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Published online 1 March 2007
Published in Crop Sci 47:703-708 (2007)
© 2007 Crop Science Society of America
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CROP BREEDING & GENETICS

Disease Expression and Ecophysiological Yield Components in Sunflower Isohybrids with and without Verticillium dahliae Resistance

C. Creusa,c, M. E. Bazzalob,c,*, M. Grondonab, F. Andradea and A. J. Leónb

a Unidad Integrada INTA Balcarce, Facultad de Ciencias Agrarias UNMP, Ruta 226 Km 60.5 (7620) Balcarce, Buenos Aires, Argentina
b ADVANTA SEMILLAS SAIC Ruta 226 Km 60.5 (7620) Balcarce, Buenos Aires, Argentina
c contributed equally to this work

* Corresponding author (maria-eugenia.bazzalo{at}advantasemillas.com.ar).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Verticillium wilt (Verticillium dahliae Kleb.) is an important disease affecting sunflower (Helianthus annuus L.) in most production areas in Argentina, Canada, and the United States. Estimation of yield losses produced by the disease is difficult because of the absence of highly efficient chemical control and resistant hybrids of comparable yield potential. In this work nine pairs of isohybrids with and without resistance to V. dahliae were sown at five different locations and evaluated under natural infection to determine disease induced yield reductions across environments with varying V. dahliae incidence. The effects of the disease on physiological components of growth were studied in a separate trial with three pairs of isohybrids. Disease incidence and severity for resistant isohybrids (R) were zero or nearly zero independently of their genetic background in all locations, indicating the isohybrids were highly resistant or immune. In the most severe environments, grain and oil yield of susceptible isohybrids (S) were nearly 30% less than those of the resistance couterparts. The incorporated resistance resulted in an increased percent radiation interception and radiation use efficiency (RUE). However, the effect of the disease on crop growth rate was evident after flowering. Up to that phenological stage, leaf expansion, radiation interception and photosynyhetic rate did not differ between R and S isohybrids.

Abbreviations: DAS, days after sowing • DI, Disease Incidence • DS, Disease Severity • PAR, Photosynthetically active radiation • R isohybrid, isohybrid with V. dahliae resistance • RUE, Radiation use efficiency • S isohybrid, isohybrid susceptible to V. dahliae • SE, standard error.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Verticillium wilt (Verticillium dahliae Kleb.) is an important disease in many production areas in Argentina, Canada, and the United States (Sackston, 1980; Gulya et al., 1997). The appearance of interveinal chlorotic lesions, which become necrotic and progress systemically from the lower to the upper leaves results in a reduction of photosynthetic foliar surface with the consequent decline of photoasimilate availability (Sadras et al., 2000).

Verticillium dahliae is a soil-borne widely distributed pathogen which has a broad host range. In Argentina this disease is considered a major problem causing up to 30% of yield reduction in susceptible commercial sunflower hybrids (Pereyra and Escande, 1999). Nevertheless, the absence of efficient chemical controls and resistant hybrids of comparable yield potential had made the accurate evaluation of yield losses produced by this disease difficult.

Molecular markers associated with a gene of interest may be useful for selection in breeding programs, especially for agronomic traits as disease resistance (Michelmore, 1995). By means of marker-assisted selection, near-isogenic lines (NILs) were obtained from backcrossing programs. These NILs differ in their response to the predominant Argentinean V. dahliae race under artificial inoculation method (Creus, unpublished results).

The use of isohybrids obtained from the cross between NILs and the same female line constitutes a good experimental system to study the effect of a specific portion of DNA on the response of a crop to a pathogen (Young et al., 1988). This type of experimental system would also constitute an excellent approach to evaluate disease-induced yield reductions across environments with contrasting V. dahliae incidence.

V. dahliae was considered as a turgor reducing pathogen, according to Boote et al. (1983). Sadras et al. (2000), working on artificially inoculated plants, judged that the effect of this pathogen on physiological components was comparable to drought stress. Thus, V. dahliae would affect radiation interception by the crop and radiation use efficiency (Andrade and Sadras, 2000). Previous works on the effect of the disease on crop growth and yield were conducted with cultivars of contrasting resistance (Sadras et al., 2000). The availability of isohybrids with and without V. dahliae resistance presents an excellent opportunity to study the effects of the disease on physiological components of growth, independent of the genetic background.

The objectives of this work were: (i) to study the effect of incorporated resistance in different genetic backgrounds on V. dahliae symptom expression at different environmental conditions and (ii) to determine yield losses and their related ecophysiological components in susceptible isohybrids compared to their resistant counterparts under V. dahliae natural infection.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Plant Materials and Experimental Design
Crosses between two pairs of NIL (A and B male lines with resistance (R) and without resistance (S) to V. dahliae) and different female lines were performed to obtain 9 pairs of isohybrids.

The isohybrids and three commercial hybrids, used as environmental descriptors of known response to V. dahliae, were sown at 5 different locations in Buenos Aires province: San Cayetano (38°23‘ S, 59°30’ W; 29 Oct. 2000), Orense (38°23‘ S, 60°16’ W; 29 Oct. 2000), Daireaux (36°60‘ S; 61°75’ W, 17 Nov. 2000), Balcarce (37°45‘ S, 58°18’ W; 1 Nov. 2000) and 9 de Julio (35°12‘ S; 60° 25’ W, 19 Nov. 2000) (Trial 1). Final plant density was 3.3 pl m–2. The check hybrids were CF11 (Advanta) (Resistant), ACA884 (ACA) (Susceptible 1), and Mycogen2 (Dow-Morgan) (Susceptible 2).

Rainfall for November, December, January, February, and March was 35, 83, 119, 119, and 105 mm in Balcarce; 120, 72, 148, 72, and 414 mm in 9 de Julio; 37, 18, 134, 37, and 107 mm in Orense; 41, 29, 141, 46, and 165 mm in Daireauxand 22, 0, 151, 83, and 128 mm in San Cayetano.

A randomized complete block design with three replicates and four factors: location, female, male, and resistance (presence vs. absence) was employed. In each location, the experimental unit consisted of three rows 6 m long, 0.7 m apart.

A separate experiment (Trial 2) was conducted during 1999–2000 at Balcarce (37°45‘ S, 58°18’ W) to study the effect of V. dahliae on the ecophysiological yield components. The soil was a fine-loam with a minimum effective depth of 1.5 m and with an organic matter content of approximately 6% in the top 20 cm of depth. The area is characterized by moderate temperatures from December to March (20.3°C) and a frost-free period of about 150 d.

Rainfall was 66, 122, 224, and 164 mm for December, January, February, and March, respectively. Irrigation (40 mm) was applied to mitigate December and early January water deficits. The trial was conducted without any appreciable water or nutrient limitation.

The experimental design was the same as Trial 1 except that plots were comprised of 4 rows, 14 m long. Pairs of isohybrids 2A, 4B, and 5B were sown on 25 Nov. 1999 with a final plant density of 5.5 pl m–2.

In all experiments, weeds were controlled with herbicide before planting (1.5 L acetoclor + flurocloridona) and manually thereafter. No infestation of damaging insects occurred during the experiments.

Disease Evaluation
Middle row plants of each plot were evaluated for Disease Incidence (DI = % of plants with visible symptoms within the row) and Disease Severity (DS = row average of % of necrosed foliage per plant). These measurements were determined between 68 and 75 d after sowing (DAS). In trial 2, DI and DS were evaluated at 50, 62, 68, 75, 88, and 101 DAS.

Yield Evaluation
The central row (Trial 1) and 7.15 m of one of the two central rows (Trial 2) of the plots were harvested. Seed yield was adjusted to 11% moisture content. Yield loss attributable to V. dahliae was calculated for each pair of isohybrids as the diference between the average yield of the R isohybrid and the average yield of the S isohybrid, and expressed as a percentage of the R isohybrid.

Oil concentration in all samples was measured utilizing nuclear magnetic resonance (Analyser Magnet Type 10, Newport Oxford Instruments, Buckinghamshire, England). Samples were measured in triplicate and average oil concentration was calculated.

Ecophysiological Components
Green foliar area evolution was estimated in isohybrid pair 2A using a nondestructive method (Pereyra, 1978). The estimations were done in marked leaves in three plants per experimental unit at 43, 67, and 90 DAS.

Photosynthetically active radiation (PAR) interception, expressed as percentage of the incident radiation, was estimated in 2A and 4B pairs, according to Gallo and Daughtry (1986) using a linear sensor (LI-COR, Lincoln, NE, USA). Radiation interception was calculated as 100 * [1–(It/I0)], where It is the incident radiation just below the lowest green layer of leaves and I0 is the incident radiation at the top of the canopy. At least 5 determinations per plot were made at noon (+- 1 h) placing the sensor perpendicular to the rows. Measurements were taken at 39, 48, 63, 89, and 102 DAS.

Daily total incident photosynthetically active radiation obtained from a nearby experimental station was multiplied by the corresponding fraction of PAR interception and accumulated to obtain total PAR intercepted by the crop during a period. Daily values of fraction PAR interception were obtained by linear interpolation between measurements. Radiation use efficiency (RUE) was calculated as dry matter accumulated in a period divided by the total amount of PAR intercepted during the same period.

Aboveground dry matter accumulation was estimated in pairs 2A and 4B by harvesting plants from each experimental unit at different periods during the growing cycle. In each sampling date at least five plants per plot were cut at ground level, oven dried to constant weight and weighed.

Leaf photosynthesis was estimated in expanded, fully exposed leaves at the 4-sixth node from the top in three plants per replicate in pair 2A, at 49, 62, and 71 DAS. Measurements were taken on cloudless days at solar noon (±1h) using a portable photosynthesis system (LiCOR 6200, LiCOR, Lincoln, Nebraska, USA). Measurements were discontinued after flowering because of technical problems with the equipment.

In pairs 2A and 4B, grain samples were taken every week starting at flowering. In each sampling, a total of 50 grains were taken from five randomnly selected plants from the central rows per plot from two positions of the capitullum, the peripheral and intermediate region. Grains were oven dried to constant weight and weighed. From these data, individual grain weight and grain filling rate and duration were estimated by adjusting linear-plateau functions.

Statistical Analysis
Data was analyzed by ANOVA test. A significance level of 0.05 was used. Response variables defined as percentages were transformed for statistical analyses (arcsine of square root) but data without transformations are presented in the tables and figures. Covariance analysis was performed between percentage yield loss and DS at different environments.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Trial 1
Environmental Severity Description
In all locations natural V. dahliae infections were present and typical symptoms of the disease could be observed. No other important disease interfered with the evaluation. Disease incidence in the resistant check (CF11) was low for all the locations (0–14.5%). In contrast, DI in the susceptible checks had different values at the different locations (Table 1). All plants of the susceptible hybrids had visible symptoms in Orense and San Cayetano, indicating both uniform distribution of the fungus and proper conditions for fungal infection. Susceptible hybrids exhibited intermediate behavior at Balcarce and 9 de Julio. In Daireaux the susceptible checks were less affected than in the other locations.


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  		Table 1. Disease incidence (DI) and disease severity (DS) (%) and standard errors in three control hybrids at five different locations under Verticillium natural infections. Resistant: CF11; Susceptible 1: ACA884; Susceptible 2: Mycogen2.

 
With respect to DS, approximately 40% of the foliar tissue was affected in the susceptible checks and close to 0% in the resistant control in Orense and San Cayetano. In the other locations, intermediate DS were recorded. From these results, environments with different disease levels were identified.

Disease Evaluation of Isohybrids at Different Locations
Means of DI and DS for the 9 pairs of isohybrids at 5 locations are presented in Table 2. When S isohybrids were analyzed, significant location x male interaction for DS and DI was found, indicating that the expression of the disease between males varied across environments (P < 0.0031 for DS and P < 0.0006 for DI). When individual ANOVA was performed for S isohybrids in each location, statistical differences on DI were found only at Daireaux and Balcarce (P < 0.05). DI was higher in the materials derived from the A male than in those derived from the B male. In the more severe environments all S isohybrids showed incidences up to 100% as expected. In DS, significant differences between parental males were found at Balcarce (P < 0.001), San Cayetano (P < 0.012), and Orense (P < 0.001) and only at Orense for parental female effect (P < 0.008).


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  		Table 2. Disease incidence (DI) and disease severity (DS) (%) and standard errors for nine pair of sunflower isohybrids with resistance (R) and susceptible (S) to V. dahliae, at five different locations under natural infections. Crosses between two pairs of near-isogenic lines (NILs) (A and B male lines with [R] and without [S] V. dahliae resistance) and different female lines (1–5) were performed to obtain nine pairs of isohybrids (cross between B male NILs and female 4 not performed).

 
DI and DS for R isohybrids were zero or near zero independently of their genetic background in all locations indicating that these materials were highly resistant or immune (Table 2). In none of the isohybrids with resistance, the proportion of necrosed leaves was higher than 6% of the total foliage, which could be confounded with the natural senescence of lower leaves. From these results we concluded that R isohybrids could be considered as healthy controls for studies of yield losses due to V. dahliae attack.

Grain and Oil Yield of Isohybrids at Different Locations
Means of grain and oil yield for 9 pairs of isohybrids at 4 locations are presented in Table 3. Significant interactions were found for location x resistance both for grain and oil yield (P < 0.0005 and 0.0001, respectively), so each location was study separately. Location 9 de Julio was excluded from this analysis because of an inadequate plant stand in some plots. At San Cayetano, Orense, and Daireaux, significant effects of resistance on grain and oil yield were detected (P < 0.014 and 0.0075; 0.0001 and 0.0001; 0.0004 and 0.0001, respectively) and no interactions male x resistance and female x resistance were found. At Balcarce, a significant male x resistance interaction on oil yield (P < 0.0005) was detected, indicating that the effect of resistance on oil concentration depends on the background of the male parent. A significative reversion in the damage produced by the fungus was observed when resistance was included. In cases of strong disease severity environments, grain and oil yields of S isohybrids were nearly 30% less than those of the resistant counterparts. (Table 4).


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  		Table 3. Grain and oil yield means (kg/ha) and standard errors for nine pair of sunflower isohybrids with (R) and without (S) resistance to V. dahliae, at 4 different locations under natural infections. Crosses between two pairs of near-isogenic lines (NILs) (A and B male lines with (R) and without (S) V. dahliae resistance) and different female lines (1–5) were performed to obtain nine pairs of isohybrids (cross between B male NILs and female 4 not performed).

 

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  		Table 4. Mean values of grain and oil yield (kg/ha) and % yield losses for resistant (R) and susceptible (S) isohybrids at four different locations under V. dahliae natural infections. Yield loss attributable to V. dahliae was calculated for each pair of isohybrids as the diference between the average yield of the R isohybrid and the average yield of the S isohybrid, and expressed as a percentage of the R isohybrid.

 
Yield Loss Percentage and Disease Severity in Susceptible Isohybrids
A linear regression between yield losses and DS was fitted in all locations (P < 0.0048) (Fig. 1 ). The significant effect (P < 0.05) of location on yield loss indicates the influence of the environment on both the disease and yield losses. The absence of a significant interaction between site and DS suggests that a general equation of yield losses as a function of DS could be assumed. The slope of the regression lines indicated that 10% increase in necrosed foliage resulted in a 2.5% yield reduction.


Figure 1
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  		Figure 1. Relation between yield losses (%) and V. dahliae disease severity (DS) (%) at four different locations, under natural infection, in nine sunflower susceptible (S) isohybrids without resistance (R). Yield losses were calculated as (R yield–S yield)/R yield. Data from Trial 1.

 
Trial 2
Disease Evaluation
Disease expression at flowering was higher in the susceptible check 1 (70% DI and 29% DS) than in the resistant one (9% DI and 1.5% DS). These differences were observed throughout the growing period. In this trial, the disease level could be considered of intermediate severity because necrotic foliar tissue was limited to the lower third of the foliage.

Average DI and DS values were higher for S isohybrids than for R ones during the crop-growing period (P < 0.01) (Fig. 2 ). Disease incidence reduction over time in R cultivars could be due to the fact that (i) disease was limited to lower leaves that senesced progressively or (ii) senescence confounded disease symptoms. Resistant isohybrids and the resistant check showed a high level of resistance with DI and DS near zero.


Figure 2
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  		Figure 2. Disease progression over time in terms of disease incidence (DI) and disease severity (DS) for resistant (R) and susceptible (S) isohybrids under V. dahliae natural infections. Data are mean values of isohybrids 2A, 4B, and 5B. Data from Trial 2.

 
Yield and Ecophysiological Components
Average grain yield was 25% lower in S than in R isohybrids (2299 vs. 3077 kg.ha–1; P < 0.02). No significant interaction between resistance and isohybrid pair was observed (P = 0.41).

Percent radiation interception did not differ among hybrids before flowering. All cultivars reached or were very close to the critical leaf area index (95% radiation interception). Accordingly, green leaf area values of pair 2A did not show statistically significant differences between S and R isohybrids during the vegetative period, indicating that the leaf expansion process was unaffected by the disease. After that stage, radiation interception decreased more sharply for S isohybrids. A trend toward higher values of green leaf area for R isohybrids at flowering (430 vs. 400 cm2.leaf–1) and at early seed filling (360 vs. 260 cm2.leaf–1) stages was observed. Statistically significant differences were found for percent radiation interception between R and S isohybrids at 89 (p < 0.01) and 102 (p < 0.02) days after sowing. At the last sampling date, which was close to physiological maturity, R cultivars intercepted 50% of the incident radiation whereas S cultivars intercepted less than 30% (Fig. 3 ). These data indicate that V. dahliae induced an acceleration of leaf senescence in susceptible cultivars.


Figure 3
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  		Figure 3. Percent radiation interception as a function of time expressed as days after sowing (DAS) for resistant (R) and susceptible (S) isohybrids under V. dahliae natural infections. Standard error (SE) was 1.8 and 3.6% for 89 and 102 days after sowing (DAS), respectively. Data are means of isohybrid 2A and 4B. Flowering took place at 70 DAS and physiological maturity at approximately 105 DAS. Data from Trial 2.

 
Aboveground dry matter did not differ between S and R cultivars until the grain filling stages. Accordingly, leaf photosynthesis (measured at the third leaf from the top) during vegetative stages did not show statistical differences between R and S isohybrids, indicating that this process was unaffected by V. dahliae during that period (P > 0.75). Average leaf photosynthesis during the vegetative period was 46.2 and 45.8 µmol m–2 s–1 for R and S isohybrids, respectively.

At 92 DAS however, aboveground dry matter of S cultivars was 15% lower (P < 0.04) than that of their respective R isohybrids (Fig. 4 ). No resistance by hybrid pair interaction was found (P = 0.27). Maximal aboveground dry matter accumulation was obtained at this sampling date. After this moment, cumulative dry matter decreased (Fig. 4) in accordance to data presented in the literature (Andrade, 1995 and Dosio et al., 2000). At harvest, total aboveground dry matter was 18% lower for S isohybrids than for their R counterparts (P < 0.01).


Figure 4
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  		Figure 4. Aboveground dry matter accumulation as a function of time expressed as days after sowing (DAS) for resistant (R) and susceptible (S) isohybrids under V. dahliae natural infections. Differences were statistically significant at 92 DAS (SE = 79 g m–2) and at the final harvest (SE = 32 g m–2). Data are means of isohybrid 2A and 4B. Beginning of flowering took place at 70 DAS and physiological maturity at approximately 105 DAS. Data from Trial 2.

 
The resistance resulted in an increased radiation interception (370 vs. 340 MJ m–2, SE = 3.2 MJ m–2, p < 0.004) and RUE (2.59 vs. 1.82 g MJ–1, SE = 0.07 g MJ–1 p < 0.003) during the 56 to 92 DAS period. Thus, dry matter accumulation during this critical period was significantly lower in S than in R isohybrids (620 vs. 960 g m–2, SE = 48 g m–2 p < 0.04). Based on the data presented in Fig. 3 and 4, the differences in dry matter accumulation were mainly established after flowering.

S isohybrid 2A showed a 20% decrease in individual grain weight compared with the R counterpart (isohybrid) (36 vs. 45 mg; p < 0.01; Fig. 5 ). Grainfilling rate was 1.26 mg d–1 and 1.41 mg d–1 (r2 = 0.99) for S and R cultivars, respectively. Grainfilling duration was 4 d shorter for the former cultivar.


Figure 5
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  		Figure 5. Individual grain weight as a function of time expressed as days after flowering (DAF) for resistant (R) and susceptible (S) isohybrids under V. dahliae natural infections. Differences were statistically significant after 20 DAF (SE = 0.74, 1.26, 2.31, 1.74, and 2.00 mg at 14, 21, 29, 37, and 45 DAF, respectively). Data for isohybrid 2A. Grainfilling rate estimated from 14 to 29 DAF was 1.41 and 1.26 mg d–1 for R and S cultivars, respectively. Data from Trial 2.

 
Considering the complete growing cycle and taking into account differences in grainfilling duration, S isohybrids showed lower cumulative radiation interception (8.3%) and RUE (12%) than R isohybrids. Harvest index was 9% lower for the former cultivars. No differences in individual grain weight were observed within pair 4B isohybrids (data not shown), probably due to the small sample size and the random distribution of plants with symptoms.


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Effect of the Incorporated Resistance on Symptomatological Expression and Yield
A gradient of V. dahliae natural infection and the use of sunflower isohybrids provided experimental conditions to accurately assess the impact of Verticillium wilt on yield. Microsclerotia distribution and density in soil and the presence of different virulence pathotypes are factors that may affect the level of disease in sunflower and in other host species (Pereyra and Escande, 1994; Grogan et al., 1979; Smith and Rowe, 1984; Xiao and Subbarao,1998). V. dahliae races that differ in host range and pathogenecity have been cited in the USA and Argentina (Gulya et al., 1997). In Argentina, Bertero de Romano and Vázquez (1982), Quiroz et al. (2001) and Galella et al. (2004) described the presence of different races but there has not been a systematic monitoring of their distribution in this country. Nevertheless, our own unpublished work showes that there currently is one predominant race present in the five locations

Commercial sunflower hybrids selected as environmental indicators and S isohybrids confirmed the extensive genetic variability range of the behavior against V. dahliae.

Several authors found different reactions in commercial materials against V. dahliae and identified resistance sources in different materials (Hoes and Putt, 1964; Fick and Zimmer, 1974; Gulya, 1985) and wild species (Hoes et al., 1973; Seiler, 1992 Pustovoit and Krasnokutskaya, 1976). Putt (1964) studied the heritability of resistance in inbred lines and found qualitative or complete resistance in some lines, and additive resistance in others. Fick and Zimmer (1974) identified dominant resistance of one simple gene in different lines. In Argentina, experiments with 37 commercial sunflower hybrids evaluated under natural infection in six environments indicated the existence of high and stable resistant materials (Escande et al., 2000).

Our hypothesis was that the resistance incorporated into different backgrounds conferred similar protection against V. dahliae, and the resistance was expressed in a similar way across environments. Incorporation of V. dahliae resistance decreases the disease manifestations to a level near zero in all the environments. Relationships between disease severity level and yield loss percentage were established for the first time employing proper checks of similar genetic background. In this study a 10% increase in foliar symptoms in S cultivars resulted in a 2.5% mean yield loss. In previous works, Bertero de Romano et al. (1992) and Pereyra and Escande (1999) assessed the yield loss in commercial cultivars in relationship to their susceptibility under field conditions. Nevertheless, accurate determinations could not be obtained because checks of equal yield potential without disease symptoms were not available at that moment. Assuming that the predominant race was the same in the different locations, the different behavior between S isohybrids belonging from the crosses with different males showed that certain partial resistant genes were present in the B male. The detection of these types of resistance genes and combination (pyramiding) in sunflower would allow reaching more durable resistance levels against V. dahliae.

Ecophysiological Components
Plant disease could affect one or more ecophysiological yield components. The type of injuries produced by diseases could be classified according to their physiological effects on the crop (Boote et al., 1983). These physiological effects include decreased photosynthetic rate, leaf senescence acceleration, reduction of light interception, among others. Johnson (1987) considered that most of the effect of V. dahliae on potato crops was associated with reduction in leaf area. In this study, V. dahliae did not affect crop growth until flowering. Up to that phenological stage, leaf expansion, radiation interception and photosynyhetic rate did not differ between R and S isohybrids. These results differ from those reported by Sadras et al. (2000). These authors compared two commercial hybrids, artificially inoculated by V. dahliae in a field, and detected two main responses to the disease (i) an early reduction in plant leaf area explained by a reduction in leaf expansion and by accelerated senescence as the disease progressed and (ii) early reduction in leaf photosynthetic rate. These more severe effects would be explained by the magnification of the disease due to the inoculation method.

V. dahliae effects on yield of S isohybrids were related to lower plant growth rates during reproductive growth compared with R isohybrids. The lower growth rate of the S isohybrids cultivars was explained in part by a decrease in cumulative radiation interception. These differences in radiation interception during reproductive growth were mostly attributable to accelerated senescence in leaves of S isohybrids since leaf area expansion was not affected by the disease. The accelerated senescence was associated with an increase in DS after flowering (Fig. 2).

V. dahliae also reduced radiation use efficiency. The effect on this ecophysiological component was greater than that observed for radiation interception. The absence of the resistance resulted in lower RUE (30%) and cumulative radiation interception (8%) during the period from 56 to 92 DAS.

The differences between R and S isohybrids in radiation interception, RUE and dry matter accumulation were mainly established after flowering and the disease effects were more pronounced as the growing season progressed (Fig. 3, and 4).

Reductions in growth rate after flowering are generally related to decreased grain weight, grain number and grain quality (Andrade and Ferreiro, 1996; Cantagallo et al., 1997; Dosio et al., 2000). However, considering the grain weight data obtained for isohybrid pair 2A and the Verticillum induced yield reductions, the disease affected individual grain weight more than grain number. This observation is in agreement with the evolution of aboveground dry matter (Fig. 3). Differences in this variable are not evident before the effective grainfilling period, supporting the fact that the disease had a stronger impact on grain weight than on grain number. Finally, the lower harvest index of the S isohybrids reflects the stronger effect of the disease on reproductive than on vegetative growth.

The disease effects on radiation interception and RUE agree with the way this pathogen interacts with the plant. The pathogen colonizes the roots and produces a systemic infection and occlusion of the vascular system by conidia and mycelia that result on stomatal closure and leaf senescence (Mol et al., 1996). V. dahliae effects on sunflower growth were studied by comp aring isohybrids with and without resistance that are similar (almost identical) for the rest of the genome. Thus the results obtained through this experimental approach are reliable since they are not confounded by other traits that would also affect the ecophysiological components under consideration.


   ACKNOWLEDGMENTS
 
We want to thank Fabian Bianchini for collaborating in the field trials.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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Received for publication May 10, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





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The SCI Journals Agronomy Journal Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome