Crop Science 42:1180-1185 (2002)
© 2002 Crop Science Society of America
CROP PHYSIOLOGY & METABOLISM
Low Red to Far-Red Ratios Reaching the Stem Reduce Grain Yield in Sunflower
S. Libensona,
V. Rodrigueza,
M. López Pereirab,
R. A. Sáncheza and
J. J. Casal*,a
a IFEVA, Buenos Aires, Argentina
b Cátedra de Cultivos Industriales, Facultad de Agronomía, Univ. de Buenos Aires, Av. San Martín 4453, 1417 Buenos Aires, Argentina
* Corresponding author (casal{at}ifeva.edu.ar)
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ABSTRACT
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The red light (R) to far-red light (FR) ratio reaching the stem of crop plants is reduced by increasing canopy densities. We sought to investigate whether this light signal that increases resource allocation to the stem affects yield in sunflower (Helianthus annuus L.) plants. The stem of sunflower plants grown outdoors in pots at very low densities was exposed to low R/FR ratios by placing selective plastic filters around this organ. Control plants had a clear filter around the stem and received the R/FR ratio of unfiltered sunlight. In experiments with the open pollinated cultivar Guayacán II, the hybrid ‘Dekalb G100’, and the line MA 566, low R/FR ratios promoted stem extension growth and dry matter accumulation without affecting the ratio between stem dry matter and length. In comparison with the sunlight R/FR ratio controls, low R/FR ratios reduced grain yield and grain number per plant, without affecting grain dry weight. No significant interaction of the R/FR ratio with cultivar or sowing date was observed. The negative correlation between stem growth and yield did not extend to the genetic variability observed for plant stature and grain yield. We propose that the promotion of stem growth by the low R/FR ratios typical of dense stands could reduce the resources available for grain yield in sunflower crops.
Abbreviations: FR, far-red light • INTA, National Institute of Agricultural Technology of Argentina • PAR, photosynthetically active radiation • R, red light
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INTRODUCTION
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GRAIN YIELD IN IRRIGATED SUNFLOWER crops can show considerable stability across a wide range of plant densities (5–10 plants m-2; Villalobos et al., 1994). A comparable situation is found in many crops as a result of compensation between plant density and yield per plant within the range of stable yields. The negative effect of plant density on yield per plant has a component associated with the reduced availability of resources per plant. Competition for water, nutrients, and photosynthetically active radiation (PAR) become more intense when the plants are grown more densely.
The architecture of the canopy not only alters the availability of resources but also generates photomorphogenic signals. The R/FR ratio of the radiation propagated horizontally decreases due to selective reflection on nearby vegetation even before competition for PAR (i.e., mutual shading) is detectable (Ballaré et al., 1987). This R/FR ratio signal is established earlier in dense than in sparse canopies (Ballaré et al., 1987). These early signals reach the stem of dicot seedlings, where it is perceived by phytochromes and promotes stem elongation (Ballaré et al., 1987, 1988). Low R/FR ratios are also perceived by phytochromes at the base of the shoot in grass plants, causing reduced tillering (Casal et al., 1986, 1987; Skinner and Simmons, 1993).
The responses to the low R/FR ratios (i.e., R/FR ratios lower than those provided by unfiltered sunlight), collectively called shade avoidance syndrome are predicted to increase the competitive ability of individual plants within a canopy (Smith, 1982; Casal and Smith, 1989; Schmitt, 1997). Under certain conditions, however, these responses could be negative in terms of crop productivity, particularly because the enhanced stem growth observed under low R/FR ratios could reduce the resources available for the growth of harvestable organs (Ballaré et al., 1992, 1997; Smith, 1992; Sánchez et al., 1993; Ballaré and Casal, 2000). Under field conditions, the abscission of reproductive structures can be reduced in soybean [Glycine max (L.) Merr.] plants by increasing the R/FR ratio through supplementing the lower strata of the canopy with R during flowering and early pod set (Heindl and Brun, 1983). Mild overexpression of the photoreceptor phytochrome A in transgenic tobacco (Nicotiana tabacum L.) causes density-dependent dwarfing (the plants become shorter and not taller with increasing plant density) and increases leaf-to-stem dry weight ratio (Robson et al., 1996). Overexpression of phytochrome B in transgenic potato (Solanum tuberosum L.) plants grown in pots in the glasshouse increases both photosynthesis and, depending on the duration of the cycle, tuber yield per plant (Thiele et al., 1999). Whether the low R/FR ratios reaching the stem of crop plants can affect yield has not been established. The objective of this work was to investigate whether low R/FR signals reaching the stem affect stem growth and yield in sunflower plants.
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MATERIALS AND METHODS
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Experimental Design and Culture
Seeds of sunflower were sown on moistened cotton wool in clear plastic boxes incubated in darkness at 25°C. Shortly after germination the seedlings were transplanted to 80-L pots (two per pot) filled with a mixture of soil and perlite. The pots were placed outdoors, keeping at least a 1-m distance between adjacent pots, in the experimental area of the Faculty of Agronomy, University of Buenos Aires (34°35' S, 58°29' W). The seedlings were thinned to one per pot after establishment. Each plant received two applications of fertilizer (grade 12-12-17, NPK) to provide 3 g of nitrogen before the fifth pair of leaves was visible. Insecticide (Carbofurán, 2,3-dihydro-2,2-dimethyl-7-benzefuranyl-methylcarbamate) was applied to the soil at transplanting. A preventive fungicide (Iprodione, 3-(3,5-dichlorophenyl)-N-isopropyl-2,4-dioxoimidazolidine-1-carboxamide) was applied twice during the cycle. Each pot daily received dripping water (1–5 L, according to the needs).
Two experiments with a completely randomized design were conducted with plants of the cultivar Dekalb G100, a hybrid released to the Argentinean market in 1984. For the first sowing (30 Dec. 1994), average temperature and radiation levels during the growing season were 19.3°C and 19.5 MJ m-2 d-1. For the second sowing (30 Jan. 1995), average temperature and radiation levels during the growing season were 22.9°C and 16.0 MJ m-2 d-1. Twelve (first sowing) or 18 (second sowing) plant replicates were used for each light condition (i.e., control and low R/FR stem treatment described below) and sowing date was included in the ANOVA as a main plot. The treatments were applied starting with the first internode. Stem length was measured every 2 or 3 d from the cotyledonary node to the apex of the plant. After senescence, the shoots and the heads were harvested and dried at 70°C until constant weight.
In a separate experiment, plants of the cultivar Dekalb G100 were grown without R/FR treatments until the seventh internode began to expand (0.46 ± 0.047 cm). The seventh internode was grown with either high (control) or low R/FR ratios in factorial combination with or without a plastic mesh placed on top of the plants to reduce PAR to 40% of incoming sunlight without affecting the measured spectral composition. The internode was harvested immediately after finishing extension growth and dried at 70°C until constant weight. Six plants were used for each combination in a completely randomized design. The analysis was repeated in unreported experiments with younger plants.
Another two experiments were conducted with Guayacán II, Dekalb G100, and MA 566. Guayacán II is an open pollination cultivar produced by the National Institute of Agricultural Technology of Argentina (INTA) and released to the market in 1966. MA 566 is a recent (1997) experimental line from Morgan Company (Colón, Province of Buenos Aires, Argentina). Each pot was randomly assigned to a genotype and light treatment; treatments started with the first internode. In a completely randomized design, 11 or 12 plant replicates were used per genotype and light condition for each sowing date (that was included in the ANOVA as a main plot). For the first sowing (25 Oct. 1998), average temperature and radiation levels were 22.3°C and 21.3 MJ m-2 d-1 during the growing season. For the second sowing (6 Jan. 1999), average temperature and radiation levels were 23.0°C and 18.4 MJ m-2 d-1 during the growing season. Stem length was measured every 2 or 3 d from the cotyledonary node to the apex of the plant. The dates of visible flowering bud and anthesis were recorded for each plant. The heads were harvested when the grains reached a water content of 
300 g kg-1 and dried at 70°C until constant weight.
Filters Used to Modify the Red Light to Far-Red Light Ratio Reaching the Stem
To modify the R/FR ratio reaching the stem without affecting PAR reaching the leaves, selective filters were placed around the stem. The tube made by the filters had a variable diameter (depending on the age of the plants) but always 2 cm in excess of the diameter of the stem to ensure air circulation. Clear filters were made of cellophane or acetate (depending on the experiments), and these filters allowed sunlight to reach the stem without modifications of the R/FR ratio (1.1). Blue cellophane or acrylic filters were used (in different experiments) to absorb R and reduce the R/FR ratio to 0.3. New filters were added every other day (or more frequently) to reduce the exposure of new stem to high R/FR ratios in the low R/FR ratio treatment. The addition of several filter segments to each internode (along with its extension) also helped to maintain air circulation.
Analysis of the Genetic Correlation Between Stature and Yield
The correlation between cultivar final plant height and cultivar grain yield was analyzed independently for twelve sets of field data provided by INTA (trial results of experimental stations). These sets included four different locations: Junín and Pergamino (province of Buenos Aires, Argentina), Paraná (province of Entre Ríos, Argentina), and Las Breñas (province of El Chaco, Argentina). Up to four sets of data differing in sowing dates, trials, and groups of cultivars with long or short cycles (analyzed separately) were analyzed per location. Each set of data included a minimum of 17 and a maximum of 41 cultivars. The crops were sown at commercial densities at normal sowing dates for the different localities and were not irrigated or fertilized.
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RESULTS AND DISCUSSION
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Stem Growth
Selective light absorption and reflection by green tissues is known to reduce the R/FR ratio within plant canopies (Smith, 1982). The low R/FR ratio signal is first perceived by vertical stems before actual competition for PAR is established (Ballaré et al., 1987). Plants of sunflower were grown isolated in pots placed outdoors. Half of the plants of each genotype received low R/FR ratios (0.3) in the stem, while the rest of the plants were exposed to the R/FR ratio of unfiltered sunlight (1.1). The contribution of the stem to total carbon fixation is negligible (Connor and Sadras, 1992). Thus, the treatments are predicted not to affect plant photosynthesis. Low ratios, compared with control R/FR ratios reaching the stem, increased stem length in all the experiments and cultivars (Table 1, Fig. 1)
. This observation is consistent with previous results using selective light filters placed around the stem in other species (Ballaré and Casal, 2000).
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Table 1. Effects of low red/far-red (R/FR) ratios (0.3) compared with control R/FR ratio (1.1) reaching the stem on stem length and dry weight of the head of ‘Dekalb G100’ sunflower plants. Data are means of 12 (first sowing date) or 18 (second sowing date) plants. Standard errors are shown in parentheses.
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Fig. 1. Time course of stem length increment in three sunflower cultivars that received unfiltered sunlight red light/far-red light (R/FR) ratios (1.1, control) or low R/FR ratios (0.3) at the stem. First sowing date was 25 Oct. 1998. Second sowing date was 6 Jan. 1999. Bars indicate standard errors.
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A significant correlation between stem length and stem dry weight was observed across different cultivars, sowing dates, and R/FR conditions (Fig. 2)
, indicating that low R/FR ratios increased not only stem length but also stem dry weight. To investigate the relationship between stem length and dry weight in further detail, the selective light filters were applied only to the seventh internode and this internode was harvested immediately after cessation of extension growth. Neutral shade was applied (only during the growth of the seventh internode) to the whole plant in factorial combination with the R/FR stem treatments to investigate the effects of reduced PAR. Low R/FR reaching only the seventh internode increased internode length and internode dry weight without affecting the ratio between dry weight and length (Fig. 3)
. Reduced irradiance reaching the whole plant had no statistically significant effects on internode length or dry weight but reduced internode dry weight per unit length. No interactions between the stem R/FR ratio and whole plant irradiance were observed (Fig. 3).
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Fig. 2. Relationship between final stem length and stem dry weight at grain harvest in sunflower plants that received sunlight red light/far-red light (R/FR) ratios (1.1, control) or low R/FR ratios (0.3) at the stem. Data include three genotypes (Guayacán II, Dekalb G100, and MA 566) and two sowing dates (25 Oct. 1998 and 6 Jan. 1999). Regression line corresponds to least squares fit significant at P < 0.01.
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Fig. 3. Length, dry weight, and dry weight/length ratio of the seventh internode of sunflower plants as affected by sunlight [control, red light/far-red light (R/FR) = 1.1] or low R/FR ratios (0.3) reaching the stem, and normal or reduced (40%) PAR reaching the whole shoot. The internode was harvested after reaching its maximum length. Bars indicate standard errors and the results of two-way ANOVA are shown (NS, not significant at the 0.05 level).
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Grain Yield
Low R/FR treatment applied to the stem reduced the weight of the capitulum in plants of the cultivar Dekalb G100 (Table 1). To investigate this effect in further detail, two experiments were conducted with the cultivar Guayacán II (1966), the hybrid Dekalb G100 (1983), and the line MA 566, where grain yield, weight per grain, and number of grains were recorded. The only significant interaction obtained in the three-way ANOVA (sowing date, genotype, R/FR ratio) was that observed between sowing date and cultivar for grain yield and grain number (Table 2). Data are also presented as the average for the two sowing dates and the three cultivars to concentrate the attention on the effects of R/FR ratios. Low ratios, in comparison with high R/FR ratios, reduced grain yield and grain number, but had no significant effects on the dry weight per grain (Table 2).
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Table 2. Grain yield per plant, grain weight, and number of grains per plant of sunflower as affected by low (0.3) compared to control (1.1) red/far-red (R/FR) ratios reaching the stem. Sowing dates: 25 Oct. 1998 and 6 Jan. 1999. Data are means of 11 or 12 plants. Standard errors are shown in parentheses.
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Changes in grain yield were not associated with changes in phenology as the number of leaves per plant, the date of visible flowering button, and the date of anthesis were unaffected by the stem R/FR conditions (data not shown). Taking into account that low R/FR ratios increased dry matter allocation to the stem and that sunflower plants allocate a significant proportion of plant dry weight to the stem (Trapani et al., 1994), the reduced yield could be the consequence of competition for resources between the stem and the head. The average effect of stem R/FR ratios on yield (mean of three cultivars and two sowing dates, Table 2) was 16 g plant-1, whereas the average effect of stem R/FR ratios on final stem dry weight was 7.1 g plant-1 (Fig. 2). This observation and the lack of significant differences on dry weight per grain (Table 2) suggest that competition for resources between the stem and reproductive structures was critical during the generation of grain number, and this in turn conditioned the full expression of grain filling capacity.
Implications for Sunflower Crops
Testing the impact of canopy R/FR ratios by experimental manipulation of these signals at crop level is not feasible with available technology (at least without interfering with PAR). In principle, however, yield responses to R/FR ratio observed here could be an underestimation of those taking place at commercial densities for two reasons. First, while only the light environment of the stem was experimentally modified here, at commercial densities the R/FR ratio signal reaches the stem at early stages of canopy growth and both the stems and the leaves when the leaves mutually shade each other (Rousseaux et al., 1999). At least in other species, the leaves are also photoreceptive organs that contribute to the stem growth response (Casal and Smith, 1988). Second, in the present experiments, competition for PAR was negligible, whereas in commercial density crops, the low R/FR signals partially overlap in time with reduced PAR. Lowering PAR does not reduce stem responses to R/FR ratio (Fig. 3), but scarcity of photoassimilates due to mutual plant shading could exacerbate the competition between the stem and reproductive structures.
Low R/FR ratios increased stem growth and reduced grain yield, but this negative correlation between both variables does not apply to genetic variation. Plants of the cultivar Guayacán II were the tallest (Fig. 1) but did not yield consistently less than the other cultivars (Table 2). We have extended a similar analysis to a large number of cultivars grown at commercial densities without finding a negative correlation between cultivar yield and cultivar stature. The analysis was repeated for 12 sets of data, including different locations and sowing dates. Actually, in two trials we observed a positive correlation (Fig. 4) . This result indicates that, at least under certain field conditions, genetic variability in sunflower yield could be the reflection of overall plant vigor, and yield could eventually be improved via changes in harvest index. The latter possibility is consistent with the observation that low R/FR ratios increase stem dry weight and reduce grain dry weight.
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Fig. 4. Yield of sunflower crops of different cultivars plotted against their average plant height at harvest. Open symbols correspond to 34 cultivars grown in Junín, Argentina (1995–1996). Closed symbols correspond to 24 cultivars grown in Pergamino, Argentina (1996–1997). Regression lines correspond to least squares fit significant at P < 0.01 (Junín) and P < 0.05 (Pergamino).
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Since low R/FR ratio signals promote stem growth and reduce grain yield, cultivars with impaired stem growth responses to R/FR ratio could be more productive. Sunflower yield shows stability across a range of plant densities due to a balance between number of plants per unit area and yield per plant (Villalobos et al., 1994). Reducing the impact of negative responses to R/FR ratios would elevate this balance by increasing yield per plant. It must be noted that selection for reduced responses to low R/FR ratio is not equivalent to selection for short stature. First, stature may involve a vigor component. Second, a reduced response to low R/FR ratios could have a stronger effect on stature when the canopy is closed and the R/FR ratio signals are more intense, whereas a constitutive (i.e., R/FR ratio independent) reduction in stature should also modify canopy architecture at early stages. Semidwarf sunflower cultivars exhibit enhanced leaf overlapping and impaired light interception at early stages of canopy development (Sadras et al., 1991). Selection for maintained grain/stem dry weight ratio at high population densities could be one means of indirectly selecting against a negative effect on yield via photomorphogenic processes.
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CONCLUSIONS
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Low R/FR ratios reaching the stem of plants grown at very low densities caused an 8 to 20% reduction (95% confidence interval) of yield. This reduction in yield (mainly grain number) was likely the consequence of enhanced stem growth (length and dry matter) and competition for resources between the reproductive structures and the stem. Thus, the light environment of commercial crops could have a negative effect on yield via photomorphogenic processes that have not been eliminated by selection. These photomorphogenic responses could be reduced by transgenic transformation to increase the steady-state levels of photoreceptors (Robson et al., 1996; Thiele et al., 1999). Alternatively, variability across a wide range of cultivars could be explored, and desirable alleles of photomorphogenic genes could be identified and introduced by marker-assisted introgression. Regardless of the strategy, the results presented here encourage the pursuit of efforts to improve crop yield via modifications in the way plants perceive the crop environment.
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ACKNOWLEDGMENTS
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Financially supported by SECyT (BID 802/OC-AR PID/PVT 1-282), Fundación Antorchas (A-13622/1-40) and University of Buenos Aires (G067).
Received for publication September 10, 2001.
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ABSTRACT
INTRODUCTION
