Effects of moderate shade and irrigation with eutrophicated water on the nitrogen economy of Mediterranean oak seedlings

We evaluated the effects of moderate shade (43% vs. 100% of full sunlight) and irrigation with eutrophicated river water (daily vs. alternate-day watering) on growth and nitrogen economy of seedlings of three Mediterranean oak species, two evergreen (Quercus coccifera, Quercus ilex subsp. ballota) and a deciduous (Quercus faginea), grown in pots outdoors. Seedling biomass, N pool, N concentration and N losses by litter fall were measured at the beginning (March 2002) and end (November 2002) of a growing season. All species showed an increase of biomass and N pool under shade and/or high irrigation, while only Q. coccifera – from more arid regions – did the same under full sunlight and low irrigation. At the end of the experiment, biomass of the evergreens was higher in shade than in sun, and in high than in low irrigation, while Q. faginea – from more humid zones – responded to irrigation only. Shade-induced growth was accompanied by a decline in N concentration in the evergreens, but irrigation reduced N concentration only of Q. faginea. Shade, but not irrigation, reduced above-ground N loss. We conclude that both treatments differentially affected the evergreen and the deciduous oaks, probably due to differences in plant hydraulic and stomatal conductance. Although both treatments have similar effects on the growth of evergreens, they produced different effects on seedling N economy, which may have important consequences on future field seedling performance.     

Drought stress effects on three cultivars of Eragrostis curvula: photosynthesis and water relations

Water stress effects were studied on three cultivars ofEragrostis curvula. Leaf water potential, RWC, total plantleaf area, green dry weight mass percentage and CO₂ gas-exchangeweremeasured during the onset of stress and after recovery. After 3 days of waterstress, RWC of cv Tanganyika plants was around 30–40% of controls,while RWC of cvs Ermelo and Consol was around 50–60% of controls.However midday and predawn water potentials were lower in cvs Tanganyka andErmelo than in cv Consol. After re-watering, RWC and water potentials recoveredonly in Consol plants. A strong decrease of leaf area was recorded in cvsErmeloand Consol during water stress (about 91–94% less than the leafarea of controls). Photosynthesis decreased as a function of the degree ofwaterstress severity in all cultivars. Also, light saturated photosynthesis,CO₂ quantum yield and light at which saturated photosynthesisoccurred, were strongly reduced by water stress. Recovery of photosynthesis wasfound in cv Consol after five days re-watering. Cv Consol showed a betterconservation of water and higher resistance to water stress than the other twocvs.     

Changes in leaf δC and δN for three Mediterranean tree species in relation to soil water availability

A rain exclusion experiment simulating drought conditions expected in Mediterranean areas for the following decades (15% decrease in soil moisture) was conducted in a Mediterranean holm oak forest to study the response of leaf δ¹³C, δ¹⁵N, and N concentrations to the predicted climatic changes for the coming decades. Plant material was sampled in 2000, 2003, 2004, and 2005 in eight plots: four of them were control plots and the other four plots received the rain exclusion treatment. Although there was a negative relationship between δ¹³C and soil moisture, for each species and year, the rain exclusion treatment did not have any significant effect on δ¹³C, and therefore on the intrinsic water use efficiency (iWUE) of the three dominant species: Phillyrea latifolia, Arbutus unedo, and Quercus ilex. On the other hand, rain exclusion clearly increased the δ¹⁵N values in the three species studied, probably indicating higher N losses at the soil level leading to a ¹⁵N enrichment of the available N. It suggested that rain exclusion exerted a greater effect on the nitrogen biogeochemical cycle than on the carbon assimilation process. δ¹⁵N values were inversely correlated with summer soil moisture in Q. ilex and A. unedo, but no relationship was observed in P. latifolia. This latter species showed the lowest iWUE values, but it was the only species with no decrease in annual basal increment in response to the rain exclusion treatment, and it also had the highest resistance to the hot and dry conditions projected for the Mediterranean basin in the coming decades. The different strategies to resist rain exclusion conditions of these species could induce changes in their competitive ability and future distribution. The losses of N from the ecosystem may further limit plant growth and ecosystem functioning.     

Quercus ilex facing water stress: a functional equilibrium hypothesis

A boundary line analysis of the scatterplot relating pre-dawn leaf water potential to pre-dawn minus minimum leaf water potential was applied to study the efficiency of the Mediterranean evergreen oak species to buffer the effects of variability in water resources. The results are discussed in term of stomatal regulation and changes in soil-to-leaf hydraulic conductance of water flow, both induced by changes in leaf water potential. For this purpose, we used data for leaf water potential measured in three stands of Quercus ilex across a soil water availability gradient in Southern France, and two others obtained in California and Arizona for Quercus turbinella and Portugal for Quercus coccifera. A classification of plant responses from mesic to xeric sites is proposed.     

Plant morphology and root hydraulics are altered by nutrient deficiency in Pistacia lentiscus (L.)

The plants in arid and semiarid areas are often limited by water and nutrients. Morpho-functional adjustments to improve nutrient capture may have important implications on plant water balance, and on plant capacity to withstand drought. Several studies have shown that N and P deficiencies may decrease plant hydraulic conductance. Surprisingly, studies on the implications of nutrient limitations on water use in xerophytes are scarce. We have evaluated the effects of strong reductions in nitrogen and phosphorus availability on morphological traits and hydraulic conductance in seedlings of a common Mediterranean shrub, Pistacia lentiscus L.. Nitrogen deficiency resulted in a decrease in aboveground biomass accumulation, but it did not affect belowground biomass accumulation or root morphology. Phosphorus-deficient plants showed a decrease in leaf area, but no changes in aboveground biomass. Root length, root surface area, and specific root length were higher in phosphorus-deficient plants than in control plants. Nitrogen and phosphorus deficiency reduced both root hydraulic conductance and root hydraulic conductance scaled by total root surface area. On the other hand, nutrient limitations did not significantly affect root conductance per unit of foliar surface area. Thus, adaptation to low nutrient availability did not affect seedling capacity for maintaining water supply to leaves. The implications for drought resistance and survival during seedling establishment in semi-arid environments are discussed.     

Spatial pattern of seedlings 1 year after fire in a Mediterranean pine forest

Summary The spatial distribution of seedlings of the dominant perennial plant species (Pinus halepensis, Cistus salviifolius, Rhus coriaria) and may annual species was studied after a wild fire in an eastern Mediterranean pine forest. The spatial distribution of all seedlings is affected by the location of the old burned pine trees. Seedling density of Pinus and Cistus is higher at a distance from the burned pine canopy and lower near the burned pine trunk. It is also higher beneath small burned pine trees than under big ones. Rhus seedling density is higher under big burned pine trees and also near the burned trunks. Seedlings of Pinus, Cistus and Rhus growing under the burned canopy of big pine trees tend to be taller than seedlings under small ones or outside the burned canopy. Most annual species germinate and establish themselves outside the burned canopies, and only a few annual species are found beneath them. It is suggested that variation in the heat of the fire, in the amount of ash between burned pine trees of different sizes, and in the distance from the burned canopy are responsible for the observed pattern of seedling distribution. The possible ecological significance of the spatial pattern of seedlings distribution and their differential growth rate are discussed.     

Seasonal changes in net photosynthesis rates and photosynthetic capacity in leaves of Cistus salvifolius,a European mediterranean semi-deciduous shrub

Summary During five different periods between Nov. 1982 and Aug. 1983, the diurnal patterns exhibited in photosynthetic CO₂ uptake and stomatal conductance were observed under natural conditions on twigs of Cistus salvifolius, a Mediterranean semi-deciduous shrub which retains a significant proportion of its leaves through the summer drought. During the same periods, net photosynthesis at saturating CO₂ partial pressure was measured on the same twigs as a function of irradiance at different temperatures. From these data, photosynthetic capacity, defined here as the CO₂- and light-saturated net photosynthesis rate, was obtained as a function of leaf temperature. C. salvifolius is a winter growing species, shoot growth being initiated in Nov. and continuing through May. Photosynthetic capacity was quite high in Nov., March and June, exceeding 40 mol m^-2 s^-1 at optimum temperature. In Dec., photosynthetic capacity was somewhat reduced, perhaps due to low night-time temperatures (<5°C) during the measurement period. In Aug., capacity in oversummering shoots at optimum temperature fell to less than 8 mol m^-2 s^-1, due to water trees and perhaps leaf aging. Seasonal changes in maximal photosynthetic rates under ambient conditions were similar, and like those found in co-occurring evergreen sclerophylls. Like the evergreens, Cistus demonstrated considerable stomatal control of transpirational water loss, particularly in oversummering leaves. During each measurement period except Aug. when capacity was quite low, the maximum rates of net photosynthesis measured under ambient conditions were less than half the measured photosynthetic capacities at comparable temperatures, suggesting an apparent excess nitrogen investment in the photosynthetic apparatus.     

Resprouting vigour of two mediterranean shrub species after experimental fire treatments

Experimental fire treatments were carried out by applying a propane torch flame to individual stumps ofArbutus unedo andErica arborea, two dominant ericaceous shrubs living in relatively moist maquis of the western Mediterranean Basin. No mortality was observed in either species. Individual plant size, measured as the individual stump area, was the most important factor determining both number and biomass of resprouts for all fire treatments. The number of resprouts at 3 months was less at the higher temperature, but no difference could not be detected at 18 months. Duration of flame application had no effect on resprouting success. This effect was not statistically significant 18 months after treatments were applied. Biomass of resprouts was not clearly affected by different fire treatments either 3 months or 18 months after the start of the resprouting process.     

Comparison of proline accumulation in two mediterranean shrubs subjected to natural and experimental water deficit

The effect of water stress on proline accumulation was tested in two contrasted species of Mediterranean scrub: Halimium halimifolium (L.) Willk and Pistacia lentiscus L. Leaf water potential, stomatal resistance and proline content have been measured both in experimental and in natural water stress conditions. Both species accumulated proline in their leaves when leaf water potential dropped below a threshold value of –3.0 MPa, under natural as well as under experimental conditions. In the field, however, a time-lag between decrease of leaf water potential and proline accumulation could be observed. In Halimium halimifolium, proline accumulation appeared to be associated with severe stress conditions as most plants with high proline contents suffered irreversible wilting, especially in the greenhouse. P. lentiscus showed a different pattern, accumulating proline at two different times of the year, as a response to cold or to drought. The results of our study indicated that the role of proline in this species, rather than an osmotic agent, seems to be more related to a protective action in cases of severe stress conditions.     

Growth response of barley and tomato to nitrogen stress and its control by abscisic acid,water relations and photosynthesis

Barley (Hordeum vulgare L.) and tomato Lycopersicon esculentum Mill.) were grown hydroponically and examined 2, 5, and 10 d after being deprived of nitrogen (N) supply. Leaf elongation rate declined in both species in response to N stress before there was any reduction in rate of dryweight accumulation. Changes in water transport to the shoot could not explain reduced leaf elongation in tomato because leaf water content and water potential were unaffected by N stress at the time leaf elongation began to decline. Tomato maintained its shoot water status in N-stressed plants, despite reduced water absorption per gram root, because the decline in root hydraulic conductance with N stress was matched by a decline in stomatal conductance. In barley the decline in leaf elongation coincided with a small (8%) decline in water content per unit area of young leaves; this decline occurred because root hydraulic conductance was reduced more strongly by N stress than was stomatal conductance. Nitrogen stress caused a rapid decline in tissue NO ₃ ^- pools and in NO ₃ ^- flux to the xylem, particularly in tomato which had smaller tissue NO ₃ ^- reserves. Even in barley, tissue NO ₃ ^- reserves were too small and were mobilized too slowly (60% in 2 d) to support maximal growth for more than a few hours. Organic N mobilized from old leaves provided an additional N source to support continued growth of N-stressed plants. Abscisic acid (ABA) levels increased in leaves of both species within 2 d in response to N stress. Addition of ABA to roots caused an increase in volume of xylem exudate but had no effect upon NO ₃ ^- flux to the xylem. After leaf-elongation rate had been reduced by N stress, photosynthesis declined in both barley and tomato. This decline was associated with increased leaf ABA content, reduced stomatal conductance and a decrease in organic N content. We suggest that N stress reduces growth by several mechanisms operating on different time scales: (1) increased leaf ABA content causing reduced cell-wall extensibility and leaf elongation and (2) a more gradual decline in photosynthesis caused by ABA-induced stomatal closure and by a decrease in leaf organic N.Abbreviation and symbols ABA abscisic acid - c_i leaf internal CO₂ concentration - L_p root hydraulic conductance     

Field water relations of a wet-tropical forest tree species,Pentaclethra macroloba (Mimosaceae)

Summary The water relations of Pentaclethra macroloba (Willd.) Kuntze, a dominant, shade-tolerant, tree species in the Atlantic lowlands of Costa Rica, were examined within the forest canopy. Pressure-volume curves and diurnal courses of stomatal conductance and leaf water potential were measured in order to assess differences in water relations between understory, mid-canopy and canopy leaves. Leaves in the canopy had the smallest pinnules but the largest stomatal frequencies and stomatal conductances of the three forest levels. Osmotic potentials at full turgidity decreased with height in the forest; in the canopy and midcanopy they were reduced relative to those in the understory just enough to balance the gravitational component of water potential. Consequently, maximum turgor pressures were similar for leaves from all three canopy levels. Bulk tissue elastic modulus increased with height in the canopy. Leaf water potentials were lowest in the canopy and highest in the understory, even when the gravitational component was added to mid-canopy and canopy values. As a result, minimum turgor pressures were also lowest in the canopy compared to those at lesser heights, and approached zero in full sunlight on clear days.Osmotic potentials at each canopy level were similar for both wet and dry season samples dates suggesting that seasonal osmotic adjustment does not occur. Despite lowered predawn water potentials during the dry season, turgor was maintained in the understory by reduced stomatal conductances.     

Comparative water relations of four Mediterranean oak species

The water relations and responses of two evergreen (Quercus ilex L. and Q. suber L.) and two deciduous (S. afares Pomel. and Q. faginea Will.) Quercus species were studied under experimental conditions. Two-year old seedlings grown in 30 l. pots were subjected to a drying period during which stomatal conductance, pre-dawn potential and minimum foliar potential were measured.The results shows that, for all species, the daily course of stomatal conductance agrees with the patterns proposed by Hinckley et al. (1978 & 1983). Concurrent with the species responses to short-term variation in water availability, it was found that pre-dawn leaf water potential controlled the maximum daily leaf conductance. There was a strong correlation between pre-dawn leaf potential and maximum daily conductance as described by the reciprocal function g_{srmax for}=(-0.47+2.61._p)^-1 the evergreen oaks and g_{srmax for}=(-1.94+7.39._p)^-1 for the deciduous species. These differences between the two groups may partialy explain their geograhic distributions, and suggest general questions concerning the mechanisms which optimize water-use efficiency in Mediterranean oak species.     

Daily variations in water relations of apricot trees under different irrigation regimes

Mature apricot (Prunus armeniaca L. cv. Búlida) trees, growing under field conditions, were submitted to two drip irrigation treatments: a control (T1), irrigated to 100 % of seasonal crop evapotranspiration (ETc), and a continuous deficit (T2), irrigated to 50 % of the control throughout the year. The behaviour of leaf water potential and its components, leaf conductance and net photosynthesis were studied at three different times during the growing season, when they revealed a diurnal and seasonal pattern in response to water stress, evaporative demand of the atmosphere and leaf age. The deficit-irrigated trees showed, among other effects, a pronounced decrease in leaf water potential (ψ_w), decreased in leaf conductance (g_s) and no osmotic adjustment. For this reason, g_l and ψ_w can be considered good indicators of mature apricot tree water status and can therefore be used for irrigation scheduling.     

Flooding and drought tolerance in seeds and seedlings of two Mora species segregated along a soil hydrological gradient in the tropical rain forest of Guyana

Mora excelsa and M. gonggrijpii are well segregated along a soil hydrological gradient. M. excelsa is positively associated with soil hydromorphic characteristics such as gley, mottling and groundwater within 1.20 m, whereas M. gonggrijpii is negatively associated with these characteristics. Growth and mortality of artificially installed seedlings were studied in both species in occasionally flooded forest and dryer uphill forest. In a moderate year (1992, no pronounced flooding, no drought), there was no difference between the two species in growth or mortality in the two forest types. M. gonggrijpii was larger in both forest types. Flooding tolerance of seeds and seedlings were studied under controlled water regimes. Seeds of M. gonggrijpii appeared to be very intolerant to flooding, since germination in this species dropped to 50% after only 11 days of flooding. Seeds of M. excelsa floated and 80% of the seeds were viable after as much as 50 days of flooding. Artificially submerged seeds of the latter species had an intermediate survival response. Flooding in seedlings resulted in cessation of growth in both species. Mortality was nil in most treatments, but all M. gonggrijpii individuals died after a treatment of 8 weeks of continuous flooding. Drought tolerance of seedlings of M. excelsa and M. gonggrijpii was studied in a drying-out experiment. Seedlings of both species of approximately equal size differed widely in a number of characteristics: total leaf area, leaf dry weight, leaf thickness, leaf size and leaf area ratio (LAR) were all larger in M. gonggrijpii, while stomatal density and specific leaf area (SLA) were smaller in this species. Seedlings did not differ in stem hydraulic conductivity. M. excelsa showed lower osmotic potential at full hydration. Turgor potential loss points were not nearly approached in the forest during the middle of the dry season in either species. M. gonggrijpii had much lower stomatal conductance than M. excelsa, due to lower stomatal density. Boundary layer conductance was of the same magnitude as stomatal conductance, especially in the morning. In a drying-out experiment, total plant transpiration was higher in M. gonggrijpii, as the lower conductance observed in this species was compensated for by its larger leaf area. M. gonggrijpii was able to extract water from dryer soils than M. excelsa and may be able to utilize its higher leaf water content under moderate drought in the forest understorey.     

Role of water relations and photosynthesis in the release of buds from apical dominance and the early reinvigoration of decapitated poplars

A decrease in xylem pressure potential starting 1 h after decapitation of young hybrid poplars ( Populus deltoides Bartr. × Populus nigra L. cv. DN22) reduced stomatal conductance and transpiration rates for the first 3 days after decapitation. This early moisture stress was alleviated 4 to 5 days after decapitation, resulting in substantial increases in stomatal aperture, transpiration and net photosynthetic rates which continued for the remainder of the one week measurement period. The results suggest the following sequence of events in the decapitated plant: After a brief moisture stress, decapitation increases moisture availability by increasing the root/shoot ratio and by reducing shoot competition for moisture. Improvement in hydration releases buds from apical dominance and increases stomatal conductance and rates of net photosynthesis. This, in turn, leads to the acceleration of growth observed when plants are reinvigorated by decapitation.     

狭叶红景天幼苗对水分及遮阴的生长及生理生化响应

研究植物对水分和遮阴胁迫的响应及其生理机制对制定合理的栽培管理措施十分必要。以红景天属植物为研究对象,设置土壤含水量分别为田间持水量的80%(过湿水分)、70%(正常水分)、60%(轻度干旱)、40%(中度干旱)、20%(重度干旱)5个水分梯度;设置2个遮阴处理,以全光照(遮阴率为0)为对照、黑色遮阴网遮阴(遮阴率为85%),研究狭叶红景天生长及生理生化指标的变化特征。结果表明:在不同水分处理下,与对照相比,叶绿素含量、茎干重和茎重比(SMR)显著增加(P0.05),株高、总生物量、叶面积、叶干重、叶重比(LMR)、比叶面积(SLA)、叶面积比(LAR)和叶面积根干重比(LARMR)增加,根冠比和根重比(RMR)减少;随着干旱程度加剧,丙二醛(MDA)、脯氨酸(Pro)和可溶性糖(Ss)含量增加,超氧化物歧化酶(SOD)活性总体呈先增加后减小的趋势。在遮阴处理下,株高、SMR、SLA、LAR和LARMR显著增加(P0.05),叶绿素SPAD值和叶面积增加,总生物量、根干重、根冠比和LMR显著减少(P0.05),茎干重和叶干重减少,MDA含量显著增加,Pro含量略有下降,Ss含量减少。在水分胁迫下,狭叶红景天中度干旱时通过增加酶活性抵御伤害,重度干旱超过其阈值,SOD活性下降,植物体受到伤害,Ss可能是主要的渗透调节物质。在遮阴处理下,狭叶红景天通过增加SLA避免遮阴伤害。狭叶红景天在受到环境胁迫时会通过形态改变、调节MDA含量、抗氧化酶活性和渗透调节物质来保证自身正常的生长发育。     

Growth and nitrogen fixation ofAeschynomene under water stressed conditions

Summary Studies on the tolerance ofAeschynomene americana L. to periods of flooding or soil moisture deficit were conducted in an attempt to elucidate nitrogen fixation as affected by soil moisture. Nitrogenase activity was not reduced significantly in pot-grown Aeschynomene plants subjected to flooding in greenhouse conditions. After 20 days of withholding water from the soil, nitrogenase activities of the drought-stressed plants were much lower than those of either the well-watered or flooded plants. Leaf water potentials were similar in flooded and control plants; however, the droughted plants had leaf water potentials that were 4 bars lower than those of the control plants. Aeschynomene plants were tolerant to long-term periods of flooding, but exhibited a reduction in nitrogenase activity and leaf water status when subjected to soil moisture deficits.     

Soil water dynamics,transpiration, and water losses in a crested wheatgrass and native shortgrass ecosystem

The status of water in soil and vegetation was monitored in a stand of crested wheatgrass (Agropyron cristatum) and a nearby shortgrass steppe during a growing season. This was done to determine if water use and losses were similar among two very different communities in a similar climate. Precipitation was similar throughout the study period for both the crested wheatgrass and native shortgrass communities. However, the native shortgrass community with greater root biomass had consistently greater soil water depletion in the deeper soil horizons than was found in the crested wheatgrass community. Greater depletion of soil water by native shortgrass species suggests that they might be more competitive than crested wheatgrass in a water-limited environment.Crested wheatgrass maintained high leaf water potential early in the season, but lower water potential during the latter part of the growing season as compared with the major species of the shortgrass steppe, blue grama (Bouteloua gracilis) and western wheatgrass (Agropyron smithii). Leaf conductance was lower for crested wheatgrass than for the native grasses during the later part of the growing season. Consequently, seasonal transpiration for crested wheatgrass was lower when compared with blue grama or western wheatgrass. Lower conductance allowed crested wheatgrass to maintain relatively high internal water potential and may have accounted for less soil water use at deeper soil depths during the latter part of the growing season.Water loss through transpiration was less for western wheatgrass than for either blue grama or crested wheatgrass because western wheatgrass had less leaf area. However, western wheatgrass was as efficient as the other species in its use of water. Crested wheatgrass transpired more water than blue grama early in the growing season, but less than either native species for the remainder of the growing season. Estimated seasonal transpiration loss was greater in the shortgrass ecosystem than in the established crested wheatgrass stand.     

Canopy structure within a Quercus ilex forested watershed: variations due to location,phenological development,and water availability

Spatial and temporal changes in canopy structure were studied in 1988 and 1989 in a Mediterranean Quercus ilex forest in north-eastern Spain. Due to differences in precipitation patterns the 1989 growing season was drier than the 1988 growing season. Sampling was conducted in parallel at two sites which represent endpoints along a slope gradient within a watershed (ridge top at 975 m, and valley bottom at 700 m). At both sites, similar inter-annual changes in canopy structure were observed in response to differences in water availability. Samples harvested in the upper 50 cm of the canopy during 1989 exhibited a decrease in both average leaf size and the ratio of young to old leaf and stem biomass relative to samples obtained in 1988. At the whole canopy level, a decrease in leaf production efficiency and an increase in the stem to leaf biomass ratio was observed in 1989. Temporal changes in canopy leaf area index (LAI) were not statistically significant. Average LAI values of Q. ilex at the two sites were not significantly different despite differences in tree stature and density (4.6 m² m^–2 at the ridge top, and 5.3 m² m^–2 at the valley bottom). Vertical distribution of leaves and stems within the canopy was very similar at the two locations, with more than 60% of the total LAI in the uppermost metre of the canopy. The possible significance of such an LAI distribution on the canopy carbon budget is discussed.