Reduced Water Potential Research Articles

Functional and evolutionary correlations of steep leaf angles in the mexical shrubland

In the evergreen shrubland vegetation of Mexico (mexical), most of the species are sclerophyllous woody plants with steep leaf angles. This architectural pattern has been interpreted as a strategy to cope with water shortages and high radiation. However, the current association between evergreenness and steep leaf angles across mexical plant species could be the result of an adaptive association achieved through correlated evolutionary change between both traits or, alternatively, may be the result of common evolutionary ancestry. In this study, we quantified leaf angle in 28 dominant species under a phylogenetic framework and evaluated the functional implications of the observed range of leaf angles in terms of leaf temperature, water potentials and transpiration by combining manipulative experiments restraining leaves horizontally with microclimatic and stomatal conductance measurements in selected species and energy balance calculations. Horizontally restrained leaves exhibited reduced water potentials and stomatal conductances, and significantly increased temperatures and transpiration rates. Steeply inclined leaves operated near air temperatures and could sustain relatively high stomatal conductances during the dry season since they were associated with low transpiration rates. Phylogenetic analyses showed that steep leaf angles evolved in a correlated fashion in evergreen species. The functional consequences of leaf angle together with the phylogenetic analysis indicate the adaptive nature of this trait which allows the evergreen species to cope with arid conditions and therefore to persist within the mexical community.

THE ROLE OF ANTIOXIDANTS IN INDUCING WHEAT FLAG LEAF OSMOTIC ADJUSTMENT UNDER SALINITY STRESS

Salt stress up to 11.5 dSm-1 causes a significant reduction in water potential, osmotic potential, as well as relative water content, and water content. On the other hand, turgor potential and osmotic adjustment were significantly increased due to inducing increasing the higher accumulation of compatible osmolytes which leads to decreasing flag leaf area and grain yield per plant. Application of both antioxidants, in particular, ascorbic acid increased significantly flag leaf area, and grain yield per plant due to osmotic adjustment and maintaining leaf turgor potentials as a consequence of increasing leaf water potential, water content and relative water content as compared to control plants. On the other hand, application of both antioxidants under all salinity levels, nullify the harmful effect of salinity effects on flag leaf area and grain yield per plant due to increasing osmolyte accumulation, maintaining turgor potential and osmotic adjustments which in turn increasing flag leaf area and grain yield per plant. Anatomically, increasing salinity levels decreased thickness of leaf blade at midrib region, thickness of mesophyll tissue, tangential dimension of midrib vascular bundle, thickness of upper epidermis, thickness of lower epidermis, thickness of big motor cell, and tangential dimension of big xylem vessel. Treatment with either ascorbic acid or tocopherol at 100 mg/L and their interactions with salinity increased all the above mentioned parameters in both nonsalinized and salinized plants. Ascorbic acid is the most effective in this concern. In conclusion, wheat plants responded to an increased ion influx in their cells by increasing the osmolytes synthesis and accumulation under salt stress, which further increased with antioxidants treatment and helped in maintaining the osmotic balance.

Critical conditions for successful regeneration of an endangered annual plant, Cryptantha minima: A modeling approach

Tiny cryptanthe ( Cryptantha minima Rydb.) is an endangered annual plant adapted to prairie sand dunes of Canada. Thermal time and hydrothermal time models were used to quantify the effect of temperature and water potential on seed germination and to relate it to field conditions. Tiny cryptanthe seeds had a base temperature of −3.9 °C for germination. Final germination percentage was highest at temperatures near the freezing point. Seeds required approximately 130.6 °C d to reach 50% germination under optimal water conditions. Seed germination was sensitive to water stress and large seeds had lower germination than small ones especially at reduced water potentials and higher temperatures. The high potential germination and low seed dormancy indicate that tiny cryptanthe lacks a dormancy mechanism, but the unique germination response to temperature enables it to fully utilize water from spring snowmelt before drought in the summer. Future climate change with higher temperatures could reduce regeneration success of this species unless there is an associated increase in winter and spring precipitation.

Differences in ecophysiological responses to summer drought between seedlings of three deciduous oak species

Seasonal variations in water status and gas exchange were assessed in 2-year-old seedlings of Quercus pubescens Willd., Quercus ithaburensis Decaisne subsp. macrolepis (Kotsch) Hedge & Yalt, and Quercus frainetto Ten. during the 2004 growing season characterized by the typical summer drought conditions observed in Greece. The seedlings were grown under field conditions. After establishment half of the seedlings were well watered, while the rest received no irrigation other than natural rainfall. Measurements were conducted from April to October. Progressive drought reduced water potentials and gas exchange in all species. However, Q. ithaburensis displayed significantly higher water and osmotic potentials and assimilation rate than the other species. In contrast, non-watered plants of Q. frainetto exhibited the lowest leaf conductance and finally leaf loss after predawn water potentials dropped to as low as −5.8 MPa. Predawn water potentials were affected by the temperature of the day before measurements and afternoon water potentials of stressed plants were controlled by humidity. Stomatal conductance correlated positively with water potentials of non-watered seedlings of Q. pubescens and Q. frainetto. The results suggested that water potentials in Q. ithaburensis appeared to be independent of stomatal regulation. Based on interspecific variations in water relations and gas exchange patterns under drought, Q. ithaburensis exhibits drought-avoidance characteristics and seems the most suitable candidate among studied Quercus species for growth in xerothermic environments. Despite its substantially lower water potentials Q. pubescens also shows drought resistance potential, hence suitability for reforestation purposes by recovering to pre-drought patterns, which could indicate desiccation tolerance response promoted by osmotic adjustment. Finally, Q. frainetto appears to be most prone to summer drought.

The impact of salt stress on the water status of barley plants is partially mitigated by elevated CO 2

With the changing climate, plants will be facing increasingly harsh environmental conditions marked by elevated salinity in the soils and elevated concentrations of CO 2 in the atmosphere. These two factors have opposite effects on water status in plants. Therefore, our objective was to determine the interaction between these two factors and to determine whether elevated [CO 2] might alleviate the adverse effects of salt stress on water status in two barley cultivars, Alpha and Iranis, by studying their relative water content and their water potential and its components, transpiration rate, hydraulic conductance, and water use efficiency. Both cultivars maintained their water status under salt stress, increasing water use efficiency and conserving a high relative water content by (1) reducing water potential via passive dehydration and active osmotic adjustment and (2) decreasing transpiration through stomatal closure and reducing hydraulic conductance. Iranis showed a greater capacity to achieve osmotic adjustment than Alpha. Under the combined conditions of salt-stress and elevated [CO 2], both cultivars (1) achieved osmotic adjustment to a greater extent than at ambient [CO 2], likely due to elevated rates of photosynthesis, and (2) decreased passive dehydration by stomatal closure, thereby maintaining a greater turgor potential, relative water content, and water use efficiency. Therefore, we found an interaction between salt stress and elevated [CO 2] with regard to water status in plants and found that elevated [CO 2] is associated with improved water status of salt-stressed barley plants.

Germination and radicle growth in unprimed and primed seeds of sweet sorghum as affected by reduced water potential in NaCl at different temperatures

The effects of reduced water potential ( ψ) in NaCl and pre-osmopriming in PEG, on seed germination and early radicle growth at different temperatures were assessed in the laboratory for sweet sorghum [ Sorghum bicolor (L.) Moench] cv. Roce. Five salt solutions (with a ψ of 0-control, −0.22, −0.42, −0.82 or −1.23 MPa) and four temperatures of 10, 15, 25 or 35 °C were used for germination tests. Daily and final germination, seed moisture at 4 and 24 h of imbibition and radicle length 2 days after initial germination were recorded. Sorghum seeds seem to be more tolerant to reduced ψ at optimum temperature. Indeed, at 25 °C, down to −0.82 MPa seed germination was not affected. The increase in incubation temperature to 35 °C determined a greater seed water content and a faster germination than at 25 °C at all ψ s, despite the lower final germination percentage. With the lowering of temperature to 15 °C, germination percentage significantly declined with the decrease of ψ and at −0.82 MPa it reduced to less than 50%. At 10 °C seeds failed to germinate at ψ < −0.22 MPa due to combined depressive effects of reduced ψ and low temperature. Seed priming enhanced germination and shortened the delay in germination time due to the increase in saline stress, at suboptimal temperatures only. This effect could be explained by the faster water absorption occurring in primed seeds as compared to those unprimed, at these temperatures, regardless of salt concentration of the solution. Root growth was more sensitive than germination to salt stress. Indeed, the increase in NaCl concentration adversely affected root elongation at all temperatures. The beneficial effects of PEG-osmopriming were evident on root growth at all temperatures except the optimal one. From a practical point of view, when early sowings of sweet sorghum are requested, the use of primed seeds is suggested, as PEG-osmopriming is helpful in overcoming the negative effect imposed by reduced water potential upon seed germination under suboptimal thermal conditions. When saline water is available for irrigation at sowing, it may be useful to sow when optimal or nearly optimal temperature may be encountered in the soil in order to avoid the combined stress of temperature and salinity.

Germination responses to water potential in neotropical pioneers suggest large-seeded species take more risks.

In neotropical forests, very small-seeded pioneer species (<0.1 mg seed mass) recruit preferentially in small tree fall gaps and at gap edges, but large-seeded pioneers do not. Since water availability is related to gap size, these differences in microsite preference may reflect in part species-specific differences in germination at reduced water potentials. For 14 neotropical pioneer species, the hypothesis is tested that small-seeded species, with shallow initial rooting depths, reduce the risks associated with desiccation by germinating more slowly and at higher water potentials than large-seeded species. Germination occurred both more quickly and at lower water potentials with increasing seed mass. For example, Ochroma pyramidale (seed mass 5.5 mg) had a time to 50 % germination (T50) of 2.8 d and a median base potential for germination (psi(b50)) of -1.8 MPa while Clidemia quinquenervia (seed mass 0.017 mg) had a T50 of 17.6 d and psi(b50) of -1.1 MPa. These data suggest that small-seeded species germinate only in comparatively moist microsites, such as small canopy gaps, which may reduce the risk of drought-induced mortality. Conversely, large-seeded species are able to germinate in the drier environment of large gaps, where they benefit by enhanced seedling growth in a high irradiance environment. The positive association of seed size and canopy gap size for optimal seedling establishment is maintained by differential germination responses to soil water availability coupled with the scaling of radicle growth rate and seed size, which collectively confer greater drought tolerance on large-seeded species.

Adaptations of higher plant cell walls to water loss: drought vs desiccation

Water-deficit stress poses unique challenges to plant cells dependent on a hydrostatic skeleton and a polysaccharide-rich cell wall for growth and development. How the plant cell wall is adapted to loss of water is of interest in developing a general understanding of water stress tolerance in plants and of relevance in strategies related to crop improvement. Drought tolerance involves adaptations to growth under reduced water potential and the concomitant restructuring of the cell wall that allow growth processes to occur at lower water contents. Desiccation tolerance, by contrast, is the evolution of cell walls that are capable of losing the majority of cellular water without suffering permanent and irreversible damage to cell wall structure and polymer organization. This minireview highlights common features and differences between these two water-deficit responses observed in plants, emphasizing the role of the cell wall, while suggesting future research avenues that could benefit fundamental understanding in this area.

Growth, water status and nutrient accumulation of seedlings of Holoptelea integrifolia (Roxb.) Planch in response to soil salinity

Greenhouse experiments were conducted to assess the effects of soil salinity on emergence, growth, water status, proline content and mineral accumulation of seedlings of &lt;I&gt;Holoptelea integrifolia&lt;/I&gt; (Roxb.) Planch (&lt;I&gt;Ulmaceae&lt;/I&gt;). NaCl was added to the soil and salinity was maintained at 0.3, 3.9, 6.0, 7.9, 10.0, 12.1 and 13.9 dS/m. Salinity caused reduction in water potential of tissues, which resulted in internal water deficit to plants. Consequently, seedling growth significantly decreased with increase in soil salinity. Proline content in tissues increased with increase in soil salinity. There were no effective mechanisms to control net uptake of Na transport to shoot tissue. Potassium content increased in leaves to avoid Na toxicity to this tissue. Nitrogen content significantly increased in tissues in response to salinity. Phosphorus, calcium and magnesium content in tissues significantly decreased as salinity increased. Changes in tissues and whole-plant accumulation patterns of other nutrients, as well as possible mechanisms to avoid Na toxicity in this species in response to salinity, are discussed.

Germinação de sementes de Drymaria cordata (L.) Willd. ex Roem &amp; Schult.: efeito do potencial hídrico

Modelos matematicos baseados no conceito de graus dia (thermal-time) e Ψw dia (hydrotime) podem ser usados para a elaboracao de modelos mais gerais sobre a germinacao e emergencia de plântulas no campo, podendo ser uma importante ferramenta para estudos sobre a biologia de plantas daninhas e seu controle. Neste trabalho, avaliou-se a germinacao de sementes de D. cordata em resposta ao potencial hidrico (Ψw), usando-se o modelo Ψw dia. Tanto a germinabilidade como a velocidade de germinacao decresceram linearmente com a reducao do Ψw, atingindo valores proximos a zero em -0.8 MPa. Em temperatura otima, a taxa de queda na germinacao foi maior em comparacao com as temperaturas sub- e supra-otima. O Ψw base (Ψwb) mediano foi similar entre as temperaturas sub-otima (19 oC) e supra-otima (32 oC), mas foi maior (menos negativo) a temperatura otima (25 oC), mostrando que sementes de D. cordata sao menos sensiveis a reducao do potencial hidrico a 19 oC do que a 25 oC. O Ψw dia foi maior para sementes germinadas a temperatura sub-otima do que a temperatura otima, mostrando que a velocidade de germinacao num dado potencial hidrico e maior em temperatura otima. A quantidade de Ψw dia necessaria para a germinacao foi maior em temperatura supra-otima do que em temperatura otima, e menor em temperatura supra-otima do que em sub-otima. Em geral, Ψw dia foi relativamente constante entre as diferentes sub-populacoes. O modelo de Ψw dia pode descrever bem o efeito do potencial hidrico sobre as curvas de germinacao (porcentagem acumulada x tempo), especialmente as temperaturas sub-otima e supra-otima.

Abscisic Acid, High-Light, and Oxidative Stress Down-Regulate a Photosynthetic Gene via a Promoter Motif Not Involved in Phytochrome-Mediated Transcriptional Regulation

In etiolated seedlings, light perceived by phytochrome promotes the expression of light-harvesting chlorophyll a/b protein of photosystem II (Lhcb) genes. However, excess of photosynthetically active radiation can reduce Lhcb expression. Here, we investigate the convergence and divergence of phytochrome, high-light stress and abscisic acid (ABA) signaling, which could connect these processes. Etiolated Arabidopsis thaliana seedlings bearing an Lhcb promoter fused to a reporter were exposed to continuous far-red light to activate phytochrome and not photosynthesis, and treated with ABA. We identified a cis-acting region of the promoter required for down-regulation by ABA. This region contains a CCAC sequence recently found to be necessary for ABI4-binding to an Lhcb promoter. However, we did not find a G-box-binding core motif often associated with the ABI4-binding site in genes promoted by light and repressed by ABI4. Mutations involving this motif also impaired the responses to reduced water potential, the response to high photosynthetic light and the response to methyl viologen but not the response to low temperature or to Norflurazon. We propose a model based on current and previous findings, in which hydrogen peroxide produced in the chloroplasts under high light conditions interacts with the ABA signaling network to regulate Lhcb expression. Since the mutation that affects high-light and methyl viologen responses does not affect phytochrome-mediated responses, the regulation by retrograde and phytochrome signaling can finally be separated at the target promoter level.

Seed dormancy in red rice. XII: Population-based analysis of dry-afterripening with a hydrotime model

AbstractRed rice (Oryza sativaL.) dispersal units (florets) were dry-afterripened for 0–8 weeks and subsequently incubated at 30°C in polyethylene glycol (PEG) solutions with water potentials from 0 to − 1.6 MPa. Germination percentages and rates increased with dry-afterripening and water potential of the incubation medium. The seed population exhibited a normal distribution of base water potentials (Ψb, i.e. minimum water potential allowing germination) among individual seeds, characterized by three parameters: the hydrotime constant (θH), the mean base water potential (Ψb) and the standard deviation of the base water potential distribution (σΨb). Changes in germination during afterripening could be described by modifications of such parameters, particularlyΨb, which was employed to derive an index, DH(ARX = Ψb(ARX) − Ψb(ARN), where DH(ARX) represents a measure of dormancy of the seed population (in MPa) based on the hydrotime model,Ψb(ARX) is the mean base water potential of the seed population at any afterripening timeX, andΨb(ARN) is the mean water potential of the non-dormant (fully afterripened) population. The introduction of this index permitted interpretation of afterripening as a measurable reduction in the dormancy status of the seed, with progressive acquisition of both full germinative capacity and maximum germination rate, as anticipated by the hydrotime model. Moreover, secondary dormancy was induced proportionally to the reduction in water potential in the dark. Susceptibility to secondary dormancy induction was defined through DI(ARX), an index analogous to DH(ARX). These indices revealed that, in red rice, both breaking of primary dormancy and the inducibility of secondary dormancy followed decay kinetics with different sensitivities to the duration of dry storage.

Emergência e crescimento inicial de plantas de arroz e capim-arroz em função do nível de umidade no solo

Em anos de baixa precipitação durante o período pós-semeadura, as plantas de capim-arroz tendem a emergir primeiro que as de arroz, promovendo maior competição inicial com a cultura. O objetivo deste estudo foi avaliar o efeito do nível de umidade do solo, expresso em potenciais hídricos, no percentual e na velocidade de emergência, bem como no crescimento inicial de plantas de arroz e capim-arroz, em condições controladas. O experimento foi instalado em delineamento experimental de blocos casualizados, com quatro repetições. O solo utilizado foi Planossolo Hidromórfico eutrófico típico, para o qual foi previamente construída a curva de retenção de umidade. As unidades experimentais constaram de copos plásticos de 0,5 L, contendo exatamente 400 g de solo seco, e dos tratamentos dos seguintes potenciais hídricos (psiw): -0,03, -0,07, -0,1, -0,3 e -0,5 MPa, aos quais foram submetidas plantas de arroz e capim-arroz. Foi construída a curva de emergência para cada potencial hídrico e espécie; 20 dias após o início da emergência para cada espécie, três plantas de cada unidade experimental foram cortadas rente ao solo, sendo avaliados massa fresca e seca, conteúdo de água e emergência total. Plantas de capim-arroz foram mais eficientes na emergência sob potencial hídrico reduzido e menos eficientes no acúmulo de massa no início do desenvolvimento. As plantas não apresentaram estresse com potencial hídrico reduzido, e a faixa de umidade ótima para emergência do arroz foi mais estreita que para capim-arroz, situando-se ao redor de -0,07 MPa.

Diurnal and seasonal trends of water relations in five co-occurring chasmophytic species

The present study examines the seasonal and diurnal patterns of water management by plant species inhabiting the wall fissures of the ancient castle of Patras. Their water status (water potential), stomatal behaviour (leaf resistance and transpiration rate) as well as tissue water relations (turgor loss point, osmoregulation capability and cell wall elasticity) were recorded in relation to season, daytime and respective environmental conditions. Despite some minor deviations, all five species exhibited a water spending strategy with high diurnal transpiration rates through seasons, limited only by the generally low light intensities prevailing at the northwest-facing vertical walls. Progressive shortage of water during summer resulted in the reduction of transpiration in four species. Diurnal water losses caused a reduction of water potential until midday or dusk, which further decreased with the progress of the dry period. However, predawn water potential remained high through all seasons. The above finding, together with the high transpiration, was unexpected for species growing in a very hostile environment as far as water supply is regarded. It could be partly explained by the recently found ability of all five species to absorb dew from leaf surfaces. However, such plants should also possess mechanisms to take up water efficiently from a rather dry substrate. Indeed, pressure–volume analysis revealed substantial seasonal changes in osmoregulatory capacity and minor changes in cell wall elasticity of leaf tissue. Both changes facilitate (the mechanism differs) water uptake from the wall-fissure substrate during the dry period. Although the relative contribution of the two mechanisms was different, they both allowed plants to maintain turgor and thus growth throughout their growing season.

Biophysical and morphological leaf adaptations to drought and salinity in salt marsh grasses

Leaf energy budgets were constructed for 13 species of estuarine C 4 grasses (Poaceae) to elucidate the biophysical effects of drought and salinity on the interception and dissipation of solar energy. Spartina alterniflora, S. anglica, S. argentinensis, S. bakeri, S. cynosuroides, S. densiflora, S. foliosa, S. foliosa × S. alterniflora hybrids, S. gracilis, S. patens, S. pectinata, S. spartinae, and Distichlis spicata plants were grown under controlled soil water potential gradients in a greenhouse. Species were grouped into four major ecological functional types, based on elevational zonation ranges: low marsh species, middle marsh species, high marsh species, and freshwater species. Different functional types are adapted to different environmental conditions, and responded differently to reduced water potentials. Latent heat flux decreased similarly across species in response to decreasing water potential. Latent heat loss was found to decrease by as much as 65% under decreasing water potential, leading to an increase in leaf temperature of up to 4 °C. Consequently, radiative and sensible heat losses increased under decreasing water potential. Sensible heat flux increased as much as 336% under decreasing water potential. Latent heat loss appeared to be an important mode of temperature regulation in all species, and sensible heat loss appeared to be more important in high marsh species compared to low marsh species. High marsh species are characterized by narrower leaves than middle and low marsh species, leading to a smaller boundary layer, and providing higher conductance to sensible heat loss. This may be an adaptation for high marsh species to regulate leaf temperature without access to large amounts of water for transpirational cooling. Stomatal conductance decreased with decreasing water potential across species: leaf conductances to water vapor and CO 2 decreased as much as 69% under decreasing water potential. Additionally, oxidative stress appeared to increase in these plants during times of drought or salinity stress. Ascorbate peroxidase activities increased with decreasing soil water potential, indicating increased cellular reactive oxygen species. High marsh species had higher ascorbate peroxidase activities compared to low marsh species, indicating higher tolerance to drought- or salinity-induced stresses. It was concluded that different species of marsh grasses are adapted for growth in different zones of salt marshes. Adaptations include biophysical, biochemical, and morphological traits that optimize heat exchange with the environment.

A Tolerant Behavior in Salt-Sensitive Tomato Plants can be Mimicked by Chemical Stimuli

Lycopersicon esculentum plants exhibit increased salt stress tolerance following treatment with adipic acid monoethylester and 1,3-diaminepropane (DAAME), known as an inducer of resistance against biotic stress in tomato and pepper. For an efficient water and nutrient uptake, plants should adapt their water potential to compensate a decrease in water soil potential produced by salt stress. DAAME-treated plants showed a faster and stronger water potential reduction and an enhanced proline accumulation. Salinity-induced oxidative stress was also ameliorated by DAAME treatments. Oxidative membrane damage and ethylene emission were both reduced in DAAME-treated plants. This effect is probably a consequence of an increase of both non enzymatic antioxidant activity as well as peroxidase activity. DAAME-mediated tolerance resulted in an unaltered photosynthetic rate and a stimulation of the decrease in transpiration under stress conditions without a cost in growth due to salt stress. The reduction in transpiration rate was concomitant with a reduction in phytotoxic Na+ and Cl- accumulation under saline stress. Interestingly, the ABA deficient tomato mutant sitiens was insensitive to DAAME-induced tolerance following NaCl stress exposure. Additionally, DAAME treatments increased the ABA content of leaves, therefore, an intact ABA signalling pathway seems to be important to express DAAME-induced salt tolerance. Here, we show a possibility of enhance tomato stress tolerance by chemical induction of the major plant defences against salt stress. DAAME-induced tolerance against salt stress could be complementary to or share elements with induced resistance against biotic stress. This might be the reason for the observed wide spectrum of effectiveness of this compound.

Evaluating the effects of gross nitrogen mineralization, immobilization, and nitrification on nitrogen fertilizer availability in soil experimentally contaminated with diesel

Sandy clay loam soil was contaminated with 5000 mg kg(-1) diesel, and amended with nitrogen (15.98 atom% (15)N) at 0, 250, 500, and 1000 mg kg(-1) to determine gross rates of nitrogen transformations during diesel biodegradation at varying soil water potentials. The observed water potential values were -0.20, -0.47, -0.85, and -1.50 MPa in the 0, 250, 500, and 1000 mg kg(-1) nitrogen treatments respectively. Highest microbial respiration occurred in the lowest nitrogen treatment suggesting an inhibitory osmotic effect from higher rates of nitrogen application. Microbial respiration rates of 185, 169, 131, and 116 mg O(2) kg(-1) soil day(-1) were observed in the 250, 500, control and 1000 mg kg(-1) nitrogen treatments, respectively. Gross nitrification was inversely related to water potential with rates of 0.2, 0.04, and 0.004 mg N kg(-1) soil day(-1) in the 250, 500, and 1000 mg kg(-1) nitrogen treatments, respectively. Reduction in water potential did not inhibit gross nitrogen immobilization or mineralization, with respective immobilization rates of 2.2, 1.8, and 1.8 mg N kg(-1) soil day(-1), and mineralization rates of 0.5, 0.3, and 0.3 mg N kg(-1) soil day(-1) in the 1000, 500, and 250 mg kg(-1) nitrogen treatments, respectively. Based on nitrogen transformation rates, the duration of fertilizer contribution to the inorganic nitrogen pool was estimated at 0.9, 1.9, and 3.2 years in the 250, 500, and 1000 mg kg(-1) nitrogen treatments, respectively. The estimation was conservative as ammonium fixation, gross nitrogen immobilization, and nitrification were considered losses of fertilizer with only gross mineralization of organic nitrogen contributing to the most active portion of the nitrogen pool.

(208) Effects of Temperature During Seed Development in Lactuca sativa and Helianthus debilis

Among the factors affecting germinability of a seed lot are the environmental conditions under which the seeds are produced. The objective of this study was to determine the effects of temperature during seed development on seed quality of two Asteraceae species. Seeds of lettuce cv. Tango and Helianthus debilis cv. Vanilla Ice and sp. cucumerifolius were produced in a greenhouse under one of two treatments: i) hot (27, 40, and 20 °C temperatures average, max, and min, respectively), and ii) cool (23, 33, and 18 °C temperatures average, max, and min, respectively). In both species, heavier seeds were produced under the cool conditions and no differences were observed in standard germination. In lettuce, germination percentage and rate were both affected by increased levels of exogenous ABA concentrations and reduced water potential (PEG solutions), and, in both cases, seeds from cool treatments were more affected. Germination at 30 °C and constant light was higher for seeds from the hot treatment. Lettuce seed showed a strong light requirement for germination. However, seeds from the hot treatment gave better dark germination at 13 and 19 °C. Seeds of H. debilis did not required light for germination, and the germination percentage and rates were evaluated at 13, 21, and 29 °C. For both lines, seeds from each treatment behave similarly; however, the germination of H. debilis cv. Vanilla Ice at 29 °C was higher when seeds were produced in the hot conditions. The results showed that temperature during seed development affected aspects of seed quality that are not detectable by the standard germination, but by germination at suboptimal conditions. Within the Asteraceae family, differences varied among and within species.

Water Balance and Proximate Composition in Cowpea (&lt;i&gt;vigna unguiculata&lt;/i&gt; (l). Walps) Seedlings exposed to Drought and Flooding Stress

Studying the water balance and proximate composition in plants subjected to these stresses compared the influence of drought and flooding on cowpea seedlings. In drought plants the leaf water potential, its components and the proximate composition were markedly reduced by the end of the experimental period. In contrast, flooded cowpea seedlings did not show marked reduction in water potential and other parameters studied. From these results, it was concluded that cowpea strives better in a flooded environment than in a water stressed environment. Journal of Applied Sciences and Environmental Management Vol. 8 (2) 2004: 55-57

Control of purple spot of loquat fruit (Eriobotrya japonica) by means of mineral compounds

SummaryTreatments to control purple spot of loquat fruit (Eriobotrya japonica) were tested based on the hypothesis that the disorder appears as a consequence of water unbalance between flesh and epidermal tissues caused by their different ability for sugar accumulation. Calcium nitrate, calcium chloride, Ca‐EDTA, ammonium nitrate and potassium nitrate at a concentration of 150 mm applied 2 weeks before fruit colour break reduced significantly the proportion of purple‐spotted fruit, giving rise to a reduction of water potential of the epidermal tissue that allows it to retain water. The most favourable date of treatment was during the 2 weeks prior to fruit colour break.