Responses of eggplant to drought. I. Plant water balance

Eggplant (Solanum melongena L.) is known to have a higher resistance to drought than have other vegetables. Studies were made to see whether this property can be explained by its physiological responses. Water was withheld from plants and measurements were made of sap osmotic potential (ψ_os), leaf pressure potential (ψ_p), stomatal diffusive resistance to water vapour (r_s), relative water content of leaf (RWC), whole plant transpiration, leaf water potential (ψ_leaf), and soil water potential (ψ_soil). The first 4 parameters varied linearly with ψ_leaf and transpiration varied exponentially. There was a parabolic relation between ψ_leaf and ψ_soil. The severely stressed plants were rewatered and underwent the same measurements 1 day later (“recovery treatment”). Compared with some other vegetables reported in the literature, ψ_p and RWC remained higher in droughted eggplant. This species has also a better stomatal control on transpiration. There was an after-effect of drought on the stomates. In recovery treatment, ψ_p values exceeded those of the control. This may have functional significance in resumption of growth after stress is alleviated.     

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.     

Response of “Sangiovese” grapevines to partial root-zone drying: Gas-exchange,growth and grape composition

Our study focuses on the physiological response and yield-quality performance of split-root potted Sangiovese grapevines under a partial root-zone drying (PRD) regime from pre-veraison to harvest by withholding water from one of the two pots and comparing the results to a well-watered control (WW). While predawn water potential (ψ_pd) tended to equilibrate in PRD with the soil moisture level of the wet pot, both stem (ψ_st) and mid-day leaf-water potential (ψ_l) were markedly lower in PRD as compared to WW vines, indicating that Sangiovese shows anisohydric response. On the other hand, the seasonal reduction of leaf assimilation rate (A) in PRD over the 6-week stress period versus WW was 16% as compared to a 41 and 25% for leaf stomatal conductance (g_s) and transpiration (E), respectively. As a consequence, intrinsic WUE (A/g_s) was markedly increased in the half-stressed vines, suggesting a response more typical of an isohydric strategy. Shoot growth was promptly checked in PRD vines, which had no limitation in yield and better grape composition as per soluble solids and total anthocyanins. These responses occurred in spite of sub-optimal leaf photosynthesis rates and lowered leaf-to-fruit ratio and qualify Sangiovese as a good candidate for adapting to regulated deficit irrigation strategies.     

Water status,leaf area and fruit load influence on berry weight and sugar accumulation of cv. ‘Tempranillo’ under semiarid conditions

Berry weight (BW) and sugar concentration (SC) are relevant indices in viticulture since they can be easily measured and, when considered together, give a relatively fair estimation of grape quality. This work aims to evaluate the influence of water availability, leaf area and fruit load on BW and SC, estimating the relative importance of these factors. Leaf area (LA), berry number (BN), yield (YLD), water potential in summer (ψ_pd-s), BW and SC were measured in 16 and 17 ‘Tempranillo’ vineyards in 1999 and 2000, respectively. In all the vineyards, according to local practices, the irrigation amount decreased as summer progressed. The study vineyards comprised a broad range of situations concerning leaf area, fruit load and water status in summer. Average leaf water potential in summer and LA/BN ratio, when considered together, estimated properly BW (R² = 0.91; P < 0.001) and, in a similar way, ψ_pd-s and LA/YLD ratio estimated SC (R² = 0.74; P < 0.001). The relative weight of ψ_pd-s in both relationships was much higher than that of any of the LA:fruit ratios, showing that, under semiarid conditions, water availability plays the main role in regulation of berry growth and sugar accumulation and, therefore, the highest attention should be paid in these areas to irrigation management, seeking the degree of stress that allows optimizing the combination of yield and berry quality in each situation.     

Effect of different drip irrigation methods and fertilization on growth,physiology and water use of young apple tree

A pot experiment was conducted to investigate the effect of three drip irrigation methods (i.e. conventional drip irrigation (CDI), both sides of the root-zone irrigated with full watering, alternate drip irrigation (ADI), both sides of the root-zone irrigated alternatively with half of the full watering, and fixed drip irrigation (FDI), only one side of the root-zone irrigated with half of the full watering) on growth, physiology, root hydraulic conductance and water use of young apple tree under different nitrogen (N) or phosphorus (P) fertilization (i.e. CK (no fertilization), N₁ (0.2 g N/kg), N₂ (0.4 g N/kg), P₁ (0.2 g P₂O₅/kg) and P₂ (0.4 g P₂O₅/kg)). Results show that compared to CDI, ADI and FDI reduced mean root dry mass, daily transpiration, root hydraulic conductance (K_r), leaf photosynthesis rate, transpiration rate and stomatal conductance of young apple tree by 6.9 and 27.7, 29.3 and 45.0, 6.8 and 37.9, 2.5 and 4.8, 32.6 and 33.0, 22.1 and 22.3%, but increased leaf water use efficiency (WUE) by 31.3 and 29.8%, respectively when they saved irrigation water by 50%. Compared to the CK, N or P fertilization significantly increased K_r, and K_r was increased with the increased N or P fertilization level. There were parabolic correlations between K_r and root dry mass, daily transpiration and stomatal conductance. Our results indicate that ADI reduced transpiration rate significantly, but it did not reduce photosynthesis rate and K_r significantly, thus alternate drip irrigation improved WUE and the regulation ability of water balance in plants.     

Water relations of olive trees cultivated under deficit irrigation regimes

The experiment was carried out at the Experimental Field ‘Taoues’, southern Tunisia (34°N, 10°E) to examine the effect of irrigation schedules on water relations for young olive trees, cultivars Chétoui, Chemlali, Coratina, Picholine and Manzanille. Plants were cultivated at 7 m × 7 m spacing and drip irrigated from April to September. Irrigation amounts (IA) of 20%ET_c, 50%ET_c and 100%ET_c were applied, where ET_c is the FAO crop evapotranspiration. The effect of IA on midday leaf water potential (Ψ_leaf), stomatal resistance (R_s) and conductance (g_s), soil (H_v) and relative leaf (RWC) water contents was studied. Results showed that a further increase in IA was not systematically followed by an increase of water potential, production and WUE values. Some controversial responses were observed following to a variety with large seasonal variations. At the beginning of the growing season, differences between treatments were not very important because soil water content was still high enough to prevent important changes in Ψ_leaf values. The most stressed trees showed potentials of −1.53 MPa for Picholine cv. and ranging between −2.30 MPa and −2.10 MPa for the other varieties depending on IA. The less stressed trees provided potentials of −0.97 MPa for Picholine cv. and varying between −1.63 MPa and −2.13 MPa for cultivars Coratina, Manzanille, Chétoui and Chemlali. Then, as the season progresses, and when IA was increased from 20%ET_c to 50%ET_c, Ψ_leaf values decreased significantly for cultivars Chemlali (−3.05 MPa), Coratina (−3.75 MPa), Manzanille (−3.0 MPa) and Chétoui (−3.5 MPa). At 100%ET_c, Manzanille and Picholine cultivars show better water status with respective potentials of −2.7 MPa and −2.6 MPa. Stomatal resistance monitoring showed maximums at midday for all cultivars with pick value of 4.45 s/cm recorded for Chétoui cultivar irrigated at 50% ET_c. The most important variations were recorded for cultivar Chemlali which seem to be the ablest to regulate stomata aperture. Close coordination between Ψ_leaf and g_s and Ψ_leaf and RWC measurements was found. But, the response varied following to treatment. At low irrigation levels (20%ET_c and 50%ET_c), g_s was found to be linearly and positively correlated to Ψ_leaf. It increased linearly and positively with increasing values of Ψ_leaf (r of 0.84 and 0.96, respectively). At 100%ET_c, Ψ_leaf is found to be correlated to g_s following to a polynomial function with an optimum g_s value of 450 mmol/m²/s and Ψ_leaf of about −2.5 MPa.     

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.     

Seasonal trends in hydraulic conductance of field-grown ‘Valencia’ organge trees

Leaf water potential (ψ₁), whole tree transpiration (F) and leaf surface conductance (g), together with a number of environmental parameters, were measured from early summer until late winter in a well-watered mature ‘Valencia’ orange (Citrus sinensis (L.) Osb.) orchard during the growing season of 1977/78. F and g showed a seasonal trend, with maximum values reached during the late summer. Tree conductance (C_tree), calculated as the ratio between transpiration and the water potential difference between the wet soil and the sunlit leaves, was found to vary seasonally and showed maximum values during late summer. The soil temperature (T_s) showed a seasonal trend similar to C_tree. Analysis of this and other data from the literature confirmed that the sensitivity of tree conductance to soil temperature is related to the climatic origin of the plant species.The linear regression fitted to the relationship between C_tree and T_s was used to calculate soil temperature at apparent zero C_tree. This datum can be used as an index of plant sensitivity to root medium temperature.     

Effect of pruning and plant spacing on the growth of cherry rootstocks and their influence on stem water potential of sweet cherry trees

Summary

The aims of this work are to describe the effects of pruning and planting density on growth and water relations of ungrafted and grafted sweet cherry trees. A trial with cherry rootstocks ‘Prunus avium’, ‘CAB 11E’, ‘Maxma 14’, ‘Gisela 5’ and ‘Edabriz’ was begun in 1997. Pruning severities were applied to the rootstocks (0, 30, 60 and 90% of the vegetative growth was removed corresponding to P1, P2, P3 and P4 treatments, respectively) after planting to two plant spacings (S1 = 0.25 × 1.0 m and S2= 0.45 × 1.5 m). Canopy, root growth and leaf water potential (ψ_leaf) were quantified throughout the growing season. Pruning significantly affected root length and root weight of the rootstocks. Uncut plants (P1) showed a heavier and expanded root biomass (231 g and 108 m) than the intensively pruned plants (P4) (187 g and 75 m). The greater root biomass was obtained with the spacing/pruning combination, S1/P1 (285 g), and the smaller with S1/P4 (180 g) and S2/P4 (176 g). ψ_leaf varied significantly between the rootstocks and plant spacing but not with pruning. ‘Maxma 14’ and ‘P. avium’ attained the lowest values of midday ψ_leaf, –2.28 and –2.04 MPa, but the highest values of predawn ψ_leaf, –0.29 and –0.25 MPa, respectively. Generally, with high density (S1), the rootstocks exhibited lower predawn and midday ψ_leaf. In 1998, cultivars ‘Burlat’, ‘Summit’ and ‘Van’ were grafted onto rootstocks and a trial was installed in 1999. Predawn and midday stem water potential (ψ_stem) on cherry trees, measured in 2002, were affected significantly by the rootstock/genotype combination. Cultivars grafted on ‘P. avium’ and ‘Maxma 14’ showed the less negative midday ψ_stem, –1.36 and –1.42 MPa respectively, so these rootstock genotypes perhaps induced a higher drought resistance to the scion. Recorded data show that the scion-rootstock interaction with regard to production performance under water deficits may be an important consideration in cherry tree planting strategies.     

Effect of combined water and salinity stress factors on evapotranspiration of Sedum kamtschaticum Fischer in relation to green roof irrigation

In dryland areas, secondarily treated municipal wastewater could be used in extensive green roof systems. In this study, the effects of water and salt stress on a crassulacean acid metabolism (CAM) plant, Sedum kamtschaticum Fischer, was evaluated under intermittent saline irrigation. The salinity of irrigation water varied from 6.0 to 18.0 dS m^?1. A reduction in soil water content and an increase in soil water electrical conductivity (EC) were observed during the irrigation interval (5–17 d) as a result of evapotranspiration (ET). The effect of soil water potential (SWP) on reduction of the ET ratio (ET_r) was successfully described with an equation that could be applicable to a wide range of soil salinities and water contents, to estimate ET. In this study, the stress factor was defined as the integration of solute potential, and matric potential less soil water content for optimum growth (less than ?0.1 MPa) with elapsed time. The stress factor rapidly reduced total ET in CAM plants but effectively increased water-use efficiency (WUE). Thus, by using CAM plants for green roofs under intermittent saline irrigation, the need for irrigation water is reduced without a considerable loss of plant biomass.     

Plant growth,water relations and transpiration of two species of African nightshade (Solanum villosum Mill. ssp. miniatum (Bernh. ex Willd.) Edmonds and S. sarrachoides Sendtn.) under water-limited conditions

The adaptation to drought stress of two African nightshade species, Solanum villosum and S. sarrachoides was investigated in pot and field experiments between 2000 and 2002. Two genotypes of S. villosum (landrace and commercial) and one accession of S. sarrachoides were grown under droughted, moderate stress and well-watered conditions. Leaf expansion, stem elongation and transpiration began to decline early in the drying cycle with fraction of transpirable soil water (FTSW) thresholds of 0.46–0.64. Osmotic adjustment (OA) of both species was in the range of 0.16–0.19 MPa and could not maintain positive turgor below water potentials of −1.80 to −2.04 MPa. The responses evaluated were similar in the three genotypes suggesting similar strategies of adaptation to drought stress. Under field conditions, the S. sarrachoides accession showed a higher leaf area than the S. villosum commercial genotype. It is concluded that the three African nightshade genotypes have limited OA capacity and adapt to drought mainly by regulating transpiration. This was achieved by reduction of leaf area. In general, it is necessary to maintain FTSW above 0.5–0.6 to prevent decline in leaf expansion, stem elongation, and transpiration.     

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.     

Models of water competition between fruits and leaves on spurs of ‘Bartlett’ pear trees and its measurement by a heat-pulse method

Water streams through the conducting systems in spurs of ‘Bartlett’ pear trees were modelled in piped water-streams or electric currents, and measured by a heat-pulse method. Water-potential measurements showed that water potentials of stems (ψ_s), leaves (ψ_l) and fruits (ψ_f) were highest, lowest and intermediate, respectively, at mid-day throughout fruit growth. Retaining this condition (ψ_s >ψ_f >ψ_l), a model was constructed where streams of water through pipes connecting 3 tanks with different water-levels was analogous to water streams among 3 organs of a spur. It was then possible to estimate the possibility of water flowing from the fruit into the leaf against a reverse gradient of water-potential between the stem and the fruit. Another model used electric currents passing through 3 connected electric cells with different voltages. As the electric resistances of the 3 paths were equal, the voltage gradients toward each cell in the circuit were calculated using Kirchhoff's law. Replacing voltages by water potentials allowed the gradients of water potential toward each organ to be estimated, $\text{2}\text{3}\text{ψ}{}_{\mathrm{<mtext>l</mtext>}}\text{?}\text{1}\text{3}\text{(ψ}{}_{\mathrm{<mtext>s</mtext>}}\text{+ ψ}{}_{\mathrm{<mtext>f</mtext>}}\text{)}$ toward the leaf, $\text{2}\text{3}\text{ψ}{}_{\mathrm{<mtext>f</mtext>}}\text{?}\text{1}\text{3}\text{(ψ}{}_{\mathrm{<mtext>s</mtext>}}\text{+ ψ}{}_{\mathrm{<mtext>l</mtext>}}\text{)}$ toward the fruit and $\text{2}\text{3}\text{ψ}{}_{\mathrm{<mtext>s</mtext>}}\text{?}\text{1}\text{3}\text{(ψ}{}_{\mathrm{<mtext>l</mtext>}}\text{+ ψ}{}_{\mathrm{<mtext>f</mtext>}}\text{)}$ toward the stem of the spur. Adverse water fluxes from fruits (for instance, ?0.5 ml peduncle^?1 h^?1 at the maximum rate) were determined by a heat-pulse method at mid-day in early August. Demonstrations of water fluxes through the peduncles and the petioles by this method gave close agreement with results from the 2 models in relation to the diurnal shifts of direction of the water streams. The gradients of water potential estimated by the electric current model was closely related to water fluxes during the day (r = ?0.9832 for peduncles, r = ?0.9604 for petioles). Hydraulic conductivities (L_p) for petioles or peduncles were in the order of 10^?4 cm s^?1 bar^?1.     

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.     

Effect of the elevated ozone on greening tree species of urban: Alterations in C-N-P stoichiometry and nutrient stock allocation to leaves and fine roots

Anthropogenic ground-level ozone (O₃) pollution can alter the phosphorus (P), carbon (C), and nitrogen (N) of terrestrial plants’ ecological stoichiometry, which in turn affects forest productivity, nutrient utilization, and carbon sink capacity. However, there is still quite a lot of uncertainty regarding the impact of high O₃ levels on C-N-P stoichiometry in organs with a rapid turnover (i.e., fine roots and leaves) across varied functional types. This study investigated the effects of O₃ on the stoichiometry of C-N-P nutrient allocation of stocks to various plant organs, with a special focus on tree species frequently employed for urban greening. The impact of O₃ on C-N-P stoichiometry among different functional tree types was subsequently evaluated by reviewing the published literature. Under a pooling of all species, elevated O₃ decreased and leaf C and P concentrations increased, thereby decreasing the leaf C: P ratio. Elevated O₃ increased the N concentration in fine roots, thereby decreasing the C: N ratio, although no significant impact was observed in leaves. Elevated O₃ significantly reduced the leaf stocks of C (CS_leaf) and N (NS_leaf), however, there was no observed variation in these stocks in fine roots. The content of P, C, and N in fine roots and leaves in evergreen broadleaf species exceeded those in deciduous species. Elevated O₃ significantly reduced CS_leaf, NS_leaf, and PS_leaf in deciduous broadleaf species, whereas there was a significant reduction for the same in evergreen species. The literature analysis further demonstrated a larger O₃-induced increment in leaf P concentration in deciduous species as compared to evergreen species. Elevated O₃ significantly increased the difference in C and N stocks between fine roots and leaves in deciduous broadleaf species, whereas this difference was observed to decrease in evergreen species. The results of this study can facilitate an improved understanding of ecological stoichiometric responses of urban greening tree species under O₃ stress and the resulting nutrient use strategies.     

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.     

Water relations and keeping-quality of cut gerbera flowers. IV. Internal water relations of ageing petal-tissue

The relationship between relative water content (R.W.C.) and water potential (ψ_total) of Gerbera petals was studied when cut inflorescences aged in a vase. The regression of sap osmotic potential (ψ_osm), matric potential (ψ_m) and pressure potential (ψ_press) on R.W.C. appeared to be the same for young and old inflorescences.Computing ψ_press as the difference between ψ_total measured with a pressure chamber and ψ_osm + ψ_m gave erroneous results for old cut inflorescences because of the increasing leakage of ions from petal cells. As there existed a correlation between ψ_press and the bending of petals (B) caused by a small weight, it was possible to obtain time courses of ψ_press by measuring B.Time courses of ψ_osm and ψ_press of petals were completely different when inflorescences aged on the plant as compared to ageing in a vase. For inflorescences ageing in a vase without stem plugging, ψ_osm increased during the first 6 days, followed by a decrease, and ψ_press decreased during the entire vase period. When inflorescences were left on the plant, ψ_osm was steady during the first 6 days and increased thereafter, whereas ψ_press was steady until day 6 and then decreased. A hypothesis to explain the difference between inflorescences in the vase and on the plant is given.     

Water relations of pistachio (Pistacia vera L.) as affected by phenological stages and water regimes

Pistachio is a drought tolerant fruit tree that can be cultivated in rainfed and irrigated conditions. The water requirements of the tree, however, are considerable so in most of the commercial orchards deficit irrigation is a common practice. Regulated deficit irrigation in pistachio trees has been described in several works, which reported that the phenological stage of shell hardening, so called stage II, is the most drought tolerant. This paper proposes that such drought resistance is related to changes in water relations linked to the phenological stages, even in conditions of no water stress. In order to evaluate such changes, the daily pattern of stem water potential and gas exchange (net photosynthesis, P_n, and leaf conductance, g_s) was measured, determining also the pressure–volume curves, in three different phenological stages of mature pistachio trees (Pistacia vera cv Kerman on P. terebinthus L. rootstock.). The daily pattern of stem water potential and gas exchange were performed in three different irrigation treatments: control, regulated deficit irrigation and rainfed. The pressure–volume curves were made only in the control and rainfed treatments. Significant differences were found in the daily pattern of stem water potential in all the phenological stages considered, while only in the last one the net photosynthesis was affected by water stress. The daily pattern of gas exchange at the beginning of the season was not affected by the evaporative demand, with a constant value when radiation was not limiting. Moderate levels of water stress during the last measurement date reduced the maximum values of g_s and P_n resulting also in a clear change in the pattern of the daily curve, with maximum values only at the beginning of the day. The relationships between stem water potential and gas exchange parameters were different during stage II and almost the same in stages I and III. The parameters drawn from the pressure–volume curves also indicated a change in the elastic modulus of the leaf cells in stage II. In addition, differences in the osmotic adjustment (OA) index suggested different degree of osmotic adjustment of the phenological stages in the response to water stress. The results showed that different mechanisms of drought resistance are operating in the different phenological stages in pistachio trees.     

Citrus rootstock responses to water stress

Tolerance to drought-stress (DS) of the citrus rootstock Forner–Alcaide no. 5 (FA-5) was tested and compared with that of its parents, Cleopatra mandarin (CM) and Poncirus trifoliata (PT). Nine-month-old seedlings of CM, PT and FA-5 and 15-month-old grafted trees of ‘Valencia’ orange scions on these three rootstocks were cultivated in sand under glasshouse conditions and irrigated with a nutrient solution. Plants were drought-stressed by withholding irrigation until leaves were fully wilted. Survival time of both seedlings and grafted trees under DS was linked to the water extraction rate from the soil, which depended mainly on leaf biomass and on transpiration rate. Seedling responses to DS affecting leaf water relationships and gas exchange parameters varied among genotypes. FA-5 seedlings survived longer than the other seedlings, maintaining the highest levels of water potential, stomatal conductance, transpiration rate and net CO₂ assimilation towards the end of the experiment, when water stress was most severe. Thus, FA-5 was more resistant to DS than its parents (CM and PT). Moreover, rootstock affected the performance of grafted trees under water stress conditions. The higher drought tolerance induced by FA-5 rootstock could be related to the greater osmotic adjustment (OA), which was reflected by smaller reductions in leaf relative water content (RWC) and in higher turgor potentials and leaf gas exchange than the other rootstocks.     

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.