Replacement studies were conducted under greenhouse conditions to determine if available soil water influences the competitive interaction between soybean and Florida beggarweed. Stomatal conductance and leaf water potential were determined for both species under different levels of available soil water to identify possible mechanisms involved in changes in the relative competitiveness induced by the water deficits. Soybean leaf area and aboveground biomass were greater than for Florida beggarweed under optimum water, but equal to or less than Florida beggarweed with water stress. Soybean was more competitive than Florida beggarweed when there was adequate soil water but less competitive than Florida beggarweed under water stress. Stomatal conductance was higher for soybean with optimum soil water (at high leaf water potentials) but equal to Florida beggarweed as soil water became limiting (low leaf water potentials). These data indicated that water stress differentially affected soybean and Florida beggarweed.

The influence of soil moisture on the competitive relationship between yellow nutsedge and cotton was determined in an additive competition experiment where main plots were irrigation regimes and subplots were nutsedge densities. Increasing nutsedge density caused a significant linear decrease in seed cotton yield. Regression analysis indicated that approximately 9 (1993), 12 (1994), or 37 (1995) kg ha−1 of seed cotton yield was lost for each additional initial nutsedge tuber per meter of crop row. Both postharvest stem biomass, an indicator of total aboveground biomass, and seed cotton yield increased with increasing soil moisture in all 3 yr of the study. Increasing yellow nutsedge density reduced midseason cotton height in both wet and dry soil moisture regimes but only slightly reduced postharvest stem biomass in the wet treatment in 1995. There was an interaction between soil moisture regime and yellow nutsedge density only with respect to midseason cotton leaf water potentials. Soil moisture availability did not interact with the effect of yellow nutsedge density with respect to end of the season stem biomass or seed cotton yield.

Intraspecific and interspecific interference effects on growth, gas exchange, and water potential of wild oat and spring barley were measured under greenhouse conditions using a 1:1.06 barley to wild oat replacement series. Intraspecific barley interference affected barley growth more than interspecific wild oat interference. Interspecific wild oat interference with barley reduced wild oat growth more than intraspecific interference. Wild oat plant height surpassed barley plant height near barley anthesis. Growth and gas exchange of barley and wild oat responded the same to short-term water stress.

In a 2-yr field study conducted on a Weswood silt loam soil (Fluventic Ustochrepts), interspecific competition between soybeans [Glycine max (L.) Merr. ‘Hutton′] and velvetleaf (Abutilon theophrasti Medik. # ABUTH) resulted in greater than 40 and 50% reductions in soybean and velvetleaf seed yield, respectively. Leaf area index, number of mainstem nodes, total number of leaves, and plant dry weight of monocultured and intercropped velvetleaf differed significantly as early as 4 weeks after emergence. Interspecific competition had litttle or no effect on soybean morphology before 8 weeks after emergence. Soil water extraction occurred to 1-m depths in a monoculture of velvetleaf (five plants/m2) in 1984 and 1985. Monocultured soybeans (32.5 plants/m2) extracted water from a 1.5-m or greater depth of the soil profile during the same years. Soil water extraction in the intercropped plots resembled that of the monocultured velvetleaf treatment until soybeans attained R6, when soil water was extracted to a 1.5-m depth. The potential for interspecific competition for water existed early in the season before late-season soybean root development. Relative water content and leaf water potential (Ψw1) did not differ (0.05) between monocultured and intercropped soybeans in 1984 or 1985. In 1985, Ψw1 differed between monocultured and intercropped velvetleaf during anthesis. Leaf water potential values in the youngest, fully expanded leaves were approximately 0.3 and 0.4 MPa lower during midmorning and midday hours, respectively, in intercropped and monocultured velvetleaf. Transpiration and stomatal conductance did not differ between monocultured and intercropped soybeans or velvetleaf at any time during 1984. Photosynthetic and transpiration rates, stomatal conductance, and Ψw1 were lower in intercropped than in monocultured velvetleaf during anthesis in 1985, suggesting interspecific competition for soil water. Soybean water relations were not affected in either year. The data suggest that soybean yield reductions in soybean-velvetleaf interspecific competition are attributable to resource limitations other than water in south-central Texas.

Field studies were conducted to evaluate the effect of quackgrass [Agropyron repens (L.) Beauv. ♯ AGRRE] density and soil moisture on corn (Zea mays L.) growth and yield. Quackgrass densities ranging from 65 to 390 shoots/m2 reduced corn yield 12 to 16%. A quackgrass density of 745 shoots/m2 reduced corn yields an average of 37% and significantly reduced corn height, ear length, ear-fill length, kernels/row, rows/ear, and seed weight. In the soil moisture study, quackgrass was shorter than corn throughout the growing season, and analyses of corn leaf tissue indicated that quackgrass did not interfere with the nutrient status of the corn. In 1979, soil moisture was not limiting and corn yields were similar in all treatments regardless of irrigation or the presence of quackgrass. In 1980, soil moisture was limited and irrigation increased the yield of quackgrass-free corn. Irrigation also increased the yield of quackgrass-infested corn to a level similar to irrigated corn. When light and nutrients are not limiting factors, an adequate supply of soil moisture can eliminate the effects of quackgrass interference on the growth, development, and yield of corn.

Growth dynamics, water relations, and photosynthesis of soybean [Glycine max (L.) Merr. ‘Ransom’], common cocklebur (Xanthium pensylvanicum Wallr.), jimsonweed (Datura stramonium L.), prickly sida (Sida spinosa L.), sicklepod (Cassia obtusifolia L.), smooth pigweed (Amaranthus hybridus L.), spurred anoda [Anoda cristata (L.) Schlect.], and velvetleaf (Abutilon theophrasti Medic.) were compared in a controlled-environment greenhouse programmed for 32C day and 23C night temperatures. Net photosynthetic rates, net assimilation rates, and water-use efficiency on a whole-plant or single-leaf basis were greatest in the C4-plant, smooth pigweed. Total dry-matter production at 29 days after planting was greatest in common cocklebur and least in jimsonweed. Interspecific differences in dry-matter production were highly positively correlated with leaf area duration and negatively correlated with net assimilation rate. Threshold leaf water-potential levels causing stomatal closure varied among species. The stomata of jimsonweed were the most sensitive to water stress and those of prickly sida were the least sensitive.

Research was conducted at Rosemount, Minnesota in 1979 and 1980 to evaluate the effect of soil moisture on quackgrass [Agropyron repens (L.) Beauv. # AGRRE] interference in soybeans [Glycine max (L.) Merr.]. Four treatments were studied: 1) nonirrigated soybeans, 2) irrigated soybeans, 3) nonirrigated soybeans, quackgrass-infested, and 4) irrigated soybeans, quackgrass-infested. Percent N, P, and K of soybeans was not affected by quackgrass interference. Irrigation did not increase the yield of quackgrass-free soybeans. The yield of quackgrass-infested soybeans was increased by irrigation when soil moisture was limited, but yields were always less than quackgrass-free soybeans. Quackgrass was nearly the same height or taller than the soybeans at all stages of soybean development and offered substantial competition for light throughout the growing season. This research indicates that an adequate supply of soil moisture can reduce quackgrass interference in soybeans, but cannot eliminate it, since quackgrass apparently continues to compete with soybeans for light.

Greenhouse experiments were conducted to determine the effects of drought stress length and relief on velvetleaf growth and bentazon efficacy with crop oil concentrate (COC) and 28% urea ammonium nitrate (UAN). Drought stress caused leaf water and osmotic potentials to decline linearly over time. Leaf water potential ΨL) declined 0.02 and 0.08 MPa day−1 for velvetleaf subjected to −0.03 and −0.4 MPa soil water potential (ΨP), respectively. Osmotic potential (ΨO) declined 0.02 and 0.21 MPa day−1 for velvetleaf subjected to −0.03 and −0.4 MPa ΨP respectively. Bentazon did not reduce drought-stressed compared to non-drought-stressed velvetleaf dry weight 21 d after treatment with either COC or no adjuvant but did decrease dry weight with UAN. Reduced velvetleaf dry wights in drought-stressed velvetleaf compared to non-drought-stressed velvetleaf could not be accounted for by increased bentazon penetration in the drought-stressed plants. Rewatering velvetleaf subjected to 7 d of −0.4 MPA ΨP caused them to be more sensitive to bentazon than velvetleaf subjected to −0.03 MPa ΨP except when UAN was the adjuvant.

Pigweed is an increasingly aggressive weed in semiarid environments such as Mediterranean areas, and in general the control of all Amaranthus species is becoming more and more difficult. Increasing pigweed aggressiveness could be a result of its ability to keep a high water use efficiency under drought conditions. An experiment was conducted to study the effect of water stress on the photosynthetic capacity, growth, and leaf water potential of pigweed at the field level and assess if this species, as a model for C4 weeds, is CO2-saturated at the current level of atmospheric CO2 in a Mediterranean area. Pigweed was studied within a naturally occurring weed population in a bell pepper field in southern Italy where a rain-fed treatment (V0) was compared to a fully irrigated one (V100) corresponding to the restoration of 100% of the maximum crop water evapotranspiration. Soil water content was measured periodically, and net assimilation rate, stomatal conductance, transpiration rate, and intercellular CO2 concentration were determined on pigweed leaves. Photosynthetic rates of 37.6 µmol m−2 s−1 in V100 and 13.9 µmol m−2 s−1 in V0 were recorded, with higher transpiration rates in V100; consequently stomatal conductance was significantly lower in rain-fed conditions (0.08 mol m−2 s−1)) compared to the irrigated treatment (0.30 mol m−2 s−1). Photosynthesis in pigweed is not completely CO2-saturated at the current atmospheric CO2 level in the Mediterranean area and this could affect competition and increase of aggressiveness toward crops at the actual CO2 atmospheric concentration in agro-ecosystems. This occurs because unlike other C4 crops already saturated for CO2, weeds that are not CO2-saturated will remain CO2-sensitive to higher ambient CO2 levels. Thus, when they are grown in mixed stands where competition occurs, they can still suppress the slower-growing species.

Experiments were conducted to examine the use of spectral reflectance curves for discriminating between plant species across moisture levels. Weed species and soybean were grown at three moisture levels, and spectral reflectance data and leaf water potential were collected every other day after the imposition of moisture stress at 8 wk after planting. Moisture stress did not reduce the ability to discriminate between species. As moisture stress increased, it became easier to distinguish between species, regardless of analysis technique. Signature amplitudes of the top five bands, discrete wavelet transforms, and multiple indices were promising analysis techniques. Discriminant models created from data set of 1 yr and validated on additional data sets provided, on average, approximately 80% accurate classification among weeds and crop. This suggests that these models are relatively robust and could potentially be used across environmental conditions in field scenarios.

We determined seasonal variation in soil matric potentials (ψsoil) along a topographical gradient and with soil depth in a Bolivian tropical dry (1160 mm y−1 rain) and moist forest (1580 mm y−1). In each forest we analysed the effect of drought on predawn leaf water potentials (ψpd) and drought response (midday leaf water potential at a standardized ψpd of −0.98 MPa; ψmd) of saplings of three tree species, varying in shade-tolerance and leaf phenology. ψsoil changed during the dry season and most extreme in the dry forest. Crests were drier than slopes and valleys. Dry-forest top soil was drier than deep soil in the dry season, the inverse was found in the wet season. In the moist forest the drought-deciduous species, Sweetia fruticosa, occupied dry sites. In the dry forest the short-lived pioneer, Solanum riparium, occupied wet sites and the shade-tolerant species, Acosmium cardenasii drier sites. Moist-forest species had similar drought response. The dry-forest pioneer showed a larger drought response than the other two species. Heterogeneity in soil water availability and interspecific differences in moisture requirements and drought response suggest great potential for niche differentiation. Species may coexist at different topographical locations, by extracting water from different soil layers and/or by doing so at different moments in time.

Osmotic adjustment (OA) and increased cell-wall extensibility required for expansive leaf growth are well defined components of adaptation to water stress in dry soil, which might interact with soil phosphorus (P) concentration and defoliation frequency for intensively grazed white clover in legume-based pastures. Experiments were conducted with frequently and infrequently defoliated mini-swards of white clover growing in dry soil with low and high P concentrations. The higher yielding high-P plants were able to dry the soil to greater soil water suctions; their leaves had lower water potential values, yet they showed fewer water stress symptoms and underwent a more complete recovery from the water stress symptoms on rewatering, than the low-P plants. High- P plants had greater OA, proline concentration and leaf expansion rate. On the other hand, low-P plants showed an increased osmotic concentration when there was no change in the total solute content per unit of leaf d. wt, indicating more loss of water from the leaf tissue. The key measures that appeared to be directly associated with plant recovery over a short period following water stress were increased proline concentration and leaf expansion rate, probably resulting from increased cell-wall extensibility rather than increased production of cells for the high-P plants.

Leaf water and osmotic potentials and gas exchange were monitored during a prolonged El Niño drought in 1998 for saplings of seven species in a Bornean heath forest and compared with measurements taken during a subsequent wet period. The four dipterocarp species maintained reasonably good water status throughout the drought, especially Dipterocarpus borneensis which had thick and deep tap roots. In contrast, two of three non-dipterocarp species, Cleistanthus baramicus and Tristaniopsis obovata displayed predawn leaf water potentials approaching their turgor-loss points. During the drought, all species except D. borneensis displayed strongly reduced stomatal conductance after a brief exposure to sun, and all displayed lower maximum rates of stomatal conductance and net photosynthesis than during the wet period. Only Cotylelobium burckii displayed significant osmotic acclimation to the drought. T. obovata possessing a superficial root system suffered a high mortality due to the drought, but recovered faster after the first rains than the other species all of which had tap roots. Deep roots and strong stomatal control favour trees in tropical heath forests where water deficits probably occur regularly.