Sunflower agronomy – manipulating the crop for best yield and oil content outcomes. Which leaves contribute the most to yield and oil?

Author: Loretta Serafin, Mark Hellyer and Neroli Graham, NSW DPI, Tamworth | Date: 02 Mar 2018

Take home messages

  • The removal of all leaves from a sunflower plant at the start of flowering or at petal drop had the greatest impact on grain yield (80% and 65% respectively).
  • Oil content was most affected (reduced by 13% compared to the control) by removing the top 2/3 of leaves at budding.
  • Retaining the top 2/3 of sunflower leaves at budding, start of flowering and petal drop can prevent potential significant reductions in yield of 49, 40 and 35% respectively.
  • Loss of the bottom one third of sunflower leaves did not affect grain yield or oil content at budding or the start of flowering.
  • Based on the data obtained it is suggested that the middle 1/3 of leaves on a sunflower plant may contribute the most to grain yield.

Introduction

Sunflower plants have the ability to produce a large amount of leaf area; in the order of 2- 6,000 cm2/plant, largely depending on soil moisture and plant nutrition. Maximising a sunflower crop’s grain yield and oil content is paramount to ensure maximum economic returns. However, critical information on how many of and which leaves make the greatest contribution to grain yield and oil content has been missing.

Several factors such as hail, insect damage (e.g. heliothis or loopers) and diseases (e.g. alternaria and powdery mildew) have the potential to reduce leaf area. Insects and diseases can be managed through chemical application; however, we need to be able to confidently make decisions around which leaves to protect in order to obtain the best return on our investment in chemical and application costs.

Sunflowers have a growth and development scale similar to one of the Zadok based systems used in cereals. In sunflowers, the vegetative stages are given the preceding letter “V” and then the leaf number, e.g. V6 means the 6th leaf stage. The reproductive stages; from bud initiation are preceded by the letter “R”. The reproductive stages are measured on a scale of 1 – 9.

Three growth stages are referred to in this paper:

  1. R2: when the flower bud is 0.5-2.0 cm above the nearest leaf attached to the stem; commonly called budding or bud initiation
  2. R5.1: the start of flowering
  3. R6: when flowering is complete and the ray flowers (large petals surrounding the head) are wilting; commonly called petal drop.

Treatment timings were selected based on their importance in sunflower growth stages. Bud initiation is when the final leaf number is present and from this point onwards, leaves will continue to unfold and expand but no new leaves will be formed. A plant develops about 50% of its total leaf area by the start of head development and more than 75% by the start of flowering (Warmington, 1981). At bud initiation (R2), plants also make critical decisions on the maximum number of florets (and potential seed number) to be produced. Flowering (R5) is an important stage to ensure florets are fertilised to start the process of developing seeds (the hull and kernel) and R6 corresponds with the end of flowering and roughly the start of kernel development.

Previous research has found that in a plant’s vegetative stages, most assimilates produced by photosynthesis are directed towards development of the plant’s root system. While after bud initiation, the plant diverts assimilates towards the top of the plant for the developing head (Merrien, 1986). The retention of leaves is important, as a reduction in the photosynthetic area means a reduction in the amount of photosynthate available to the developing sunflower seeds.

Younger leaves, which are higher up the plant, have better access to light. As such there is also an age relationship to photosynthetic capacity. Therefore, bottom leaves, which senesce earlier are thought to have less impact on photosynthetic capacity, when compared to the middle and top leaves.

Since we are targeting higher yields and oil content, in this research we focused on leaf contribution at three of the reproductive growth stages; budding, flowering and petal drop.

Research methods

A series of five experiments were conducted between the 2014/2015 and 2016/2017 seasons in northern New South Wales. The single site/year experiments were conducted at “Boonery Park” (Curlewis) in 2016/2017, “Kyntyre” (Gurley) in 2015/2016, “Parraweena” (Blackville) in 2016/2017 and “Windy Station” (Pine Ridge) in both 2014/2015 and 2015/2016.

Each site/year experiment was designed as a randomised complete block design with three replicates. The monounsaturated hybrid Ausigold 62 was used in each of the experiments. Plots were sown with a Monosem precision planter and harvested with a KEW small plot header.

Grain yield (t/ha), oil content (%), oil yield (kg/ha), plant height (cm), test weight (kg/hL), thousand seed weight (g), head arc length (cm) and head diameter (cm) were measured at a selection of the single site/year experiments.

Twelve leaf removal treatments were used across each of the trials. These were targeted at 3 key growth stages; budding (R2), start of flowering (R5.1) and petal drop (R6). Each experiment included a control treatment where all leaves remained in-tact plus eleven leaf removal treatments which involved removing 10 leaves from various parts of the plant, e.g. top, middle or bottom and referred to as top 1/3, middle 1/3 or bottom 1/3.

As sunflower plants typically produce between 30-40 leaves, the 10 leaves removed were equivalent to removing around 1/3 of plant leaves. Treatments were a combination of growth stage and the section on the plant where the leaves were removed from, e.g. the bud 2/3 treatment had the top 20 leaves removed at budding. The treatments were imposed by cutting off the leaf with secateurs, but leaving the leaf petiole (leaf stem) in-tact.

Results

The final grain yield and oil content varied significantly between the five site/years. Grain yields when averaged across the leaf removal treatments ranged from over 1 t/ha at “Windy Station” in each year, to a low of 0.22 t/ha at “Boonery Park” in 2016/2017 (Table 1). However, when plant material was removed from the bottom third of leaves at budding in 2014/2015 at “Windy Station” the yield exceeded 2.0 t/ha.

When averaged across treatments, oil content of the harvested seed was highest at “Windy Station” and “Kyntyre” in 2015/2016 with 42.7 %, while the other three trial sites/years were below the accepted grain receival standard of 40 % (Table 1).

The difference in seed yields and oil content between sites/years was due in part to variable environmental conditions, over the three years.

Table 1. Seed yield at 9 % moisture (t/ha) and oil content (%) for the five site/years when averaged for leaf loss treatments at three growth stages.

Site/Year

Year

Grain Yield (t/ha)

Oil Content (%)

Windy Station

2014/2015

1.04

39.6

Kyntyre

2015/2016

0.23

42.7

Windy Station

2015/2016

1.05

42.7

Parraweena

2016/2017

0.63

39.8

Boonery Park

2016/2017

0.22

37.1

l.s.d. (P=0.05)

0.3

2.0

The leaf removal treatments affected grain yield, oil content, grain quality and plant characteristics within the five sunflower experiments.

At the budding stage, the reduction in yield corresponded to the number of leaves removed. Removing 1/3 of leaves reduced yields by 17%, removing the top 2/3 of leaves reduced yields by 49% and removing all the leaves reduced yields by 71%. There was no impact on grain yield when the bottom 1/3 of leaves were removed during this stage of development

At budding, the impact of leaf loss on oil content was only apparent when 2/3 of the leaves were removed. This resulted in a 13 % loss in oil content and a reduction in oil levels from 41.4 % in the control, to 37.4 % (Table 2)

Removing leaves at the start of flowering resulted in a 40% yield loss when the top 2/3 of leaves were removed and an 80 % yield loss when all leaves were removed. There was no impact on yield when the top or bottom third of leaves were removed at the start of flowering.

The impact of removing the bottom 1/3, top 1/3 and top 2/3 of leaves on oil content was negligible, (>1 %) when compared to the control (Table 2). In contrast, removing all leaves reduced oil content by 9 % (Table 2).

When the leaves were removed at the petal drop growth stage, the impacts on grain yield had started to diminish compared to the start of flowering, but were still very large. Similar to the start of the flowering growth stage, removing the top 1/3 of leaves at petal drop did not impact on grain yield. There was significant loss in yield when the top 2/3 or all leaf matter was removed, with 35 % and 65 % losses respectively.

When all of the leaf material was removed at petal drop, the oil content was 39.3 %, which was lower than the receival standard of 40 %, as well as 5 % less than the control (41.4 %) (Table 2).

There were additional impacts on grain quality resulting from the defoliation treatments. Test weight was reduced by removing the bottom 1/3 of leaves at budding and by removing all of the leaves at both the start of flowering and petal drop stages. However, all treatments still remained well above the 32 kg/hl minimum trading standard (Table 2).

Thousand grain weights also showed a reduction of 22 - 24 % resulting from the total leaf defoliation treatments for each of the three growth stages (Table 2). There was no impact on thousand grain weight when the bottom 1/3 of leaves were removed at budding or flowering.

Table 2. Seed yield at 9 % moisture (t/ha), oil content (%), test weight (kg/HL) and thousand seed weight (g) for the five site/years for leaf loss treatments at three growth stages.

Treatment

Grain Yield (t/ha)

Oil Content (%)

Test weight (kg/HL)

Thousand grain weight (g)

Bud 1/3

0.75

41.8

39.6

40.4

Bud 2/3

0.46

37.4

38.8

37.9

Bud bottom 1/3

0.98

40.5

37.5

43.1

Bud Total

0.26

41.0

42.1

32.9

R5.1 1/3

0.89

41.5

39.0

39.8

R5.1 2/3

0.54

41.1

40.4

36.7

R5.1 bottom 1/3

0.90

40.8

40.1

42.3

R5.1 Total

0.18

37.7

36.6

33.8

R6 1/3

0.86

40.9

38.9

39.1

R6 2/3

0.58

41.3

39.0

37.8

R6 Total

0.32

39.3

36.6

33.6

Control

0.91

41.4

39.5

43.3

l.s.d (P=0.05)

0.13

0.9

1.4

2.4

The removal of leaves also affected other plant characteristics such as head size, measured as head diameter and the length across the front arc. Head diameter was reduced to 9.5 cm and 5.3 cm, a reduction of 38 % and 65 % when defoliation occurred on the top third or bottom third of leaves at budding, respectively. Smaller head size occurred after loss of one third of leaf material. Head diameter reduction also occurred when defoliation occurred during seed development. The reduction in head diameter was smaller as the growth stages progressed (data not shown) towards plant maturity.

The only impact on plant height occurred when leaves were removed at budding, the earliest stage of growth, where either the top 2/3 or total leaves were removed. (data not shown).

Discussion

Defoliation through leaf removal mimics the effect of insect infestation, disease pressure or hail damage through the reduction in healthy leaf material. The reduction in leaf material can result in a reduction in energy production (and therefore stored energy sources) which may lead to reduced yield and oil contents, as well as impacting on grain quality and other plant characteristics.

The removal of plant material significantly decreased grain yield at each of the three growth stages, with the largest reduction resulting from total leaf loss at the start of flowering (80 % yield reduction).

The experiment did not include a treatment where the middle third of leaves were removed, so the impact of these leaves cannot be accurately reported. However, comparison of the losses resulting from the removal of the top 1/3 of leaves compared to the removal of the top 2/3 of leaf material, suggest that the middle 1/3 of leaves on the plant have the overall greatest impact on yield.

For example, the loss of the top third of plant material at budding resulted in yield loss of 0.16 t/ha or 17 %, while the loss of the top 2/3 of leaves at budding resulted in a 49% yield loss (Table 2). Therefore it could be suggested that the difference between these two treatments which was 32 %, could be attributed to the middle 1/3 of leaves.

Further removing the top 1/3 of leaves at the start of flowering or petal drop did not have a significant impact on yield, which adds to the evidence suggesting the top 1/3 of leaves do not contribute as much to yield.

As the amount of leaf material removed increased; from 1/3, 2/3 or all of the leaf material; the grain yield and oil content also reduced. At bud development, grain yield reduced by 17 % to 49 % to 71 %, compared to the control when 1/3, 2/3 or all leaf material was removed, respectively.

Increased intensity of leaf removal from one third to two-thirds’ up to all leaf material resulted in increased yield and oil yield loss at each of the three timings of plant material removal. Early in bud development there was a reduction from 17 % to 49 % to 71 %, when compared to the control for one third, two-thirds’, all leaf material, respectively. In addition, there was a similar trend in oil content when plant material was removed. When all leaves were removed from plants at each of the three stages, oil content was reduced when compared to the control (or no plant removal).

Grain test weight and thousand grain weight were also affected by leaf removal treatments; however the impact on test weight was not sufficient to result in an economic impact to growers by lowering the receival grade.

Head diameter and arc lengths were reduced when a higher proportion of leaf material was removed. Loss of all leaf material during bud formation reduced the size of the head diameter from 15.4 cm to 5.3 cm.

Conclusions

Leaf loss in sunflower crops can cause loss in yield and oil content. Loss of plant material could be caused by insect infestation, disease pressures or hail damage throughout the growing season. Any reduction in plant material has the potential to negatively impact carbohydrate production (through the loss of leaf material, for example) as well as energy stores (through losses in the stem and head of the plant).

The timing of leaf loss and the amount of leaves lost, markedly affects yield and oil content and also affects physical attributes such as plant height, head diameter and arc length.

The largest reductions in yield resulted from removal of all leaf material, however this is an unlikely event in most crop situations. It is more likely that a proportion of the leaves will be removed through disease, insect or environmental impacts.

Where management and maintenance of the leaves of a sunflower plant is possible (such as insect or disease management), the results of this research suggest that growers and advisors should be careful to maintain the middle 1/3 of leaves on the sunflower plant. Removing the top 1/3 of leaves had a significant impact on grain yield at the budding stage (17 % yield reduction), however, no impact was recorded when the top 1/3 of leaves were removed at the start of flowering or petal drop.

Oil content was mainly affected by the removal of the top 2/3 of leaves at the budding stage. Surprisingly, there was little impact on oil content from removal of the top 1/3 or 2/3 of leaves at the start of flowering or petal drop growth stages, which are closer to when oil content develops in the kernel. However, removal of all leaves (at both the start of flowering and petal drop) did have a major impact on reducing oil content by 9 % and 5 % respectively, in comparison to the control.

Acknowledgements

The research undertaken in this project was made possible by the significant contributions of growers, through trial cooperation, and the support of the NSW DPI and GRDC. The author would like to thank them for their continued support. The author would also like to thank the following NSW DPI staff for their assistance in conducting these trials: Peter Perfrement, Delphi Ramsden, Alice Bowler and James Filby.

Thanks especially to the cooperating growers, Geoff Barker “Boonery Park”, Curlewis, Doug Clark “Kyntyre” Gurley, Joe Fleming “Parraweena” Blackville and Peter Winton “Windy Station” Pine Ridge. Thanks also to their advisors; Gavin McDouall, Jim Hunt and Peter McKenzie for their assistance with the sites. Thanks to Nuseed for supplying the Ausigold 62 seed utilised in the trials.

Thank you to Tendo Mukasa Mugerwa for proof reading.

References

Merrien, A. 1986. Cahier technique cetiom tournesol-phisiologie. Ed. Cetiom, p.1-47.

Warmington C.R. 1981 Sunflowers in Australia. Pacific Seeds, Toowoomba

Contact details

Ms Loretta Serafin
NSW Department of Primary Industries, Tamworth
4 Marsden Park Road, Calala, NSW, 2340
0427 311 819
Loretta.serafin@dpi.nsw.gov.au

GRDC Project code: DAN00197