Does nitrogen availability have greater control over the formation of tropical heath forests than water stress?A hypothesis based on nitrogen isotope ratios

Global scale analyses of soil and foliage δ15N have found positive relationships between δ15N and ecosystem N loss (suggesting an open N cycle) and a negative relationship between δ15N and water availability. We show here that soils and leaves from tropical heath forests are depleted in 15N relative to 'typical' forests suggesting that they have a tight N cycle and are therefore limited by N rather than by, often suggested, water availability.

Photosynthetic parameters and growth in seedlings of Bertholletia excelsa and Carapa guianensis in response to pre-acclimation to full sunlight and mild water stress

Light and water are important factors that may limit the growth and development of higher plants. The aim of this study was to evaluate photosynthetic parameters and growth in seedlings of Bertholletia excelsa and Carapa guianensis in response to pre-acclimation to full sunlight and mild water stress. I used six independent pre-acclimation treatments (0, 90 (11h15-12h45), 180 (10h30-13h30), 360 (09h00-15h00), 540 (07h30-16h30) and 720 min (06h00-18h00)) varying the time of exposure to full sunlight (PFS) during 30 days, followed by whole-day outdoor exposure for 120 days. Before PFS, the plants were kept in a greenhouse at low light levels (0.8 mol m-2 day-1). The PFS of 0 min corresponded to plants constantly kept under greenhouse conditions. From the beginning to the end of the experiment, each PFS treatment was submitted to two water regimes: moderate water stress (MWS, pre-dawn leaf water potential (ΨL) of -500 to -700 kPa) and without water stress (WWS, ΨL of -300 kPa, soil kept at field capacity). Plants under MWS received only a fraction of the amount of water applied to the well-watered ones. At the end of the 120-day-period under outdoor conditions, I evaluated light saturated photosynthesis (Amax), stomatal conductance (g s), transpiration (E) and plant growth. Both Amax and g s were higher for all plants under the PFS treatment. Stem diameter growth rate and Amax were higher for C. guianensis subjected to MWS than in well-watered plants. The contrary was true for B. excelsa. The growth of seedlings was enhanced by exposure to full sunlight for 180 minutes in both species. However, plants of B. excelsa were sensitive to moderate water stress. The higher photosynthetic rates and faster growth of C. guianensis under full sun and moderate water stress make this species a promissory candidate to be tested in reforestation programs.

Effect of water stress on nitrogen fixation and nodule structure of common bean

The aim of this work was to investigate the effect of water stress on N2 fixation and nodule structure of two common bean (Phaseolus vulgaris L.) cultivars Carioca and EMGOPA-201. Plants were harvested after five and eight days of water stress. Carioca had lower nodule dry weight on both water stress periods; shoot dry weight was lower at five days water stress and did not differ from control after eight days stress. Both cultivars had lower nitrogenase activity than control after five and eight days water stress. For both cultivars, after eight days stress bacteroid membranes were damaged. Carioca presented more pronounced damage to infected tissue, with host cell vacuolation and loss of the peribacteroid membrane at five days after stress; at eight days after stress, there was degradation of cytoplasm host cells and senescence of bacteroids, with their release into intercellular spaces. Intensity of immunogold-labeling of intercellular cortical glycoprotein with the monoclonal antibodies MAC 236/265 was different for both cultivars.

Water stress before harvest of pepper-rosmarin

The objective of this work was to assess the effect of different periods of water stress before harvest of pepper-rosmarin (Lippia sidoides) on the contents of essential oil and flavonoids. The experiment was carried out during 270 days of cultivation, with drainage lysimeters, in a completely randomized block design with five treatments: 0, 2, 4, 6, and 8 days of water suppression before harvest, with four replicates. Fresh and dry matter yield, essential oil content, total flavonoids content, and water potential and temperature of leaves were determined. There was a decrease of approximately 50% in oil content and of 60% in total flavonoid content with the reduction of leaf water potential in 0.3 MPa. Essential oil is more sensitive to water stress than total flavonoids.

Citrus root distribution under water stress grown in sandy soil of central Florida

The experiment was conducted in an orchard located in University of Florida (Citrus Research and Education Center), Lake Alfred, Polk County, Florida, USA. The objective of this study was to evaluate the effects of water stress in root distribution of 'Valencia' orange tree on 'Swingle' citrumelo rootstock. Three treatments were imposed on the trees: 1) normal irrigation with microsprinklers, 2) no irrigation in winter (November through mid-March) and 3) rainfall exclusion by placing a water repelling fabric (Tyvek) under the trees. Trees in treatments 1 and 2 received normal rainfall during the winter, but treatment 3 received no rain. Normal irrigation was resumed on all treatments in mid March. Soil was collected using root auger head (0.09 m diameter and height 0.25 m) in two opposing quadrants (West and East at 3 horizontal distances from tree trunk (1, 2 and 3 m) and 4 depths (0.0-0.15; 0.15-0.30; 0.30-0.60 and 0.60-0.90 m). The results from root sampling showed that there was a significant difference in root distribution between irrigated treatment and non irrigated/non rainfall.

Competition between drought-tolerant upland rice cultivars and weeds under water stress condition

The objective of this study was to evaluate the competitiveness of two cultivars of upland rice drought-tolerant, cultured in coexistence with weed S. verticillata, under conditions of absence and presence of water stress. The experiment was conducted in a greenhouse at the Experimental Station of the Universidade Federal de Tocantins, Gurupi-TO Campus. The experimental design was completely randomized in a factorial 2 x 2 x 4 with four replications. The treatments consisted of two rice cultivars under two water conditions and four densities. At 57 days after emergence, were evaluated in rice cultivars and weed S. verticillata leaf area, dry weight of roots and shoots and total concentration and depth of roots. Was also evaluated in rice cultivars, plant height and number of tillers. Water stress caused a reduction in leaf area, the concentration of roots and vegetative components of dry matter (APDM, and MSR MST) of rice cultivars and Jatoba Catetão and weed S. verticillata. The competition established by the presence of the weed provided reduction of all vegetative components (MSPA, and MSR MST) of cultivars and Jatoba Catetão. It also decreased the number of tillers, the concentration of roots and leaf area. At the highest level of weed competition with rice cultivars, a greater decrease in vegetative components and leaf area of culture, regardless of water conditions.

Interactions between soybean and weeds in a replacement series system, considering the effects of water stress

Due to the increase of water deficiency in many farm regions and its meaning on weed interference, competitive interactions between soybean and three weeds were evaluated under water stress (20 to 40 days after transplanting) and no stress conditions. Three independent experiments were carried out in a growth chamber, being each one composed by the weeds Alternanthera tenella, Tridax procumbens or Digitaria ciliaris, along with the crop, in which soil water condition and plant composition effects were evaluated while in competition. A replacement series system was used, including both monoculture of each species and a mixture with a ratio of 50% between weed and soybean. A completely randomized design was used in factorial arrangement, with treatments distributed in three levels for plant composition factor (soybean and weeds monocultures, in addition to the soybean + weed mixture) and two levels for the water factor (with or without stress), amounting six treatments in each experiment. Soybean dry mass was higher than weed dry mass, when growing without water stress. However, under water stress conditions, the dry mass of soy was reduced in all experiments, mainly in the D. ciliaris comparative experiment. Water restriction was also significant in the plants' photosynthesis reduction in most of the experiments, reducing leaf area duration and efficiency of water use. Analysing all variables shows greater weed tolerance than soybean when submitted to water deficit and with distinct changes of their interactions and mechanism of competition, in each experiment.

The effect of water stress on seed germination of three terrestrial bromeliads from restinga

Bromeliad seedlings are rarely found on sandy coastal plains (restinga), limited, probably, by stressful conditions and/or specific abiotic requirements for germination. The effect of water stress on rate, time, synchronicity and spreading of germination was evaluated here for three terrestrial bromeliads from the restinga of Maricá using osmotic solutions of polyethyleneglicol 6000 (PEG 6000), from 0.0 to -0.26 MPa for 30 days. Water stress induced by PEG lowered rate and increased time and synchronicity values, besides the number of daily events of bromeliad seed germination, under water potentials between 0.00 to -0.14 MPa. No seeds germinated under water potentials lower than -0.14 MPa. These results reinforce a constant and/or high moisture requirement for bromeliad seeds to germinate. We conclude that bromeliads are not able to act as pioneer plants through germination outside the vegetation islands of the restinga of Maricá, due to the inability of seeds to germinate under lower water potential.

Least limiting water range in assessing compaction in a Brazilian Cerrado latosol growing sugarcane

In the south-central region of Brazil, there is a trend toward reducing the sugarcane inter-harvest period and increasing traffic of heavy harvesting machinery on soil with high water content, which may intensify the compaction process. In this study, we assessed the structural changes of a distroferric Red Latosol (Oxisol) by monitoring soil water content as a function of the Least Limiting Water Range (LLWR) and quantified its effects on the crop yield and industrial quality of the first ratoon crop of sugarcane cultivars with different maturation cycles. Three cultivars (RB 83-5054, RB 84-5210 and RB 86-7515) were subjected to four levels of soil compaction brought about by a differing number of passes of a farm tractor (T0 = soil not trafficked, T2 = 2 passes, T10 = 10 passes, and T20 = 20 passes of the tractor in the same place) in a 3 × 4 factorial arrangement with three replications. The deleterious effects on the soil structure from the farm machinery traffic were limited to the surface layer (0-10 cm) of the inter-row area of the ratoon crop. The LLWR dropped to nearly zero after 20 tractor passes between the cane rows. We detected differences among the cultivars studied; cultivar RB 86-7515 stood out for its industrial processing quality, regardless of the level of soil compaction. Monitoring of soil moisture in the crop showed exposure to water stress conditions, although soil compaction did not affect the production variables of the sugarcane cultivars. We thus conclude that the absence of traffic on the plant row maintained suitable soil conditions for plant development and may have offset the harmful effects of soil compaction shown by the high values for bulk density between the rows of the sugarcane cultivars.

Effects of soil water depletion on the water relations in tropical kudzu

Tropical kudzu (Pueraria phaseoloides (Roxb.) Benth., Leguminosae: Faboideae) is native to the humid Southeastern Asia. Tropical kudzu has potential as a cover crop in regions subjected to dryness. The objective of this paper was to evaluate the effect of soil water depletion on leaflet relative water content (RWC), stomatal conductance (g) and temperature (T L) in tropical kudzu. RWC of waterstressed plants dropped from 96 to 78%, following a reduction in SWC from 0.25 to 0.17 g (H2O).g (dry soil)-1.Stomatal conductance of stressed plants decreased from 221 to 98 mmol.m-2.s-1, following the reduction in soil water content (SWC). The day after re-irrigation, g of water stressed plants was 15% lower than g of unstressed plants. Differences in T L between waterstressed and unstressed plants (deltaT L) rose linearly from 0.1 to 2.2ºC following progressive water deficit. RWC and T L of waterstressed plants paralled RWC and T L of unstressed plants the day after reirrigation. The strong decrease in SWC found in this study only induced moderate water stress in tropical kudzu. In addition, tropical kudzu recover rapidly from the induced water stress after the re-irrigation.

Differences in ureide and amino acid content of water stressed soybean inoculated with Bradyrhizobium japonicum and B. elkanii

The objective of this work was to study the response to water stress of a drought sensitive soybean cultivar inoculated with Bradyrhizobium japonicum (strain CB1809, Semia 586) and B. elkanii (strain 29W, Semia 5019). CB1809 nodulated plants produced a significantly higher root fraction (19%) than 29W (14.6%). Plants inoculated with CB1809 produced less nodules and accumulated more nitrogen than those inoculated with 29W. In general, low amounts of ureides in nodules were found in watered plants inoculated with either CB1809 or 29W strains, but those levels were five-fold increased in stressed plants inoculated with CB1809. Nodules formed by strain CB1809 had aspartate and glutamate as major amino acids, while those formed by 29W had glutamate, asparagine and alanine. In nodules of plants inoculated with CB1809 aspartate showed the highest accumulation (5 µmol g-1); in stressed plants this amino acid reached a value of 26 µmol g-1, and asparagine was not detected. Nodules formed by the strain 29W accumulated 1 µmol g-1 of aspartate, whether plants were stressed or not. Asparagine was the major amino acid found in nodules from watered plants (6 µmol g-1) and the amount of this amino acid was six-fold increased when plants were water stressed.

Transpiration response of upland rice to water deficit changed by different levels of eucalyptus biochar

The objective of this work was to evaluate the effect of eucalyptus biochar on the transpiration rate of upland rice 'BRSMG Curinga' as an alternative means to decrease the effect of water stress on plant growth and development. Two-pot experiments were carried out using a completely randomized block design, in a split-plot arrangement, with six replicates. Main plots were water stress (WS) and no-water stress (NWS), and the subplots were biochar doses at 0, 6, 12 and 24% in growing medium (sand). Total transpirable soil water (TTSW), the p factor - defined as the average fraction of TTSW which can be depleted from the root zone before water stress limits growth -, and the normalized transpiration rate (NTR) were determined. Biochar addition increased TTSW and the p factor, and reduced NTR. Consequently, biochar addition was able to change the moisture threshold (p factor) of the growing medium, up to 12% maximum concentration, delaying the point where transpiration declines and affects yield.

Physiological parameters to select upland rice genotypes for tolerance to water deficit

Abstract: The objective of this work was to evaluate the feasibility of using physiological parameters for water deficit tolerance, as an auxiliary method for selection of upland rice genotypes. Two experiments - with or without water deficit - were carried out in Porangatu, in the state of Goiás, Brazil; the water deficit experiment received about half of irrigation that was applied to the well-watered experiment. Four genotypes with different tolerance levels to water stress were evaluated. The UPLRI 7, B6144F-MR-6-0-0, and IR80312-6-B-3-2-B genotypes, under water stress conditions, during the day, showed lower stomatal diffusive resistance, higher leaf water potential, and lower leaf temperature than the control. These genotypes showed the highest grain yields under water stress conditions, which were 534, 601, and 636 kg ha-1, respectively, and did not differ significantly among them. They also showed lower drought susceptibility index than the other genotypes. 'BRS Soberana' (susceptible control) was totally unproductive under drought conditions. Leaf temperature is a easy-read parameter correlated to plant-water status, viable for selecting rice genotypes for water deficit tolerance.

Water status and gas exchange of umbu plants (Spondias tuberosa Arr. Cam.) propagated by seeds and stem cuttings

The experiment was carried out at the Embrapa Semi-Árido, Petrolina-PE, Brazil, in order to study the physiological responses of umbu plants propagated by seeds and by stem cuttings under water stress conditions, based on leaf water potential and gas exchange measurements. Data were collected in one-year plants established in pots containing 30 kg of a sandy soil and submitted to twenty-day progressive soil water deficit. The evaluations were based on leaf water potential and gas exchange data collection using psychrometric chambers and a portable infra-red gas analyzer, respectively. Plants propagated by seeds maintained a significantly higher water potential, stomatal conductance, transpiration and photosynthesis under decreasing soil water availability. However, plants propagated by stem cuttings were unable to maintain a favorable internal water balance, reflecting negatively on stomatal conductance and leaf gas exchange. This fact is probably because umbu plants propagated by stem cuttings are not prone to formation of root tubers which are reservoirs for water and solutes. Thus, the establishing of umbu plants propagated by stem cuttings must be avoided in areas subjected to soil water deficit.

Heliotropism and water availability effects on flowering dynamics and seed production in Macroptilium lathyroides

The experiment was carried out in pots in a glasshouse, with one plant per pot and nine repetitions per treatment. The treatments consisted of free or restricted leaves, submited to 90-100% or 60-70% soil field capacity (FC). Only independent effects of water availability or leaf movement were observed on yield components. Plants under well-watered conditions and with freely orienting leaves were taller, and had a larger number of ramifications. The greater development favored the setting of a higher number of inflorescences per plant in these treatments. This behavior resulted in a high number of flowers, green and mature legumes per plant, thus resulting in high seed production which was the most evident response to water availability. Although individual seed weight was higher in the water stress treatment, total seed production was higher for well-watered plants, with no statistically significant effect of leaf movements.