Additional nitrogen fertilization affects salt tolerance of lemon trees on different rootstocks

Irrigation with saline water is one of the major problems in citrus crop in arid and semi-arid regions. Because rootstock and fertilization play an important role in citrus salt tolerance, we investigated the influence of the nitrogen fertilization and rootstock on salt tolerance of 2-year-old potted Fino 49 lemon trees. For that, trees grafted on Citrus macrophylla (M) or Sour orange (SO) rootstocks were watered for 12 weeks with complete nutrient solution containing either 0 mM NaCl (control, C), 50 mM NaCl (S), 50 mM NaCl with an additional 10 mM potassium nitrate (S + N), or 50 mM NaCl with a 1% KNO₃ (S + Nf) foliar spray application. Trees on M were more vigorous than trees on SO and saline treatments reduced leaf growth similarly in trees on both rootstocks. Trees on SO had a lower leaf Cl⁻ and Na⁺ concentration than those on M. Additional soil nitrogen (S + N) decreased leaf Cl⁻ concentration and increased leaf K⁺ concentration in salinized trees on both rootstocks. However, the salinity-induced reduction leaf growth was similar in S + N and S trees. This was due to osmotic effect, beside leaf Cl⁻ and Na⁺ toxicity, played an important role in the growth response of Fino 49 lemon to the salt stress. Additional foliar nitrogen in the S + Nf treatment also reduced leaf Cl⁻ concentration relative to the S treatment but trees from S + Nf treatment had the lowest leaf growth. Net assimilation of CO₂ (ACO₂$A_{\text{C}{\text{O}}_2}$), stomatal conductance (g_s) and plant transpiration were reduced similarly in all three salt treatments, regardless rootstock. Salinity reduced leaf water and osmotic potential such that leaf turgor was increased. Thus, the salinity-induced ACO₂$A_{\text{C}{\text{O}}_2}$ reductions were not due to loss of turgor but rather due to high salt ion accumulation in leaves.     

Some physiological changes in strawberry (Fragaria × ananassa ‘Camarosa’) plants under heat stress

Summary

The effects of heat injury induced by long exposures were evaluated in strawberry (Fragaria × ananassa ‘plants’) Camarosa in this study. Seedlings were grown in 14 × 12 cm pots using perlite for three weeks at 25/10°C day/night temperature, and watered daily by modified 1/3 Hoagland nutrient solution. Half of the plants were transferred to a growth chamber with a constant 25°C, 16/8 h (light/dark) photoperiod regime and 1200 lux light intensity for a week to acclimate the plants. Temperature was increased stepwise (5 K per 48 h) to 30, 35, 40°C and finally to 45°C. In addition to others, plants were transferred from the outside to the growth chamber, at each temperature step to impose a heat shock. Leaf relative water content (RWC, %), loss of turgidity, chlorophyll content (Spad value) and heat-stress tolerance (HTS; LT₅₀) were measured in control and stressed plants. Total soluble proteins and total DNA were extracted from the leaves following the above treatments using standard procedures and total protein contents were determined using a Bradford assay. In general, effects of gradual heat stress (GHS) and shock heat stress (SHS) on the variables studied were mostly significant, except for chlorophyll content, while the effect of temperatures was significant for all the variables. Interaction between the heat stress type and temperature treatments was not significant for leaf RWC, loss of turgidity and chlorophyll content. Data also indicated that total protein and DNA contents were changed significantly by heat stress types (GHS and SHS) and/or temperature treatments. The plants exposed to GHS exhibited a significant increase in HST compared with the plants exposed to SHS (LT₅₀ of 41.5°C and 39°C, respectively). Consequently, gradual heat stress increased HST in strawberry leaves. Increased HST may be associated with the accumulation of several heat-stable proteins in GHS plants.     

Effect of two irrigation rates on yield,incidence of blossom-end rot,mineral content and free amino acid levels in tomato cultivated under drip irrigation using saline water

Summary

An experiment investigated the effects of two rates of irrigation, one of which reflected a substantial degree of water stress, on the mineral content, free amino acid levels and incidence of blossom-end rot (BER) in tomato (Lycopersicon esculentum Mill. `Durinta'). The plants were grown in the open with drip irrigation using saline water from a well (mean EC_w 5.2 dS m²¹). The yield per plant was higher and fewer fruit were affected by BER in the treatment involving the higher level of irrigation. The fruit of the first and fifth truss, and the leaves immediately above, were analysed for their macronutrient, micronutrient and free amino acid content. The macronutrient leaf and fruit content hardly showed any difference, only the N concentration in fruit being significantly affected in the water stressed plants, in which the levels were higher. The Ca concentration in the stylar portion of mature fruit, which is related with the incidence of BER, was not significantly affected by the level of irrigation. As regards micronutrients, only the Fe (in leaf and fruit of the first truss), Cu (in leaf of the first truss), Zn (in leaf and fruit of the first truss, and leaf of the fifth truss) and Mn (in leaf of the first truss) concentrations differed significantly. The total free amino acid leaf content was similar in both irrigation treatments. However, the total free amino acid content of fruit, significantly those of the first truss, was higher in the less irrigated treatment. The amino acids: aspartic acid (only from the first.truss), glutamic acid, proline and alanine had high concentrations in the fruit of the less irrigated plants, while the g-aminobutyric acid and phenylalanine (only from the fifth truss) concentrations were higher in fruit of the more irrigated plants.     

Spring growth of almond nursery trees depends upon nitrogen from both plant reserves and spring fertilizer application

Summary

June-budded ‘Nonpareil’/‘Nemaguard’ almond (Prunus dulcis (Mill) D. A. Webb) trees were fertigated with one of five nitrogen (N) concentrations (0, 5, 10, 15, or 20 mM) from July to September. The trees were sprayed with either water or 3% urea in October, then harvested bareroot after natural leaf fall, and stored at 2°C. One set of trees was destructively sampled for total N content; the remaining trees were transplanted into N-free media in the spring after cold storage. After budbreak, these trees were supplied for 70.d with either N-free Hoagland’s solution or Hoagland’s solution containing ¹⁵N-NH₄NO₃. Nitrogen concentrations in both stem and root tissues were positively correlated with the N-fertigation concentration. Fall foliar urea applications increased levels of stem and root N regardless of the N-fertigation concentration. During the first 70 d of spring growth, the trees utilized nitrogen from both their reserves and spring fertilizer applications. The amount of N reserves used for growth of new shoots and leaves was proportional to the total amount of reserves. Trees with low N reserves relied primarily on the spring fertilizer as their source of nitrogen. We conclude, therefore, that both reserve N and spring-applied N fertilizers are important for enhancing the regrowth of bareroot almond nursery trees during establishment after transplanting. Nitrogen fertilization in the spring can especially improve the performance of trees with low N reserves.     

Response to drought and salt stress of lemon ‘Fino 49’ under field conditions: Water relations,osmotic adjustment and gas exchange

Drought and salinity are two of the most important factors limiting the lemon yield in south-eastern Spain. The effects of drought and salt stress, applied independently, on water relations, osmotic adjustment and gas exchange in the highest evapotranspiration period were studied to compare the tolerance and adaptive mechanisms of 13-year-old ‘Fino 49’ lemon trees, in immature and mature leaves. The study was carried out in an experimental orchard located in Torre Pacheco (Murcia). Three treatments were applied: Control, well-irrigated; drought-stress (DS), non-irrigated from 15th May to 7th July and salinity, irrigated with 30 mM NaCl from 1st March to 7th July. At the end of the experiment, only DS trees showed a decreased leaf stem water potential (Ψ_md). Under DS conditions, both types of leaf lost turgor and did not show any osmotic or elastic mechanism to maintain leaf turgor. Osmotic adjustment was the main tolerance mechanism for maintenance of turgor under salt stress, and was achieved by the uptake of Cl⁻ ions. Gas-exchange parameters were reduced by DS but not by salinity, stomatal closure being the main adaptive mechanism for avoidance of water loss and maintenance of leaf turgor. Salinity gave rise to greater Cl⁻ accumulation in mature than in immature leaves. The increase of proline in immature leaves due to DS indicates greater damage than in mature leaves.     

Transpiration,photosynthetic responses,tissue water relations and dry mass partitioning in Callistemon plants during drought conditions

Callistemon is an Australian species used as ornamental plant in Mediterranean regions. The objective of this research was to analyse the ability of Callistemon to overcome water deficit in terms of adjusting its physiology and morphology. Potted Callistemon laevis Anon plants were grown in controlled environment and subjected to drought stress by reducing irrigation water by 40% compared to the control (irrigated to container capacity). The drought stress produced the smallest plants throughout the experiment. After three months of drought, the leaf area, number of leaves and root volume decreased, while root/shoot ratio and root density increased. The higher root hydraulic resistance in stressed plants caused decreases in leaf and stem water potentials resulting in lower stomatal conductance and indicating that water flow through the roots is a factor that strongly influences shoot water relations. The water stress affected transpiration (63% reduction compared with the control). The consistent decrease in g_s suggested an adaptative efficient stomatal control of transpiration by this species, resulting in a higher intrinsic water use efficiency (P_n/g_s) in drought conditions, increasing as the experimental time progressed. This was accompanied by an improvement in water use efficiency of production to maintain the leaf water status. In addition, water stress induced an active osmotic adjustment and led to decreases in leaf tissue elasticity in order to maintain turgor. Therefore, the water deficit produced changes in plant water relations, gas exchange and growth in an adaptation process which could promote the faster establishment of this species in gardens or landscaping projects in Mediterranean conditions.     

Regulated deficit irrigation in potted Dianthus plants: Effects of severe and moderate water stress on growth and physiological responses

The purpose of this study was to analyze the physiological and morphological response of carnation plants to different levels of irrigation and to evaluate regulated deficit irrigation as a possible technique for saving water through the application of controlled drought stress. Carnations, Dianthus caryophyllus L. cultivar, were pot-grown in an unheated greenhouse and submitted to two experiments. In the first experiment, the plants were exposed to three irrigation treatments: (control); 70% of the control (moderate deficit irrigation, MDI) and 35% of the control (severe deficit irrigation, SDI). In the second experiment, the plants were submitted to a control treatment, deficit irrigation (DI, 50% of the control) and regulated deficit irrigation (RDI). After 15 weeks, MDI plants showed a slightly reduced total dry weight, plant height and leaf area, while SDI had clearly reduced all the plant size parameters. RDI plants had similar leaf area and total dry weight to the control treatment during the blooming phase. MDI did not affect the number of flowers and no great differences in the colour parameters were observed. RDI plants had higher flower dry weight, while plant quality was affected by the SDI (lower number of shoots and flowers, lower relative chlorophyll content). Leaf osmotic potential decreased with deficit irrigation, but more markedly in SDI, which induced higher values of leaf pressure. Stomatal conductance (g_s) decreased in drought conditions more than the photosynthetic rate (P_n). Osmotic adjustment of 0.3 MPa accompanied by decreases in elasticity in response to drought resulted in turgor less at lower leaf water potentials and prevented turgor loss during drought periods.     

Comparison of growth,leaf water relations and gas exchange of Cistus albidus and C. monspeliensis plants irrigated with water of different NaCl salinity levels

Four-month-old potted Cistus albidus and Cistus monspeliensis plants growing in a greenhouse were submitted to saline stress from 9 August to 2 December, using irrigation water containing 0, 70, and 140 mM NaCl. C. monspeliensis plants are more tolerant to saline irrigation water than C. albidus plants, mainly due to their capacity to resist stress with a lower plant biomass and canopy area; furthermore, they showed no leaf necrosis symptoms. Under saline stress conditions the main growth limiting factor in both species was photosynthesis. Both Cistus species responded to saline stress by developing avoidance and tolerance mechanisms. The avoidance mechanisms took place at a morphological and physiological level. Morphologically, the reduction in the canopy area can be considered a mechanisms for regulating water loss via transpiration. Treated C. monspeliensis plants showed a greater capacity to absorb water and were able to conserve it more efficiently than C. albidus plants. Tolerance mechanisms included Na⁺ and Cl⁻ inclusion and osmotic adjustment. However, the reaction of each species to osmotic adjustment was different, because in C. monspeliensis plants the osmotic adjustment was unable to prevent a decrease in leaf turgor. The curvilinear relationship between P_n and g_l observed in C. monspeliensis plants indicated stomatal limitation of photosynthesis below a leaf conductance of about 160 mmol m⁻² s⁻¹. In C. albidus plants, a linear relationship between photosynthesis and leaf conductance rather a curvilinear model was significant, indicating limitation of the photosynthetic capacity.     

Floral induction in tropical fruit trees: Effects of temperature and water supply

Summary

Floral induction in tropical trees generally follows a check in vegetative growth. However, it is not easy to identify the environmental factors involved in flowering, which normally occurs during the dry season when temperatures are also often lower. The separate and combined effects of temperature and water supply on floral induction were investigated in ‘Hass’ avocado (Persea americana), ‘Lisbon’ lemon (Citrus limon). ‘Wai Chee’ litchi (Litchi chinensis) and ‘Sensation’ mango (Mangifera indica). Low temperatures (15°/10°C or 15°/10°C and 20°/15°C compared with 30°/25°C and 25°/20°C) generally decreased vegetative growth and induced flowering in well-watered avocado, litchi and mango. A pre-dawn leaf water potential (ψ_L) of ?1.7 to ?3.5 MPa compared with ?0.4 to ?0.7 MPa in control avocado and litchi, and a pre-dawn relative water content (R.W.C.) of 90-93% compared with 97% or above in control mango plants also reduced or eliminated vegetative growth, but did not induce flowering. Low temperatures (15°/10°C compared with 20°/5°C, 25°/20°C or 30°/25°C) and water stress (pre-dawn ψ_L of ?2.0 to ?3.5 MPa compared with ?0.7 to ?0.8 MPa in controls) reduced or eliminated vegetative growth in lemon. In contrast to the response in avocado, litchi and mango, flowering in lemon was very weak in the absence of water stress at 15°/10°C or outdoors in Brisbane in subtropical Australia (Lat. 28°S), and was greatest after a period of water stress. The number of flowers increased with the severity and duration of water stress (two, four or eight weeks) and was generally greater after constant rather than with cyclic water stress. In lemon and litchi, net photosynthesis declined with increasing water stress reaching zero with a midday ψ_L of ?3.5 to ?4.0 MPa. This decline in carbon assimilation appeared to be almost entirely due to stomatal closure. Despite the reduction in midday CO₂ assimilation, starch concentration increased during water stress, especially in the branches, trunk and roots of lemon. Leaf starch was uniformly low. The number of flowers per tree in lemon was strongly correlated with starch in the branches (r²=77%, P<0.01) and roots (r²=74%, P<0.001). In litchi, starch was lower than in lemon roots and was not related to flowering.

In separate experiments to test the interaction between temperature and water supply, low day/night temperatures (23°/18° and 18°/15°C compared with 29°/25°C) reduced vegetative growth and induced flowering in avocado, litchi and mango. None of these species flowered at 29°/25°C or as a result of water stress (ψ_L of ?1.5 MPa compared with ?0.3 MPa for avocado and ?2.0 MPa compared with ?0.5 MPa for litchi, and R.W.C, of 90-93% compared with 95-96% in mango). In contrast, in lemon, flowering was very weak (<10 flowers per tree) in the absence of water stress (pre-dawn ψ_L of ?2.0 MPa compared with ?0.5 MPa) and was only heavy (>35 flowers per tree) after stressed trees were rewatered. There were slightly more flowers at 18°/15°C than at 23°/18° and 29°/25°C in control plants, but no effect of temperature in stressed plants. Starch concentration in the roots of avocado, lemon, litchi and mango was generally higher at 18°/15°C and 23°/18°C than at 29°/25°C. Water stress increased the starch concentration in the roots of lemon and litchi and decreased it in avocado. There was no effect in mango. There was a weak relation (r²=57%, P<0.05) between the number of flowers per tree in lemon and the concentration of starch in the roots. In contrast, there was no significant relationship between flowering and starch levels under the various temperature and water regimes in the other species. In another experiment, only vegetative growth in litchi and mango occurred at 30°/25°C and only flowering at 15°/10°C. Six weeks of water stress (pre-dawn ψ_L of ?2.5 MPa compared with ?1.0 MPa or higher in litchi, and R.W.C, of 90-93% compared with 95% or higher in mango) in a heated glasshouse (30°C days/20°C night minimum) before these temperature treatments did not induce flowering.

Temperatures below 25°C for avocado and below 20°C for litchi and mango are essential for flowering and cannot be replaced by water stress. The control of flowering in lemon over the range of day temperatures from 18°C to 30°C differed from that of the other species in being mainly determined by water stress. Flowering was generally weak in well-watered plants even with days at 18°C. Starch did not appear to control flowering.     

Salinity and shading effects on leaf water relations and ionic composition of strawberry plants grown on rockwool

Summary

Strawberry plants were grown in rockwool under glasshouse conditions at NaCl salinities of 2.6,5.9 and 8.6 mS cm^-1 and at total irradiances of 2.1 and 4.9 MJ m^?2d^?1. Increasing salinity and irradiance reduced leaf water potential (ψω), osmotic potential (ψπ) and turgor potential (ψτ). There was an interaction between salinity and irradiance on with the lowest ψπ recorded for the unshaded leaves. Increased salinity altered the ionic composition of the leaf. Chloride concentration was increased from 0.03 to 0.61 % (D.W.) while NO₃ content in the leaf sap was reduced from 10.51 to 3.60 mg ml^-1 as salinity increased from 2.6 to 8.6 mS cm^?1. Expressed on a fresh-weight basis, the concentration of K was reduced at high irradiance whereas Ca and Mg were enhanced. On a dry-weight basis K, Na, Ca and Mg were unaffected by salinity treatment. Net photosynthesis was reduced by high salinity but only in plants grown in unshaded conditions.     

Calcium starvation increases salt susceptibility in olive plants but has no effect on susceptibility to water stress

Summary

The responses of leaf water status, growth, and ion concentrations to water or to saline stresses were compared in olive cuttings of different Ca²⁺ status. Mist-rooted ‘Picual’ olive cuttings were grown in a greenhouse in 2 l plastic pots containing perlite. A nutrient solution with or without 2.5 mM CaCl₂ was initially used to irrigate the plants. When the Ca²⁺-starved plants differed in height from the Ca²⁺-treated plants, water or saline stress (i.e., no irrigation or 75 mM NaCl, respectively) were applied. The results indicated that Ca²⁺ increased growth in saline-treated plants, but not in water-stressed plants. After 98 d growth, the stresses were relieved and the plants were irrigated again with or without Ca²⁺. Growth increased and leaf water status was increased during this recovery period, but no direct effects of Ca²⁺ were observed in the response of plants to stress-relief. We suggest that the beneficial effect of Ca²⁺ on tolerance to salt stress in olive plants was related to protection against Na⁺ toxicity, because there was no response of water-stressed plants to the supply of Ca²⁺.     

Indicators of nitrogen status for ornamental woody plants based on optical measurements of leaf epidermal polyphenol and chlorophyll contents

Indicators of plant nitrogen (N) status adapted to woody ornamental plants are essential for the adjustment of fertilization practices in nurseries. The objective of this study was to investigate whether optical measurements of leaf epidermal polyphenol (EPhen) and chlorophyll (Chl) contents could be used as N status indicators for woody deciduous and evergreen ornamental plants. One-year-old plants of Lagerstroemia indica, Callicarpa bodinieri and Viburnum tinus were grown outdoors in containers. They received low (TN₁, 4 mg L⁻¹) or high (TN₂, 105 mg L⁻¹) levels of N during 2 months in spring and summer. TN₁ treatment limited shoot growth from 28 to 37 days after treatment initiation in Lagerstroemia and Callicarpa, respectively. Shoot growth was unaffected until day 176 in Viburnum. The mass-based leaf N content (N_M) of a sample of young expanded leaves exposed to direct sunlight was tightly correlated with shoot N content and differentiated treatments several weeks before shoot growth reduction for the three species. N_M was therefore used as an index of plant N status. EPhen and Chl contents were recorded with Dualex™ and SPAD-502 leaf-clip meters, respectively. Dualex values were strongly and negatively correlated with N_M, and differentiated the treatments early in the experiment, in all three species. SPAD values were positively correlated with N_M for Lagerstroemia and Callicarpa, but not for Viburnum, because large variations in leaf mass per area (LMA) in this species compensated for variations in leaf dry mass invested in Chl. The SPAD/Dualex ratio was used to assess changes in the proportion of leaf dry mass allocated to proteins and polyphenols in response to fertilization. It differentiated between the treatments early in the experiment and was correlated with N_M in all three species.     

Root damage affects nitrogen uptake and growth of young Fuji/M.26 apple trees

Summary

Effects of root damage during the transplant process on growth and nitrogen (N) uptake were studied with one-year-old bench-grafted Malus domestica Borkh ‘Fuji’ on M.26 rootstock apple nursery plants. Plants were potted after grafting and grown outside for one season. At the end of the season uniform trees were selected and randomly divided into four groups. One group of plants were moved into a 2°C cold room with soil and container intact (IR Treatment). Plants in other groups were removed from pots and stored as bareroot in the same cold room for three months. In the spring, bareroot plants were either: (1) transplanted with about 10% of the root system damaged during transplant (TP Treatment and Control-CK); or (2) root pruned by 25% (by volume) prior to transplant (RP treatment). Five trees from each treatment received 1 g of ¹⁵NH₄¹⁵NO₃ at 12, 41 and 76 d after repotting. Control (CK) trees received no N. Trees were harvested 10 d after each N application, and plant growth and total N and ¹⁵N content of different tissues were determined. Root pruning reduced plant total biomass and root biomass at the first two harvests, but the plants from the RP treatment had highest total plant biomass and root biomass at the third harvest. There was no significant difference in the new stem and leaf growth among IR, RP and CK treatments at harvests but the TP treatment reduced new shoot biomass. Plants with intact roots (IR) had the higher total N content while control plants (CK) had the lowest. Root pruning reduced ¹⁵N uptake rate at the first two harvests but promoted it at the third harvest. Our results suggest that plant growth and nutrient uptake was suppressed by root pruning/damage during transplanting only in the early season, and the negative effects on growth and N uptake were offset later in the season by compensative root regeneration.     

Photosynthetic and growth responses of juvenile Chinese kale (Brassica oleracea var. alboglabra) and Caisin (Brassica rapa subsp. parachinensis) to waterlogging and water deficit

Chinese kale (Brassica oleracea var. alboglabra) and Caisin (Brassica rapa subsp. parachinensis) are leafy vegetable crops grown in south-east Asian countries where rainfall varies dramatically from excess to deficit within and between seasons. We investigated the physiological and growth responses of these plants to waterlogging and water deficit in a controlled experiment in a glasshouse. Juvenile plants were subjected to waterlogging or water deficit for 19 days in case of Chinese kale and 14 days in case of Caisin and compared with well-watered controls. Caisin tolerated waterlogging better than Chinese kale because it produced hypocotyl roots and gas spaces developed at the stem base. In Chinese kale, waterlogging reduced plant fresh weight (90%), leaf area (86%), dry weight (80%) and leaf number (38%). In contrast, waterlogging had no impact on leaf number in Caisin and reduced plant fresh and dry weights and leaf area by 60–70%. Water deficit reduced leaf area, fresh weight and dry weight of both species by more than half. Leaf number in Chinese kale was reduced by 38% but no effect occurred in Caisin. Water deficit increased the concentration of nitrogen in the leaf dry matter by more than 60% in both species and the leaf colour of water deficient plants was dark green compared with the leaf colour of well-watered plants. Soil water deficit delayed flowering of Caisin while waterlogging accelerated it. Thickening and whitening of the cuticle on the leaves of Chinese kale probably increased its ability to retain water under drought while Caisin adjusted osmotically and Chinese kale did not. Waterlogging and water deficit had strong effects on leaf gas exchange of both Brassica species. Water deficit closed the stomata in both species and this was associated with a leaf water content of 9 g g⁻¹ DW. In contrast, waterlogging reduced conductance from 1.0 to 0.1 mol H₂O m⁻² s⁻¹ in direct proportion to changes in leaf water content, which fell from 11 to 5 g g⁻¹ DW. This separation of the effects of water deficit and waterlogging on conductance was reflected in transpiration, internal CO₂ concentration and net photosynthesis. In conclusion, Chinese kale and Caisin showed rather different adaptations in response to waterlogging and water deficit. Caisin was more tolerant of waterlogging than Chinese kale and also showed evidence of tolerance of drought. There is genetic variation to waterlogging within the Brassica genus among the leafy vegetables that could be used for cultivar improvement.     

Effects of water and salt stresses on growth,water relations and gas exchange in Rosmarinus officinalis

Summary

Plants of Rosmarinus officinalis were submitted to water and salt stress, independently. The effects of water or salt stress on growth, water relations and gas exchange were investigated in order to understand the tolerance and adaptative mechanisms of R. officinalis to these types of stress. Under both stress conditions, plants developed avoidance mechanisms to minimise water loss based on morphological and physiological changes (e.g., reduction of plant biomass and leaf area, stomatal closure). Only under salt stress conditions were treated plants able to maintain turgor via osmotic adjustment, which was achieved by the uptake of Na⁺ and Cl^– ions. Osmotic adjustment was not observed in R. officinalis plants submitted to water stress. The results indicate that high accumulation of Na⁺ and Cl^– ions was responsible for the growth reduction observed in salinised plants. However, the growth reduction observed in water-stressed plants was caused by a dehydration process.     

Regulated deficit irrigation in ‘Clementina de Nules’ citrus trees. II: Vegetative growth

Summary

An experiment on Regulated Deficit Irrigation (RDI) was performed during 1995 and 1996 in an orchard planted with drip-irrigated ‘Clementina de Nules’/Carrizo Citrange in Moncada (Valencia) Spain. Treatments consisted of a control, irrigated during the whole year at 125% ET_lys and RDI treatments where irrigation was reduced to 25% or to 50% of crop evapotranspiration measured by a weighing lysimeter (ET_lys) during one of the following periods: I) flowering and fruit set (spring); II) initial fruit enlargement phase (summer) and III) final fruit growth and maturation phases (end of summer-autumn). An additional treatment, denominated 50%-Year, was irrigated at 50% ET_lys during the whole year. The effects of RDI treatments in relation to tree water status (pre-dawn Ψ^pd and midday Ψ^md leaf water potential, as well as their integral with time) show a good relation between total shoot emergence in the different growth flushes and the stress intensity reached (Ψ^pd) (r² = 0.80). This correlation was mainly due to the number of floral shoots (r² = 0.86) and not to vegetative ones (r² = 0.22). Similar results were observed between the stress integral at pre-dawn in each period and the former sprouting variables. In all cases, correlation was better with pre-dawn leaf water potential or with pre-dawn stress integral than with those at midday. RDI during spring reduced shoot length of the first growth flush (A1) and increased fruitlet fall after restarting normal irrigation. It also produced “off-season” flowering in the second flush growth (A2) and increased shoot emergence of the third flush growth (A3) with about 10% of them being floral. Summer RDI treatments did not alter vegetative growth, and although they produced off-season flowering (A3) it was much smaller than that of autumn RDI treatments, which in addition reduced vegetative growth with respect to the control. These effects, together with those of yield and fruit quality presented elsewhere, show that summer is the more appropriate period to apply RDI in “Clementina de Nules” mandarin trees.     

Orange varieties as interstocks increase the salt tolerance of lemon trees

Summary

We investigated the ability of interstocks to increase salt tolerance in lemon trees. We compared 2-year-old ‘Verna’ lemon trees [Citrus limon (L.) Burm.; VL] grafted on Sour Orange (C. aurantium L.; SO) rootstock either without an interstock (VL/SO), or interstocked with ‘Valencia’ orange (C. sinensis Osbeck; VL/V/SO), or with ‘Castellano’ orange (C. sinensis Osbeck; VL/C/SO). Trees were grown under greenhouse conditions and supplied with nutrient solutions containing 0, 30, or 60 mM NaCl. Reductions in leaf growth caused by salt treatment were greatest in non-interstocked (VL/SO) trees, followed by VL/C/SO trees, and were the least in VL/V/SO trees. Although the levels of Cl^? and Na⁺ ions in the roots and stems were not affected by either interstock, leaf concentrations of Cl^? and Na⁺ were higher in VL/SO trees than in VL/C/SO or VL/V/SO trees, suggesting that an interstock in Citrus trees could limit the uptake and transport of such ions to the shoots. Saline-treated VL/SO trees also tended to have the lowest shoot:root (S:R) ratios; so, overall, there was a negative relationship between S:R ratio and leaf Cl- ion concentration. Leaf transpiration (E_leaf) may also be involved in the reduction in leaf Cl^? concentration, as interstocked trees had lower E_leaf values at mid-day than non-interstocked trees. Salinity increased leaf concentrations of Ca²⁺ in VL/C/SO trees and increased both leaf K⁺ and N concentrations in all trees, regardless of interstock. Salinity reduced leaf water potentials and osmotic potentials, such that leaf turgor was increased in all trees.     

Influence of water deficit and low air humidity in the nursery on survival of Rhamnus alaternus seedlings following planting

Summary

The effect of irrigation and air humidity on the water relations and root and shoot growth of Rhamnus alaternus L. during the nursery phase was considered to evaluate the resulting degree of hardening obtained by these treatments. R. alaternus seedlings were pot-grown in two greenhouses of equal characteristics. In one of these greenhouses air humidity was controlled using a dehumidifying system, while in the other one the environmental conditions were not artificially modified. In each greenhouse, two irrigation treatments were used. Thus, four different treatments were applied during the nursery phase (January-May): 1) control air humidity + control irrigation; 2) control air humidity + deficit irrigation; 3) low air humidity + control irrigation; 4) low air humidity + deficit irrigation. In May, plants of all treatments were transplanted and grown in good environmental and irrigation conditions for one month (17 May–20 June), after which they received no irrigation until the end of the experiment (14 July). Low air humidity and water deficit reduced all shoot growth parameters during the nursery phase, however the root growth was not significantly affected by air humidity and even increased under the water deficit. The reduction in leaf water potential under water stress was induced by tissue dehydration since leaf turgor potential also decreased and non-osmotic adjustment was observed. The drought effects on water relations were similar in both low and high air humidity. The leaf stomatal conductance was also reduced by both types of stress, leading to a decrease in the rate of photosynthesis at the end of the nursery phase. Both water deficit and low air humidity showed their value as nursery acclimation processes, improving the survival of seedlings following transplanting and non-irrigation conditions (establishment phase). The stomatal regulation and a shift in the allocation of assimilates from shoot to root were the acclimation mechanisms showed by R. alaternus under both types of stress. The accumulated effects in low air humidity and water deficit plants could explain the highest percentage of survival at the end of the establishment period (97%) for the combined treatment.     

Determining morphological and physiological parameters for the selection of drought-tolerant geraniums (Pelargonium x hortorum L. H. Bailey)

Summary

Greenhouse experiments were carried out to study the effect of drought stress on morphological and physiological parameters of Pelargonium hortorum, to define the most sensitive indicators that could be used to evaluate genotypes for tolerance to water stress. Nine genotypes of P. hortorum and two genotypes of the parental species (P. inquinans L. and P. zonale L.) were examined. Drought stress was induced by withholding water until the soil water potential reached –80 kPa, followed by recovery and applying another drought stress cycle. Growth decreased with drought stress with regard to all parameters and all genotypes, compared to well-watered plants. Production and loss of leaves were sensitive indicators of drought stress to discriminate between genotypes. A classification of genotypes was obtained from these parameters. Changes in leaf area, the density of stomata (mm^–2 or per leaf), leaf water content and leaf cell osmolarity during the drought cycle and after recovery were also studied. Changes in stomata mm^–2, leaf surface and cell osmolarity were found to be sensitive indicators of drought stress. Correlations between different parameters made it possible to obtain a robust classification of genotypes using only a few parameters. Significant variation was found between genotypes of P. hortorum in their response to drought stress.The different mechanisms which can be used by tolerant genotypes to adapt to drought stress are discussed. The present study provides a simple method to evaluate and select for water stress-tolerance in P. hortorum genotypes.     

Morpho-Physiological Responses of Grape Rootstock ‘Dogridge’ to Arbuscular Mycorrhizal Fungi Inoculation Under Salinity Stress

Effects of arbuscular mycorrhizal fungi (AMF) alone or in combination with bacterial consortium (AMF+BC) inoculation prior to induced salinity (NaCl @ 150 or 250 mM) were studied on root growth; plant biomass; leaf area; Na⁺ and K⁺ contents; leaf water potential (Ψ_w); osmotic potential (Ψ_π); photosynthesis rate (P_n); and contents of chlorophyll, phytohormones, and polyamines in the grape rootstock ‘Dogridge’, popular among Indian vine growers. AMF inoculation in the NaCl untreated rootstocks plants increased root growth, root and shoot biomass, and leaf area and improved leaf Ψ_w, Ψ_π, P_n, and chlorophyll content, and also countered the stress-induced decline in the NaCl treated plants. The abscisic acid (ABA), cytokinins, and polyamine-spermidine and spermine contents in the leaves of NaCl untreated or treated were significantly increased by the AMF inoculation. Among the treatments, AMF with BC was relatively more effective than AMF alone with respect to changes in above morpho-physiological characters. The results depicted that AMF (AMF alone or AMF+BC) inoculation significantly improved salinity tolerance of grape rootstock and tolerance is induced by improvements in plant water balance, K⁺:Na⁺ ratio, and P_n, besides distinct accumulations in ABA and polyamines-spermine and spermidine. The above findings have potential in suggesting the AMF usefulness in improving the efficacy of ‘Dogridge’ rootstock in grape cultivation under salt affected soils.