptake in +Fe and –Fe seedlings (29.8 and 66.2%, respectively) and 57Fe distribution to aerial part (30.6 and 72.3%, respectively). This evidences that the synergistic action of both enzymes H+-ATPase and FC-R is the preferential regulator of the Fe acquisition system under flooding conditions and, hence, their inactivation implies a limiting factor of citrus in their Fe-deficiency tolerance in waterlogged soils.

METHYL XANTHINE AS A POTENTIAL ALTERNATIVE TO GIBBERELLIC ACID IN ENHANCING FRUIT SET AND QUALITY IN CLEMENTINE CITRUS TREES IN SPAIN

Citrus clementine cv. ‘Nules’ is a seedless mandarin with excellent quality for consumption as fresh fruit; however, its poor fruit set leads to an excess of small-sized fruits. Currently, the only available practice to increase fruit set is the use of the growth regulator gibberellic acid (GA 3 ) as a foliar spray. A trial was conducted to compare an environmentally friendly new bioregulator (MX), containing methyl-xanthine, to GA 3 in terms of increasing fruit set and yield of clementine cv. Nules mandarin trees. Fruit yield was significantly increased in treated trees from 17.4 to 25.7% for GA 3 and MX treatments, respectively, compared to untreated control trees, without any significant differences between the growth regulators. At the tested doses, MX performed similarly to GA 3 in terms of fruit quality and nutrient content, while colour index was not affected significantly by this new bioregulator. A single foliar application of MX was enough to increase fruit set. Spray application of MX on clementine mandarins at the end of the flowering period, when all petals have fallen showed a similar response to GA 3 treatments.

Correction: Physiological and Molecular Responses to Excess Boron in Citrus macrophylla W.

Fig 6, “Boron concentration ([Bf], μg g-1 DW) and boron content (Bf, μg) in (A) soluble in water, (B) soluble in organic solvents and (C) insoluble fractions measured in roots and leaves of Citrus macrophylla seedlings grown for 25 days in B-normal (50 μM, Ct) and B-toxic (400 μM, +B) nutrient solutions,” appears incorrectly. Please see the corrected Fig 6 here. Fig 6 Boron concentration ([Bf], μg g-1 DW) and boron content (Bf, μg) in (A) soluble in water, (B) soluble in organic solvents and (C) insoluble fractions measured in roots and leaves of Citrus macrophylla seedlings grown for 25 days in B-normal ...

Molecular characterization and stress tolerance evaluation of new allotetraploid somatic hybrids between Carrizo citrange and Citrus macrophylla W. rootstocks

Polyploidy is one of the main forces that drives the evolution of plants and provides great advantages for breeding. Somatic hybridization by protoplast fusion is used in citrus breeding programs. This method allows combining the whole parental genomes in a single genotype, adding complementary dominant characters, regardless of parental heterozygosity. It also contributes to surpass limitations imposed by reproductive biology and quickly generates progenies that combine the required traits. Two allotetraploid somatic hybrids recovered from the citrus rootstocks—Citrus macrophylla (CM) and Carrizo citrange (CC)—were characterized for morphology, genome composition using molecular markers (SNP, SSR, and InDel), and their tolerance to iron chlorosis, salinity, and Citrus tristeza virus (CTV). Both hybrids combine the whole parental genomes even though the loss of parental alleles was detected in most linkage groups. Mitochondrial genome was inherited from CM in both the hybrids, whereas recombination was observed for chloroplastic genome. Thus, somatic hybrids differ from each other in their genome composition, indicating that losses and rearrangements occurred during the fusion process. Both inherited the tolerance to stem pitting caused by CTV from CC, are tolerant to iron chlorosis such as CM, and have a higher tolerance to salinity than the sensitive CC. These hybrids have potential as improved rootstocks to grow citrus in areas with calcareous and saline soils where CTV is present, such as the Mediterranean region. The provided knowledge on the effects of somatic hybridization on the genome composition, anatomy, and physiology of citrus rootstocks will be key for breeding programs that aim to address current and future needs of the citrus industry.

DETERMINATION OF FOLIAR SAMPLING CONDITIONS AND STANDARD LEAF NUTRIENT LEVELS TO ASSESS MINERAL STATUS OF LOQUAT TREE

Loquat [Eriobotrya japonica (Thunb.) Lindl.] is a minor fruit-tree species which is grown commercially in the Mediterranean region. Given the current increase in loquat cultivation, there is a need to define basic crop-management procedures in order to obtain high yields and optimal fruit quality. The aim of this work was to develop a routine for loquat farmers to follow, in order to know the nutritional status of trees in order to establish a rational fertilization regime, and to correct nutrient deficiencies as soon as possible. This paper reports three experiments aimed at establishing: 1) leaf type, 2) time of leaf sampling and 3) the standard leaf nutrient levels as a function of maximum yield. Results indicate that the summer flush leaves could be the most representative of nutritional status. Thus, the most appropriate time to sample leaves for analysis is in summer (beginning of August to the end of September), taking mature 3–4 month old leaves.