J. A. Franco

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.

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.

Effect of nursery irrigation regimes on vegetative growth and root development of Silene vulgaris after transplantation into semi-arid conditions

Summary The influence of irrigation regimes during nursery production of seedlings on development of the aerial parts and root system after being transplanted into semi-arid conditions was investigated in Silene vulgaris (Moench.) Garcke. This Mediterranean native herb is of interest for phytoremediation of contaminated soils and for edible and medicinal uses. During the 35 d nursery period, seedlings were grown in polystyrene trays. Three irrigation treatments were used throughout the nursery period:WI, well-irrigated seedlings; MS, moderately-stressed seedlings; and HS, highly-stressed seedlings. In all treatments, seedlings were overhead irrigated on 2 d per week, and the total amounts of water applied (per tray of 176 seedlings) over the whole nursery period were:WI, 16.10 l; MS, 10.75 l; and HS, 4.50 l.After the nursery period, WI and MS plants were transplanted into transparent containers (cylindrical acrylic tubes, 8 cm in diameter and 100 cm tall) and into the field, where mini-rhizotrons were used to evaluate root system dynamics. Plants transplanted into containers were watered with two irrigation regimes: normal irrigation (NI) and deficit irrigation (DI), with 0.90 or 0.45 l plant–1 week–1, respectively. All plants transplanted into the field were watered equally, with 1.16 l plant–1 week–1. Post-transplantation growth of aerial parts and roots was studied over 120 d in the transparent containers and over 60 d in the field. MS and HS treatments during the nursery period produced seedlings that showed lower midday leaf water potential and greater root:aerial parts fresh weight (FW) ratios than the WI treatment. The MS treatment produced seedlings with the greatest length and FW of roots and with the highest quality. The HS treatment produced seedlings which were too small and over-hardened. The latter were therefore not used for post-transplantation experiments. After transplantation into transparent containers, MS seedling-derived plants showed greater root growth than WI-derived plants, especially when the water content of the substrate was low (DI treatment). Also, mini-rhizotrons allowed observation of more active root growth in MS seedling-derived plants than in WI seedling-derived plants after transplantation into the field, especially in the deepest layer of soil (50-75 cm). WI-derived and MS-derived plants, under NI or DI post-transplantation treatments, showed similar FWs and dry weights (DWs) of their aerial parts, but MS-derived plants showed greater leaf:stem FW and DW ratios than WI-derived plants under DI conditions.

Nursery irrigation regimes affect the seedling characteristics of Silene vulgaris as they relate to potential performance following transplanting into semi-arid conditions

Summary The influence of pre-conditioning water regimes during nursery production on seedling characteristics was investigated in Silene vulgaris (Moench.) Garcke. Two experiments were carried out. In Experiment 1, throughout the nursery period (5 weeks), seedlings growing in polystyrene trays were overhead-watered twice a week using three irrigation treatments: WW (well-watered), irrigated to 75% of the water-holding capacity (WHC); MS (moderately-stressed), watered to 50% of the WHC; and HS (highly-stressed) watered to 15% of the WHC. At the end of the nursery period, seedlings raised under the MS treatment showed greater total root lengths, specific root lengths (SRL), degrees of root branching, root fresh weights (FW) and dry weights (DW), root DW:volume ratios, and water-use efficiencies (WUE) than those raised under the WW treatment. The MS seedlings were shorter, had a lower midday leaf water potential, and had lower shoot:root FW and DW ratios than the WW seedlings. Irrigation regime had no influence on the suberisation of roots, but the taproots of WW seedlings showed a lower cortex thickness:root radius (C:R) ratio (0.102) than taproots of MS and HS seedlings (0.125 and 0.136, respectively). HS seedlings were too small (1.9 cm high) and over-hardened, with poorly developed root systems. Experiment 2 (identical to Experiment 1 but using only two irrigation regimes: WW and MS) confirmed the differences observed in shoot and root growth between WW and MS seedlings.

Effects of salinity on the germination, growth, and nitrate contents of purslane (Portulaca oleracea L.) cultivated under different climatic conditions

Summary Interest in cultivating purslane (Portulaca oleracea L.) as a food crop has increased since its identification as an excellent source of some biologically-active compounds that are considered to be essential for the promotion of human health and the prevention of disease. The aim of this research was to determine the effects of salinity, by means of exposure to different concentrations of NaCl in the nutrient solution, on the germination, growth, yield, and nitrate contents of purslane cultivated in a hydroponic system under two different light intensities. The results indicated that higher germination percentages (approx. 50%) were obtained between saline levels with electrical conductivities (EC) of 0 – 12.5 dS m–1. Above 12.5 dS m–1, the germination percentage decreased significantly. Salinity reduced both shoot and root growth. This reduction was greater in plants grown under the lower light intensity. When the EC of the nutrient solution increased from 2.5 dS m–1 to 15.0 dS m–1, yields were reduced by 26% in plants grown in a greenhouse, and by 82.2% in plants grown under the lower light intensity in a growth chamber. In both cases, nitrate levels increased in plants grown at an EC of 5.0 dS m–1, but nitrate levels decreased with further increases in salinity. In all cases, nitrate levels were below the maximum levels established by European Regulations for lettuce.

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.