S. Bañón

Selection and nursery production of ornamental plants for landscaping and xerogardening in semi-arid environments

Summary In landscaping and xerogardening projects, under semi-arid conditions, appropriate plant selection and conditioning techniques used in the nursery during seedling production are crucial for the establishment, survival and subsequent growth of plants after transplanting. Selecting ornamental plants with appropriate morphological and physiological characteristics to improve nursery performance and tolerance of harsh environments is of vital importance. The use of native species of wild flora is of increasing interest because of their capacity to adapt to adverse local environmental conditions. However, the degree of adaptation to abiotic stresses varies considerably within a family, within a genus and even within a species. Morphological and anatomical adaptations in seedlings include reductions in shoot height and/or leaf area, rises in root-collar diameter and root growth potential and, often, a reduction in the shoot:root ratio. These occur as a result of hardening and acclimation processes (pre-conditioning) during the nursery period, and are correlated with the ability to withstand the shock of transplantation and to increase survival and plant growth following transplantation in xerogardens and semi-arid landscapes. In addition, there are physiological characteristics of seedlings related to osmotic adjustment and water-use efficiency, such as low stomatal conductance, leaf water potential, leaf turgor potential and relative water content. These provide seedlings with a considerable capacity to adapt to adverse conditions after transplantation into harsh environments. Suitable environmental conditions and cultivation techniques in the nursery are essential to produce sturdy seedlings, with the above-mentioned morphological and physiological characteristics. Deficit irrigation is the most commonly used pre-conditioning technique to produce high-quality seedlings. In addition, using large-sized containers and appropriate substrates, withholding N nutrition, inoculating arbuscular mycorrhizal fungi, applying plant growth retardants and mechanical conditioning methods (including brushing, and shoot- and/or root-pruning) are common. Varying microclimatic conditions (low temperature, low air humidity, enrichment with CO2, light intensity and photoperiod management) are also used to control growth to produce high-quality seedlings with the ability to withstand transplanting shock and be capable of rapid establishment and resumption of growth under xerogardening and semi-arid landscaping conditions.

Review Article:Root development in horticultural plants grown under abiotic stress conditions – a review

Summary Roots usually suffer greater exposure to multiple abiotic stresses than shoots. Therefore, the root system can be as affected, or even more affected, than the aerial parts of a plant by such stresses. Despite this, the influence of abiotic stresses on root development has been considerably less studied than on shoots because of limited accessibility for root observations. This work reviews the recent scientific literature on root development and the performance of root systems in horticultural plants growing under abiotic stresses such as drought, waterlogging, salinity, extreme temperature, low illumination, nutrient deficiency or excess, heavy metals, elevated atmospheric CO2, and mechanical restrictions. Changes in the shoot:root ratio are often observed when plants are subjected to various stresses. Thus, a redistribution of metabolites from shoots to roots is frequently observed under drought, salt, or sub-optimal temperature stress, as well as during some nutrient deficiencies, or elevated levels of atmospheric CO2. Conversely, reductions in solar radiation or excess nutrient usually cause an increased shoot:root ratio. Plants subjected to increased atmospheric CO2 concentrations, or to low-moisture regimes, may develop a more extensive root system; however, the other stresses reviewed here commonly inhibit root growth, and cause significant modifications to the architecture of the root system, often giving rise to more branched root systems with shorter roots. Colonisation by arbuscular mycorrhizal fungi can also induce changes in the root system of the host plant that may improve its resistance to several abiotic stresses. A wide variety of hormones and biochemical processes are involved in the regulation of root growth under abiotic stress. Essential regulatory functions have been attributed to abscisic acid, ethylene, reactive oxygen species, and reactive nitrogen species.

Changes in leaf water relations, gas exchange, growth and flowering quality in potted geranium plants irrigated with different water regimes.

Geranium plants are an important part of urban green areas but suffer from drought, especially when grown in containers with a limited volume of medium. In this experiment, we examined the response of potted geraniums to different irrigation levels. Geranium (Pelargoniumxhortorum L.) seedlings were grown in a growth chamber and exposed to three irrigation treatments, whereby the plants were irrigated to container capacity (control), 60% of the control (moderate deficit irrigation, MDI), or 40% of the control (severe deficit irrigation, SDI). Deficit irrigation was maintained for 2 months, and then all the plants were exposed to a recovery period of 112 month. Exposure to drought induced a decrease in shoot dry weight and leaf area and an increase in the root/shoot ratio. Height and plant width were significantly inhibited by the SDI, while flower color parameters were not affected by deficit treatment. The number of wilting and yellow leaves increased, coinciding with the increase in the number of inflorescences and open flowers. Deficit irrigation led to a leaf water potential of about -0.8MPa at midday, which could have caused an important decrease in stomatal conductance, affecting the photosynthetic rate (Pn). Chlorophyll fluorescence (Fvm) values of 0.80 in all treatments throughout the experiment demonstrate the lack of drought-induced damage to PSII photochemistry. Pressure-volume analysis revealed low osmotic adjustment values of 0.2MPa in the SDI treatment, accompanied by increases in the bulk tissue elastic modulus (epsilon, wall rigidity) and resulting in turgor loss at lower leaf water potential values (-1.38MPa compared with -1.0MPa for the control). Leaf water potential values throughout the experiment below those for Psitlp were not found at any sampling time. By the end of the recovery period, the leaf water potential, stomatal conductance and net photosynthesis had recovered. We infer from these results that moderate deficit irrigation in geranium reduced the consumption of water, while maintaining the good overall quality of plants. However, when SDI was applied, a reduction in the number of flowers per plant was observed.

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.

Water relations, nutrient content and developmental responses of Euonymus plants irrigated with water of different degrees of salinity and quality.

For 20xa0weeks, the physiological responses of Euonymus japonica plants to different irrigation sources were studied. Four irrigation treatments were applied at 100xa0% water holding capacity: control (electrical conductivity (EC)xa0<0.9xa0dSxa0m−1); irrigation water normally used in the area (irrigator’s water) IW (EC: 1.7xa0dS m−1); NaCl solution, NaCl (EC: 4xa0dS m−1); and wastewater, WW (EC: 4xa0dS m−1). This was followed by a recovery period of 13xa0weeks, when all the plants were rewatered with the same amount and quality of irrigation water as the control plants. Despite the differences in the chemical properties of the water used, the plants irrigated with NaCl and WW showed similar alterations in growth and size compared with the control even at the end of the recovery period. Leaf number was affected even when the EC of the irrigation water was of 1.7xa0dS m−1 (IW), indicating the salt sensitivity of this parameter. Stomatal conductance (gs) and photosynthesis (Pn), as well as stem water potential (Ψstem), were most affected in plants irrigated with the most saline waters (NaCl and WW). At the end of the experiment the above parameters recovered, while IW plants showed similar values to the control. The higher Na+ and Cl+ uptake by NaCl and WW plants led them to show osmotic adjustment throughout the experiment. The highest amount of boron found in WW plants did not affect root growth. Wastewater can be used as a water management strategy for ornamental plant production, as long as the water quality is not too saline, since the negative effect of salt on the aesthetic value of plants need to be taken into consideration.

Regulated deficit irrigation in different phenological stages of potted geranium plants: water consumption, water relations and ornamental quality

The irrigation water requirements and sensitivity to water deficits of ornamental plants is of great interest to horticultural producers for planning irrigation strategies. The effect of different deficit irrigation strategies on physiological and morphological parameters in geranium plants was studied in different growth phases to evaluate how such strategies can be safely used and to ascertain whether the flowering phase is sensitive to deficit irrigation. Pelargoniumxa0×xa0hortorum L.H. Bailey plants, grown in a controlled growth chamber, were subjected to four irrigation treatments: control (100xa0% water field capacity throughout the experiment), sustainable deficit irrigation (75xa0% water field capacity throughout the experiment), and two regulated deficit irrigation treatments that included water stress during the vegetative growth phase or during the flowering development phase. Although the total amount of irrigation water was similar in the three deficit irrigation treatments (around 80xa0% of the control value), the lowest values for both height and flowering were found when deficit irrigation was applied during flowering. This indicates that plant quality does not only depend on the amount of water applied but also on the time when the reduction is applied, and that flowering is the most sensitive phase to water stress. Evapotranspiration was related to the formation of inflorescences and to increased plant height. When the irrigation strategy was changed, plants increased or decreased their water consumption and stomatal conductance to adjust to the new conditions by regulating stomatal opening, although, in general, the values of both parameters remained below those observed in the control plants.

Changes in tissue-water relations, photosynthetic activity, and growth of Myrtus communis plants in response to different conditions of water availability

Summary The influence of different levels of water deficit on physiological and morphological alterations in Myrtus communis plants was investigated to evaluate their adaptability to such conditions. M. communis plants, growing under greenhouse conditions, were subjected to three irrigation treatments between February and August 2007: a control, and two water deficit treatments. Plants submitted to severe water deficit showed reduced shoot and root dry weights, leaf numbers, leaf areas, and plant heights, while moderate water deficit reduced only plant height. Leaf colour was not affected by either water deficit treatment. Root hydraulic resistance increased in proportion to the level of drought, and lower values of leaf water potential (Ψl) were observed at pre-dawn in both deficit treatments. The absence of osmotic adjustment could explain the reduction of leaf turgor potential at midday. Photosynthesis decreased in both water deficit treatments and was related to stomatal factors, since no significant changes in chlorophyll fluorescence, chlorophyll content, or ion leakage were observed. The highest Ψl values were found in the early morning, and the lowest at midday, in all treatments. The latter coincided with the minimum values of stomatal conductance. Significant differences in Ψl values during the day were noted between treatments, but were always highest in the control. Cell wall rigidity, measured as an increased bulk modulus of elasticity, increased under severe water stress, resulting in a loss of turgor at lower Ψl values.

Relationships among electrical conductivity measurements during saline irrigation of potted Osteospermum and their effects on plant growth

SUMMARY Potted Osteospermum hybrida plants grown in a greenhouse during the Winter were irrigated with water having electrical conductivities (EC) of 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 or 5.0 dS m−1. The following relationships were studied in order to improve the management of saline irrigation systems using soil moisture and EC sensors: pore water EC (ECPW) using the pour-through method (ECPWPT) vs. ECPW estimated using a GS3 sensor (ECPWGS3); ECPWPT vs. bulk EC (ECB); ECPWPT vs. leachate EC (ECL); and ECPWGS3 vs. ECB. The tolerance of Osteospermum plants to salinity was also determined. Bulk EC, ECPWGS3, and ECL values were closely and positively correlated with ECPWPT. ECL over-estimated ECPWPT, while ECPWGS3 underestimated ECPWPT. Estimation of ECPWGS3 (by the Hilhorst Model) was not accurate due to the influences of humidity, salinity, and temperature. The higher the irrigation water EC (ECIW), the greater the variability in all measurements made of EC. Increases in ECIW reduced plant height, diminished the aerial dry biomass, and encouraged the presence of basal leaves with necrotic damage. These results highlight the moderate salinity tolerance of Osteospermum, which was related to the efficient accumulation of Cl− and Na+ ions in its leaves.