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Crop Science 40:1637-1644 (2000)
© 2000 Crop Science Society of America

CROP PHYSIOLOGY & METABOLISM

Solar Radiation Intercepted during Seed Filling and Oil Production in Two Sunflower Hybrids

Guillermo A.A. Dosio, Luis A.N. Aguirrezábal, Fernando H. Andrade and Víctor R. Pereyra

Unidad Integrada Facultad de Ciencias Agrarias (UNMdP), Estación Experimental Agropecuaria INTA Balcarce, C.C. 276, 7620 Balcarce, Argentina

laguirre{at}mdp.edu.ar


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 Conclusion
 REFERENCES
 
The expression of the components of oil yield in sunflower (Helianthus annuus L.) could depend on environmental conditions during the seed filling period. In this work, we investigated the effect of intercepted PAR (photosynthetically active radiation) during seed filling on the weight of individual seeds and oil concentration in two hybrids, one with low and one with high oil concentration potential. Three experiments were carried out in three different years under good water and nutrient conditions. Intercepted PAR was modified by shading or thinning to reduce plant population and by the combination of shading and thinning. The treatment application date corresponded to 46 ± 2°C days (base temperature = 6°C) after R6. Greater intercepted PAR increased weight per seed in both hybrids. Oil concentration was affected in `Dekalb G-100' (G-100, high oil concentration and black hull) but remained unaffected in `Northrup King Tordillo' (NKT, low oil concentration and stripped hull) although the thinned treatments intercepted up to 7.5 times the amount of radiation per plant than the shaded ones. About 84 (G-100) and 80% (NKT) of the variability among treatments and experiments in weight per seed was accounted for by the PAR intercepted during seed filling. The variation in oil concentration in G-100 was related to intercepted solar radiation (r2 = 0.93). Our results suggest that intercepted PAR during seed filling plays a primary role in determining oil production in sunflower, and considering genotypic differences may be important when using relationships between weight per seed or oil concentration and intercepted PAR as modeling tools.

Abbreviations: ADM, per plant aboveground dry matter • DAE, days after emergence • HI, harvest index • PAR, photosynthetically active radiation • PM, physiological maturity • TA, treatment application date


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 Conclusion
 REFERENCES
 
SUNFLOWER is mainly cultivated for its oil. Oil yield per plant is the result of number of seeds per capitulum, weight per seed, and oil concentration. These three components are determined by genetic factors (see reviews of Merrien, 1992; Sadras and Villalobos, 1994; Connor and Hall, 1997) but they can be highly modified by the environment and growth conditions (Cardinali et al., 1982; Steer et al., 1984; Hall et al., 1985; Andrade and Ferreiro, 1996; Bange et al., 1997).

Incident and intercepted PAR during the seed filling period may affect weight per seed and oil concentration (Andrade and Ferreiro, 1996). Moreover, these effects could vary depending on the hybrid. Hybrids could differ in their oil concentration potential. Some (usually with black fruit pericarp [hull], as the one studied by Andrade and Ferreiro, 1996) are potentially able to produce high oil concentrations (480 g kg-1 or more), whereas others (usually with stripped hull) typically produce low oil concentrations (440 g kg-1 or less). There are no data about the effect of intercepted PAR during seed filling on oil yield and its components in sunflower hybrids with low oil concentration potential.

Daily mean intercepted PAR from 30 d before to 20 d after flowering was a good predictor of seed number (Cantagallo et al., 1997). However, relationships between weight per seed or oil concentration and intercepted PAR during the seed filling period have not been reported in previous studies and could differ with type of hybrid. These relationships could be useful for simulation purposes, and could eventually be incorporated into growth models (Texier, 1992; Steer et al., 1993; Villalobos et al., 1996).

In this paper, we test the hypothesis that weight per seed and oil concentration depend on intercepted radiation during seed filling in hybrids with high and low concentrations. The objective of this work was to study the effect of intercepted PAR during seed filling on weight per seed and oil concentration in two sunflower hybrids, one with high oil concentration potential and black hulls, the other with low oil concentration potential and stripped hulls.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 Conclusion
 REFERENCES
 
Cultural Practices
Three experiments, hereafter referred to as Exp. A, B, and C, were conducted at the INTA Balcarce Experimental Station, Argentina (37°45' S, 58°18' W). Sowing dates were 30 Nov. 1993 (A), 14 Nov. 1995 (B), and 16 Dec. 1994 (C). The soil was a Typic Argiudol (USDA taxonomy). The two hybrids were `Dekalb G-100' (G-100, oil concentration from 480 to 540 g kg-1, black hull), and `Northrup King Tordillo' (NKT, oil concentration from 380 to 440 g kg-1, stripped hull). The experiments were designed as split-plots with main plots in a randomized complete block design with four (Exp. A) and three (Exp. B and C) replicates. The hybrids were assigned to main plots and the radiation treatments to subplots. Each subplot had eight rows 0.7 m apart and 6 m long.

Emergence occurred 10, 7, and 7 d after sowing in Exp. A, B, and C, respectively. Final plant density was 72 000 plants ha-1 in Exp. A and C, and 45 000 plants ha-1 in Exp. B. Soil analysis indicated that N or P fertilization was not necessary. Soil water content was monitored every 5 to 7 d by neutron probe (Troxler 4300, Troxler Electronic Laboratories, INC, Research Triangle Park, NC). Soil water was maintained by irrigation above 40% of maximum available water in the first 0.60 m of soil during the entire growing season. Weeds and insects were adequately controlled. Flowering of a plant was registered when all florets from the outer ring of the capitulum showed their stamens. In Exp. A and B, each plant was marked at flowering (with a different color each day). Flowering of a plot was registered when 95% of plants had flowered, and occurred 62, 60, and 57 d after emergence (DAE), in Exp. A, B, and C, respectively.

Treatments to vary intercepted PAR per plant during seed filling (from end of flowering to physiological maturity), were applied at 74, 73, and 70 DAE, in Exp. A, B, and C, respectively. The treatment application (TA) date corresponded to 48, 45, and 44°C d (base temperature = 6°C) after R6 (Schneiter and Miller, 1981). End of flowering corresponded to when 95% of the plants had their inner florets fertilized. The treatments were (i) 50% uniform shading with black, synthetic and neutral mesh cloth (shaded treatment), (ii) thinning to 25% (Exp. A and C) and 50% (Exp. B) of the original plant density (thinned treatment), (iii) the combination of shading and thinning (shaded-thinned treatment), and (iv) untreated control.

In Exp. A and B, physiological maturity (PM) of plants was taken as the day in which individual weight per seed did not increase compared with the previous sample. In Exp. C, PM was estimated visually from the hard yellow color of the capitulum back face and from the brown color of its bracts (Pereyra et al., 1982).

Measurements
Global daily incident radiation was measured with a pyranometer (LI-200SB, LI-COR, Lincoln, NE) located 400 m from the experiments. Daily incident PAR was calculated as 0.48 x global daily incident radiation. The proportion of PAR intercepted by the crop at noon was determined according to Gallo and Daughtry (1986), as (1 - Rb/Ro), where Rb is radiation measured below the last green leaf, and Ro is the radiation measured above the canopy. Rb and Ro were measured weekly at solar noon (± 1 h), with a line quantum sensor (LI-191SB, LI-COR). According to Charles-Edwards and Lawn (1984), the daily proportion of PAR intercepted was calculated as 2 x proportion of PAR intercepted at noon / (1+ proportion of PAR intercepted at noon). In sunflower, this correction allowed a substantial improvement on the error arising from a single measure at noon (Trapani et al., 1992). Daily proportion of PAR intercepted between two measurements was calculated by linear interpolation. Daily intercepted PAR was calculated as the product of daily incident PAR and daily proportion of PAR intercepted.

Capitulum temperature was measured in G-100, in Exp. A and B, with small thermistors (LM35 DH, Sener Electrónica, Mar del Plata, BA, Argentina) located in the seed insertion zone inside the receptacle. Measurements began at flowering and were registered every 60 s. Data were averaged every 3600 s, and recorded by a data logger (LI-1000, LI-COR).

Above-ground dry matter (ADM) accumulation per plant was followed during the growing period. In Exp. A, samples of 3 to 5 plants were taken at 23, 35, 45, 52, 63, 74, 80 (NKT), 87 (G-100), 94, 102 (G-100), and 116 (NKT) DAE. In the last 5 sampling dates the harvested plants were separated into stem, leaves, receptacle, and fruits. In Exp. B, samples were taken at 23, 35, 45, 52, 68, 88, 113 (G-100), and 122 (NKT) DAE. Dry matter partition was measured at the end of flowering (68 DAE in both hybrids) and at harvest (113 DAE in G-100 and 122 DAE in NKT). In Exp. C, samples were taken only at harvest (110 DAE in G-100 and 119 DAE in NKT). The samples were oven-dried (with air circulating at 60°C) to constant weight, and weighed. In Exp. A, the fruits were separated manually into non-empty (seed occupying at least 20% of total space in the fruit) and empty categories. The fruits from Exp. B and C were cleaned mechanically (manual ventilator) and only non-empty fruits were recovered.

Non-empty fruits (Exp. A, B, and C) and empty fruits (Exp. A) were counted and weighed. Individual fruit weight was obtained as the quotient between the weight of all non-empty fruits in each head and the number. Harvest index (HI) was calculated as the ratio between weight of fruits and total ADM. Oil concentration was measured by nuclear magnetic resonance (NMR, Analyser Magnet Type 10, Newport Oxford Instruments, Buckinghamshire, England) in duplicate and averaged.

Seed proportion in fruit (w/w) and oil concentration in seed were measured in Exp. A and B in an homogeneous fruit mixture obtained from the 5 plants harvested from each experimental unit on the last sampling date. After mechanical dehulling, seed oil concentration was measured by NMR as described for whole fruit. Seed proportion in fruit was measured in triplicate after manual dehulling. Whole fruit, seed and hull weights, and oil concentration were expressed on a dry weight basis. Hereafter, seed will refer to the entire fruit.

The energy yield of seeds per plant was calculated in A and B, according to Hernández and Orioli (1985), with the following conversion coefficients: oil = 39.8 kJ g-1, disaccharides and polysaccharide = 17.6 kJ g-1, and protein = 23.9 kJ g-1 (Westlake, 1963).

Data Analysis
Allometric relationships were calculated as the linear regression between the natural log of seed dry matter and the natural log of total ADM (Coleman et al., 1994). The slope (K) of this relationship is called the allometric constant and is the ratio of the logarithmic growth rates of the two components under study. Any value of K other than unity implies a discrepancy between these two rates (Hunt, 1978). Stem and receptacle respiration was estimated in G-100 (Exp. A) according to Hall et al. (1990).

Data of seed number, weight per seed, oil concentration, and energy yield were processed by analysis of variance procedures (General Linear Models Procedure; SAS Institute Inc., 1988). When statistical differences were detected in more than one experiment, only the highest P value is presented. Differences between treatments means were evaluated with the Tukey test (P < 0.05).

Weight per seed and oil concentration were related to intercepted PAR accumulated during the TA to PM period by negative exponential functions [y = a + b exp(-cx)] and the software Sigma Plot Scientific Graphing System, version. 4.10 (Jaendel Corporation, 1986–1991). The same software was used for regression analysis between oil concentration of the whole fruit and (i) seed oil concentration and (ii) the proportion of seed in the whole fruit; and between temperature and residuals from the functions relating weight per seed and oil concentration to intercepted PAR.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 Conclusion
 REFERENCES
 
Growth Conditions
Daily mean air temperature during grain filling in Exp. A was 1.5 and 4.3°C higher than in Exp. B and C, respectively (Table 1) . Daily mean incident radiation was highest in Exp. B, and lowest in Exp. C (Table 1).


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  		Table 1 Daily mean air temperature and daily mean incident radiation for the period from treatment application to physiological maturity. Data correspond to the untreated control in Exp. A, B, and C

 
The thinned treatment showed the highest cumulative PAR intercepted per plant from TA to PM and the shaded treatment showed the lowest (Table 2) . The untreated control and the shaded-thinned treatment showed intermediate values. The shaded-thinned treatment intercepted more PAR than the untreated control in Exp. A and C, and less in Exp. B.


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  		Table 2 Cumulative intercepted PAR for the period from treatment application to physiological maturity for each radiation treatment, in hybrids G-100 and NKT, Exp. A, B, and C

 
Physiological maturity was reached sooner in the shaded treatment than in the others (except in NKT, Exp. B). In both hybrids, the thinned treatment was generally the last to reach PM (Table 3) .


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  		Table 3 Effect of shading and thinning on the occurrence of physiological maturity in hybrids G-100 and NKT, Exp. A, B, and C

 
Capitulum mean daily temperature during the TA to PM period (measured only in G-100) was higher in non-shaded treatments (untreated control and thinned treatments) than in shaded treatments (shaded and shaded-thinned treatments, Table 4) .


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  		Table 4 Mean daily temperature of capitulum for the period from treatment application to physiological maturity, Hybrid G-100, Exp. A and B

 
Dry Matter
Thinned treatment produced the highest per plant ADM and the shaded treatment the lowest (Fig. 1) . The untreated control and the shaded-thinned treatment showed intermediate values. At the last sampling date, considering the two hybrids and all treatments in Exp. A and B, ADM ranged from 179 to 380 g plant-1.



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  		Fig. 1 Dry matter accumulation as a function of days after emergence in hybrids G-100 and NKT, Exp. A and B. The arrows show the days of the beginning of flowering (BF) and treatment application (TA). Vertical bars represent the standard error (if there is more than one treatment, largest standard error is shown)

 
Dry matter partitioning into seeds increased when intercepted PAR decreased, in both hybrids (Fig. 2) . Allometric coefficients (K, natural log of the weight of seed dry matter/natural log of the weight of total aboveground dry matter) in Exp. A were 11.4, 6.5, 5.9, and 5.2 for the shaded, control, shaded-thinned, and thinned treatments in G-100, and 5.5, 4.7, 4.1, and 2.8 for the same treatments in NKT. The K values in G-100 were greater than in NKT, indicating that partitioning into seeds was lower in the hybrid with low oil concentration potential. Relative differences between extreme values (shaded and thinned treatments) were similar in both hybrids (54 and 49%, in G-100 and NKT). In G-100, the harvest index was greater in the thinned treatment than in the shaded treatment (40 vs. 33% in Exp. A and 37 vs. 29% in Exp. B) despite of a high relative partitioning into seeds during seed filling in the latter. NKT followed the same trend (HI = 41 vs. 38% in Exp. A and 32 vs. 30% in Exp. B), but differences between treatments were not significant.



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  		Fig. 2 Seed dry matter per plant as a function of total dry matter in hybrids G-100 and NKT, Exp. A. Values were transformed to ln. The slope (K, ln g seed dry matter/ln g total dry matter) of this relationship is called the allometric constant and is the ratio of the logarithmic growth rates of the two components under study. The slope of shaded treatment for hybrid G-100 was graphically estimated

 
Receptacle weight in G-100 (Exp. A) decreased (by 3–7 g plant-1) in all treatments from 74 DAE to PM (data not shown). Stem weight also decreased in all treatments (by 15–22 g plant-1), but this decrease started later than that observed for the receptacle (87 DAE). Estimated receptacle respiration (data not shown) accounted for most of the decrease in receptacle weight, except in the shaded treatment in which it only accounted for 49% of the decrease. In contrast, estimated respiration accounted for no more than 26% of the loss in stem dry weight, suggesting that part of the stem weight loss is a consequence of dry matter export to other organs (probably seeds). In Exp. B, in which intercepted PAR was higher, stem weight decreased 7 to 24 g plant-1 in the shaded, control and shaded-thinned treatments, respectively, and increased 3 g plant-1 in the thinned treatment from 68 to 113 DAE. In NKT, no decreases in stem weight were observed. Moreover, in Exp. B it increased 29 g plant-1 in the thinned treatment. Contrarily, receptacle dry weight loss began after 80 DAE in all treatments (Exp. A).

Oil Yield Components
Average total number of seeds (empty + non-empty) per plant (Exp. A) was 1904 ± 54 in G-100 and 1898 ± 46 in NKT and it was not affected by intercepted PAR per plant during seed filling. Number of non-empty seeds increased in response to increases in intercepted PAR per plant in some cases (P <= 0.004, Table 5) . Differences among treatments were statistically significant in the three experiments for Hybrid G-100 and in Exp. A and C for Hybrid NKT. Relative differences between extreme treatments (shaded and thinned) were, on average, less than 20%. In Exp. B, in which the greatest amount of PAR was intercepted, both hybrids had a larger number of seeds than in Exp. A and C (Table 5).


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  		Table 5 Oil yield components in hybrids G-100 and NKT, Exp. A, B, and C in response to radiation treatments

 
Mean weight per seed was affected by intercepted PAR in all three experiments (P <= 0.0002). A significant hybrid x intercepted PAR interaction was only found in Exp. B (P <= 0.008). Weight per seed was generally higher in the thinned treatment and lower in the shaded treatment (Table 5). The untreated control and the shaded-thinned treatments had intermediate values. Differences between shaded and thinned treatments were as much as 24% in G-100 and 26% in NKT.

Intercepted PAR affected oil concentration in G-100, but not in NKT (Table 5). In NKT, the effect was not observed even though intercepted PAR per plant increased up to 7.5 times. The hybrid x intercepted PAR interaction was highly significant for the three experiments (P <= 0.0003). Maximum oil concentrations for hybrid G-100 were higher than for NKT in all experiments (differences up to 93 g kg-1 in Exp. B). However, oil concentrations of G-100 in the shaded treatments in Exp. A were lower than in NKT. Differences in oil concentration of the entire fruit in G-100 were explained by changes in seed oil concentration (r2 = 0.93, n = 28, P < 0.0001, in Exp. A and B together) and not by the proportion of seed in the whole fruit (r2 = 0.17, n = 28, P = 0.11).

Energy Yield
Seed yield per plant in energy units (Joules) was affected by intercepted PAR in G-100 in Exp. A and B (P <= 0.0002, Fig. 3) . Relative differences between extreme treatments (shaded and thinned) were 38 (Exp. A) and 48% (Exp. B). The hybrid x intercepted PAR interaction was significant in Exp. B (P <= 0.0024). The hybrid NKT produced statistically significant differences only in Exp. A (39%, Fig. 3c).



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  		Fig. 3 Seed yield per plant expressed in energy units (kJ) in hybrids G-100 and NKT, Exp. A and B. For each experiment, means with the same letter are not significantly different (Tukey, P < 0.05)

 
Relationships between Intercepted PAR and Oil Yield Components
Weight per seed of both hybrids followed a curvilinear relationship with intercepted PAR (Fig. 4) . Weight per seed was closely associated with intercepted PAR accumulated during the TA - PM period (r2 = 0.84 and 0.80 for G100 and NKT respectively). When intercepted PAR was low, weight per seed was higher in NKT than G-100. Differences were smaller at higher values of intercepted PAR (intercepted PAR levels above 60 MJ per plant increased weight per seed in G-100 but not in NKT, Fig. 4).



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  		Fig. 4 Weight per seed as a function of cumulative PAR intercepted from treatment application to physiological maturity, in Hybrids G-100 and NKT, Exp. A, B and C. The equations are: G-100: weight per seed = 84.00 - 51.46 exp(-0.00679 PAR); NKT: weight per seed = 68.14 - 27.96 exp(-0.04546 PAR)

 
Oil concentration in G-100 was related to intercepted PAR in all experiments (r2 = 0.93, Fig. 5) . The response was curvilinear. Oil concentration increased up to 60 MJ PAR intercepted per plant. Beyond this point the response was smaller and oil concentration tended to plateau (Fig. 5). In NKT, oil concentration did not respond to intercepted PAR. Oil concentrations of Exp. A and B did not differ significantly, showing an average for both experiments of 431 ± 5 g kg-1. This value was higher than the average of Exp. C (403 ± 9 g kg-1).



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  		Fig. 5 Oil concentration as a function of cumulative PAR intercepted from treatment application to physiological maturity, in hybrids G-100 and NKT, Exp. A, B and C. The equations are: G-100: oil concentration = 514 - 134 exp(-0.0275 · PAR); NKT: oil concentration = 431 (Exp. A and B averages) and oil concentration = 403 (Exp. C average)

 
Capitulum temperature was modified by treatments applied to vary intercepted PAR. In G-100, temperature and intercepted PAR rankings among experiments were different (Table 2 and Table 4). Shaded treatments (shaded and shaded-thinned treatments) showed lower temperatures than unshaded treatments (untreated control and thinned). In Exp. A and B, intercepted PAR explained more of the variation in weight per seed (r2 = 0.83 vs. 0.33) and in oil concentration (r2 = 0.96 vs. 0.10) than temperature. Residuals from the function relating oil concentration to intercepted PAR did not show a significant relationship with temperature (r2 = 0.10, NS). On the other hand, residuals from the function relating weight per seed to intercepted PAR were related to temperature (r2 = 0.72). This temperature effect was mostly a consequence of differences between experiments and not differences among treatments.


   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
Conclusion
 REFERENCES
 
The use of pre-anthesis reserves in seed filling is common in sunflower (Sadras et al., 1993). Stem and receptacle weight evolutions suggest mobilization of accumulated reserves in G-100, and only receptacle reserves in NKT, in Exp. A. These weight losses could not be explained by respiration, so probably they were used in seed filling. Receptacle weight losses occurred before stem weight losses. This suggests that reserves located close to the seeds may be used before those located at greater distances, as suggested by models of source-sink relationships (Gifford and Evans, 1981; Snyder and Carlson, 1984; Minchin et al., 1993).

In our work, variability in weight per seed among treatments and experiments was explained by a single function for each hybrid, which included intercepted PAR during grain filling. This suggests that, when intercepted radiation is reduced, the supply of assimilates to the seeds decreases, which causes a decrease in weight per seed. A low carbohydrate supply to the seeds could also explain the reductions in weight per seed in plants exposed to different types of stress, such as short periods of water deficit (Hall et al., 1985) or defoliation (Cardinali et al., 1982) during the seed filling period. Constants in the function relating weight per seed and intercepted radiation were different for each hybrid, showing that weight per seed responses to intercepted PAR were not the same for both genotypes. Hybrid NKT, which showed a greater weight per seed than G-100 at lower intercepted PAR, reached the maximum weight per seed with low values of intercepted PAR. Weight per seed in NKT was similar in plants that differed 62% in intercepted PAR (difference between thinned and shaded treatments, Exp. B). This suggests that, in this hybrid, physical or chemical capacity to accumulate dry matter in seed tissues was saturated in the plants that intercepted high quantities of PAR.

In both genotypes, variation in intercepted radiation affected ADM, dry matter partitioning into seeds, reserve remobilization, and weight per seed. However, oil concentration in seeds remained constant in the stripped hull hybrid (NKT). On the other hand, oil concentration was strongly modified by variations in intercepted PAR in the black hull hybrid (G-100), in agreement with results of Andrade and Ferreiro (1996). In NKT, oil concentration did not vary among treatments, but it did vary among experiments (A and B with respect to C), suggesting that oil concentration may be affected by other variables not studied in this work.

Close relationships between intercepted PAR and weight per seed in both hybrids and oil concentration in G-100 were unexpected. Despite variation in the use of preflowering reserves during seed filling in our crops and in temperature between treatments and experiments, cumulative intercepted PAR during the whole TA to PM period accounted for most of the variation in weight per seed. Temperature during this period did not affect oil concentration and only slightly modified the relationship between intercepted PAR and weight per seed. Interestingly, intercepted PAR explained oil concentration in G-100 for different plant densities (72000 plants ha-1 in Exp. A and C and 45000 plants ha-1 in Exp. B), although plant density has modified oil concentration in some reports (Villalobos et al., 1994; Riccobene et al., 1997). Close relationships between weight per seed or oil concentration and intercepted PAR were found across treatments and experiments, even though remobilization of carbohydrate reserves was highly variable. These results suggest, then, an important role of intercepted PAR in weight per seed determination in both hybrids, and in oil concentration in G-100. Variations in weight per seed can be attributed to differences in assimilate supply determined by intercepted PAR. The mechanism by which oil concentration in G-100 is modified by changes in intercepted PAR remains unknown. Variation in assimilate supply would have more effect in a hybrid in which oil synthesis is over expressed or in which source capacity is more limiting. Moreover, an effect of a different local concentration of sugar on the genes and/or the enzymes involved in oil synthesis in the seed is also possible. Such type of mechanisms has been proposed for other processes like root elongation in maize (Zea mays L., Muller et al., 1998).

Most of the differences in seed number among experiments can be attributed to different environmental conditions prior to the period studied in this work, as seed number in sunflower is defined over a long period (from 30 d before to 20 d after anthesis, Cantagallo et al., 1997). Even though the radiation treatments were applied late after anthesis to avoid changes in seed number and close to the end of the period during which Villalobos et al. (1996) considered that seed number was set, a small decrease in seed number was detected in some cases in response to reduced intercepted PAR. Andrade and Ferreiro (1996) also found that, in G-100, non-empty seed number could be affected by applying shade at R6 (Schneiter and Miller, 1981). In this work, the effect of intercepted PAR on weight per seed and oil concentration was similar in all the experiments independently of changes in seed number.


   Conclusion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 Conclusion
REFERENCES
 
Our results suggest a significant role of intercepted PAR during seed filling in determining weight per seed in both hybrids and oil concentration in the hybrid with high oil concentration potential. More than 80% of the variability among treatments and experiments in these yield components was accounted for by variability in the solar radiation intercepted.

Results reported in this work show important genotypic differences in response of oil yield components to variations in PAR intercepted during the seed filling period. Weight per seed was affected by intercepted PAR in both hybrids. Oil concentration was affected in the hybrid with high oil concentration potential and black hull but remained unaffected in the hybrid having low oil concentration potential and stripped hull although differences in intercepted PAR were more than 7.5 fold. Consideration of genotypic differences may be important when using relationships between weight per seed or oil concentration and intercepted PAR as modeling tools.SAS Institute 1988


   ACKNOWLEDGMENTS
 
This work was supported by Universidad Nacional de Mar del Plata, Instituto Nacional de Tecnología Agropecuaria, Oleaginosa Moreno Hnos. S.A., Nidera S.A., and Aceitera General Deheza S.A. The research was completed while GAAD was supported by a fellowship from FOMEC (UNMdP) and the results were taken from his M.S. thesis. FHA and LANA are members of the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET, Argentina). Dr. V.O. Sadras made helpful comments on the manuscript. Guillermo Pereyra Irujo revised the manuscript.

Received for publication August 2, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 Conclusion
 REFERENCES
 




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Crop Sci., January 1, 2003; 43(1): 152 - 161.
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A. J. de la Vega and A. J. Hall
Effects of Planting Date, Genotype, and Their Interactions on Sunflower Yield: II. Components of Oil Yield
Crop Sci., July 1, 2002; 42(4): 1202 - 1210.
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