Os efeitos de Meloidogyne javanica ou de Heterodera glycines sobre a fotossíntese, a área foliar e a produção de soja foram estudados em 4 experimentos, dois para cada patossistema, desenvolvidos em casa de vegetação. No caso de M. javanica,foram utilizadas as variedades Embrapa 133 e Coodetec 201, tidas como suscetível e tolerante, respectivamente, e no de H. glycines, Embrapa 133 e Pintado, respectivamente suscetível e resistente. A soja foi semeada em uma mistura de solo + areia, desinfestada com brometo de metila e contida em copos de plástico com capacidade de 200 mL. Cinco a oito dias após a semeadura, foi feito o desbate, deixando-se uma planta por copo e, no dia seguinte, foi realizada a inoculação de cada planta com 0, 1200, 3600, 10800, 32400 ou 97200 J₂ de M. javanica ou M. javanica, nas duas variedades testadas. Por outro lado, na variedade suscetível, o parasitismo por H. glycines causou claras reduções da taxa fotossintética e do teor de clorofila foliar, bem como o amarelecimento das folhas. Ambas as espécies de nematóides causaram uma expressiva redução na área foliar, na massa fresca das raízes e na produção de massa seca total da parte aérea e de grãos de soja. No caso de M. javanica, esses efeitos foram mais evidentes nas maiores densidades populacionais iniciais e foram verificados nas duas variedades. Em se tratando de H. glycines, no entanto, mesmo na menor densidade populacional os efeitos sobre a produção foram drásticos. No experimento de curta duração, mesmo a variedade resistente Pintado evidenciou, sob os mais altos níveis de população inicial, pequenas reduções da área foliar, da massa fresca de raízes e da produção de massa seca da parte aérea. A produção de soja, parasitada tanto por M. javanica como H. glycines, apresentou alta correlação com a integral da área foliar, ficando a indicação de inclusão dessa variável em modelos de previsão de danos que levem em consideração a fisiologia de produção de plantas doentes.

The effects of Meloidogyne javanica or Heterodera glycines on the leaf area, photosynthesis, and yield of soybeans were studied in four greenhouse trails, two to each pathosystem. Depending on the pathosystem, the varieties Embrapa 133 and Coodetec 201, respectively rated as susceptible and tolerant to M. javanica, or Embrapa 133 and Pintado, respectively susceptible and resistant to H. glycines were used. Soybean seeds were sown in 200 mL plastic pots filled with a mixture of field soil + sand (1:1), previously disinfested with methyl bromide. From five to eight days after sowing, seedlings were thinned to one per pot, and day later inoculated with 0; 1,200, 3,600; 10,800; 32,400 or 97,200 J₂ of either M. javanica or H. glycines (= P_i values). After 5 days, the inoculated soybean plants transplanted to clay pots containing the same substrate previously mentioned. The first replication of each trial was carried out until 48-49 days from the inoculation as to allow just one generation of the nematodes to be completed; in these trials 1.5 L pots were used. The second replication extended until harvest time and for that 5.0 L pots were used. Measurements of photosynthesis rate, stomatic condutance, chlorophyll fluorescence, leaf color, leaf area, and chlrophyll leaf content were taken at 10-day intervals along the experiments. Data on fresh root weight, top dry weight, grain yield, number of eggs/gram of roots, and reproduction factor were obtained at the end of the trials. A statistical design of randomized blocks with the treatments arranged according to a 2 x 6 factorial model (soybean varieties x nematode initial inoculum levels) was established. Each experimental unit, represented by one plant growing per pot, was replicated 10 times. Values of photosynthesis rate, stomatic condutance, chlorophyll fluorescence, leaf color and chlorophyll content were very slightly affected by M. javanica for both tested varieties. In opposition, there was a clear reduction in both photosynthetic rate and chlorophyll content, as well as a marked yellowing of “Embrapa 133’ leaves as affected by H. glycines. Leaf area, fresh root weight, top dry weight, and grain yield were extensively reduced by each of the nematodes, but in the case of M. javanica infection, despite of the variety, these depressive effects largely depended on the use of high P_i values. Conversely, even at the lowest P_i (= 1,200), the effects of H. glycines on the top dry weight or grain yield were much severe. Even being rated as resistant, the variety Pintado was slightly affected (leaf area, fresh root and top dry weight values) at the higher P_is of H. glycines in the short-term trial. As no complete development or reproduction of the nematode in the roots were noticed, this effect was assigned to a massive initial penetration of juveniles in the root system of the plantlets. In spite of the plants being parasitized by either M.javanica or H. glycines, soybean yield was highly correlated with the integrated leaf area, what may lead this parameter to be used in potential damage predicting models which include physiological aspects of nematode-diseased plants.