成株期叶锈病侵染对大麦植物光合作用和产量的影响

 应用¹⁴C同位素示踪分析法研究叶锈病(Puccinia hordei)在花期后(GS71)侵染对大麦植物光合作用的影响,结果表明,给叶片接菌后0至8天内无明显的光合病变,8天之后叶片的总光合量下降而单位绿叶面积光合效率提高,且这种下降%和提高%分别与锈病严重度呈显著的直线相关(r² ≥ 0.980,P<0.001);植株上单叶发病后其它器官的光合作用略有增强,全株叶片发病后穗部光合作用的增加达到显著水平(P<0.05)。
叶锈病在抽穗期(GS55)侵染后,植株籽粒产量和千粒重减低(P<0.01),而病叶干物重则显著增高(P<0.01),本文分析讨论了这些变化的生理机制。     

叶锈病侵染对大麦碳素代谢的影响

 本研究用¹⁴C同位素示踪法测定叶锈病侵染引起的大麦幼苗光合作用和光合产物分布的病变。试验以植株第一叶和第二叶所得的结果基本相同:接菌7天后病叶总光合量逐渐降低(10天后降低到健叶的60%以下)而单位绿叶面积光合率则明显提高(10天后提高约20%),且这种光合病变与锈病严重度间呈密切的线性相关(r ≥ 0.949,P<0.001);接菌3天后病叶光合产物的输出明显受阻,其截留的¹⁴C物质大量增加(为对照的3倍多)而输运到根和茎部的¹⁴C物质却严重减少。这些病变无疑是叶锈病抑制大麦植物生长发育的主要生理原因之一。     

Impact of <Emphasis Type="Italic">Erysiphe alphitoides</Emphasis> on transpiration and photosynthesis in <Emphasis Type="Italic">Quercus robur</Emphasis> leaves

Oak powdery mildew, (Erysiphe alphitoides) causes one of the most common diseases of oaks. We assessed the impact of this pathogen on photosynthesis and water relations of infected leaves using greenhouse-grown oak seedlings. Transpiration of seedlings infected by oak powdery mildew was also investigated. Altogether, E. alphitoides had a low impact on host gas exchange whether at the leaf or whole plant scale. Maximal stomatal conductance of infected leaves was reduced by 20–30% compared to healthy controls. Severely infected seedlings did not experience any detectable change of whole plant transpiration. The reduction in net CO₂ assimilation, A_n, was less than proportional to the fraction of leaf area infected. Powdery mildew reduced both the maximal light-driven electron flux (J_max) and the apparent maximal carboxylation velocity (Vc_max) although Vc_max was slightly more impacted than J_max. No compensation for the infection occurred in healthy leaves of partly infected seedlings as the reduced photosynthesis in the infected leaves was not paralleled by increased A_n levels in the healthy leaves of the seedlings. However, E. alphitoides had a strong impact on the leaf life-span of infected leaves. It is concluded that the moderate effect of E. alphitoides on oak might be related to the small impact on net CO₂ assimilation rates and on tree transpiration; nevertheless, the severe reduction in leaf life-span of heavily infected leaves may lead to decreased carbon uptake over the growth season.     

The influence of <Emphasis Type="Italic">Aphanomyces cochlioides</Emphasis> on selected physiological processes in sugar beet leaves and yield parameters

Alterations in some physiological processes in source leaves of sugar beet—such as chlorophyll and carbohydrate concentrations, stomatal conductance, rate of net photosynthesis and transpiration, and activity of the photosynthetic apparatus during root interaction with Aphanomyces cochlioides, were investigated. The influence of time of infection on plant health, yield quality and quantity was also examined. Plants were infected at different times of their growth period: on the sowing day and 4 or 8 weeks after sowing. A variation treatment, with non-pelleted seeds infected on the sowing day, was also analyzed. The experiment showed that development of disease symptoms depends on the time of infection and seed protection. A significant root yield decrease was observed in case of late infection, as compared to the yield of plants infected on the sowing day. The fresh weight of leaves was significantly increased where there was late infection. The infected plants showed a lower content of K⁺, Na⁺ and α-amino-N than did the controls. Infection by A. cochlioides induced chlorophyll degradation mostly in older leaves with the occurrence of natural senescence processes. Chlorophyll fluorescence parameters indicated that the photosynthetic apparatus of younger leaves was more sensitive to pathogen infection, when compared to older ones. The photochemical efficiency of photosystem II was reduced in young leaves mainly due to disturbance of the water-splitting system. In plants grown from non-pelleted seeds a strong impairment of PSII was observed only in those leaves which developed during early pathogen infection. In young leaves of plants infected in the fourth week after sowing, inhibition of the rate of net photosynthesis was correlated with the increase in intercellular CO₂ concentration, indicating some disturbance in the carbon assimilation phase. In mature leaves of late infected plants the reduction of photosynthesis net rate was associated with a decrease of stomatal conductance and an increase of diffusion resistance to CO₂ and H₂O, which was also the cause of the transpiration rate inhibition. When the leaves developed during early infection, an increase of specific leaf weight and accumulation of carbohydrates was observed. In mature leaves of non-protected plants infected on the sowing day, the recovery of all physiological processes was observed together with a diminution of disease symptoms.     

The influence of cyst nematodes and drought on potato growth. 1. Effects on plant growth under semi-controlled conditions

Potatoes were grown under a permanent rain shelter in mobile containers in soil with or without potato cyst nematodes (Globodera pallida). The plants were subjected to an early drought stress period starting at planting until 43 days after planting, to a late drought stress period starting at 43 days until senescence at 92 days and to a drought control. Dry matter weight and characteristics of leaves, stems, stolons and roots were determined at periodic harvests. The early drought stress and nematode infection affected many plant organ characteristics in similar ways. Numbers of leaves, specific leaf area, plant height, specific stem weight, leaf area ratio, mean tuber weight and harvest index were reduced by both stress factors at early stages of growth.Later on, interactions between both stress factors which influence the development rate of the plants led to more diverse plant reactions. Plants of all treatments rapidly senesced at about 90 days after planting. Uninfected plants had then depleted the soil nutrient supply whereas the plants grown in the inoculated soil senesced as a result of the potato cyst nematode infection.     

The effects of rust and anthracnose on the photosynthetic competence of diseased bean leaves

ABSTRACT The effects of rust (caused by Uromyces appendiculatus) and anthracnose (caused by Colletotrichum lindemuthianum) and their interaction on the photosynthetic rates of healthy and diseased bean (Phaseolus vulgaris) leaves were determined by gas-exchange analysis, in plants with each disease, grown under controlled conditions. The equation P(x)/P(0) = (1 - x)() was used to relate relative photosynthetic rate (P(x)/P(0)) to proportional disease severity (x), where beta represents the ratio between virtual and visual lesion. The beta values obtained for rust were near one, indicating that the effect of the pathogen on the remaining green leaf area was minimal. The high values of beta obtained for anthracnose (8.46 and 12.18) indicated that the photosynthesis in the green area beyond the necrotic symptoms of anthracnose was severely impaired. The impact of anthracnose on bean leaf photosynthesis should be considered in assessments of the proportion of healthy tissue in diseased leaves. The accurate assessment of the healthy portion of the leaf could improve the use of concepts such as healthy leaf area duration and healthy leaf area absorption, which are valuable predictors of crop yield. The equation used to analyze the interaction between rust and anthracnose on the same leaf was P(z) = P(0) (1 - x)(x) x (1 - y)(y), where P(z) is the relative photosynthetic rate of any given leaf, P(0) is the maximum relative photosynthetic rate, x is anthracnose severity, y is rust severity, betax is the beta value for anthracnose in the presence of rust, and betay is the beta value for rust in the presence of anthracnose. From the resulting response surface, no interaction of the two diseases was observed. Dark respiration rate increased on diseased leaves compared with control leaves. The remaining green leaf area of leaves with both diseases was not a good source to estimate net photosynthetic rate because the effect of anthracnose extended far beyond the visual lesions, whereas the effect of rust on photosynthesis was essentially limited to the pustule plus halo.     

Physiological alterations in pepper during wilt induced by Phytophthora capsici and soil water deficit

Capsicum annuum cv. Morrón inoculated at the collar with Phytophthora capsici developed cortical stem necrosis and severe wilting soon after infection. Water potential, specific conductivity, ethylene evolution, CO₂ and water vapour exchange rates and chlorophyll content were monitored during symptom development. Well watered and non-inoculated water-stressed plants were used as controls. Necrosis developed upwards from the collar 4 days after inoculation coupled with a 80% reduction in hydraulic conductivity. Leaf water potential progressively decreased from -0.17 to -2.41 MPa in 2 weeks. The rate of ethylene evolution was significantly higher in infected than in non-inoculated stressed plants for similar water potentials and originated in the necrotic segment of disease stem tissue. Net photosynthesis and leaf conductance markedly decreased (74%) 4 days after infection, coupled to a burst in dark respiration (1.5 times) but were not associated with water stress alone. This suggests that the pathogen reduced photosynthesis initially through stomatal closure which was not directly mediated by water stress. The possible implication of ethylene in impaired stem conductivity and altered gas exchange of infected plants is discussed.     

Synergism of Pratylenchus penetrans and Verticillium dahliae Manifested by Reduced Gas Exchange in Potato

ABSTRACT The effects of solitary and concurrent infection by Pratylenchus pene-trans and Verticillium dahliae on gas exchange of Russet Burbank potato (Solanum tuberosum) were studied in growth chamber experiments. Treatments were P. penetrans at low, medium, and high density; V. dahliae alone at one initial density; the combination of the nematode at these three densities and V. dahliae; and a noninfested control. Gas exchange parameters of leaf cohorts of different ages in the different treatments were repeatedly measured with a Li-Cor LI-6200 portable photosynthesis system. At 45 days after planting, joint infection significantly reduced net photosynthesis, stomatal conductance, and transpiration of 1- to 25-day-old leaf cohorts. Intercellular CO(2) levels were significantly increased by co-infection, especially in older leaves. The synergistic effect of co-infection on gas exchange parameters was greater in the oldest cohort than in the youngest cohort. No consistent effects on leaf gas exchange parameters were observed in plants infected by the nematode or the fungus alone. The relationship between the assimilation rate and stomatal conductance remained linear regardless of solitary or concomitant infection, indicating that stomatal factors are primarily responsible for regulating photosynthesis. The significant reduction of gas exchange in leaves of co-infected plants without reduction in intercellular CO(2) concentrations suggests that nonstomatal factors also play a role when both organisms are present.     

The occurrence of rust disease,and biochemical and physiological responses on <Emphasis Type="Italic">Apocynum venetum</Emphasis> plants grown at four soil water contents,following inoculation with <Emphasis Type="Italic">Melampsora apocyni</Emphasis>

We studied the development of rust disease, and biochemical and physiological responses, on Apocynum venetum plants inoculation with Melampsora apocyni that were growing in a greenhouse at four relative soil water contents. The soil conditions were 25% (severe drought), 50% (mild drought), 75% (optimal) and 100% (waterlogging) relative soil water content. Plants exposed to drought and waterlogging stress had a lower number of open stomata before inoculation, corresponding with the disease index on the 10th day after inoculation being lower than that of the optimal soil water condition. Inoculated plants exposed to severe and mild drought stress had a gradually enhanced resistance to the rust disease from the 10th day after inoculation, corresponding with the enhanced activity of polyphenol oxidase and phenylalanine ammonialyase. For the inoculated plants exposed to severe drought stress, hydrogen peroxide always remained at the highest level for any treatment, and they had a rapidly enhanced activity of peroxidase, two factors that were associated with suppression of disease development. A. venetum plants exposed to double stress of waterlogging and disease had a high activity of peroxidase that not only removed reactive oxygen to prevent or reduce cell injury but also enhanced resistance to the rust disease. In addition, a rapidly enhanced activity of phenylalanine ammonialyase in the waterlogging condition from the 25th day after inoculation was also associated with an enhanced resistance to the rust disease. Drought and waterlogging stress had a negative effect on the leaf photosystem, and in particular, there was a significant decrease in the net photosynthetic rate with an increase in the duration and degree of drought stress, and this lead to a statistically significant decrease in the weight of aboveground tissue compared with that of plants under optimal soil water condition (P < 0.05). Inoculating with M. apocyni had a slight effect on photosynthesis of plants during early disease development, but the physiological function of diseased leaves under the drought stress was damaged more seriously than that of non-inoculated plants in later disease development, leading to a large reduction in the net photosynthetic rate. However, this reduction did not cause a statistically significant (P > 0.05) decrease in the weight of aboveground tissue compared with that of non-inoculated plants under drought stress.     

The influence of cyst nematodes and drought on potato growth. 2. Effects on plant water relations under semi-controlled conditions

Potatoes were grown under a permanent rain shelter in mobile containers in soil with and without potato cyst nematodes (Globodera pallida). The plants were either subjected to an early drought stress period from planting until 43 days after planting, to a late drought stress period during tuber bulking or to a drought control. Leaf water potentials, stomatal diffusion resistances for water vapour, transpiration rates, dry matter accumulation and water use efficiencies of the plants were determined periodically. Both drought and nematodes decreased leaf water potential and increased stomatal resistance.Drought led to a higher water use efficiency, cyst nematode infection, however, reduced the water use efficiency at early stages of growth, and increased it at later stages. It is concluded that at least two main growth reducing mechanisms exist of which the relative importance varies with time. Firstly, reduced apparent assimilation rates, which are unrelated to a change in the water balance caused by the initial attack by the cyst nematodes. Secondly, a reduced dry matter accumulation resulting from a decrease of water uptake. Effects of drought and cyst nematode infection on plant growth and water relations were not always additive mainly because infected plants used up less water leading to less drought stress.     

Relationship between physiological response of French beans of different age to Meloidogyne incognita and subsequent yield loss

A study was made of the effect of a single generation of the root-knot nematode Meloidogyne incognita on the growth of potted French bean plants ( Phaseolus vulgaris ) inoculated at different stages of plant maturity. In separate experiments. 3-, 11- and 13-day-old plants were inoculated before primary leaf expansion (BPLE). at the appearance of trifoliate leaves (TRIF) and at the flower bud (BDS) stages respectively, with 0, 2000, 4000 or 8000 second-stage juvenile nematodes and maintained in a growth chamber under controlled conditions. The photosynthetic rate of the plants inoculated at the TRIF and BDS stages decreased significantly with increasing inoculum level 7 days after inoculation. Although the respiration rate did not significantly change throughout the experimental period, the ratio of photosynthetic to respiration rate decreased significantly with increasing nematode inoculum level and duration of infection. Chlorophyll content, plant dry weight and the numbers of buds, flowers, pods and seeds were significantly lower in infected plants than in the controls; this effect increased with increasing levels of nematode inoculum for all three plant stages. The leaf area was significantly smaller only when nematode infection occurred at the BPLE stage. The plants which were youngest at the time of nematode infection produced the lowest yield; this appeared to result from the effect of nematodes on photosynthesis and related physiological processes.     

Biochemical and physiological responses to long‐term Citrus tristeza virus infection in Mexican lime plants

Reactions that occur when a plant is subjected to Citrus tristeza virus (CTV) infection often result in triggering of numerous defence mechanisms to fight the infection. The reactions vary according to virus strain, host genotype, time of exposure to the infection and environmental conditions. To date, no study has examined in detail the consequences of 10‐year exposure to CTV infection on the biochemical and physiological status of susceptible Mexican lime plants (Citrus aurantifolia). To understand the reaction of such plants, changes in nutrient status, total proteins, enzyme activity involved in scavenging of reactive oxygen species, photosynthetic and transpiration rates, chlorophyll content, membrane permeability and water content were analysed in plants infected with different CTV isolates and in healthy plants. The activity of superoxide dismutase and polyphenol oxidase significantly decreased in the infected leaves, and membrane permeability was lower in the infected plants. Macro‐ and micronutrient elements were significantly changed: concentrations of leaf nitrogen, zinc, magnesium and iron were elevated but potassium concentration depressed in comparison to noninfected control leaves. Levels of other analysed nutrient elements, enzymes, photosynthesis and stomatal conductance, chlorophyll content and relative water content were unchanged. Clear physiological changes were found among infected and noninfected control plants but none between plants infected with different CTV isolates. The data suggest that some of the defence mechanisms investigated here were suppressed due to the continuous and long‐term pressure of biotic stress.     

The effect of fungal infection and nitrogen fertilization on the carbohydrate composition of Rumex obtusifolius leaves

The effect of infection by the rust fungus Uromyces rumicis and fertilization by different concentrations of nitrate or ammonium solutions on the concentrations of alcohol-soluble carbohydrates, fructans and starch in Rumex obtusifolius leaves was investigated. In leaves from healthy plants there was an increase in concentration of alcohol-soluble carbohydrates and fructans as the concentration of nitrate given was decreased, with the exception of nitrogen-stressed plants (those fed less than 10 mM l^-1 nitrate) which had a lower concentration of alcohol-soluble carbohydrates than plants fed 10 mM l^-1 nitrate. There was an increase in fructan and starch concentration in leaves as the concentration of ammonium solution given was decreased. The concentration of alcohol-soluble carbohydrates was reduced in infected leaves and increased in healthy leaves on infected plants, while the fructan concentration increased in infected leaves, compared to healthy leaves. The effects of infection were consistent over the range of nitrogen concentrations used, and thus were additive to the effects of fertilization. These results confirm the known effects of fertilization or fungal infection, singly, and indicate that, when combined, nitrogen deficiency and fungal infection may produce an additive stress on the plant.     

Effects of Simplicillium lanosoniveum on Phakopsora pachyrhizi, the soybean rust pathogen, and its use as a biological control agent

The fungus Simplicillium lanosoniveum was isolated from soybean leaves infected with Phakopsora pachyrhizi, the soybean rust pathogen, in Louisiana and Florida. The fungus did not grow or become established on leaf surfaces until uredinia erupted, but when soybean rust signs and symptoms were evident, S. lanosoniveum colonized leaves within 3 days and sporulated within 4 days. Development of new uredinia was suppressed by about fourfold when S. lanosoniveum colonized uredinia. In the presence of S. lanosoniveum, uredinia became increasingly red-brown, and urediniospores turned brown and germinated at very low rates. Assays using quantitative real time polymerase chain reaction revealed that the fungus colonized leaf surfaces when plants were infected with P. pachyrhizi, either in a latent stage of infection or when symptoms were present. However, when plants were inoculated before infection, there was no increase of DNA of S. lanosoniveum, suggesting that the pathogen must be present in order for the antagonist to become established on soybean leaf surfaces. We documented significantly lower amounts of DNA of P. pachyrhizi and lower disease severity when soybean leaves were colonized with S. lanosoniveum. These studies documented the mycophilic and disease-suppressive nature of S. lanosoniveum.     

Mature plant resistance of potato against some virus diseases. I. Concurrence of development of mature plant resistance against potato virus X,and decrease of ribosome and RNA content

Four of five weeks after planting a group of potato plants ‘Bintje’ was inoculated with potato virus X (PVX). Other groups were inoculated at intervals of 14 days. Tubers produced by plants inoculated 35 days after planting were all infected. The plants inoculated 49 days or later after planting produced few infected tubers. The latter had developed mature plant resistance against PVX infection. The ribosome and RNA contents of leaves were measured by application of adsorption chromatography. A rapid decrease in ribosome and RNA contents occurred in plants at the time of rapid increase in the rate of mature plant resistance. The decrease was most distinct in the fifteenth leaf and therefore the contents in this leaf seem to give a good indication of the rapid increase in resistance.     

Salicylic acid alleviates growth inhibition and oxidative stress caused by zucchini yellow mosaic virus infection in Cucurbita pepo leaves

The effects of zucchini yellow mosaic virus (ZYMV) infection and pretreatments with salicylic acid (SA) on biomass accumulation of pumpkin (Cucurbita pepo cv. Eskandarani) were investigated. The response of photosynthesis, transpiration and the activities of antioxidant enzymes in leaves was also considered. Significant reductions in growth parameters (i.e. leaf area, biomass and shoot height), photosynthesis and chlorophyll a and b content were detected in ZYMV-infected leaves in comparison to healthy controls. Antioxidant enzyme activities were increased up to 3-fold for peroxidase (POD), 2-fold for ascorbate peroxidase (APX) and catalase (CAT) activities and 1.3-fold for SOD activity by virus infection. ZYMV infection also caused increases in H₂O₂ and malondialdehyde (MDA) contents. These results suggest that ZYMV infection causes oxidative stress in pumpkin leaves leading to the development of epidemiological symptoms. Interestingly, spraying pumpkin leaves with SA led to recovery from the undesirable effects of ZYMV infection. Leaves treated with 100 μM SA three days before inoculation had the appearance of healthy leaves. No distinct disease symptoms were observed on the leaves treated with 100 μM SA followed by inoculation with ZYMV. In non-infected plants, SA application increased activities of POD and superoxide dismutase (SOD) and inhibited APX and CAT activities.In contrast, SA treatment followed by ZYMV inoculation stimulated SOD activity and inhibited activities of POD, APX and CAT. In addition, MDA displayed an inverse relation, indicating inhibition of lipid peroxidation in cells under SA treatment. It is suggested that the role of SA in inducing plant defense mechanisms against ZYMV infection might have occurred through the SA-antioxidant system. Such interference might occur through inhibition or activation of some antioxidant enzymes, reduction of lipid peroxidation and induction of H₂O₂ accumulation following SA application.     

磁化水对不同土壤水分下黄瓜幼苗生长、光合和养分吸收的影响

采用二因素二水平完全试验设计,研究了砂培条件下磁化处理（分别为0.3T磁场强度处理的磁化蒸馏水和蒸馏水处理）对不同土壤水分条件下（正常水分处理：田间持水量的80%~85%;干旱处理：田间持水量的40%~45%,于黄瓜4~5叶期维持7 d）黄瓜幼苗生长、水分关系、光合作用和养分吸收的影响。结果表明,磁化水浇灌显著降低黄瓜幼苗的生长和水分利用,其中茎粗、地上部生物量、根生物量、总生物量、根体积、根表面积、整株耗水量和整株水分利用效率分别降低6.7%、8.9%、19.1%、9.9%、22.1%、18.5%、6.2%和10.9%;磁化水浇灌导致黄瓜幼苗叶SPAD值降低了3.7%,叶气孔导度和蒸腾速率则分别增加了21.8%和17.5%,叶瞬时水分利用效率降低了17.7%,但对净光合速率影响不大;磁化水浇灌使PSII最大光化学效率和电子传递速率分别降低了5.2%和18.6%,同时增加叶片中K含量。除叶片K含量外,干旱条件下磁化水对黄瓜生长和生理代谢影响不大,表明磁化水的作用依赖于土壤水分条件。     

Salicylic acid alleviates growth inhibition and oxidative stress caused by zucchini yellow mosaic virus infection in Cucurbita pepo leaves

The effects of zucchini yellow mosaic virus (ZYMV) infection and pretreatments with salicylic acid (SA) on biomass accumulation of pumpkin (Cucurbita pepo cv. Eskandarani) were investigated. The response of photosynthesis, transpiration and the activities of antioxidant enzymes in leaves was also considered. Significant reductions in growth parameters (i.e. leaf area, biomass and shoot height), photosynthesis and chlorophyll a and b content were detected in ZYMV-infected leaves in comparison to healthy controls. Antioxidant enzyme activities were increased up to 3-fold for peroxidase (POD), 2-fold for ascorbate peroxidase (APX) and catalase (CAT) activities and 1.3-fold for SOD activity by virus infection. ZYMV infection also caused increases in H₂O₂ and malondialdehyde (MDA) contents. These results suggest that ZYMV infection causes oxidative stress in pumpkin leaves leading to the development of epidemiological symptoms. Interestingly, spraying pumpkin leaves with SA led to recovery from the undesirable effects of ZYMV infection. Leaves treated with 100 μM SA three days before inoculation had the appearance of healthy leaves. No distinct disease symptoms were observed on the leaves treated with 100 μM SA followed by inoculation with ZYMV. In non-infected plants, SA application increased activities of POD and superoxide dismutase (SOD) and inhibited APX and CAT activities.In contrast, SA treatment followed by ZYMV inoculation stimulated SOD activity and inhibited activities of POD, APX and CAT. In addition, MDA displayed an inverse relation, indicating inhibition of lipid peroxidation in cells under SA treatment. It is suggested that the role of SA in inducing plant defense mechanisms against ZYMV infection might have occurred through the SA-antioxidant system. Such interference might occur through inhibition or activation of some antioxidant enzymes, reduction of lipid peroxidation and induction of H₂O₂ accumulation following SA application.     

The effects of beet yellows virus on the growth and physiology of sugar beet (Beta vulgaris)

The effect of beet yellows virus (genus Closterovirus , BYV) on sugar beet growth was studied in a series of field and glasshouse experiments. Infection reduced total plant weight by 20%, primarily through a 25% reduction in storage root growth. Sugar extraction efficiency was depressed by an increase in root impurities. BYV had little effect on above-ground yield or total crop cover but did decrease green cover significantly. Infection did not reduce water extraction depth in field experiments despite decreasing lateral root growth in the glasshouse. The growth reduction in infected plants resulted from both a decrease in net photosynthesis and an increase in the proportion of light intercepted by yellow leaves. Damage to the photosynthetic mechanism at least partly caused the reduction in net photosynthesis.     

Photosynthesis,transpiration and plant growth characters of different potato cultivars at various densities of Globodera pallida

Plants of four potato (Solanum tuberosum L.) cultivars were grown in pots in a greenhouse at five densities ofGlobodera pallida between 0 and 300 eggs per gram of soil. Photosynthesis and transpiration of selected leaves were measured at 30, 37, 49 and 60 days after planting. Stem length was recorded at weekly intervals. Plants were harvested 70 days afteer planting and various plant variables were determined.At 30 days after planting, when second and third stage juveniles were present in roots, both photosynthesis and transpiration rates were severely reduced byG. pallida. In the course of time these effects became less pronounced. Water use efficiency was reduced byG. pallida between 30 and 49 days, but not at 60 days after planting. The results suggest independent effects ofG. pallida on stomatal opening and on photosynthesis reactions. There were no consistent differences among cultivars in the response of leaf gas exchange rates and water use efficiency to nematode infection. Reduction of photosynthesis byG. pallida appeared additive to photosynthesis reduction due to leaf senescence.Total dry weight was reduced by 60% at the highestG. pallida density. Weights of all plant organs were about proportionally affected. Shoot/root ratio was not affected and dry matter content was reduced. Stem length and leaf area were most strongly reduced during early stages of plant-nematode interaction. The number of leaves formed was only slightly reduced byG. pallida, but flowering was delayed or inhibited. Reduction of total dry weight correlated with reduction of both leaf area and photosynthesis rate. Leaf area reduction seems the main cause of reduction of dry matter production. Tolerance differences among cultivars were evident at 100 eggs per gram of soil only, where total dry weight of the intolerant partially resistant cv. Darwina was lower than that of the tolerant partially resistant cv. Elles and of the tolerant susceptible cv. Multa. The tolerance differences were not correlated with leaf photosynthesis and transpiration. Apparently these processes are not part of tolerance of plants.