Plant Relative Growth Rate Research Articles

Photosynthetic Performance of Potato (<i>Solanum tuberosum</i>) Subject to Water Deficit Regulated with Mulched Drip Irrigation

An experiment was carried out to determine the effect of mulched drip irrigation under water deficit on leaf area index (LAI), leaf area duration (LAD), dry matter (DM) and relative growth rate (RGR) of potato in an arid environment. Five water deficit treatments and a full irrigation control were established to subject potato to various levels of soil water deficit at different crop growth stages. The result indicated that potato LAI and LAD at all the determined growth stages were not reduced under water deficit regulation. Additionally, the DM and RGR at starch accumulation were not significantly decreased under water deficit either. Therefore, medium soil water deficit regulated at 55%~65% of field capacity with mulched drip irrigation at potato tuber initiation could be used to effectively improve leaf area index, leaf area duration, dry matter and relative growth rate of plants.

Impacts of Tamarix‐mediated soil changes on restoration plant growth

AbstractQuestionDo soils impacted by Tamarix spp. affect the growth of plants used for restoration through altered soil chemistry and/or plant‐soil feedbacks?LocationThe Bighorn River, the Yellowstone River and the Fort Peck Reservoir, Montana, western USA.MethodsSoil was collected from paired subsites where Tamarix was either present or absent along three water bodies. To evaluate chemical and biological soil effects on plant growth, eight plant species (Achnatherum hymenoides, Astragalus cicer, Dalea candida, Elymus lanceolatus, Leymus cinereus, Pascopyrum smithii, Ratibida columnifera and Trifolium pratense) commonly used in restoration projects at Tamarix‐invaded sites were grown in the collected soil. Plant‐soil feedbacks were evaluated by growing two species (D. candida and P. smithii) in greenhouse soils inoculated with small amounts of the field soils. Germination, emergence and growth characteristics were compared between Tamarix‐invaded and un‐invaded subsites and across water bodies.ResultsSeedling emergence and plant relative growth rate, total biomass production and allocation of resources to roots and shoots were not negatively affected in field soils or in greenhouse soil inoculated with soil from areas where Tamarix was present. In fact, overall, plants emerged earlier and produced more biomass in soils affected by Tamarix than in soils from where Tamarix was not present. These results indicate that for sites in the northern range of Tamarix, restoration would not be inhibited by Tamarix‐induced soil changes.ConclusionsTamarix is a relatively new invader in the northern USA, and little is known about its impacts in this area or the potential implications for restoration. However, our results indicate that neither altered soil chemistry nor plant‐soil feedbacks negatively impact native plant growth, and restoration efforts would not be hindered by Tamarix‐induced changes to soil chemistry or microbiota.

Phytoremediation of atrazine by three emergent hydrophytes in a hydroponic system

A hydroponic system was used to evaluate atrazine (ATZ) removal and uptake by three emergent hydrophytes, Iris pseudacorus, Lythrum salicaria and Acorus calamus, determining their potential as phytoremediation agents for ATZ-contaminated water. After 20 days of exposure, the relative growth rate of plants in sterile conditions was less than in natural conditions. ATZ amount in a culture solution planted with emergent plants decreased significantly compared with an unplanted solution, and the removal rate of ATZ in natural conditions was greater than in sterile conditions (p < 0.05). The degradation contributions of I. pseudacorus, L. salicaria and A. calamus were 75.6, 65.5 and 61.8%, respectively. Those of the corresponding microbial population in the solution were 5.4, 11.4 and 17.4%, respectively. Emergent plants play a dominant role in reducing the ATZ level in the water body and could be used as phytoremediation agents.

THE EFFECT OF PLUG SIZE AND GRAFTING METHOD ON THE DRY MATTER PARTITIONING AND STAND ESTABLISHMENT RATE OF GRAFTED CUCUMBER SEEDLINGS UNDER SALINE CONDITIONS

ISHS V Balkan Symposium on Vegetables and Potatoes THE EFFECT OF PLUG SIZE AND GRAFTING METHOD ON THE DRY MATTER PARTITIONING AND STAND ESTABLISHMENT RATE OF GRAFTED CUCUMBER SEEDLINGS UNDER SALINE CONDITIONS

Contribution of N : P ratio and endogenous phytohormones during development of phosphorus toxicity in Brassica campestris spp. parachinensis

AbstractAgricultural practices may lead to excessive phosphorus (P) accumulation in soil. The effects of excessive P on Chinese flowering cabbage (Brassica campestris spp. parachinensis) were investigated by exposing plants for 4 weeks in solution containing 1, 2, 3, 5, and 7 mM NaH2PO4. Phosphorus concentrations [P] greater than 3 mM produced significantly stunted growth, together with reduced appearance quality due to overaccumulation of anthocyanin in the epidermis of flower stalk. Nitrate concentrations in the flower stalk decreased with increasing solution [P]. Nitrogen (N) concentrations in the roots and potassium (K) concentrations in the leaves, stems, and roots all decreased at [P] higher than 3 mM. Increasing P supply caused great enhancement of root and stem [P], but decreased total plant and root N : P ratios. A positive linear relationship between solution [P] and anthocyanin concentration and a negative linear relationship between root N : P ratio and anthocyanin concentration were also observed. In addition, 3 to 7 mM P caused decreased levels of indole‐3‐acetic acid (IAA) and gibberellin (GA3) in the leaves, but promoted ethylene production. The average gibberellin concentration was generally correlated with the plant's relative growth rates. Ethylene was negatively correlated with plant growth parameters except for the last day of the experiment. In conclusion, N : P ratio and endogenous phytohormones may be involved in the development of P toxicity in Chinese flowering cabbage plants.

Photosynthetic activity and leaf antioxidative responses of Atriplex portulacoides subjected to extreme salinity

Responses of Atriplex portulacoides upon 40-day-long exposure to salinity (0–1,000 mM NaCl) were investigated. Mother plants originated from a sabkha located in a semi-arid region of Tunisia. The plant relative growth rate and leaf expansion increased significantly at 200 mM NaCl but decreased at higher salinities. Interestingly, the plants survived salinity as high as 1,000 mM NaCl without displaying salt-induced toxicity symptoms. Despite significant increase in leaf Na+ and Cl− concentrations upon salt treatment, no significant effect on leaf relative water content was registered. Chlorophyll contents and the gas exchange parameters showed a significant stimulation at the optimal salinity (200 mM NaCl) followed by a decline at higher salinities. Extreme salinity hardly impacted the maximal efficiency of photosystem II photochemistry (Fv/Fm), but a marked decrease in the relative quantum yield of photosystem II (ФPSII) was observed, along with a significant increase in non-photochemical quenching (NPQ). Leaf malondialdehyde and carotenoid contents were generally unaffected following salt exposure, whereas those of anthocyanins, polyphenols, and proline increased significantly, being maximal at 1,000 mM NaCl. Leaf superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11), and glutathione reductase (EC 1.6.4.2) activities were significantly stimulated by salinity, whereas catalase (EC 1.11.1.6) activity was maximal in the 0–400 mM NaCl range. As a whole, protecting the photosynthetic machinery from salt-induced photodamage together with the sustained antioxidant activity may account for the performance of A. portulacoides under high salinity.

Rationality of using various physiological and yield related traits in determining salt tolerance in wheat

-1 salinity levels in pots filled with sandy clay loam soil. This study aimed to evaluate the significance of various physiological determinants commonly used for salt tolerance along with yield and yield related traits of wheat, especially grain yield. Plant relative growth rate, chlorophyll content index (CCi), water relations and biomass was recorded along with ionic (Na + and K + ) and yield related traits of wheat plant. The reduction in relative growth rate (RGR) was up to 34% in salt tolerant and 64% in salt sensitive wheat genotypes at 15 dS m -1 salinity treatment. Na + in leaf was increased by about 3 folds of control, while K in leaves decreased with increase in salinity and reduction was 34% in salt tolerant and 52% in the salt sensitive wheat genotype as compared to control plants. It was also observed that the number of spikelets per spike together with number of tillers per plant was the most sensitive parameters for salt tolerance. We conclude that these characteristics might be selected as desirable traits for a cross breeding programs or engineering such wheat that have ability to produce more tillers and/or number of spikelets per spike under stress to get varieties with better production in saline environments.

GROWTH AND BIOMASS PARTITIONING IN TOMATO IN RELATION TO RATIO OF NITROGEN:PHOSPHORUS SUPPLY

Tomato (Lycopersicon esculentum) plants were grown in hydroponic culture with nitrogen (N): phosphorus (P) supply ratios from 18:1 to 2:1. Nitrate and phosphate were supplied daily in pre-set ratios to maintain a constant P concentration, giving varying degrees of N limitation. Plant dry weight, weights of plant fractions and N and P concentrations in plant parts were measured at 9 and 18 days. Relative growth rate was related to ratio of N:P supplied, with higher ratios giving a higher growth rate, but a noticeably reduced rate was only apparent at 4:1 and 2:1 ratios. Relative growth rate was largely independent of internal N and P concentrations, although there was an optimum internal N:P ratio of approximately 14:1. The plants took up N and P at rates adjusting towards this 14:1 ratio. Depressed relative growth rate of plants inadequately supplied N was linearly correlated with decreased shoot fraction.

Physiological responses of Vallisneria spiraslis L. induced by different hydraulic conditions when exposed to copper and nitrogen

A two-flume experiment with submerged plant Vallisneria spiraslis L. was conducted to investigate the effects of different hydraulic conditions on physiological responses when exposed to water polluted with copper (Cu) and nitrogen (N). Plants were divided into two groups and grown for 120 h in hydrodynamic and hydrostatic flumes, respectively, using the same nutrient solution with fixed concentrations of 0.64 mg l –1 CuSO 4 and 4.0 mg l –1 total N. Combined pollution with Cu and N resulted in rapid increase in chlorophyll in the first hour and relative growth rate of plants in hydrodynamic water increased from 0 to 0.073 in the first 8 h. Meanwhile, peroxidase (POD) activity in shoots sharply increased to alleviate supraoptimal oxidative stress. And proteolytic degradation in shoots was observed to be catalyzed as protein contents in shoots decreased. Persistent decline in total chlorophyll and relative growth rate was noticed in longer duration. In roots, POD activity increased only at 8 h and decreased after 24 h. Malonaldehyde (MDA) content in roots also increased only at 8 h and MDA content was lower (P < 0.01), while protein content was higher (P < 0.01) after prolonged exposure than those at 0 h. Comparison of different hydraulic groups indicated that hydrodynamic condition induced higher POD activity in shoots in the first hour to alleviate the toxicity resulted by Cu and N. Thus, relative growth rate of plants was significantly higher than that in hydrostatic group (P < 0.05). While roots in hydrodynamic condition were less influenced by water flow, its protein contents in hydrodynamic group were significantly higher (P < 0.01) than those in hydrostatic group at all durations. These results suggested that hydrodynamic condition alleviated combined toxicity of Cu and N in V. spiraslis L shoots and enhanced the accumulation of protein in roots. Key words: Physiological response, hydrodynamic, copper, nitrogen, Vallisneria spiraslis L.

Effect of phosphorus (P) nutrition and leaf age on physiological growth parameters in a P-inefficient potato genotype

SUMMARYA pot experiment was conducted in a controlled environment growth chamber to investigate the effect of phosphorus (P) nutrition on the physiological growth parameters of old and young leaves in a potato genotype with inefficient P utilisation. The genotype (CGN 18233) was grown under three P regimes (10, 20, and 30 µM; rated low P, medium P, and high P, respectively). The treatments were arranged in a randomised complete block design, with three replications. The results showed that plant dry matter (DM) yield and relative growth rate (RGR) increased with increasing P. The different P treatments did not affect P concentrations and electron transport rates in young leaves, or the photosynthetic rate per unit leaf area, chlorophyll fluorescence yields, starch contents, total soluble sugar (TSS) contents, or SPAD values (chlorophyll content) of young and old leaves. Leaf age affected both leaf P concentration and electron transport rate, but chlorophyll fluorescence, SPAD values (chlorophyll content), the photosynthetic rate per unit leaf area, starch contents, and TSS contents did not differ significantly between young and old leaves. Thus, parameters such as leaf P concentration and electron transport rate need to be determined separately for young and old leaves in such studies, while measurements of parameters such as chlorophyll fluorescence, SPAD value (chlorophyll content), photosynthetic rate per unit leaf area, and starch and TSS contents need not be made separately on young and old leaves.

Evaluation of growth slowdown nursery treatments on Prunus avium seedlings by means of allometric relationships and relative growth rates

To evaluate the effects of carbonic fertilization and apical brushing (brushing of shoot apices to induce a negative thigmonastic response) on Prunus avium growth under nursery, carbon dioxide (CO2) was fed to cherry trees. Later, carbonic fertilization was withdrawn and apical brushing was applied. Finally, the original conditions were reinstated. Height and trunk diameter, allometric relations with plant biomass and relative growth rate were followed. Contrary to expectations, apical brushing resulted in increased trunk diameter and, hence, growth rate. Carbonic fertilization did not produce differences in the initial growth, probably owing to the deciduous characteristics of the species. Continuous application of carbonic fertilization maintained growth rate, whereas no fertilization or its withdrawal reduced the growth rate, but acclimation of the CO2-fed plants appeared by the end of the growing period. No differences in root growth potential at the end of the assay were observed between treatments. In conclusion, owing to the observed acclimation, carbonic fertilization is not advised throughout the complete period of growth. The tested treatments did not help to adapt supply of plant material of this species to the demand for revegetation. However, growth-slowing treatments may be applied to P. avium without any negative effects on potential root growth.

Are plant growth and photosynthesis limited by pre-drought following rewatering in grass?

Although the relationship between grassland productivity and soil water status has been extensively researched, the responses of plant growth and photosynthetic physiological processes to long-term drought and rewatering are not fully understood. Here, the perennial grass (Leymus chinensis), predominantly distributed in the Euro-Asia steppe, was used as an experimental plant for an irrigation manipulation experiment involving five soil moisture levels [75–80, 60–75, 50–60, 35–50, and 25–35% of soil relative water content (SRWC), i.e. the ratio between present soil moisture and field capacity] to examine the effects of soil drought and rewatering on plant biomass, relative growth rate (RGR), and photosynthetic potential. The recovery of plant biomass following rewatering was lower for the plants that had experienced previous drought compared with the controls; the extent of recovery was proportional to the intensity of soil drought. However, the plant RGR, leaf photosynthesis, and light use potential were markedly stimulated by the previous drought, depending on drought intensity, whereas stomatal conductance (gs) achieved only partial recovery. The results indicated that gs may be responsible for regulating actual photosynthetic efficiency. It is assumed that the new plant growth and photosynthetic potential enhanced by pre-drought following rewatering may try to overcompensate the great loss of the plant's net primary production due to the pre-drought effect. The present results highlight the episodic effects of drought on grass growth and photosynthesis. This study will assist in understanding how degraded ecosystems can potentially cope with climate change.

The influence of water level on the growth and photosynthesis of Hydrocotyle ranunculoides L.fil.

Floating Pennywort ( Hydrocotyle ranunculoides L.fil.), a native to North America and naturalized in Central and South America, is an invasive aquatic weed in western Europe and several other regions worldwide. H. ranunculoides settles primarily in stagnant to slow-flowing waters (e.g. ditches, canals, rivers, lakes and ponds). The species prefers sunny and nutrient-rich sites and forms dense interwoven mats, which can quickly cover the surface of infested waters. In this study, the effect of three different water levels on growth of Floating Pennywort was investigated. Plants were cultivated on high-nutrient soils under waterlogged, semi-drained and drained conditions. Highest relative growth rates (RGR) of 0.097±0.004 g g −1 dw d −1 were reached under waterlogged conditions. This was significantly higher than RGR of plants cultivated semi-drained and drained. Floating Pennywort showed some phenological adaptations to drained soil conditions, including significant differences in the relative amounts of leaf, petiole and shoot biomass, whilst the relative amount of root biomass was not significantly influenced by the water level. Furthermore, Floating Pennywort reached under drained conditions lower relative water contents (RWC) of leaves, petioles and shoots, a significant shorter length of internodes, a significant lower extent of shoot porosity (POR), a lower chlorophyll content and an increased Chl a:Chl b ratio. In addition, maximum gas exchange of drained cultivated plants is significantly lower, due to strongly decreased leaf conductance under reduced water availability. Overall, H. ranunculoides showed ability to grow under different water levels, but performed best under waterlogged conditions.

An Evaluation of the Effects of Soil Characteristics on Mitigation and Restoration Involving Blue Elderberry, Sambucus mexicana

We conducted field surveys and laboratory analyses to test the effects of soil characteristics in habitat mitigation sites and natural sites on the growth and condition of blue elderberry (Sambucus mexicana), which is the sole host plant for the federally threatened Valley elderberry longhorn beetle (Desmocerus californicus dimorphus). Thirty mitigation and 16 natural sites were selected throughout the range of the beetle. We found that although plant relative growth rates were comparable between mitigation sites and a natural site, mitigation sites contained substantially less soil nutrients than mitigation sites. Within mitigation sites, elderberry health and growth were positively correlated with the amount of total nitrogen in soils and less strongly with other soil nutrients and soil moisture. Analyses demonstrated reductions in the relative growth rate of elderberry as mitigation sites aged, and that soil nutrients and soil moisture became depleted over time. For mitigation sites, it took approximately seven years to develop basal stem diameters that have been linked to successful beetle colonization. Mitigation sites have smaller shrubs than natural sites and growth slows as mitigation sites age, thus delaying convergence of conditions between natural and mitigation sites. Analyses of soil particle size and whether sites were within the 100-year floodplain (as an indicator of riparian conditions) were inconclusive. We recommend investigating fertilizing and optimum planting densities for elderberry at restoration and mitigation sites, as well as increasing the duration of irrigation and monitoring.

Tolerância à inundação de Cecropia pachystachya Trec. (Cecropiaceae): aspectos ecofisiológicos e morfoanatômicos

Visando elucidar aspectos da tolerância à hipoxia, plantas de Cecropia pachystachya foram submetidas a 30 dias de inundação. O crescimento e desenvolvimento, a condutância estomática, a fotossíntese e a morfo-anatomia foram avaliadas. A taxa de crescimento relativo (TCR) da raiz e do caule foi inferior em plantas alagadas, não havendo diferenças nesta taxa para as folhas e para a planta inteira. A não variação na TCR das folhas pode ter sido em função da maior esclerofilia foliar das plantas alagadas, que apresentaram menor área. A condutância estomática e a taxa fotossintética diminuíram nas plantas submetidas ao alagamento. Foram observadas alterações morfo-anatômicas em caules e principalmente em raízes de plantas alagadas. A análise dos resultados indica que a tolerância desta espécie a períodos de inundação está relacionada à sua capacidade de manter uma satisfatória produção energética. A manutenção de um balanço energético positivo possibilita que mesmo sob estresse, com queda na taxa fotossintética, as plantas desta espécie apresentem uma considerável TCR. Portanto, C. pachystachya pode ser forte competidora em áreas sujeitas ao alagamento.

Improvement of the Growth and Yield of Lettuce Plants by Non-Uniform Magnetic Fields

Influence of pre-sowing magnetic treatments on plant growth and final yield of lettuce (cv. Black Seeded Simpson) were studied under organoponic conditions. Lettuce seeds were exposed to full-wave rectified sinusoidal non uniform magnetic fields (MFs) induced by an electromagnet at 120 mT (rms) for 3 min, 160 mT (rms) for 1 min, and 160 mT (rms) for 5 min. Non treated seeds were considered as controls. Plants were grown in experimental stonemasons (25.2 m2) of an organoponic and cultivated according to standard agricultural practices. During nursery and vegetative growth stages, samples were collected at regular intervals for growth analyses. At physiological maturity, the plants were harvested from each stonemason and the final yield and yield parameters were determined. In the nursery stage, the magnetic treatments induced a significant increase of root length and shoot height in plants derived from magnetically treated seeds. In the vegetative stage, the relative growth rates of plants derived from magnetically exposed seeds were greater than those shown by the control plants. At maturity stage, all magnetic treatments increased significantly (p < 0.05)—plant height, leaf area per plant, final yield per area, and fresh mass per plant—in comparison with the controls. Pre-sowing magnetic treatments would enhance the growth and final yield of lettuce crop.

Vegetative and reproductive plasticity of broccoli at three levels of incident photosynthetically active radiation

To study the effects of shading on the growth, development, dry matter partitioning, and plant architecture of broccoli, "Legacy" hybrid plants were grown in pots in a greenhouse under black shading meshes to generate different levels of photosynthetically active radiation (PAR). The average incident PAR was 23 mol PPF square m/d under control conditions, 15.2 under a 35% mesh, and 6.7 under a 70% mesh. The air temperature was within the range 15-22 deg C. As shading increased so did the duration of the growth cycle and the leaf area (LA). Shading did not affect the number of leaves, although the upper ones were more erect. The stem length and the accumulated intercepted PAR (IPAR) were negatively related. Inflorescence diameter and commercial fresh weight decreased only with the 70% mesh. Shading did not affect stem dry weight (DW) but altered dry matter allocation in the root and spear. The DW of the leaves maintained an average 45.7% of the total DW despite the greater LA developed under shade. The net assimilation rate diminished with the reduction of IPAR, and the LA increased; the plant relative growth rate was therefore practically constant. With increased shading, the leaves and the stem became the dominant photosynthate sinks. The commercial fresh weight achieved with 15.2 mol photosynthetic photon flux (PPF) square m/d was equivalent to that obtained with 23 mol PPF square m/d, but the cycle was extended for 4.5 days. With 6.7 mol PPF square m/d, yield diminished because of the lower DW produced in a cycle extended by 15 days, and because more dry matter was allocated to the stem than to the spear. Based on these results, broccoli could be considered a shade-tolerant plant.

Synthetic reactive dye wastewater treatment by narrow-leaved cattails (Typha angustifolia Linn.): Effects of dye, salinity and metals

Narrow-leaved cattails were studied in synthetic reactive dye wastewater (SRDW) under caustic conditions. The effects of the toxic dye were expressed in terms of relative plant growth rate and the appearance of symptoms such as necrosis, and chronic or acute wilting. The dye toxicity was 25.33 mg l(-1) which was close to approximate the concentration of dye residue from the textile effluent in the public stream. The system pH and % color removal were decreased, indicating that narrow-leaved cattail can treat wastewater. The average system pH decreased from 9 to 7. The maximum color removal was approximately 60% when cultured under soil conditions. The SEM image of narrow-leaved cattail root after treatment with SRDW indicated that the root cortex was damaged and the crystalline sodium salts deposited in the root cells which caused evaporation and transpiration decreased in SRDW. The salinity under caustic conditions also affects the growth of the plants. The maximum sodium removal was approximately 44% and was found in the SRDW under soil conditions within 14 days. A small amount of sodium could enhance the relative growth rate. However, the sodium removal of the plants was limited after the third week of treatment. It should be noted that narrow-leaved cattails are known to avoid the textile dye and salt stress conditions during SRDW treatment through special mechanisms such as salt accumulation in the roots or shedding of older leaves. In addition, elements such as silicon, calcium and iron in plants might help the plant to detoxify by forming complexes with dye molecules.

Monensin Is Not Toxic to Aquatic Macrophytes at Environmentally Relevant Concentrations

Monensin, a common livestock feed additive, has been detected in surface waters around areas of intensive agriculture. The effect of this ionophore antibiotic on floating (Lemna gibba) and submersed (Myriophyllum spicatum, Elodea canadensis, Egeria densa) freshwater macrophytes was investigated under seminatural field conditions using 12,000 l of outdoor microcosms. Exposure concentrations of 0, 12, 25, 50, and 100 mug/l (n = 3) were evaluated over a 35-day period. Submersed plants were grown individually in 115-ml plastic "cone-tainers" and assessed for various growth and pigment end points. E. canadensis and M. spicatum also were grown in assemblages to represent model populations and two-species communities. Few statistically significant differences from control organisms were observed for any of the monitored end points. Overall, monensin is deemed unlikely to cause toxicity in freshwater macrophytes at current environmental concentrations. However, the ability to characterize toxicity in macrophytes is based partially on the relative growth rates (RGRs) of the plants. The greater the RGR, the more sensitive the assay may be to contaminants. The RGRs of E. canadensis and M. spicatum grown in model populations and communities were found to be significantly higher than the RGRs of plants grown individually. This implies that the "cone-tainer" method, although simple and easy to perform, may underestimate toxicity in simulated field studies.

Density-dependent outcomes in a digestive mutualism between carnivorous Roridula plants and their associated hemipterans

Recent studies have shown that mutualisms often have variable outcomes in space and time. In particular, the outcomes may be dependent on the density of the partners with unimodal or saturating outcomes providing stability to the mutualism. We examine density-dependent outcomes of an obligate, species-specific mutualism between a South African carnivorous plant (Roridula dentata) and a hemipteran (Pameridea) that facilitates prey digestion, but also sucks plant sap. Plants occur in sandy, leached, nitrogen-poor soils and have no digestive enzymes to digest prey. Instead they rely on obligately dependent hemipterans to supply nitrogen by digesting prey for them and defecating on their leaves. We documented the densities of Pameridea on Roridula in the field. In the greenhouse, we manipulated the hemipteran densities on Roridula and measured the mean relative growth rates of plants with differing hemipteran densities. Plants exhibited a unimodal response to the density of their mutualist partners. Those with no hemipterans had negative growth rates, suggesting that hemipterans are important in facilitating nitrogen absorption. Plants with intermediate hemipteran densities had positive growth rates but growth rates were negative under very high hemipteran densities. Our research provides support for variable and unimodal outcomes in mutualism. Unimodal outcomes may be particularly important in obligate mutualisms and this is one of the few studied outside of pollinating seed parasite mutualisms. In this system, extrinsic factors such as other predators may affect the mutualism by altering the numbers of hemipterans.