Do they exist, and what difference does it make?

Day-neutral sunflowers are touted by some seed companies as something special, but seed catalogs give little explanation of why this should be so.  Furthermore, some varieties are claimed to be day-neutral, but research is showing that this is not the case.  In the paragraphs below, I will try to clear up the mystery, and classify 25 common varieties according to their daylength response.                        

First some definitions:  “Day-neutral”, with regard to flowering, means that a plant will flower at the same time after planting, regardless of the length of day under which the plant is growing, provided the temperatures and total light energy are the same in the contrasting environments.  A “short-day plant” flowers sooner under daylengths of 12 hours or shorter, as would prevail during early spring or late fall, than under daylengths of 15 hours or more, as occur in our summers.  “Long-day plants” will take fewer days to flower in summer than in early spring daylengths.  In the case of sunflowers, even those that are daylength sensitive will eventually flower when grown at an unfavorable daylength.                    

In work sponsored by the ASCFG Research Foundation, we tested the daylength response of 25 common cut flower sunflower varieties.  Previous investigations by scientists in Belgium had revealed that some sunflowers are sensitive to daylength in the first three weeks after they emerge from the ground after sowing.  It is during that time that the flower is formed, or prevented from being developed.  We therefore started the varieties in 72-cell seedboxes, and exposed them to either 12 or 16 hours of daylength in a greenhouse by using blackout curtains at the end of the day, or shining artificial light on them after dark.  Plants were kept under these treatments for three weeks from emergence, and then transplanted to the field.  Plants were planted in beds using a square spacing of 9 by 9 in., with 4 rows per bed, and 6 feet between bed centers.  Black plastic covered the beds for weed control, and supplementary irrigation was supplied when needed by two trickle lines.  The experiment was repeated three times in the summer of 2006, transplanting June 7, and three and six weeks later. There were about 24 seedlings per variety in each planting.         

Giving the emerging seedlings short-day conditions for the first three weeks caused an amazing shortening in the time to flower of some varieties, while others did not show any effect (Table 1).  Along with the earlier flowering, the plants were much smaller than normal, and the flower heads were much reduced in comparison to the same variety grown under long-day conditions (Fig. 1).  Many of the varieties that we found to show this sensitivity to daylength have been widely reported in seed catalogs to be daylength neutral, unfortunately.  For instance, ‘Sunbright’ and ‘Sunbright Supreme’, and the Sunrich lines (‘Gold’ and ‘Orange’), flower an average of 2 to 3 weeks earlier with the short-day treatment than when exposed to long days, yet are widely designated as daylength-neutral by the seed catalogs (Table 2).  The varieties that flowered extra early when given short-day treatment also showed another trait that detracts from the from the appearance of the flower: plants tended to produce several flower buds in the upper part of the stem (Fig. 2).
    

Fig. 1.  Influence of daylength under which sunflower plants (‘Premier Light Yellow’) were grown during the first three weeks, on plant appearance at flowering.  Long-day plants of the first planting (right) are shown with short-day plants grown in the second planting, because the short-day plants from the first planting flowered much sooner than the long-day plants.

Our tests did identify other varieties that do not react to daylength, namely most of the Procuts (Table 2), as well as ‘Sonja’, ‘Soraya’, and ‘Chianti’, among others.  Curiously enough, ‘Procut Bicolor’ and ‘Double Quick Orange’ flowered about a week earlier after long-day treatment than when exposed to 3 weeks of short days, so these would be classified as long-day varieties.
    
So does this make any difference to a hard-working cut sunflower grower?  It sure can, especially if you are trying to be the first one on the market with your sunflowers.  For instance, if you are in Zone 5 5 (like us), and plan to transplant sunflowers into a high tunnel by April 15, you will be starting your seed in the greenhouse at about March 21, when daylength is close to 12 hours.  If daylength on those seedlings is not extended by artificial light, those plants will flower early, but they will be small and short, and perhaps unacceptable to your customers.  To counteract the dwarfing effect of the short days, giving 4 hours of artificial light at the end of the day will restore the daylength sensitive varieties to their summer size, but also makes them about 17 days later.
    
When producing early sunflowers in lower latitudes, or in Florida in the winter, one would need to watch out for similar problems.  Sowing these photosensitive varieties in the middle of the summer makes them behave similarly to our long-day treatment. Use of daylength-neutral varieties avoids this problem.  It is therefore also important that the seed trade be honest with growers, and indicate accurately in their catalogs on which varieties are sensitive and insensitive to photoperiod.

Fig. 2.  Immature flower buds on the upper stem of a strongly daylength-sensitive variety of sunflowers, when exposed to short days for the first 3 weeks after emergence.

Table 1.  Reaction to daylength of 25 sunflower varieties when exposed to either 12 (SD) or 16 (LD) hours during the first three weeks of growth in a greenhouse, before transplanting to the field.  Flower diameter measures the diameter of the flower disk, when petals are perpendicular.

 NumberDaysDaysPlantPlantFlowerFlower
 oftotoHeightHeight DiameterDiameter
Sensitivity typeLinesFlowerFlower(Inches)(Inches) (Inches)(Inches)
  ShortLongShortLong ShortLong
Day neutral106566 52 52 2.2 2.4
Slightly sensitive SD26066 43 46 1.6 1.9
Strongly sensitive SD115370 30 54 1.6 3.0
Slightly sensitive LD27566 6653 3.22.9


Table 2.  Flowering classification of 25 varieties of sunflowers with regard to their sensitivity to daylength.  See Table 1 for characteristics of each category of plants.

 Day neutral Slightly sensitive Slightly sensitive Strongly sensitive
  long day short day long day short day
    
 ‘Procut Lemon’ ‘Chianti’ ‘Sunrich Orange’ ‘Procut Bicolor’
 ‘Procut Yellow Lite’ ‘Valentine’ ‘Sunrich Gold’ ‘Double Quick Orange’
 ‘Procut Peach’  ‘Sunrich Orange Summer’ 
‘Sonya’  ‘Sunbright’ 
 ‘Strawberry Blonde’  ‘Sunbright Supreme’ 
‘Soraya’  ‘Premier Yellow’ 
 ‘Ring of Fire’  ‘Premier Light Yellow’ 
 ‘The Joker’  ‘Solara’ 
 ‘Florenza’  ‘Sunny’ 
 ‘Full Sun Improved’  ‘Moonbright TH 472’ 

Chris Wien

Professor

Chris Wien is recently retired Professor of Horticulture at Cornell University. Contact him at [email protected]