The New Zealand Institute for Plant & Food Research Limited

Alternate bearing in ‘Hass’ avocado: possible role of carbohydrates and boron Grant Thorp, Helen Boldingh, Andrew Barnett and Peter Minchin Presentation to 4th Australian and New Zealand Avocado Growers Conference in Cairns, 23-24 July 2009

Alternate bearing in ‘Hass’ avocado Does supply, transport and/or utilisation of carbohydrate and/or boron limit fruit set in avocado? The projects are examining: 1. seasonal availability of carbohydrate and boron in the tree 2. transport of carbohydrate and boron to the flower via the phloem and xylem 3. utilisation of carbohydrate and boron by the flower

Related projects being reported at this conference include: • • •

Avocado tree growth cycles – seasonal partitioning Pollination – to confirm that pollen transfer was not limiting fruit set Transpiration during avocado flowering – role of xylem transport The New Zealand Institute for Plant & Food Research Limited

Project team Plant & Food Research

Avocado Industry Council

Grant Thorp Peter Minchin Sam Ong Eng Andrew Barnett Mike Clearwater Bill Snelgar Helen Boldingh Nick Gould Mark Goodwin Lisa Evans Jem Burden

Jonathan Dixon Henry Pak Toni Elmsley Bart Hoftsee Cindy Cotterel

Mt Albert Te Puke Te Puke Te Puke Te Puke Te Puke Ruakura Ruakura Ruakura Ruakura Mt Albert

University of California Dr Mary Lu Arpaia (avocado carbohydrates) Dr Patrick Brown (boron transport)

University of Malaga (Spain) Dr Inaki Hormaza (starch and fruit set)

The New Zealand Institute for Plant & Food Research Limited

Why are carbohydrates important? Carbohydrates are the major products of photosynthesis • Structural carbohydrates are found within cell walls and provide the structural support for plant growth • Non-structural carbohydrates provide the energy for plant growth

Major “non-structural” carbohydrates in avocado are: • Starch (reserves) • Sucrose, glucose, fructose (6-carbon sugars) • D-mannoheptulose, perseitol (7-carbon sugars) The 7-carbon sugars appear to play an important role in overall tree physiology (Mary Lu Arpaia, University of California). Their availability and/or utilisation could be a limiting step in achieving successful fruit set The New Zealand Institute for Plant & Food Research Limited

Boron transport in plants Boron is important for pollination and fruit set It is generally accepted that boron is not mobile within plants and that once boron accumulates in a plant tissue it can not be remobilised to other plant tissues However, in some plants (eg apple) boron is known to complex with sugar-alcohols, eg sorbitol, resulting in transport of a sorbitol-boronsorbitol complex in the phloem from leaves to flowers (Patrick Brown, University of California) What about perseitol in avocado? It is a sugar alcohol. Does perseitol have a role in facilitating the transport of boron from leaves to flowers in avocado?

The New Zealand Institute for Plant & Food Research Limited

Alternate bearing in ‘Hass’ avocado: possible role of carbohydrates and boron Our hypothesis is that the seasonal accumulation and utilisation of 7carbon sugars, in combination with a possible role in facilitating boron (B) transport, is a major regulatory process critical to achieving successful fruit set in avocado. Key experiments in Year 1: 1. Demonstrate that boron follows the same transport pathway as carbohydrates, from leaves to flowers 2. Compare carbohydrates and boron in ON and OFF cropping trees 3. Confirm that treatments which increase carbohydrate content of flowers also increase boron content, and thus increase fruit yields

The New Zealand Institute for Plant & Food Research Limited

Boron concentration in leaves If boron is transported in the xylem then it would accumulate in the old leaves as these leaves have transpired the most water. In avocado, boron concentration decreased with leaf age. This suggests that boron does not accumulate in the leaves but is being exported. 70 off year on year

Boron concentration (mg/kg)

60

50

40

30

20

10

0

very immature

half-mature

mature The New Zealand Institute for Plant & Food Research Limited

Boron transport Boron isotope (B10) applied to leaves before flowering, then analysed at mid-bloom (24 October) in leaf and floral tissues. These studies confirmed the transport of boron from leaves to flowers (via the phloem) early 4 September

8 October late

(50 days before mid-bloom)

(16 days before mid-bloom) 4

off flowering on flowering

3

2

1

0 labelled leaf

flower

B10/B11 ratio

B10/B11 ratio

4

off flowering on flowering

3

2

1

0 labelled peduncle leaf

flower

new leaf

The New Zealand Institute for Plant & Food Research Limited

Carbohydrates in ON and OFF flowering trees starch

B

sucrose glucose A

D

fructose D-mannoheptulose

C

perseitol E

F

(boron) ON bearing → OFF flowering OFF bearing → ON flowering

The New Zealand Institute for Plant & Food Research Limited

Avocado flower carbohydrates A v o c a d o flo w e r b u d c a rb o h y d ra te s o ff /o n y e a r ( C H O = m g /g m d w t) M % dry% Dmatter

S ta r c h starch 16

30

P e r s e ito l perseitol 40

14

25

12 20

30

10 8

15

20

6

10

4 5

2

0

0

10 0

ON flowering

D-mannoheptulose M a n n o h e p tu lo s e 40

glucose g lu c o s e

35

20

40

25 30

20 15

10

20

10 10

5 0

OFF flowering

50

30 30

fructose fr u c to s e

0

0

T o ta l C H O S u c ro s e sucrose total carbohydrates 20

160 140

15

o n y e a r = o n flo w e r in g

120 100

10

80 60

5

40 20

0

0

The New Zealand Institute for Plant & Food Research Limited

Avocado flower carbohydrates • Total carbohydrates are the same in flowers from ON and OFF bearing trees • Flowers on trees with a heavy crop load (few flowers) have low levels of starch • Flowers on trees with a low crop load (many flowers) have high levels of starch Agrees with work of Inaki Hormaza in Spain which indicates that flowers with high levels of starch are more likely to set fruit than flowers with low starch content

The New Zealand Institute for Plant & Food Research Limited

Can we increase the carbohydrate content of flowers? Branch girdling studies

Thank you to Ron and Chris Bailey The New Zealand Institute for Plant & Food Research Limited

Branch girdling study Paired branches (2-year-old growth units), one pair per tree

Treatments applied to one branch from each pair in April 2008

Treatment (n = 18 trees)

Basal Diameter (mm)

Length primary axis (m)

Girdle

60.0

2.4

No Girdle

62.1

2.4

Even mix of ON and OFF bearing trees (average 107 and 68 kg/tree)

The New Zealand Institute for Plant & Food Research Limited

Tissue sampling

B

Sample F

Sample E Sample A

A

D

Sample B

C Sample C (wood) and D (bark)

E Black = 2005/06 branch Red = 2006/07 shoot and old leaves Blue = 2007/08 terminal shoot, fruit and new leaves Green = spring 2008 inflorescence

The New Zealand Institute for Plant & Food Research Limited

F

Leaves

ON Bearing

OFF Bearing

Intact

Girdled

Intact

Girdled

Significance 1

Starch

April (autumn) October (mid-bloom)

16.4 7.5

18.2 17.4

17.5 8.8

11.5 31.1

NS (*)

Perseitol

April (autumn) October (mid-bloom)

25.2 13.9

25.2 13.3

25.5 15.4

26.2 12.5

NS NS

Boron

April (autumn) October (mid-bloom)

39.0 25.6

35.1 20.3

36.8 28.6

40.8 22.5

NS (*)

Girdling treatments applied in April 2008 The New Zealand Institute for Plant & Food Research Limited

Flowers

ON Bearing Intact

OFF Bearing

Girdled

Intact

Girdled

Significance 1

October 2008 (mid-bloom) Starch

mg/g

22.4

22.7

20.5

18.8

NS

Perseitol

mg/g

17.9

17.1

19.0

18.3

NS

Boron

mg/kg

39.3

58.7

41.2

58.7

*

Girdling treatments applied in April 2008

The New Zealand Institute for Plant & Food Research Limited

60 40

Boron (mg/kg) Boron (mg/kg)-log scale

80

100

Variable response to girdling

OFF-G Girdled

NotOFF-N girdled

ON-G Girdled

NotON-N girdled

OFF cropping Bearing-Girdling ON cropping The New Zealand Institute for Plant & Food Research Limited

Summary and questions • As expected, girdling maintained high levels of starch in leaves on girdled branches, especially in OFF cropping trees • Boron was exported from leaves during April to October, with increased export from leaves on girdled branches – why? • Neither girdling nor crop load affected the starch and perseitol content of flowers, but girdling did increase the boron content of flowers on both ON and OFF cropping trees – why? • Not all branches responded to girdling – why? • Variable response to girdling suggests opportunities to improve the efficiency of this technique and thus improve its usefulness to growers wanting to achieve more consistent yields

The New Zealand Institute for Plant & Food Research Limited

The New Zealand Institute for Plant & Food Research Limited

Thank you

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