Einfluß von Licht,Temperatur und Außenmedium auf das elektrische Verhalten von Acetabularia crenulata

Summary 1. In sea water at 25°C cells of Acetabularia crenulata exhibit a resting potential (RP) of-170 mV between cytoplasm and external medium. At temperatures below 10°C, or upon addition of 10^-3m dinitrophenol in darkness, the cell shows a second steady potential (RP) of about-70 mV. Among the cations of sea water, i.e. K⁺, Na⁺, Mg⁺⁺, only K⁺ was found to affect RP and RP. If the ionic strength of the medium is reduced by addition of isotonic mannitol solution, RP decreases, while RP is not influenced. RP is explained as a potassium diffusion potential, while for the existence of RP an electrogenic chloride pump is inferred which is driven by ATP of the photo- or oxidative phosphorylation (system X).-2. Starting from RP, the current-voltage relationship consists of two linear portions for inward (R _e ) and outward current (R _a ), respectively, merging at RP (Fig. 3). Presumably they represent potassium conductances. For a given cell, the expression RT/F ln R _e /R _a yields a value which fits the RP of the cell (Fig. 20).-3. Starting from RP, a N-shaped current-voltage relationship was obtained for depolarisation (Fig. 3). The deviation from the potassium conductance is supposed to be due to the shunt of the potassium channel and the system X (voltage-dependent resistance). An electric circuit diagram was derived from voltage and current clamp experiments (Fig. 21); the elements of the circuit were tentatively analogized with cell functions.-4. Action potentials of about 120 mV, lasting from 30 to 300 sec may arise spontaneously. They can be triggered by lowering the temperature or depolarisation (voltage clamp, current clamp, light-off-cf. Figs. 2,11). The mechanism of the action potential can be derived from the properties of the chloride pump. Action currents were recorded upon different depolarizing steps by voltage clamp to yield current-coltage curves at different times after stimulation (Fig. 13).-5. Pulses of white light shift the potential off RP: light-on elicits a small depolarisation, light-off a large transient hyperpolarisation. The primary event of this response is a change of current (Fig. 19), the voltage change being its consequence. This result is interpreted on the basis of the circuit diagram.

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Abkürzungen AP Aktionspotential - RP stabiles Potential bei ca.-170 mV - RP stabiles Potential bei ca.-70 mV Dissertation der Math.-Nat. Fakultät der Universität Tübingen.     

Erosion,phosphorus and phytoplankton response in rivers of South-Eastern Norway

The development of P fractions and phytoplankton was studied in three rivers with varying concentrations of seston.Less than 1% of the yearly TP transport may take place during periods with high algal biomass.The observation of a high growth rate of phytoplankton in the rivers coinciding with high concentrations of RP, low content of seston and high TP:Chl a ratio, indicate that the growth was often not P-limiting. During short periods with high phytoplankton biomass the ratio TP:Chl a may be low, indicating that a high fraction of TP was available.The content of P in soil samples and in samples with high seston content was about 0.1% of dry weight, and the algal availability of P often varied between 25 and 75% of TP for both types of samples.Decreasing biomass or low growth rates were observed at secchi depths less than 0.5 m and seston concentrations less than about 25 mg dry weight 1^–1. High flow rate also depressed the development of the total phytoplankton biomass. The assimilation of available P is incomplete under such conditions, i.e. under conditions of light limitation and high dilution rate.The availability of P for phytoplankton in rivers with different length, light conditions and stream velocity is discussed.     

Seasonal and vertical variation in the C : P ratio of suspended and settling seston of lakes

In the absence of field data it is often assumed that the C : P ratio of the seston in lakes corresponds to the ideal ratio of 106 : 1, reported by Redfield et al. (1963) for marine phytoplankton. However, we found out that in nearly all lakes studied C : P ratios deviate systematically from this value especially during periods, when phosphorus is growth limiting. Thus, during summer the sestonic C : P ratio generally exceeds 106 in the epilimnion, but decreases with depth in the hypolimnion. Possible mechanisms which may lead to these seasonal and spatial changes in the C : P ratio are discussed.     

Responses of phytoplankton to experimental fertilization with ammonium and phosphate in an African soda lake

Summary Phytoplankton abundance in tropical lakes is more often judged to be limited by nitrogen than phosphorus, but seldom does the evidence include controlled enrichments of natural populations. In January 1980 we performed the first experimental fertilization in an equatorial African soda lake, Lake Sonachi, a small, meromictic volcanic crater lake in Kenya. During our study the natural phytoplankton abundance was ca. 80 g chl a/l, and the euphotic zone PO₄ and NH₄ concentrations were less than 0.5 M. In the monimolimnion PO₄ reached 180 M and NH₄ reached 4,600 M. Replicate polyethylene cylinders (5 m long, 1.2 m³) were enriched to attain 10 M PO₄ and 100 M NH₄. Phytoplankton responses were measured as chlorophyll, cell counts and particulate N, P and C. After two days, the chlorophyll increase in the P treatment was significantly higher than the control (P<0.01) while the N treatment was not. After five days the molar N/P ratio of seston was the same in the N treatment and control (23) but only 6 in the P treatment. The molar N/P ratio of seston in an unenriched Lake Sonachi sample was 21 and in samples from Lakes Bogoria and Elmenteita, two shallow soda lakes in Kenya, the ratios were 12 and 70 respectively. We conclude that limitation of phytoplankton abundance by phosphorus can occur even in some tropical African soda lakes.     

Seasonal seston stoichiometry: effects on zooplankton in cyanobacteria-dominated lakes

Seasonal dynamics in elemental composition [carbon (C), nitrogenand phosphorus (P)] of seston and zooplankton were studied overseveral years in three hypereutrophic Dutch lakes with persistentdominance and high biomass of cyanobacteria. In all three lakes,there was a strong pattern with decreased P-content and increasedC:P ratio in seston (<150 µm) coinciding with the increasein water temperature. The seston C:P ratios (at:at) were morethan doubled with the rising temperature, i.e. from 200 (at:at)in winter to 500 in summer. Sestonic C:P ratios increased overthe growing season, suggesting that seasonal dynamics amongautotrophs with high P-uptake in winter and support of subsequentphytoplankton growth by consumption of internal cellular P (P-quota)was the main cause of low sestonic P contents in late summer.This could, however, occur in concert with a physiologicallydriven decrease in cell-specific P at higher temperatures insummer. In contrast, the annual variation of C:P ratios of thezooplankton fraction was only 10% of that of seston. The variationsof C:P ratios of the zooplankton were, nevertheless, stronglycorrelated with those of seston. For most of the summer, sestonC:P ratios were far above the threshold ratio for P-limitationin Daphnia and other P-demanding species. This will pose furtherconstraints on growth performance of Daphnia in these lakes,thus adding to the fish predation pressure and the poor foodquality of cyanobacteria per se. The low grazing pressure causesa high biomass of low-quality autotrophs, promoting a stablestate with low trophic transfer efficiency.     

The Effect of Reservoirs on Phosphorus Concentration

The annual average degree of phosphorus retention (both TP and OP) in reservoirs was found to depend mainly on the theoretical retention time (RT). The phosphorus retention (RP) is shown for some reservoirs in Czech Republic to be well approximated (standard deviation = 10.9) with RP_(TP) = 74.7* (1-exp(-0.0255* RT)) (N = 38, 7 ≤ RT ≤ 604 days). The inclusion of 13 additional reservoirs for which data were assembled from literature resulted in an almost identical relationship: RP_(TP) = 76.1* (1-exp(-0.0282* RT)) (N = 51, range of RT nearly unchanged). The retention of soluble reactive phosphorus RP_(OP) studied on data from literature seems to be higher than that of TP. Statistical comparison with data for RP_(TP) of lakes has shown a significantly lower retention in lakes than in reservoirs for comparable RP, resulting in RP_(TP) = 66.0* (1-exp(-0.00419* RT)) (N = 59, 5.8 ≤ RT ≤ 260000 days). The differences between RP_(OP) and RP_(TP) and differentiation of lakes and reservoirs are discussed and a need for more thorough theoretical analysis stressed. Consequences for watershed management are summarized.     

Seston quality drives feeding,stoichiometry and excretion of zebra mussels

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Physiological indicators of nutrient deficiency in phytoplankton in southern Chilean lakes

We assessed the nutrient status of phytoplankton in 28 lakes in southern Chile using two types of physiological indicators: specific alkaline phosphatase activity, and the elemental composition (carbon, nitrogen, and phosphorus) of seston. Alkaline phosphatase activity ranged from 0.001 to 0.11 mol P g chl^–1 h^–1, with P-deficiency indicated in about one-half the study lakes. C:N ranged from 3.9 to 24, C:P ranged from 86 to 919, and N:P ranged from 8.7 to 99. C:P and N:P ratios greater than the Redfield ratio were common, suggesting P deficiency in many of the lakes. C:N ratios were not generally indicative of N deficiency. Previous studies have suggested N may be the primary limiting nutrient in southern Chilean lakes, but our results indicate that P should not be discounted as a limiting nutrient.     

Patterns and sources of variation in Daphnia phosphorus content in nature

It has recently been shown that Daphnia can vary in the phosphorus (P)-content of their body tissues, but the relative importance of genetic versus environmental causes for this variation is unexplored. We measured variation in P-content (as % body mass) of Daphnia from eight lakes and conducted experiments to contrast three sources of variation: interspecific variation, clonal variation and phenotypic plasticity. Daphnia P-content decreased with increasing seston C:P ratio across lakes. This relationship reflected both inter- and intraspecific variation. Daphnia parvula and D. dubia exhibited high P-content and were found in shallow lakes with low C:P seston, whereas D. pulicaria had low P-content and was found in deep, stratified lakes having high C:P seston. Populations of D. dentifera spanned this lake gradient and exhibited P-content that was negatively related to seston C:P. Evidence for phenotypic plasticity came from experiments with D. pulicaria and D. dentifera collected from a lake with P-deficient seston and fed a P-sufficient diet in the laboratory. In addition, populations of D. dentifera differed in P-content even after 7 d of feeding on P-sufficient resources, suggesting within-species clonal variation. However, mesocosm experiments revealed broad and surprisingly continuous variation in the P-content of individual clones of D. pulex (range 1.54–1.05%) and D. mendotae (1.51–1.07%) over a gradient in dietary C:P. The broad range in P-content exhibited by individual clones, acclimated for generations, suggests that variation in Daphnia P-content from laboratory experiments needs to be interpreted with caution. These results also show that phenotypic variation in response to environment can be a larger source of variation in P-content than genetic differences within or among species.     

Linking major nutrients (C,H, N,and P) to trace metals (Fe,Mn, and Cu) in lake seston in southern Brazil

We investigated the relationships between major nutrients (C, H, N, and P) and trace metals (Cu, Fe, and Mn) in seston samples from ten lake/lagoon systems in southern Brazil. The systems were characterized by a diverse set of limnological features, including surface areas from 10⁻¹ to 10² km², water color, a _CDOM(440), from 1.4 to 12.9 m⁻¹, and electrical conductivity from 50 to 100 000 μS cm⁻¹. Seston concentrations also varied a great deal, 32-fold. The elemental (C: N, C: P, and N: P) and C: Chl-a ratios in the seston samples indicated, however, common features; i.e., most of the lakes were N-and/or P-limited, and the seston organic fraction was composed of nonvascular plants (e.g., phytoplankton). Our intersystem comparison revealed that the relative content of organic matter in seston and seston concentrations in lake water tended to correlate positively and negatively, respectively, with trace metal concentrations across the seston samples. Possible influences of elemental and C: Chl-a ratios on the association of metals with seston matrices, although theoretically important, were only partially evidenced here; positive correlations were found between C: N and also Org-H: N ratios with trace metal concentrations. We speculate that such results could be circumstantial, as the nature of the seston matrices appeared to be very similar among them. This hypothesis should thus be the theme of further research. In short, our findings suggest that C: N and Org-H: N ratios as well as the relative content of major nutrients in seston and seston concentrations can be importantly related to trace metal concentrations in seston samples. In discussing the results, however, we consider that metal-seston relationships depend on a variety of physical, chemical, and biological factors and/or variables other than those measured in this study, which could also contribute for defining and explaining variations in metal-seston concentrations in lake ecosystems.     

Mercury content in lake sediments and suspended matter — temporal variation and relation to water chemistry

Twenty-five small, mainly oligotrophic, forest Swedish lakes were treated in different ways (e.g. liming) during 1987 in order to reduce the Hg levels in fish. The temporal variation of Hg concentration in sediments and water, and the extent that this variation can be related to normal lake characteristics and/or altered chemical conditions due to lake remedial measures was studied. Yearly mean alkalinity was increased from 0.07 to 0.18 meq l^–1 and pH from 6.0 to 6.5 due to the treatments. The Hg concentration, as well as the Hg:C ratio, in seston was reduced after treatment; the median Hg/C ratio in seston decreased from 1.6 g g^–1 during 1986 to 0.8 g g^–1 during 1988. In the water column the concentration of the reactive plus non-reactive Hg fraction (RIHg, detected after reduction with NaBH₄) showed a similar inter-year variation to the Hg content in seston, in contrast to the total Hg concentration in water column which was not decreased after the treatments. The increase in pH due to the remedial measures appeared to be the main cause of the decreased RIHg concentration, although the humic content (measured as colour) also explained a large part of the variation in RIHg concentration between the lakes. The decrease in the RIHg concentration in the lake water is the probable reason for the reduced Hg levels observed in the seston.     

Daphnia galeata life history response to heterogeneous food conditions and dissolved chemicals in the Rímov Reservoir

Laboratory cultivation experiments were carried out in order to determine the response of D. galeata to food conditions and dissolved chemicals in the water collected in three contrasting sites of the Rímov Reservoir. There were two types of cultivation media: (1) lake water with natural seston, filtered through a 40 m mesh size sieve, (2) lake water filtered through a membrane filter 0.45 m (or 0.22 m), with Scenedesmus subspicatus culture added to a concentration of 1.0 mg POC l^–1. In the treatment with natural seston, the parameters investigated (body length, filtering setae length, clutch size, egg volume and postembryonic development time) were determined mainly by the food level, which was higher in the upper part of the reservoir than near the dam. In one series, significantly larger clutches and smaller eggs together with longer postembryonic development (PED) were detected in daphnids cultivated in hypolimnetic water, compared to those in epilimnetic water. As particulate carbon concentrations were similar, this indicates that in that case food quality played a role shaping life histories in Daphnia. In some of the experiments with the filtered water, there was a detectable effect of dissolved chemical(s), which induced shorter PED and smaller primiparae in the animals cultivated in water from the upper part of the reservoir.     

Seston composition and the potential for Daphnia growth

A field survey was conducted to study the relationships amongst the composition of the seston, the nutritional value of the seston for herbivorous zooplankton (Daphnia), and selected water clarity parameters. Sixteen ponds in a wetland area and seven larger lakes, all located in south central Saskatchewan, Canada, were sampled for seston. The phytoplankton species were identified, and various biochemical seston variables were measured. A biotest using the zooplankter Daphnia magna, was employed to assess the nutritional value of the seston. The best seston variable to explain Daphnia growth was the phospholipid content (simple linear regression analysis: R ² _adj = 0.50). The water absorbance ratio A250/A365 was a good predictor of lipid content of the seston. Both the absorbance ratio A250/A365 and the dissolved organic carbon (DOC) concentration were negatively correlated with Daphnia growth. We hypothesize that the penetration of visible and ultraviolet radiation is an important determinant of seston quality, especially the phospholipid content, and that this has important implications for determining ultimate growth rates of herbivorous zooplankton.     

<Emphasis Type="Italic">Daphnia</Emphasis> distribution in Andean Patagonian lakes: effect of low food quality and fish predation

Food quality in terms of carbon (C):phosphorus (P) ratio can constrain the success of highly demanding P herbivores as Daphnia. North Andean Patagonian lakes are ultraoligotrophic with low nutrient concentrations and well-developed euphotic zones. We investigated the distribution of the large Daphnia commutata in relation with food quality (sestonic C:P ratio) and predation risk in these lakes. The predation risk was estimated based on the fish species present and their relative eye diameter and transparency of the lake. The C:P ratios in the lakes were high, varying from 350 to >1,200. The lakes with D. commutata had significantly lower C:P ratio than those without these daphnids. On the other hand, those lakes where Daphnia is present have the lower predation risk than those were Daphnia is absent. In addition, we carried out growth experiments with neonates and natural seston of three lakes with different C:P ratio. The growth rates were inversely related with C:P of the food. Food quality and predation risk together determined the success or failure of large Daphnia populations in these Andean clear ultraoligotrophic lakes.     

Responses of tropical cladocerans to a gradient of resource quality

1. The response of three tropical cladocerans to a gradient of resource quality was compared in a series of growth bioassays using seston collected from five lakes of different depth and trophic structure in Michigan, U.S.A. To assess the food quality in terms of digestibility, assimilation experiments were performed with ³²P‐labelled seston from the same lakes. Animals were also analysed for P‐content in their tissues at the end of these assays. 2. In general, assimilation efficiency was higher when animals fed on seston from shallow compared to deep lakes, and was significantly correlated with growth rates, suggesting that shallow lakes have the best food resources in terms of digestibility and P availability. 3. Results also showed that all cladoceran species responded similarly to the resource gradient, with lower growth rates in deep lakes and higher growth rates in shallow lakes, although the strength of response (sensitivity) was different among the species tested. 4. The cladoceran Moina micrura was the most sensitive species, and also displayed the highest P‐content and maximal growth rate, a pattern consistent with the growth rate hypothesis. 5. However, seston C : P ratio and growth rates in the different resources did not correlate with the animals’ P‐contents, showing an uncoupling between RNA‐phosphorus demands for growth and seston food quality. 6. In conclusion, our results support the idea that digestion resistance in algae is a major constraint to cladocerans in natural plankton communities.     

On the selection of thresholds for predicting species occurrence with presence‐only data

Presence‐only data present challenges for selecting thresholds to transform species distribution modeling results into binary outputs. In this article, we compare two recently published threshold selection methods (maxSSS and maxF_pb) and examine the effectiveness of the threshold‐based prevalence estimation approach. Six virtual species with varying prevalence were simulated within a real landscape in southeastern Australia. Presence‐only models were built with DOMAIN, generalized linear model, Maxent, and Random Forest. Thresholds were selected with two methods maxSSS and maxF_pb with four presence‐only datasets with different ratios of the number of known presences to the number of random points (KP–RP_ratio). Sensitivity, specificity, true skill statistic, and F measure were used to evaluate the performance of the results. Species prevalence was estimated as the ratio of the number of predicted presences to the total number of points in the evaluation dataset. Thresholds selected with maxF_pb varied as the KP–RP_ratio of the threshold selection datasets changed. Datasets with the KP–RP_ratio around 1 generally produced better results than scores distant from 1. Results produced by We conclude that maxF_pb had specificity too low for very common species using Random Forest and Maxent models. In contrast, maxSSS produced consistent results whichever dataset was used. The estimation of prevalence was almost always biased, and the bias was very large for DOMAIN and Random Forest predictions. We conclude that maxF_pb is affected by the KP–RP_ratio of the threshold selection datasets, but maxSSS is almost unaffected by this ratio. Unbiased estimations of prevalence are difficult to be determined using the threshold‐based approach.     

Elemental stoichiometry of lower food web components in arctic and temperate lakes

Lakes were surveyed to assess the potential patterns of latitudinalvariation in carbon:nitrogen:phosphorus (C:N:P) stoichiometryof lower food web components. Thirty-four lakes were surveyedat an arctic latitude (68°38'N, 149°38'W) and 10 lakesat a temperate latitude (46°13'N, 89°32'W) during 1997.The temperate data set was augmented with earlier survey resultsemploying similar methods. It was hypothesized that differencesin environmental variables across latitude would cause differencesin community C:N:P ratios, leading to differences in trophicinteractions. Physical measurements (light, temperature), sestonand zooplankton were collected from each lake. Seston and zooplanktonwere analyzed for C, N and P content, and zooplankton were countedand measured for biomass estimates. The degree of trophic interactionbetween seston and zooplankton was assessed by (i) measuringelemental imbalances between seston and zooplankton and (ii)calculating the potential recycling ratio by the zooplanktoncommunity available for seston. Seston C:nutrient, but not N:P,ratios were higher in temperate than arctic lakes. Conversely,arctic zooplankton had higher C:nutrient, but not N:P, ratiosthan zooplankton in temperate lakes. Elemental imbalances weregreater in temperate than in arctic lakes, but N:P stoichiometryof potential zooplankton recycling was nearly identical betweenthe two latitudes. Zooplankton community C:N:P ratios were notrelated to either latitude or seston C:N:P. In accordance withstoichiometric theory, relative abundances of calanoid copepodswere positively correlated with seston C:N in temperate lakes.Additionally, relative abundances of Daphnia were negativelycorrelated with seston C:N ratios in temperate and arctic lakes,and positively correlated with N:P ratios in the arctic. Ingeneral, these results suggest that seston and zooplankton communitystoichiometry differ across latitude, and these differenceshave the potential to affect trophic interactions.     

Stoichiometric relationships among producers,consumers and nutrient cycling in pelagic ecosystems

Most ecosystem models consolidate members of food-webs, e.g. species, into a small number of functional components. Each of these is then described by a single state variable such as biomass. When a multivariate approach incorporating multiple substances within components is substituted for this univariate one, a stoichiometric model is formed. Here we show that the Nitrogen:Phosphorus ratio within zooplankton herbivores varies substantially intraspecifically but not intraspecifically. By using stoichiometric theory and recent measurements of the N:P ratio within different zooplankton taxa, we calculate large differences in ratios of nutrients recycled by different zooplankton species. Finally, we demonstrate that N:P stoichiometry can successfully account for shifts in N- and P-limitation previously observed in whole-lake experiments. Species stoichiometry merges food-web dynamics with biogeochemical cycles to yield new insights.Abbreviations b N:P in zooplankton biomass - f N:P in algal biomass - L maximum accumulation eficiency - N:P ratio of nitrogen to phosphorus (moles:moles) - s N:P supply ratio from grazers - TN Total nitrogen = seston N + dissolved N (µmoles/liter) - TP Total phosphorus = seston P + dissolved P (µmoles/liter)     

Microsatellite diversity and population genetic structure of redfin culter ( Culter erythropterus ) in fragmented lakes of the Yangtze River

Redfin culter (Culter erythropterus) is a small lethic freshwater fish and widely distributed in the adjacent lakes of the Yangtze River of China. Five microsatellite loci were applied to investigate the genetic variation and population structure of redfin culter from seven lakes in the middle-and-lower reaches of the Yangtze River. The gene diversity was high among the populations (H > 0.9), the average number of alleles among seven populations was low with a range from 2.00 to 3.87. The mean observed (H _O) and expected (H _E) heterozygosity ranged from 0.111 to 0.419 and from 0.162 to 0.750, respectively. Significant deviations from Hardy-Weinberg Equilibrium expectation were found in 50% of the total locus-population combination tests in which heterozygote deficits were apparent. The analysis of molecular variance (AMOVA) indicated that the percentage of variance among and within these populations were 6.18 and 93.82, respectively. The Fst values (0.062, P < 0.001) among studied populations indicated that there were significant genetic differentiations among redfin culture populations from the scattered lakes with different connections to the Yangtze River. These results are useful for the evaluation and conservation of small freshwater fishes. The factors that may be involved in low intra-population polymorphism and the pattern of the population genetic structure of redfin culter from the Yangtze River were discussed. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: C. Sturmbauer     

Inheritance of heat-stable resistance to Meloidogyne incognita in Lycopersicon peruvianum and its relationship to the Mi gene

Summary The inheritance of heat-stable resistance to the root-knot nematode, Meloidogyne incognita (Kofoid and White) Chitwood, was studied in crosses between different accessions and clones of Lycopersicon peruvianum L. F₁, F₂ and BC₁ generations were evaluated for their index of resistance based on numbers of eggs and infective second-stage juveniles (J₂) per gram of root, and the segregation ratios were determined in experiments carried out at constant soil temperatures of 25 °C and 30 °C. L. peruvianum P.I. 270435 clones 3 MH and 2R2 and P.I. 126443 clone 1 MH, all heatstable resistant, were crossed with L. peruvianum P.I. 126440 clone 9 MH, which is susceptible at both 25 °C and 30 °C. All F₁ progeny were resistant at 25 °C and 30 °C; F₂ and BC₁ generations at 25 °C gave resistant: susceptible (RS) ratios of 151 and 31, respectively, which suggests that resistance is conditioned by two independently assorting genes. However, at 30 °C, RS ratios of 31 and 11 were observed for the F₂ and BC₁ generations, respectively. These results indicate that heat-stable resistance is conferred by a single dominant gene expressed at 30 °C, while the second resistance gene is heat unstable and not expressed at 30 °C. P.I. 270435 clones 2R2 and 3 MH and P.I. 126443 clone 1 MH were crossed with P.I. 128657 clone 3 R4 (source of gene Mi), which is resistant at 25 °C but susceptible at 30 °C. All of the F₁ progeny were resistant at 25 °C and 30 °C.TC₁ progeny of 270435-2 R2 x 128657-3 R4, 270435-3 MH x 128657-3 R4 and 126443-1 MH x 128657-3 R4 crossed with susceptible 126440-9 MH were all resistant at 25 °C and segregated in a 11 ratio at 30 °C. These results also suggest that the heat-stable resistance is monogenic and that it is non-allelic to gene Mi. The non-segregation of TC₁ progenies at 25 °C, suggests that the heat-unstable resistance factor in L. peruvianum P.I. 270435 clones 2 R2 and 3 MH and in P.I. 126443 clone 1 MH is allelic to or the same as gene Mi. We propose the symbol Mi-2 for the gene in P.I. 270435 that confers heat-stable resistance to M. incognita.