Controlled Glasshouse Experiment Research Articles

Insights into chickpea (Cicer arietinum L.) genotype adaptations to terminal drought stress: Evaluating water-use patterns, root growth, and stress-responsive proteins

Chickpea (Cicer arietinum L.), a rainfed crop in semi-arid regions, experiences frequent intermittent drought stress, hindering growth. This study explored morpho-physiological and molecular responses of six chickpea genotypes (ICC 4958, BG 4005, Pusa 362, BGM 10218, RSG 888, ICC 1882) under terminal drought to enhance yield and adaptability. Drought stress was imposed during early podding in a controlled glasshouse experiment. Various physiological, root growth, fertility, and yield parameters were measured. Proteomics analysis was conducted on leaf, pod, and root tissue samples, collected at ∼0.3 fraction of transpirable soil water (FTSW). Significant differences occurred for various traits, including phenologies, water relations, pollen viability and germination, flower and pod numbers, abortion rates, photosynthetic apparatus and pigments, osmotic adjustments, canopy temperatures, root length, and densities, and overall yield and biomass. FTSW dynamics can aid genotype screening during early drought. Characterizing stress-responsive proteins can provide insights into cellular processes and adaptations. Terminal drought affected leaf central shoot metabolism, reducing photosynthetic capacity, while roots and pods reconfigured enzyme abundances to promote root growth and pod development. The extensive genotypic variation in proteome data, morpho-physiological traits, and water-use patterns presents opportunities for developing drought-adapted cultivars.

Consistently high heat tolerance acclimation in response to a simulated heatwave across species from the broadly distributed Acacia genus.

When leaves exceed their thermal threshold during heatwaves, irreversible damage to the leaf can accumulate. However, few studies have explored short-term acclimation of leaves to heatwaves that could help plants to prevent heat damage with increasing heatwave intensity. Here, we studied the heat tolerance of PSII (PHT) in response to a heatwave in Acacia species from across a strong environmental gradient in Australia. We compared PHT metrics derived from temperature-dependent chlorophyll fluorescence response curves (T-F 0 ) before and during a 4-day 38°C heatwave in a controlled glasshouse experiment. We found that the 15 Acacia species displayed surprisingly large and consistent PHT acclimation responses with a mean tolerance increase of 12°C (range, 7.7-19.1°C). Despite species originating from diverse climatic regions, neither maximum temperature of the warmest month nor mean annual precipitation at origin were clear predictors of PHT. To our knowledge, these are some of the largest measured acclimation responses of PHT from a controlled heatwave experiment. This remarkable capacity could partially explain why this genus has become more diverse and common as the Australian continent became more arid and suggests that the presence of Acacia in Australian ecosystems will remain ubiquitous with climate change.

Recipient and donor characteristics govern the hierarchical structure of heterospecific pollen competition networks

Abstract Pollinator sharing can have negative consequences for plant fitness via competition for visits as well as with the arrival of heterospecific pollen. Plant traits and relatedness of donor and recipient species have been suggested to drive the observed variation in plant fitness effects of both processes, but how they shape the structure of interspecific pollen competition networks has been overlooked at the community level. To understand the importance of reproductive traits and relatedness to the impact of heterospecific pollen, we conducted a controlled glasshouse experiment with an artificial co‐flowering community. We performed 2,200 hand pollination crosses by experimentally transferring conspecific pollen alone or with 50% and 100% foreign pollen among 10 species belonging to three different plant families. Relative to conspecific pollen alone, there was a significant reduction in seed set with 50% heterospecific pollen for 67% of the crosses. This effect is driven largely by recipient traits and the interaction between recipient and donor traits under specific circumstances of trait matching. In general, species with shorter styles, smaller stigmas and lower pollen:ovule ratios were more impacted by heterospecific pollen. These traits and their differences among species led to a hierarchical (or transitive) structure of pollen competition with clear winners and losers. However, phylogenetic distance among recipient and donor species did not explain the effects. Synthesis. Our study shows that specific traits and trait combinations between donor and recipient species are important in determining seed production outcomes with heterospecific pollen receipt. Moreover, the differences in traits between species lead to a competitive structure with clear ‘winners’ or ‘losers’ species. The results of this study highlight the importance of specific traits in understanding the position of the competitive hierarchy of the species and the mechanisms underlying heterospecific pollen impacts upon plant reproductive success.

The resprouting response of co‐occurring temperate woody plant and grass species to elevated [CO2]: An insight into woody plant encroachment of grasslands

AbstractThe phenomenon of woody plant thickening in grasslands has been observed globally and is likely to have widespread ecological consequences. It has been proposed that woody plant thickening is driven in part by rising atmospheric [CO2] enhancing the resprouting ability of woody plants relative to grasses so they respond more strongly after disturbances such as herbivory and fire. The aim of this study was to examine the CO2 effect on the resprouting ability of 16 co‐occurring temperate woody plant and grass species (eight species from each growth form). Plants were grown in a controlled glasshouse experiment under ambient (400 ppm) and elevated [CO2] (600 ppm) for 14 weeks after which their resprouting ability was measured. Root non‐structural carbohydrate (NSCmass) and nitrogen (Nmass) storage was used as proxies to measure the resprouting ability of woody plants while for the grasses it was measured directly. We found that both the woody plants (22% on average; P = 0.003) and grasses (20% on average; P = 0.003) produced more biomass under elevated [CO2]. Despite the woody plants not allocating additional carbon to belowground storage under elevated [CO2], they had significantly greater root NSCmass (23% on average; P = 0.007) due to increased root biomass production (8% on average; P = 0.007). In contrast, root Nmass of the woody plants did not differ between CO2 treatments (P = 0.373). Surprisingly, the resprouting ability of the grasses did not significantly differ between the CO2 treatments (P = 0.067). These results provide evidence that the differing resprouting response of woody plants and grasses under elevated [CO2] may be contributing to woody plant encroachment of grasslands worldwide.

Evidence for a shift in defence driving the invasion success of Acacia longifolia in Australia

The enemy release hypothesis (ERH) outlines the most widely tested and accepted invasion mechanism. Within the ERH there are two hypotheses, the evolution of increased competitive ability hypothesis (EICA) and the shift in defence hypothesis (SDH), which describe how this mechanism may work. To our knowledge these two hypotheses are yet to be comprehensively tested in a single study. Therefore, the aim of this study was to test them for three Acacia species (A. cyclops, A. longifolia and A. saligna) that have become highly invasive in their introduced ranges. We grew these species in a controlled glasshouse experiment from seed collected from multiple populations in their native and introduced ranges in Australia. We then measured their growth (proxy for competitive ability) as well as their qualitative (terpenes) and quantitative (phenolics) defences. We found that A. longifolia plants grown from introduced range seed produced more biomass (marginally non-significant) and terpenes and less phenolics than their native range counterparts, providing strong evidence for the SDH. In contrast, there was no difference in growth or chemical defences for A. cyclops and A. saligna between ranges. This may be because these two species were introduced to eastern Australia ~ 50 years prior to A. longifolia being introduced to western Australia, giving more chance for natural enemies to adapt to them in their introduced range. If this is true, we can conclude that A. longifolia currently poses the greatest invasion risk of our Acacia study species and therefore warrants the closest management attention.

Genes controlling legume nodule numbers affect phenotypic plasticity responses to nitrogen in the presence and absence of rhizobia.

We investigated the role of three autoregulation of nodulation (AON) genes in regulating of root and shoot phenotypes when responding to changing nitrogen availability in the model legume, Medicago truncatula. These genes, RDN1-1 (ROOT DETERMINED NODULATION1-1), SUNN (SUPER NUMERIC NODULES), and LSS (LIKE SUNN SUPERNODULAOR), act in a systemic signalling pathway that limits nodule numbers. This pathway is also influenced by nitrogen availability, but it is not well known if AON genes control root and shoot phenotypes other than nodule numbers in response to nitrogen. We conducted a controlled glasshouse experiment to compare root and shoot phenotypes of mutants and wild type plants treated with four nitrate concentrations. All AON mutants showed altered rhizobia-independent phenotypes, including biomass allocation, lateral root length, lateral root density, and root length ratio. In response to nitrogen, uninoculated AON mutants were less plastic than the wild type in controlling root mass ratio, root length ratio, and lateral root length. This suggests that AON genes control nodulation-independent root architecture phenotypes in response to nitrogen. The phenotypic differences between wild type and AON mutants were exacerbated by the presence of nodules, pointing to resource competition as an additional mechanism affecting root and shoot responses to nitrogen.

Soil water content variability drives productivity responses of a model grassland system to extreme rainfall events under elevated CO2

Climate change is significantly altering rainfall patterns globally and will likely cause increases in extreme rainfall events. Grassland systems are particularly vulnerable to these changes as their productivity is strongly mediated by soil water content (SWC). SWC mean and variability are driven by the amount of rainfall received as well as the distribution through time of that rainfall. In this study, we used a model grassland system in a controlled glasshouse experiment to identify whether SWC mean or variability is a stronger driver of productivity. We then examined how extreme rainfall events alter this driver and the resulting effect this has on productivity and biomass allocation under ambient and elevated carbon dioxide (CO2). Rainfall amount was held constant, but distribution through time varied (control, one in 20 years event, one in 100 years event). SWC variability was a stronger driver of productivity (mesocosm biomass) than SWC mean, with increasing extreme rainfall event magnitude resulting in greater SWC variability. Surprisingly, elevated CO2 only had a small effect on these productivity and biomass allocation responses which may be due to the relatively small CO2 difference tested. Our results suggest that distribution of rainfall in time is an important driver of grassland productivity and that increases in extreme rainfall events, for a given total rainfall, will result in reduced grassland productivity.

Wild bradyrhizobia that occur in the Burdekin region of Queensland are as effective as commercial inoculum for mungbean (Vigna radiata (L.)) and black gram (Vigna mungo (L.)) in fixing nitrogen and dry matter production

Anecdotal reports have suggested wild bradyrhizobia (Bradyrhizobium spp.) were as effective as commercial inoculum for mungbean (Vigna radiata (L.)) grown in the Burdekin region of far north Queensland, Australia. To test this hypothesis, we sampled the diversity of bradyrhizobium strains at two field sites in the Burdekin region of north Queensland, and from plants grown in inoculated or uninoculated soil collected from one field site in Millmerran, southern Queensland. We then compared two wild bradyrhizobium with the commercial strain for their ability to fix nitrogen and promote biomass production on plants in a glasshouse experiment. Ten mungbean nodules were collected from each of four plants, from each of two sites in the Burdekin region. One site had been treated with commercial bradyrhizobium inoculum strain CB1015, the other had never been treated with commercial inoculum. Protein analysis of the nodules using matrix assisted desorption ionization time of flight mass spectrometry (MALDI-TOF MS) showed a total of eight different strains of bradyrhizobia, distinctly different to CB1015, were present in the nodules of these plants. Of these eight strains, two strains dominated, one at each of the two collection sites. On the other hand, mungbean plants grown in soils from Millmerran, on the Darling Downs, hosted only CB1015 where inoculated, and did not nodulate when not inoculated. The two dominant wild bradyrhizobia were cultured from nodules from these field collected plants, and used in a controlled glasshouse experiment. In the glasshouse experiment, two mungbean cultivars and one black gram (Vigna mungo (L.)) cultivar were: (i) not inoculated and not supplied with nitrogen, (ii) not inoculated and supplied with nitrogen, (iii) inoculated with the commercial bradyrhizobia strain CB1015, (iv) inoculated with the dominant wild bradyrhizobia strain from Site 1, or (v) inoculated with the dominant wild bradyrhizobia strain from Site 2. Cultivars inoculated with either of the two dominant wild bradyrhizobia strains were similar in biomass and fixed a similar amount of nitrogen, to those inoculated with CB1015. There was no significant difference between the three inoculated treatments for shoot biomass nor nodule biomass per plant. Nodule number did not differ significantly from CB1015 for either of the wild bradyrizobia treatments. For root biomass per plant, the two mungbean cultivars did not differ between the three inoculated treatments, but the black gram had significantly less root biomass compared to the CB1015 treatment for one of the two wild bradyrhizobia treatments. For nitrogen fixed per plant, none of the cultivars showed significant difference between inoculated treatments. Our findings are consistent with the hypothesis that in some parts of Queensland some wild bradyrhozobia may be as effective as commercial inoculum for mungbean and black gram, potentially abrogating the need for artificial inoculation, and perhaps offering new options for the future development of better adapted commercial inoculum.

Interactive effects of temperature and drought on cassava growth and toxicity: implications for food security?

Cassava is an important dietary component for over 1 billion people, and its ability to yield under drought has led to it being promoted as an important crop for food security under climate change. Despite its known photosynthetic plasticity in response to temperature, little is known about how temperature affects plant toxicity or about interactions between temperature and drought, which is important because cassava tissues contain high levels of toxic cyanogenic glucosides, a major health and food safety concern. In a controlled glasshouse experiment, plants were grown at 2 daytime temperatures (23°C and 34°C), and either well-watered or subject to a 1month drought prior to harvest at 6months. The objective was to determine the separate and interactive effects of temperature and drought on growth and toxicity. Both temperature and drought affected cassava physiology and chemistry. While temperature alone drove differences in plant height and above-ground biomass, drought and temperature×drought interactions most affected tuber yield, as well as foliar and tuber chemistry, including C:N, nitrogen and cyanide potential (CNp; total cyanide released from cyanogenic glucosides). Conditions that most stimulated growth and yield (well-watered×high temperature) effected a reduction in tuber toxicity, whereas drought inhibited growth and yield, and was associated with increased foliar and tuber toxicity. The magnitude of drought effects on tuber yield and toxicity were greater at high temperature; thus, increases in tuber CNp were not merely a consequence of reduced tuber biomass. Findings confirm that cassava is adaptable to forecast temperature increases, particularly in areas of adequate or increasing rainfall; however, in regions forecast for increased incidence of drought, the effects of drought on both food quality (tuber toxicity) and yield are a greater threat to future food security and indicate an increasing necessity for processing of cassava to reduce toxicity.

Quantity over quality: light intensity, but not red/far-red ratio, affects extrafloral nectar production in Senna mexicana var. chapmanii.

Extrafloral nectar (EFN) mediates food-for-protection mutualisms between plants and insects and provides plants with a form of indirect defense against herbivory. Understanding sources of variation in EFN production is important because such variations affect the number and identity of insect visitors and the effectiveness of plant defense. Light represents a potentially crucial tool for regulating resource allocation to defense, as it not only contributes energy but may help plants to anticipate future conditions. Low red/far-red (R/FR) light ratios can act as a signal of the proximity of competing plants. Exposure to such light ratios has been shown to promote competitive behaviors that coincide with reduced resource allocation to direct chemical defenses. Little is known, however, about how such informational light signals might affect indirect defenses such as EFN, and the interactions that they mediate. Through controlled glasshouse experiments, we investigated the effects of light intensity, and R/FR light ratios, on EFN production in Senna mexicana var. chapmanii. Plants in light-limited conditions produced significantly less EFN, and leaf damage elicited increased EFN production regardless of light conditions. Ratios of R/FR light, however, did not appear to affect EFN production in either damaged or undamaged plants. Understanding the effects of light on indirect defenses is of particular importance for plants in the threatened pine rockland habitats of south Florida, where light conditions are changing in predictable ways following extensive fragmentation and subsequent mismanagement. Around 27% of species in these habitats produce EFN and may rely on insect communities for defense.

Resistance to Spongospora subterranea induced in potato by the elicitor β-aminobutyric acid

Induction of resistance in potato to Spongospora subterranea (which causes powdery scab on tubers) was studied in two controlled glasshouse experiments. β-aminobutyric acid (BABA) was applied at different concentrations (1, 2 or 4 mM) to potato plants which were then inoculated with S. subterranea. Amounts of the pathogen in roots and intensity of Spongospora root galling were assessed. Infection was measured at different times during plant growth, at 3 or 6 weeks after inoculation (first experiment), and 2, 4 or 6 weeks after inoculation (second experiment). In the first experiment, growth of plants was reduced in proportion to the BABA application rate, and the plants had high levels of S. subterranea infection. In the second experiment, at 4 weeks after inoculation, plants treated with BABA had less S. subterranea in their roots and fewer root galls compared with non-treated plants. Six weeks after inoculation, plants treated with 1 mM BABA developed S. subterranea infection, almost to the level of non-treated plants. In contrast, 2 and 4 mM BABA treatments greatly reduced the amount of S. subterranea in roots and numbers of root galls compared with non-treated plants. Repeated applications (at 2 weeks intervals) increased the effectiveness of BABA. A third test indicated that BABA was not directly toxic to S. subterranea resting spores. These results demonstrate that chemically induced resistance reduces host root infection by S. subterranea, and may have disease management potential. This is the first report of chemically induced host resistance to Spongospora.

Consequences of interspecific hybridization and virus infection on the growth and fecundity of three threatened coastal Lepidium (Brassicaceae) species from New Zealand

AbstractLepidium castellanum, L. juvencum and L. oleraceum are threatened coastal cresses endemic to New Zealand. These three species were selfed and interspecific hybrids generated for examination of hybrid fitness and inbreeding depression. In controlled glasshouse experiments, the interspecific hybrids and selfed progeny were inoculated with a strain of the introduced Turnip mosaic virus (TuMV) previously isolated from wild populations of L. aegrum. Experiments tested the hypothesis that heterosis in the interspecific hybrids provides a gain in TuMV resistance in comparison to selfed plants. We show that interspecific hybrids of three genetically distinct species of Lepidium increased plant performance and reduced susceptibility to the effects of the TuMV. We suggest that interspecific hybridization could be implemented as a conservation management strategy and that a broader outlook may be required to mitigate the negative impacts of introduced pathogens on threatened species.

Modulation of flavonoid and tannin production of Carpobrotus rossii by environmental conditions

Dietary supplementation with plant-derived polyphenolic compounds such as the flavonoids epigallocatechin-3-gallate (EGCG), quercetin and resveratrol can result in a beneficial effect on degenerative disease processes through both radical scavenging and activation of cellular ion channels. Preliminary investigations have shown that extracts from the halophyte species Carpobrotus rossii have high in vitro antioxidant and in vivo low-density-lipoprotein-lowering activities. In this study, we have investigated the environmental conditions responsible for inducing flavonoid production in C. rossii in an attempt to maximise production of these compounds. Both field surveys and controlled glasshouse experiments were conducted. Flavonoid production appears to be related to conditions known to cause oxidative stress in plants such as exposure to excessive light, reduced water availability, and low soil potassium levels. Flavonoid production was minimal under salinity levels optimal for C. rossi growth (around 50mM NaCl) and increased dramatically at sub- and supra-optimal salinities. Flavonoids were clearly concentrated in metabolically active palisade mesophyll tissue rather than the spongy parenchyma. Non-optimal (outside 50–100mM range) NaCl levels significantly increased flavonoid concentration on a per leaf basis. However, the reduction in biomass production at sub and supra optimal salinities diminished absolute flavonoid production per plant. As a result, saline conditions favouring optimal plant growth appear to be most suitable for maximising production of flavonoids in C. rossii.

Application of inorganic carrier-based formulations of fluorescent pseudomonads and Piriformospora indica on tomato plants and evaluation of their efficacy

Fluorescent pseudomonads are widely used as bioinoculants for improving plant growth and controlling phytopathogenic fungi. Piriformospora indica (Pi), a symbiotic root endophyte, also has beneficial effects on a number of plants. The present study focuses on the improvement of growth yields of tomato plants and control of Fusarium wilt using inorganic carrier-based formulations of two fluorescent pseudomonad strains (R62 and R81) and Pi. The inorganic carrier-based formulations of pseudomonad strains and Pi were tested for plant growth promotion of tomato plants under glass house and field conditions. In controlled glass house experiments, 8·8-fold increase in dry root weight and 8·6-fold increase in dry shoot weight were observed with talcum powder-based consortium formulation of R81 and Pi. Field trial experiments ascertained the glfass house results with a considerable amount of increase in plant growth responses, and amongst all the treatments, R81 + Pi treatment performed consistently well in field conditions with an increase of 2·6-, 3·1- and 3·9-fold increase in dry root weight, shoot weight and fruit yield, respectively. The fluorescent pseudomonad R81 and Pi also acted as biocontrol agents, as their treatments could control the incidence of wilt disease caused by Fusarium oxysporum f.sp. lycopersici in tomato plants under glass house conditions. The culture broths of pseudomonads R62, R81 and Pi were successfully used for development of talcum- and vermiculite-based bioinoculant formulations. In controlled glasshouse experiments, the talcum-based bioinoculant formulations performed significantly better over vermiculite-based formulations. In field experiments the talcum-based consortium formulation of pseudomonad R81 and Pi was most effective. This study suggests that the formulations of pseudomonad strains (R62 and R81) and Pi can be used as bioinoculants for improving the productivity of tomato plants. The application of such formulations is a step forward towards sustainable agriculture.

Effect of clover root weevil larval feeding on growth of clover progenies from parents selected for tolerance in field trials

The effects of larval herbivory by clover root weevil (CRW, Sitona lepidus) on seven white clover (Trifolium repens) and two red clover (Trifolium pratense) progenies from plants showing tolerance of CRW in field trials were tested in two controlled glasshouse experiments. CRW larvae recovered from red clover plants were, on average, fewer (6.3 larvae/g root dry weight) and shorter (5.7 mm/g root dry weight) than those from white clover (16.7 larvae, 7.1 mm length).This confirmed previous findings that red clover is an inferior host for this pest. In the first experiment, progeny of four of the white clover selections for CRW tolerance showed high tolerance (shoot dry weights −2.9% to +3.1% different from their respective weevil-free controls) compared with a 13–24% shoot weight loss for two white clover cultivars. These results confirm field trial observations that there is variation for CRW tolerance in white clover and also that CRW tolerance is hereditable in at least some populations. The second experiment, on a smaller subset of white clovers, showed little effect of CRW on either the tolerant clover selections or the cultivar controls. An index of larval feeding pressure (larval numbers×larval projected areas/root dry weight) gave much lower values for the second experiment (12–78 mm2/g) than for the first (69–102 mm2/g). The absence of negative effects of CRW on the cultivar controls in experiment 2 was assumed to result from low CRW feeding pressure, allowing compensatory growth responses. A small but consistent increase in shoot: root ratio in the +CRW treatment in experiment 1 provided supporting evidence for increased allocation to shoots as a mechanism for plants to reduce the impact of root herbivory.

Growth responses to waterlogging and drainage of woody Hakea (Proteaceae) seedlings, originating from contrasting habitats in south-western Australia

We investigated the responses of seven woody Hakea (Proteaceae) species (two populations of each), to two months of waterlogging and subsequent drainage, in a controlled glasshouse experiment. The species originated from contrasting environments (winter-wetland versus non-wetland habitats), and differed in abundance (endangered ironstone species versus common species). Waterlogging arrested growth of the main root system, and stimulated the formation of superficial adventitious roots just below the root/shoot junction in all species. Wetland species produced at least twice the amount of adventitious root dry mass of that of non-wetland species, due to differences in number, length or degree of branching. Their adventitious roots also tended to have higher porosities (7–10% versus 5–6% gas spaces). The relative amount of adventitious roots formed was strongly, positively correlated with the maintenance of shoot growth, and only the non-wetland species showed significant shoot growth reductions (19%) upon waterlogging. Dry mass percentage of stems and leaves, and leaf dry mass per area (LMA) increased considerably during waterlogging in all species (averages of 15, 29 and 27%, respectively), returning to the values of continuously drained control plants after drainage. Similarly, upon drainage, a suppression of shoot growth (average 35%) and a stimulation of root growth (average 50%) restored the root mass ratios to those of control plants. We found a negative correlation between the maintenance of growth during waterlogging versus that after waterlogging, suggesting a trade-off in functioning of the superficial adventitious roots between waterlogged and drained conditions. The rare winter-wet ironstone endemics resembled the common winter-wet species in most of their responses to waterlogging and drainage. Therefore, the results presented here cannot offer an explanation for their different distribution patterns. Our results suggest that non-wetland species may be disadvantaged in a wetland environment, due to their lower capacity to form adventitious roots resulting in stronger growth reductions.

Fluctuations in soil moisture, and plant uptake of surface applied phosphate

The mechanisms leading to decreased plant uptake of surface-applied phosphatic fertilizers during short periods of drying were investigated in controlled glasshouse experiments with subterranean clover. Phosphate and water were added differentially to surface and subsurface layers of reconstituted soil profiles. The surface layers were subjected to brief periods of drying and to partial or complete remoistening at frequencies ranging from daily to fortnightly. Phosphorus uptake from surface applications and shoot yield were proportional to the frequency of remoistening of the soil surface to field capacity, and the response to additional subsurface phosphate diminished as the amount of surface water increased. In treatments where moistening did not achieve field capacity, yield was linearly proportional to the amount of water applied to the surface. Watering the surface to field capacity twice a week led to a 50 percent reduction in phosphorus uptake compared with daily watering. Watering twice as frequently with half the amount each time did not affect plant yield or phosphorus uptake. Thus it appeared that rapid phosphorus uptake occurred only at high moisture contents, and that uptake was proportional to the volume of soil brought close to field capacity and the length of time that it remained moist. This indicates that desorption of soil-adsorbed phosphate and its diffusion to plant roots can occur rapidly only at high moisture contents. Similar results were obtained with different soil types when the extractable phosphate was concentrated near the soil surface.