Unravelling the importance of forest age stand and forest structure driving microbiological soil properties, enzymatic activities and soil nutrients content in Mediterranean Spanish black pine(Pinus nigra Ar. ssp. salzmannii) Forest

This study aimed to investigate the effects that stand age and forest structure have on microbiological soil properties, enzymatic activities and nutrient content. Thirty forest compartments were randomly selected at the Palancares y Agregados managed forest area (Spain), supporting forest stands of five ages; from 100 to 80years old to compartments with trees that were 19-1years old. Forest area ranging from 80 to 120years old and without forest intervention was selected as the control. We measured different soil enzymatic activities, soil respiration and nutrient content (P, K, Na, Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn, Pb and Ca) in the top cm of 10 mineral soils in each compartment. Results showed that the lowest forest stand age and the forest structure created by management presented lower values of organic matter, soil moisture, water holding capacity and litterfall and higher values of C/N ratio in comparison with the highest forest stand age and the related forest structure, which generated differences in soil respiration and soil enzyme activities. The forest structure created by no forest management (control plot) presented the highest enzymatic activities, soil respiration, NH4(+) and NO3(-). Results did not show a clear trend in nutrient content comparing all the experimental areas. Finally, the multivariate PCA analysis clearly clustered three differentiated groups: Control plot; from 100 to 40years old and from 39 to 1year old. Our results suggest that the control plot has better soil quality and that extreme forest stand ages (100-80 and 19-1years old) and the associated forest structure generates differences in soil parameters but not in soil nutrient content.

Soil organic carbon stocks assessment in Mediterranean natural areas: a comparison of entire soil profiles and soil control sections

Soil organic carbon (SOC) is an important part of the global carbon (C) cycle. In addition, SOC is a soil property subject to changes and highly variable in space and time. Over time, some researches have analyzed entire soil profile (ESP) by pedogenetic horizons and other researches have analyzed soil control sections (SCS) to different thickness. However, very few studies compare both methods (ESP versus SCS). This research sought to analyze the SOC stock (SOCS) variability using both methods (ESP and SCS) in The Despeñaperros Natural Park, a nature reserve that consists of a 76.8 km(2) forested area in southern Spain. Thirty-four sampling points were selected in the study zone. Each sampling point was analyzed in two different ways, as ESP (by horizons) and as SCS with different depth increments (0-25, 25-50, 50-75 and 75-100 cm). The major goal of this research was to study the SOCS variability at regional scale. The soils investigated in this study included Phaeozems, Cambisols, Regosols and Leptosols. Total SOCS in the Despeñaperros Natural Park was over 28.2% greater when SCS were used compared to ESP, ranging from 0.8144 Tg C (10,604.2 Mg km(-2)) to 0.6353 Tg C (8272.1 Mg km(-2)) respectively (1 Tg = 10(12) g). However, when the topsoil (surface horizon and superficial section control) was analyzed, this difference increased to 59.8% in SCS compared to ESP. The comparison between ESP and SCS showed the effect of mixing pedogenetic horizons when depth increments were analyzed. This indicates an overestimate of T-SOCS when sampling by SCS.

Interstock-induced mechanism of increased growth and salt resistance of orange (Citrus sinensis) trees

Interstocks improve the growth and salt resistance of lemon (Citrus limon (L.) Burm. f.) trees, but their effects on orange (Citrus sinensis (L.) Osbeck) trees are unknown. We grew 'Cleopatra' mandarin (CM) seedlings, budded trees of 'Salustiano' orange (SAO) on CM, 'Valencia Late' orange (VLO) on CM (VLO/CM), and interstock trees VLO/SAO/CM in pots of sand watered with nutrient solution containing 5 (control) or 50 mM NaCl for 12 weeks. Plants were harvested on six successive occasions and the time trends in relative growth rate (RGR) and its components were estimated by fitting a Richards function regression to the harvest data. At low and high salinities, the VLO/SAO/CM combination had higher mean RGR than VLO/CM. Under control conditions, the increase in RGR caused by the interstock was the result of an increase in leaf mass fraction (LMF; leaf dry mass/plant dry mass ratio). Increases in net assimilation rate on a leaf mass basis (NARm) and LMF contributed equally to the increase in RGR in saline conditions, their growth response coefficients being 0.52 and 0.48, respectively. The structural modifications, specific leaf area (SLA) and leaf area ratio (LAR; leaf area:plant dry mass ratio), had a slight influence on the reduction in RGR by salinity. However, NARm had a large influence on RGR, except in CM. The interstock-induced mechanism increased biomass allocation to the assimilatory organs and, under saline conditions, increased Cl- and Na+ allocations to roots. Thus, the flux of ions to the leaves was either delayed or reduced or both. The dilution of imported ions by foliar growth reduced ion concentrations in leaves, resulting in higher NARm, which together with higher LMF, increased RGR.

Influence of nitrate level on nitrate assimilation in tomato (Lycopersicon esculentum) plants under saline stress

This experiment was conducted to study the effect of salinity and different NO3– concentrations on NO3– uptake, transport, and reduction. To avoid the significant effect of plant size, the main difference from other studies was the short time period of treatment application. Tomato plants (Lycopersicon esculentum Mill. 'Asun') were grown for 11 d in half-strength Hoagland solution (with 7 mmol/L NO3–) in a growth chamber under controlled conditions. After that, the different NO3– treatments were 0.5, 2, and 4 mmol/L NO3–, and saline treatments consisted of two levels of NaCl: 0 (control) and 75 mmol/L. Under control conditions, an increase in NO3– concentration in the nutrient solution resulted in greater NO3– uptake, NO3– xylem flux, NO3- concentrations in tissues, and leaf and root nitrate reductase activity. However, when plants were exposed to salinity for a short period of time and the plant size or the shoot/root ratio was not an additional factor affecting the physiological parameters, the NO3– concentration in the solution slightly affected the different steps of NO3– assimilation.Key words: nitrate assimilation, nitrate reductase activity, nitrogen, salinity, tomato, Lycopersicon esculentum.

Improvement in growth and salt resistance of lemon (Citrus limon) trees by an interstock-induced mechanism

Interstocks can reduce toxic ion accumulations in leaves of budded citrus trees, but the mechanism is not understood. We grew sour orange (Citrus aurantium L.; SO) seedlings, budded trees of 'Salustiano' orange (Citrus sinensis L. Osbeck; SAO) on SO, 'Verna' lemon (Citrus limon L. Burm. f; VL/SO) and interstock trees (VL/SAO/SO) in pots of sand watered with nutrient solution containing 5 (control) or 50 mM NaCl (saline treatment) for 12 weeks. Plants were harvested in six successive harvests and time trends in relative growth rate (RGR) and its components were estimated by fitting a Richards function regression to the harvest data. The VL/SAO/SO trees in saline conditions had higher mean RGR than VL/SO trees in control conditions. Increases in both net assimilation rate on a leaf mass basis (NARw) and leaf mass fraction (LMF) contributed equally to a twofold increase in RGR of VL/SAO/SO trees in saline conditions. In control conditions, the increase in RGR caused by the interstock had growth response coefficients of GRC(NARw) = 0.20 and GRC(LMF) = 0.80. Structural modifications-specific leaf area, leaf area ratio and LMF-had a slight influence on the salt-induced changes in RGR, whereas NARw had a large influence. Salinity decreased root mass fraction (RMF) and increased stem mass fraction (SMF). In contrast, the interstock decreased SMF and increased LMF and RMF. The VL/SAO/SO trees had the highest RMF and proportionally higher Cl- and Na+ allocations in roots than the other plant types. In saline conditions, reductions in leaf ion transport rate and dilution of imported ions by foliar growth nearly halved ion accumulations in leaves of VL/SAO/SO trees.

Tolerance of citrus rootstock seedlings to saline stress based on their ability to regulate ion uptake and transport

Forty-five-day-old seedlings of sour orange (Citrus aurantium L.) and Citrus macrophylla Wester, the most commonly used rootstocks in lemon orchards, were grown in nutrient solutions containing 1 (control), 10, 20, 30 or 60 mM NaCl for 14 days. The effects of salinity on growth, uptake, transport and accumulation of Cl- and Na+ ions in leaves, stem and four root segments were studied. The 60 mM NaCl treatment reduced leaf dry mass more in C. macrophylla (40%) than in sour orange (20%), whereas it reduced root dry mass more in sour orange (36%) than in C. macrophylla (20%). In C. macrophylla, Cl- and Na+ uptake rates were high at the beginning of the saline treatments, but low at the end of the 14-day experiment. In contrast, sour orange showed high uptake rates at the beginning and end of the experiment. In response to increasing salinity, root and shoot concentrations of Cl- and Na+ increased in sour orange, but not in C. macrophylla. Different loading characteristics of Cl- and Na+ were observed between young and old segments of the root system. In general, old root segments reached quasi-steady-states later than young root segments. These results suggest that sour orange and C. macrophylla have different regulatory mechanisms for uptake and transport of Cl- and Na+.

Purification and kinetic characterization of an anionic peroxidase from melon (Cucumis melo L.) cultivated under different salinity conditions

The partial characterization of an anionic peroxidase in melon fruit is described. Four melon peroxidase (MPX) isoenzymes were detected in crude extracts after isoelectric focusing. The major MPX isoenzyme (pI = 3.7) was partially purified by including hydrophobic and anion-exchange chromatography in the purification scheme. The sample obtained was used to characterize MPX. This peroxidase did not show activity on ascorbic acid but oxidized guaiacol at a high rate, showing an optimum pH of 5.5 when acting on this last reducing substrate. Melon fruits grown under highly saline conditions showed slightly increased levels of this anionic isoenzyme. Kinetic studies using 2,2'-azinobis(3-ethylbenzothiazolinesulfonic acid) (ABTS) as reducing substrate showed that increased salinity in the growth medium did not modify the kinetic parameters of melon peroxidase on both hydrogen peroxide and reducing substrate.

Does calcium ameliorate the negative effect of NaCl on melon root water transport by regulating aquaporin activity?

The hydraulic conductance (L₀ ) of detached, exuding root systems from melon (Cucumis melo cv. Amarillo oro) was measured. All plants received a half-strength Hoagland nutrient solution, and plants stressed either solely with NaCl (50 mM) or with NaCl (50 mM) following treatment (2 d) with CaCl₂ (10 mM) were compared with controls and CaCl₂ -treated (10 mM) plants. The L₀ of NaCl-treated plants was markedly decreased when compared to control and CaCl₂ -treated plants, but the decrease was smaller when NaCl was added to plants previously treated with CaCl₂ . A similar effect was observed when the flux of Ca²⁺ into the xylem and the Ca²⁺ concentration in the plasma membrane of the root cells were determined. In control, CaCl₂ - and NaCl + CaCl₂ -treated plants, HgCl₂ treatment (50 μM) caused a sharp decline in L₀ to values similar to those of NaCl-stressed roots, but L₀ was restored by treatment with 5 mM DTT. However, in NaCl roots only a slight effect of Hg²⁺ and DTT were observed. The effect of all treatments on L₀ was similar to that on osmotic water permeability (P_f ) of individual protoplasts isolated from roots. The results suggest that NaCl decreased the passage of water through the membrane and roots by reducing the activity of Hg-sensitive water channels. The ameliorative effect of Ca²⁺ on NaCl stress could be related to water-channel function.

Citrus response to salinity: growth and nutrient uptake

To determine the effects of salinity on relative growth rate (RGR), net assimilation rate on a leaf weight basis (NAR(w)), leaf weight ratio (LWR), and nutrient uptake and utilization of citrus, we grew four citrus rootstocks (sour orange, Cleopatra mandarin, Carrizo citrange and Citrus macrophylla) in nutrient solutions containing 0, 10, 20, 40 or 80 mM NaCl for 20, 40 or 60 days. For each element analyzed, specific absorption rate (SAR) and specific utilization rate on a leaf basis (SUR(L)) were calculated for the period between Days 40 and 60. Relative growth rate decreased with time for all treatments and rootstocks. Salt treatment significantly reduced both RGR and NAR(w), whereas LWR showed no definite trend. In all rootstocks, NAR(w), but not LWR, was significantly correlated with RGR, indicating that NAR(w) was an important factor underlying the salinity-induced differences in RGR among the citrus rootstocks. At Day 60, salinity had a significant effect on leaf concentrations of Cl, Na, K, Ca, Mg, P, Fe, Mn and Zn and on the SAR and SUR(L) of most elements. In general, RGR was correlated with SAR and SUR(L). Therefore, in addition to osmotic effects and the inhibitory effects of high concentrations of Cl(-) and Na(+), an imbalance of essential nutrients may also contribute to the reduction in plant growth under saline conditions.