Responses of morphology and drought tolerance of Sedum lineare to watering regime in green roof system: A root perspective

The presence of drought tolerant vegetation is essential for the longevity of an extensive green roof when irrigation is not installed. Earlier studies have examined performance of green roof plants under contrasting watering regimes and found that higher watering frequency provided better growth and survival rates. The effect of early watering regimes on the subsequent response of plants to persistent drought stress in extensive green roofs, however, has not been extensively studied. In order to evaluate the effects of watering regime during the establishment period of Sedum lineare on its growth and drought tolerance, two greenhouse experiments using simulated green roofs were conducted. It was found in the first experiment that a 2-day-interval watering regimen at the early planting stage produced greater root biomass and root size than those of 6-day- and 13-day-interval watering, indicating that deficit watering tended to induce thinner roots in S. lineare. In the second experiment, the remaining plants were subsequently subjected to a 28-day drought treatment. The roots of plants watered at 13-day-interval maintained the highest respiration activity among all plants during the drought period. Results suggest that an appropriate deficit watering regimen at the early planting stage may lead to smaller root size and higher root:shoot ratios in S. lineare, and thereby improve its drought tolerance performance on extensive green roofs.     

昆明常见园林地被植物耐旱性研究

通过盆栽4种多年生观赏地被植物,对其进行连续20d的自然失水胁迫处理,研究供试植物在干旱胁迫下的土壤相对含水量以及叶片的枯叶率、游离脯氨酸、相对电导率的变化。结果表明:在干旱胁迫下各参试植物的各指标变化显著,综合分析各指标,4种观赏植物抗旱性由强至弱排序为:松叶景天萱草头花蓼紫花地丁。松叶景天、萱草具有很强的抗旱性,在园林绿化中可大力推广应用。     

Responses of eggplant to drought. II. Gas exchange parameters

Transpiration and net photosynthesis of eggplant (Solanum melongena L.) cultivar ‘Claresse’ were measured at decreasing leaf water potential (ψ_leaf). Attached single leaves were used in an open system and 6 levels of irradiance were applied. Mesophyll resistance to diffusion of Co₂ (r'_m) and stomatal resistance to diffusion of water vapour (r_s) were calculated from the gas exchange rates. As ψ_leaf declined, transpiration and photosynthesis decreased due to stomatal closure and an increase in r'_m in the case of photosynthesis. For the same degree of water stress, photosynthesis was maintained better in egg-plant than in other vegetable plants cited in the literature. Twenty four hours after the severely stressed plants were rewatered, gas exchange rates had not recovered completely. This was due to an after-effect of stress on the stomates, and to additional direct effects of drought on the photosynthetic system.     

Potential climate change impacts on green infrastructure vegetation

This study investigated the impacts of successive simulated droughts and floods on two plant species (Carex lurida and Liriope muscari) commonly installed in green-infrastructure (GI) sites built in the urban northeast USA. The instantaneous stomatal conductance, and belowground biomass growth (in a second drought experiment only) were used as metrics, since they are indicators of the ability of plants to provide ecosystem functions such as transpiration and carbon uptake. The results indicate that both species have greater tolerance for floods than for droughts. Signs of stress were only evident after a simulated flood exceeding the duration of 95% of all storms that occurred in this geographic region between 1950 and 2000. By contrast, simulated droughts had a more pronounced effect on both the instantaneous conductance measures during drought and the recovery following the cessation of drought in both species. Liriope subjected to drought treatments were all able to recover and to re-establish stomatal conductance levels similar to those displayed by a control group even after repeated drought treatments. By contrast, Carex showed reduced recovery after multiple droughts, in two separate rounds of experiments. However, regardless of moisture conditions and treatment, Carex generally displayed higher stomatal conductance than Liriope, indicating greater transpiration, and CO₂ uptake than Liriope. The belowground biomass results supported this finding, i.e. Carex gained more belowground biomass than Liriope during all experiments. At the end of the experiment, the Carex subjected to drought had less than one sixth the belowground biomass of the control treatment, whereas for Liriope this ratio was only 50% (drought to control). The drought treatments, therefore, reduced the biomass of Carex more than it did Liriope, when compared to the respective control plants. Nonetheless, both species survived repeated cycles of droughts and floods, suggesting that these particular species are both likely suitable for use in GI facilities, despite projected future increases in the frequency and intensity of floods and droughts in this geographic region. From a practical perspective, the results suggest no need for irrigation or potential replacement of plants in GI systems in a changed climate.     

Pre-conditioning ornamental plants to drought by means of saline water irrigation as related to salinity tolerance

This study examines the feasibility of using saline irrigation water for commercial pot cultivation of three ornamentals: Calceolaria hybrida, Calendula officinalis and Petunia hybrida. Two saline treatments were assayed: irrigation with low saline tap water (electrical conductivity = 1.16 dS m⁻¹), and irrigation with a high saline solution of NaCl 100 mM + CaSO₄ 10 mM + MgSO₄ 2.5 mM (electrical conductivity = 12.5 dS m⁻¹). When the control plants reached marketable size the watering was stopped and the plant response to drought was studied. Petunia and Calceolaria were tolerant to salinity. Petunia saline-treated plants reduced their growth slightly and increased N and chlorophyll contents in the leaves. Calceolaria experienced a strong reduction in growth and a delay in flowering but no toxicity symptoms or mortality was recorded. These species were moderate NaCl accumulators. Calendula was sensitive to salinity: 16% of the plants died and the surviving ones experienced a heavy reduction of growth, a decrease in chlorophyll and a large accumulation of NaCl in the leaves. Saline pre-conditioned plants of Calceolaria and Petunia were tolerant to drought. In these plants, leaf water content and, specifically, leaf relative water content were sustained longer than in non-pre-conditioned plants throughout the drought period. In Calendula, leaf relative water content decreased at the same rate in pre-conditioned and non-pre-conditioned plants. Consequently, salinization did not confer drought resistance upon this species. Possible factors determining the tolerance to drought in saline pre-conditioned plants are discussed.     

Daily and seasonal variation of water relationship in sugar apple (Annona squamosa L.) under different irrigation regimes at semi-arid Brazil

In the northeast of Brazil the drought period determines the yield period of the sugar apple (Annona squamosa L.). As a result, the use of irrigation is essential to stagger production over the course of the year. The results shown here represent an analysis of water status levels in sugar apples in daylight and seasonal periods in semi-arid regions. Two plant groups were studied: one without irrigation and the other with irrigation during drought months. This study showed that younger leaves displayed greater stomatal conductance and transpiration. In drought months, even in irrigated plants, the high air moisture deficit had a strong influence on the stomatal closure, which did not translate into a reduction in transpiration. Over the same period, the leaf water potential was −1.8 and −2.9 MPa at mid-day in irrigated and non-irrigated plants, respectively, and only the irrigated plants could recover their leaf hydration level at night. With a water deficit, plants showed greater control of transpiration through stomatal closure, with a linear relationship between stomatal conductance and transpiration.     

Response to water stress of some oleander cultivars suitable for pot plant production

Oleander (Nerium oleander L.) is an evergreen shrub of great ornamental interest which, in recent times, has been increasingly used as a flowering pot plant. Plants grown in pots undergo more frequent water stress conditions than those grown in the soil, due to the limited volume of substrate available for the roots. Oleander is a species adaptable to dry conditions and able to survive long periods of drought. It is well known that under water stress conditions all plants reduce photosynthetic activity, resulting in reduced plant growth. In case of severe water stress, leaves undergo strong wilting and senescence resulting in the loss of ornamental value. In the present work, a study was conducted to evaluate the ecophysiological response to water stress in four oleander cultivars previously recognised (on the basis of traits such as size, habit, earliness, abundance and duration of flowering, aptitude for cutting propagation and rapidity of growth) as suitable for pot plant production. Our data confirm the high drought tolerance of oleander. In the studied cultivars, plants submitted to water stress showed only minor variations in leaf gas exchange parameters [transpiration (E), stomatal conductance (g_s) and CO₂ net assimilation (A)] for at least 10 days without a change in leaf water content [assessed as relative water content (RWC)] for 22 days from the beginning of the stress treatment. During this period, non-irrigated plants maintained the same water status as control plants and were visually undistinguishable from them. Moreover, plants survived without water supply for one month. Following the supply of water again, they were able to restore RWC, gas exchange parameters and instantaneous water use efficiency [A/E ratio (WUE_inst)] to the values of control plants. Furthermore, if at the end of the stress period plants appeared withered and were pale green in colour, they regained their normal appearance after they were irrigated again. Although the four studied cultivars showed some minor differences in leaf gas exchange parameters and in the manner in which the latter parameters changed after irrigation was stopped, the response to water stress was essentially the same. Therefore, as far as drought tolerance is concerned, all these cultivars have a good aptitude for use as flowering pot plants.     

The response of different almond genotypes to moderate and severe water stress in order to screen for drought tolerance

In order to screen almond genotypes for drought tolerance, three different irrigation levels including moderate and severe stress (Ψs = −1.2 and −1.8 MPa respectively) and a control treatment (Ψs = −0.33 MPa) were applied for five weeks to six different cultivated almond seedlings. A factorial experiment was conducted with a RCBD which included 3 irrigations factors, 6 genotype factors and 3 replications. Seeds were prepared from controlled pollination of the bagged trees (after emasculation and flower isolation using isolator packets in the previous year). Genotypes included: homozygote sweet (Butte), heterozygote sweet (SH12, SH18, SH21 and White) and homozygote Bitter (Bitter Genotype). Leaf and root morphological and physiological traits including; midday relative water content, midday leaf (xylem) water potential, shoot dry weight and growth, total leaf area, leaf size, total leaf dry weight, specific leaf area, leaf greenness (SPAD), stomatal size and density, root and leaf nitrogen content and chlorophyll fluorescence were measured throughout the study. Results showed the six genotypes had different reactions to water stress but all genotypes showed an ability to tolerate the moderate and severe stresses and they showed different degrees of response time to drought stress. Almond seedling leaves could tolerate Ψ_w between −3 and −4 MPa in short periods. Water availability did not significantly affect stomatal density and size of young almond plants. The analysis of leaf anatomical traits and water relations showed the different strategies for almond genotypes under water stress conditions. Although almond seedlings even in severe stress kept their leaves, they showed a reduction in size to compensate for the stress effects. All genotypes managed to recover from moderate stress so Ψ_w = −1.2 could be tolerated well by almond seedlings but Ψ_w = −1.8 limited young plant growth. Leaf greenness, leaf size, shoot growth, shoot DW, TLDW and stomatal density were not good markers for drought resistance in almond seedlings. Root DW/LA, lower stomatal size and lower SLA might be related to drought resistance in cultivated almonds. Butte had the least resistance and White showed better performance during water stress while other genotypes were intermediate. Bitter seedlings showed no superiority in comparison with other genotypes under water stress conditions except for better germination and greater root DW which might make them suitable as rootstocks under irrigation conditions.     

Herbicide-impregnated mulches for weed control in container nursery stock

Isopropyl-m-chlorocarbanilate (chlorpropham), chlorpropham plus p-chlorophenyl-N-methylcarbamate (PPG-124), S-ethyl dipropylthiocarbamate (EPTC), N,N-dimethyl-2,2-diphenylacetamide (diphenamid) and 2,6-dichlorobenzonitrile (dichlobenil) were evaluated for plant tolerance and weed control when incorporated on milled pine bark mulch and applied at a depth of 1.27 cm to five species of container-grown nursery stock. During two seasons, all impregnated mulches except EPTC at 5.6 kg/ha and diphenamid at 5.6 kg/ha provided adequate long-season broadleaf weed control. Similarly, long-season grass weed control was acceptable with all herbicide-impregnated mulches except chlorpropham at 2.2 kg/ha, chlorpropham plus PPG-124 at 2.2 plus 0.4 kg/ha and diphenamid at 5.6 kg/ha. Plant injury appeared on all five species of nursery stock when dichlobenil was impregnated on pine bark mulch at a rate of 13.4 kg/ha.     

Water stress induces in pear leaves the rise of betaine level that is associated with drought tolerance in pear

Summary

Mild water stress signi®cantly induced the accumulation of betaine in the leaves of young pear plants (Pyrus bretschneideri Rehd. `Suly'). The betaine induced by water stress in mature leaves was maintained for about two weeks of re-watering after water stress, and gradually declined two weeks after the relief of water stress, dropping to the normal level one month later. Exogenously foliar-applied betaine slightly improved the shoot growth and the ability of the leaves to resist dehydration, alleviated wilting of leaves under water stress, and promoted recovery of leaves from wilting after re-watering. These ®ndings indicate that the accumulated betaine induced by water stress was associated with drought tolerance in pear.     

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.     

Estimation of relative water use among ornamental landscape species

Low water use plantings may enhance water conservation in dry landscapes. However, appropriate plant selection is hindered by the dearth of information available on the water needs of different species. A direct method of classification was tested under the hypothesis that relative water use by woody landscape species growing in 3.8 l containers would be representative of the water use of the same species in the landscape. Four species of distinctly different ecological origin (Leucophyllum frutescens, Spiraea vanhouteii, Viburnum tinus, Arctostaphylos densiflora) were chosen in order to obtain a wide range of responses, and their water use was measured in plants growing in 3.8 l containers and compared to that of the same species growing in drainage lysimeters, representative of landscape conditions. Half of the plants were subjected to successive cycles of stress by withholding water after irrigation to container capacity in containers, or applying a fraction of the potential evapotranspiration in lysimeters. The good fit of the regression of average daily water use by lysimeter plants on average daily water use by container plants (R²=0.87,P<0.01) reflects the consistency of relative water use of the four species. Measurement of water use at the end of nursery production may be useful for predicting the relative water use of various species after establishment in the landscape.     

Identification of drought-induced genes from the leaves of Rangpur lime (Citrus limon (L) Osbeck)

Water stress is one of the limiting factors for citrus production and Citrus species show great variation in their response to drought stress. Although the majority of Citrus rootstocks are sensitive to water stress, Rangpur lime (Citrus limon (L) Osbeck) (RL) shows a high degree of drought tolerance. Therefore, it has been used as a rootstock in drought-prone environments, but mechanisms of drought tolerance are not yet known. In this study, to explore the mechanisms of drought adaptation and tolerance, a subtractive cDNA library was constructed from the leaves of 14-day drought-stressed and non-stressed RL for identification of drought-induced genes. 285 cDNA sequences were obtained from randomly selected clones from the subtracted library containing the drought-induced genes. The expression analyses of 200 cDNAs in 14-day drought treated and untreated RL by macroarray hybridisation revealed that the expression of 56 cDNAs increased two to 11-fold. 30 non-redundant drought-induced genes were identified from these cDNAs and drought induction of eight selected genes was confirmed by a real-time RT-PCR assay suggesting that expressions of these genes were regulated by drought-stress. Genes identified in this study were mostly related with cell rescue and defense pathways involved in drought adaptation and tolerance of RL.     

Transplanting Gladiolus plants propagated in vitro

In vitro propagated Gladiolus plantlets cultivar ‘Eurovision’, subcultured to a pretransplanting low-mineral and sucrose medium under high irradiance flux, developed functional roots, and after transfer to non-aseptic conditions, continued to grow without becoming dormant.Transplanted plantlets, after hardening, produced larger-size plants and cormlets (small corms) under non-aseptic conditions than formed in vitro, thus shortening the time for the propagation of large-size cormlets.     

Drought responses of six ornamental herbaceous perennials

Although low water use landscaping is becoming common in arid regions, little is known about drought tolerance and drought responses of many ornamental plants, especially herbaceous perennials. Drought responses were assessed for six herbaceous ornamental landscape perennials in a 38 l pot-in-pot system in northern Utah over a 2-year period. The first year was an establishment period. During the second year, drought responses were evaluated for established Echinacea purpurea (L.) Moench, Gaillardia aristata Pursh, Lavandula angustifolia P. Mill., Leucanthemum × superbum (J.W. Ingram) Berg. ex Kent, ‘Alaska’, Penstemon barbatus Roth var. praecox nanus rondo, and Penstemon × mexicali Mitch. ‘Red Rocks’. Plants were irrigated at frequencies of 1 (control), 2, or 4 weeks between June and September, simulating well-watered conditions, moderate drought, or severe drought. Osmotic potential (Ψ_s), gas exchange, visual quality, leaf area, and dry weight were assessed. In a confined root zone, P. barbatus showed the greatest tolerance to all levels of drought, avoiding desiccation by increasing root:shoot ratio and decreasing stomatal conductance as water became limiting. L. angustifolia and P. × mexicali showed tolerance to moderate drought conditions, but died after exposure to the first episode of severe drought. Neither G. aristata nor L. superbum were able to regulate shoot water loss effectively. Instead, both species displayed drought avoidance mechanisms, dying back when water was limiting and showing new growth after they were watered. Compared to control plants, G. aristata shoot dry weight was reduced by 50% and 84%, and L. superbum shoot dry weight was reduced by 47% and 99% for the 2- and 4-week irrigation intervals, respectively. Root dry weights were affected similarly for both species. E. purpurea exhibited poor visual quality at all irrigation intervals, in particular wilting severely in both drought treatments, but regaining turgor when watered again. P. barbatus is recommended for ornamental landscapes that receive little or no supplemental irrigation, while E. purpurea is not recommended for low water landscapes because of low visual quality under even mild drought.     

Effect of alleviating products with different mode of action on physiology and yield of olive under drought

Two years old self-rooted Koroneiki olive trees (Olea europaea L.) were subjected to two irrigation regimes, i.e. the fully irrigated and the severely water stressed trees, while they were treated with three alleviating products of different mode of action. The products used were the osmolyte glycine betaine, the antioxidant Ambiol and the heat and irradiance reflecting kaolin clay particles. The effects of product application and water regime on leaf characteristics, shoot and root growth, photosynthesis, leaf compatible solids (carbohydrates) concentration and yield were evaluated. All products applied, exhibited significant alleviating action, based on the relative alleviation index. Irrigated trees exhibited greater growth than drought stressed ones, while the ameliorating products maintained the water content of the leaves under drought conditions and resulted in lower leaf tissue density. On the other hand carbon assimilation rate, stomatal conductance and intrinsic water use efficiency were significantly reduced under drought stress, while the opposite stood for intercellular CO₂. Drought stress resulted in elevated sucrose leaf concentration, while the application of Ambiol increased stachyose concentration and that of glycine betaine did the same with the mannitol concentration. Among the alleviating products tested in this experiment Ambiol and glycine betaine had a significant positive effect on leaf water content, photosynthesis and yield under both drought and well irrigated conditions.     

Detecting different levels of drought stress in apple trees (Malus domestica Borkh.) with selected biochemical and physiological parameters

Rational irrigation scheduling based on sensing drought stress directly in plants is becoming more important due to increasing worldwide scarcity of fresh water supplies. In order to evaluate a set of potential biochemical and physiological stress indicators and select the best drought stress markers in apple trees, two experiments with potted trees and an experiment with intensive orchard grown apple trees ‘Elstar’ and ‘Jonagold Wilmuta’ were conducted in early summer in tree following years. Biochemical parameters: ascorbic acid, glutathione, tocopherols, chlorophylls, carotenoids, free amino acids, soluble carbohydrates, and physiological parameters already known as stress indicators in apple trees: predawn and midday leaf water potential, net photosynthesis (Pn), stomatal conductance (g_s), transpiration (Tr) and intercellular CO₂ concentration (Ci) were measured in leaves of apple trees subjected to different intensities of slowly progressing drought or no drought. Our study pointed out zeaxanthin and glutathione as the best drought stress markers in apple trees. Ascorbate and sorbitol appeared to be reliable indicators of moderate drought only. Responses of other tested biochemical parameters were not consistent enough to prove their role as drought stress markers in apple trees. Relative air humidity should be taken in consideration when physiological parameters g_s, Pn, Tr and Ci are used as drought stress markers in apple trees. Our study revealed that in situations where low relative air humidity affects g_s and with g_s connected physiological parameters, biochemical markers may be better tool for determination of drought stress intensities in apple trees.     

Effects of Stockosorb and Luquasorb polymers on salt and drought tolerance of Populus popularis

The effects of two types of hydrophilic polymers on drought and salt resistance of 1-year-old cuttings of Populus popularis 35–44 were investigated in this study. The polymers used in the experiments were Stockosorb 500 XL (Stockosorb) (a granular type, cross-linked poly potassium-co-(acrylic resin polymer)-co-polyacrylamide hydrogel) and Luquasorb^® product (a powder type of potassium polyacrylate), which were manufactured by Stockhausen GmbH Krefeld and BASF Corporation in Germany, respectively. Drought or salt stress significantly decreased leaf photosynthesis and transpiration, as well as plant water-consumption and dry weight. A significant reduction occurred in Drought + NaCl-stressed plants. Soils treated by 0.5% Stockosorb or Luquasorb markedly alleviated the inhibition of plant growth and leaf gas-exchange that were caused by drought and/or salt stress treatments, and the occurrence of stress-induced leaf injury was delayed for 31 and 51 days, respectively. Experimental results showed that hydrophilic polymers in root media assisted P. popularis plants to tolerate the drought and salt stresses, due to the following reasons: (1) roots took up the retained water from hydrophilic polymers when water was deficient in the soil (Stockosorb-treated plants exhibited a higher rate of water uptake); (2) under saline conditions, Stockosorb and Luquasorb held Na⁺ and Cl⁻ in the soil solution due to their high water-holding capacity, thus limiting an excessive accumulation of toxic ions in the plant organs; furthermore, the exchangeable K⁺ that contained in Stockosorb and Luquasorb resulted in an improved K⁺/Na⁺ homeostasis in salinized plants; (3) hydrophilic polymers aided the plants to tolerate an interactive impacts of drought and salt stresses, which was mainly accounted for their water- and salt-holding capacities. In comparison, the growth and survival enhancement effects of the hydrophilic polymers on Drought + NaCl-treated plants was more evident by Luquasorb application, because it supplied water to plants at a lower rate during soil drying, thus prolonging the duration of water supply and allowed roots to grow in an environment of lower salinity for a long period of salt and drought stresses.     

Nitrogen and irradiance levels affecting net photosynthesis and growth of young coffee plants (Coffea arabica L.)

Summary

Shoot growth attributes, leaf anatomical changes and net photosynthetic rates were determined in young coffee plants fertilized at high and low levels and subjected to shade or full sunlight. High nitrogen supply increased plant height irrespective of the light level imposed during growth, and promoted a larger leaf area in plants of the shade treatment. Specific leaf weight was 15% higher in plants grown in full sunlight than in shaded plants, at both nitrogen levels. The number of leaves developed on the orthotropic stem increased significantly under full irradiance and high nitrogen availability. Leaves were 11% thicker ih unshaded plants than in shaded ones, because of the increased size of the palisade and spongy parenchyma tissues. More thylakoids per granum and more grana per chloroplast were observed in shade-grown plants, mainly in those given high nitrogen treatment. Total nitrogen content expressed on a leaf-area basis was higher in sun plants supplemented with nitrogen, whereas chlorophyll a and b and protochlorophyll contents increased in shade-grown plants. Net photosynthetic rate in high nitrogen plants reached 7.19 µmol CO₂ m^?2 s^?1 in the full irradiance treatment, and 5.46 µmol CO₂ m^?2 s^?1 in shaded plants. Maximum net photosynthetic rates in the low nitrogen plants were 5.28 and 4.90 µmol CO₂ m^?2 s^?1 in sun and shade plants, respectively. Increased photosynthetic light saturation was observed in high irradiance plants (628 µmol m^?2 s^?1) relative to shade plants (359 µmol m^?2 s^?1) in the high nitrogen treatment. The same pattern was observed in low nitrogen plants. The apparent quantum yield of shade acclimated plants was 14% higher than in those grown in full sunlight. Our results indicate that coffee may be classified as a facultative shade species, exhibiting features of sun adapted plants coupled with shade acclimation attributes, this fairly high adaptive capacity to shade:sun transition being strongly influenced by the level of nitrogen nutrition given to the plants.     

Sana Physiological responses of two grapevine (Vitis vinifera L.) cultivars to CycocelTM treatment during drought

The plant growth regulator Cycocel^TM [(2-chloroethyl)trimethylammonium chloride] can be used to produce drought tolerance in grapevine (Vitis vinifera L.) due to a reduction in the ratio between vegetative growth and fruit production. To evaluate the physiological responses of two grapevine cultivars to drought and Cycocel^TM treatment, a factorial experiment was conducted in a greenhouse. The factors included irrigation frequency (at 5-, 10-, or 15-day intervals corresponding to no, mild, or severe drought stress), Cycocel^TM concentration (0, 500, or 1000 mg l^–1), and cultivar (‘Rasheh’ or ‘Bidane-Sefid’). Stomatal conductance (g_s) the net rate of CO₂ assimilation (A_net), the rate of transpiration (T_r), and chlorophyll a and b concentrations decreased in plants exposed to mild or severe water-deficit stress, whereas carotenoid, proline, and total soluble sugar concentrations increased compared to plants with no drought stress. The relative water content (RWC) of leaves declined only under severe drought stress. A reduction in intercellular CO₂ concentrations (C_i) occurred under mild drought stress; however, under severe drought stress, C_i values increased. Under mild drought stress, the reduction in the net rate of photosynthesis was related to stomatal closure, whereas under severe drought stress, non-stomatal factors were dominant. Water-use efficiency (WUE) improved under mild drought stress relative to non-stressed plants, but under severe drought, it declined. Foliar applications of Cycocel^TM resulted in increased A_net, g_s, T_r, and WUE values, as well as proline and soluble sugar concentrations. ‘Rasheh’ was more tolerant to drought stress than was ‘Bidane-Sefid’. Foliar applications of Cycocel^TM, particularly at 1000 mg l^–1, mitigated the negative effects of drought stress by increasing A_net, WUE, RWC, compatible solute concentrations, such as proline, soluble sugar, and chlorophyll a and b concentrations.