Grant Thorp

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


Project team

Plant & Food Research Avocado Industry Council

Grant Thorp Mt Albert Jonathan Dixon
Peter Minchin Te Puke Henry Pak
Sam Ong Eng Te Puke Toni Elmsley
Andrew Barnett Te Puke Bart Hoftsee
Mike Clearwater Te Puke Cindy Cotterel
Bill Snelgar Te Puke
Helen Boldingh Ruakura University of California
Nick Gould Ruakura Dr Mary Lu Arpaia (avocado carbohydrates)
Mark Goodwin Ruakura Dr Patrick Brown (boron transport)
Lisa Evans Ruakura
Jem Burden Mt Albert University of Malaga (Spain)
Dr Inaki Hormaza (starch and fruit set)


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


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-boron-
sorbitol 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?


Alternate bearing in ‘Hass’ avocado:
possible role of carbohydrates and boron

Our hypothesis is that the seasonal accumulation and utilisation of 7-
carbon 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


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
60
on year
Boron concentration (mg/kg)

50

40

30

20

10

0
very immature half-mature mature

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
4
off flowering
3 on flowering off flowering
B10/B11 ratio

3
on flowering

B10/B11 ratio
2
2

1
1

0
0
labelled flower
leaf labelled peduncle flower new
leaf leaf


Carbohydrates in ON and OFF
flowering trees

B starch
sucrose
glucose
D A
fructose
C D-mannoheptulose
perseitol
E F (boron)

ON bearing → OFF flowering
OFF bearing → ON flowering


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)

% dry D M
% matter starch
S ta r c h perseitol
P e r s e ito l

30 16 40
14
25
12 30
20
10
15 8 20
6
10
4 10
5 2
0 0 0

ON flowering
D-mannoheptulose
M a n n o h e p tu lo s e glucose
g lu c o s e fructose
fr u c to s e

40 35 50
OFF flowering
30
30 40
25
20 30
20
15
20
10 10
10
5
0 0 0

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

160
20 o n y e a r = o n flo w e r in g
140
15 120
100
10 80
60
5 40
20
0 0


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


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

Thank you to Ron and Chris Bailey


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 Basal Diameter Length primary axis
(n = 18 trees) (mm) (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)


Tissue sampling

Sample F B

Sample E

Sample A
D A
Sample B
C
Sample C (wood) and D (bark)

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


Leaves

ON Bearing OFF Bearing
Intact Girdled Intact Girdled Significance 1

Starch April (autumn) 16.4 18.2 17.5 11.5 NS
October (mid-bloom) 7.5 17.4 8.8 31.1 (*)

Perseitol April (autumn) 25.2 25.2 25.5 26.2 NS
October (mid-bloom) 13.9 13.3 15.4 12.5 NS

Boron April (autumn) 39.0 35.1 36.8 40.8 NS
October (mid-bloom) 25.6 20.3 28.6 22.5 (*)

Girdling treatments applied in April 2008

Flowers

ON Bearing OFF Bearing
Intact 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


Variable response to girdling

100
80
Boron (mg/kg)
Boron (mg/kg)-log scale

60
40

Girdled
OFF-G NotOFF-N
girdled Girdled
ON-G Not girdled
ON-N

OFF cropping Bearing-Girdling ON cropping

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



Thank you

www.plantandfood.com

grant.thorp@plantandfood.co.nz

Grant Thorp

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