Scientia Horticulturae 127 (2011) 347–352
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Scientia Horticulturae
journal homepage: www.elsevier.com/locate/scihorti
Shading decreases the growth rate of young apple fruit by reducing
their phloem import
Brunella Morandi ∗ , Marco Zibordi, Pasquale Losciale, Luigi Manfrini,
Emanuele Pierpaoli, Luca Corelli Grappadelli
Dipartimento Colture Arboree, University of Bologna, V.le Fanin 46, 40127 Bologna, Italy
a r t i c l e
i n f o
Article history:
Received 13 October 2010
Received in revised form 3 November 2010
Accepted 5 November 2010
Keywords:
Fruit growth
Fruit transpiration
Shading net
Vascular flows
Water relations
a b s t r a c t
This study investigates the effects of shading on the biophysical mechanisms of apple (Malus Domestica
Bork.) fruit growth by assessing how vascular and transpiration flows to/from the fruit are affected by
shading. At 30 days after full bloom, a 90% neutral shading net was applied to four trees of the cv. Gala, for
seven days, while four more trees, chemically thinned, were used as control. Fruit vascular and transpiration flows were assessed from two days before, to the end of shading. The daily patterns of fruit relative
growth rate (RGR) and of phloem, xylem and transpiration flows were determined by continuous monitoring of fruit diameter by automatic fruit gauges. Before shading application, no differences between the
two groups of trees selected were found for any of the parameters measured. Despite shading induced
an immediate drop in canopy photosynthesis, both fruit daily RGR and phloem flow decreased gradually,
until reaching 20% of the before treatment values after 7 days of shading. Differences in RGR and phloem
flow appeared especially during the afternoon and night, i.e. post carbon assimilation by the tree, and
fruit growth rates were higher in control trees. In the same period no, or very small differences were
found between treatments for transpiration rates, while xylem flow was affected later than phloem and
only at specific times during the day. These results suggest that the decrease in fruit growth rate under
shading should be attributed to the reduction of canopy photosynthesis, rather than to a direct effect of
shading on fruit sink strength.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Vascular and transpiration flows represent the main incoming
and outgoing fluxes to/from a fruit. During the day, the varying
balance among these flows determines the typical fluctuations of
fruit volume, which lead to fruit shrinkage, expansion, and growth
(Higgs and Jones, 1984; Berger and Selles, 1993).
In apple, phloem and xylem flows contribute almost equally to
fruit growth during the first part of the season. However, as fruit
develop, the contribution of xylem flow is progressively reduced,
and around 90 days after full bloom (DAFB) apple growth is sustained only by the phloem (Lang, 1990). Phloem and xylem flows
are driven by hydrostatic pressure gradients along the vascular path
(Patrick, 1997), although assimilate unloading to fruit sink cells
occurs thanks to active mechanisms (Zhang et al., 2004). Fruit water
losses decrease fruit pressure potential in the xylem, enhancing the
stem-to-fruit pressure potential gradient, i.e. the force which the
Abbreviations: DAFB, days after full bloom; RGR, relative growth rate; VPD,
vapour pressure deficit.
∗ Corresponding author. Tel.: +39 051 2096432; fax: +39 051 2096401.
E-mail address: [email protected] (B. Morandi).
0304-4238/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.scienta.2010.11.002
fruit use to attract phloem and xylem saps. Epidermis transpiration
is proportional to air vapour pressure deficit (VPD) and to fruit surface conductance, which is relatively low in apple and decreases
as fruit develop (Jones and Higgs, 1982). Apples are known to
loose low amounts of water by transpiration compared to peach
(Lescourret et al., 2001; Morandi et al., 2007a), or kiwifruit in its
early stages (Montanaro et al., 2006). When the stem-to-fruit pressure potential gradient is negative, apples can also loose water via
xylem backflow from fruit to leaves, although this process occurs
only in the first half of fruit development (Lang, 1990) as xylem vessels later become dysfunctional (Drazeta et al., 2001). During the
first half of the season, when transpiration rates and xylem contribution are higher, fruit growth rates are likely to be more sensible
to changes in the environmental conditions, as these may easily
modify the daily balance between in and outflows.
Shading trees during the early stages of fruit development has
been tested as a potential low impact tool for apple thinning, as it
causes fruit drop (Byers et al., 1985, 1990, 1991; Zibordi et al., 2009).
Despite its demonstrated effectiveness, this technique is difficult to
apply to commercial orchards, as insufficient knowledge is available on the mechanism causing fruit drop, thus making it difficult
to determine the appropriate length of the shading application. The
decrease in photosynthesis induced by shading has been indicated
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B. Morandi et al. / Scientia Horticulturae 127 (2011) 347–352
as the cause of fruit drop, as the use of photosynthetic inhibitors
mimicked the response of the shade (Byers et al., 1985). Along the
same lines, a 90% reduction of available light induced an immediate
drop in tree carbon assimilation and a gradual decrease in fruit daily
growth rates, before any fruit drop was apparent (Zibordi et al.,
2009). The reduced carbon availability at the whole canopy level
likely increases the competition between vegetative and reproductive sinks, thus reducing the amount of assimilates available for the
fruit. In addition to this, a reduction in the assimilate loaded in the
phloem cells may decrease turgor pressure at the source end of the
phloem, thus negatively affecting the hydrostatic pressure gradient
necessary for phloem translocation.
At the sink end, modified environmental conditions under the
shading net (i.e. a decrease in VPD) may also affect fruit sink
strength. In fact, although the active process of phloem unloading
is unrelated to hydrostatic pressure gradients between phloem and
sink cells, a reduction in the transpiration rates of the shaded fruit
may increase their pressure potentials, thus reducing their ability to promote translocation and attract xylem and phloem saps.
Therefore, it is not clear whether the reduced fruit growth rate
under the shading net can be attributed to an effect on the source
organs (i.e. the reduction of the carbon pool available for the fruit),
or to a direct effect on the reproductive sinks (i.e. the changes in
the environmental conditions which might modify fruit water balance and/or decrease fruit sink strength). This work investigates on
the effect of shading on the vascular flows and on the water relations of apple fruit, in order to understand the biophysical processes
underpinning fruit drop in these conditions.
that: (i) xylem flow is not affected by girdling and (ii) transpiration
rate is not affected by detachment. Fishman et al. (2001) report
how assumption (i) can lead to some systematic errors, causing
under- and over-estimation of phloem and xylem flows, respectively. However, these errors seem to be limited to specific times
during the day and, to date, this is the only method which allows to
estimate vascular and transpiration flows in the field at short times
scales and on a statistically sound number of samples.
Fruit diameter variations over time were monitored at 15 min
intervals by custom-built gauges (Morandi et al., 2007b) interfaced
to CR1000 data-loggers (Campbell Scientific Ltd., Leicestershire,
UK). For each treatment, diameter variations over time were simultaneously monitored for 24 h on 3 fruit per tree, selected on 3
different spur clusters. All fruit were placed in sequence in one of
three different conditions: “intact” (with normal vascular connections), “girdled” (with a strip of bark removed below the fruiting
spur and the spur leaves removed) and “detached” (with the peduncle detached, its surface covered with glue to avoid any water loss,
and the fruit kept in their original position using thin wire). The fruit
monitored were homogeneous for size and position in the canopy
and were selected on the east side of the row. Every day the condition of each fruit was changed: “intact” fruit were girdled, “girdled”
fruit were detached and “detached” fruit were substituted with new
“intact” fruit. Two more fruit per treatment were left intact for the
whole period and served as controls so that a total of 14 fruit per
treatment were monitored simultaneously.
At each recording time (t), diameter data (D) from all fruit monitored were converted to fresh weight (FW) using the following
conversion equation:
2. Materials and methods
FW (g) = a × D (mm)b
2.1. Plant material
where a and b were 0.0006 (±SE 0.00005) and 2.9029 (±SE 0.0194).
This equation was obtained by regressing diameter and weight data
of about 300 fruit picked during the whole season from the orchard
where the experiments were set. The R2 of the relationship was
>0.99.
The relative changes of the fruit fresh weight (g g−1 ) in a given
time interval (t) were then calculated for each fruit monitored on
the three conditions: normal (N), girdled (G) and detached (D), and
per each tree, phloem (P), xylem (X) and transpiration (T) flows
were computed using the following equations:
This trial was conducted in 2008 on twelve-year-old apple trees
(Malus Domestica Bork.) of the cv. Imperial Gala, grafted on M.9
rootstock and located at the experimental farm of the University of
Bologna, in Cadriano, Bologna, Italy. Trees were trained as free spindle at a density of 2381 trees ha−1 and received standard cultural
practices.
At bloom, 8 trees were selected for blossom uniformity
and were treated as follows: four of them were chemically
thinned [12 ␮g ml−1 1-naphthaleneacetic acid (NAA) + 120 ␮g ml−1
6-benzylaminopurine (BAP)] at 14 DAFB and served as control; the
remaining trees were covered 30 DAFB with a green polypropylene
90% neutral shading net, for a duration of 7 days.
Temperature, relative humidity and rainfall data were recorded
by a weather station (A840 Base Station – Adcon Telemetry GMBH,
Klosterneuburg, Austria) located on the farm. Data were collected
at 15 min intervals during the whole seasons and from these data
VPD was calculated.
2.2. Xylem, phloem and transpiration flows
Daily patterns of fruit growth, phloem inflow, xylem in/outflow
and transpiration outflow were determined from 2 days before
shade placement until its removal (28–37 DAFB), following Lang
(1990).
This method assumes fruit diameter variation in a given time
interval as the result of the algebraic sum among phloem, xylem and
transpiration flows. Vascular and transpiration flows are then calculated as the difference between the diameter variations of fruit in
different conditions regarding vascular functionality: intact (with
normal vascular connections), girdled (with the phloem connection
severed) and detached fruit (with both phloem and ylem connections severed). This calculation is based on the further assumptions
(1)
Pt = Nt − Gt
(2)
Xt = Gt − Dt
(3)
Tt = Dt
(4)
At each recording time, and for each treatment data from the 4
trees were averaged and standard errors were computed for all the
parameters considered.
For each parameter, treatments were compared using a
repeated analysis of variance (ANOVA). This analysis was first performed for the whole experimental period, using days as time step:
on each day, RGR and flows data were averaged and compared
between treatments, as reported in Fig. 1. Then the analysis was
performed separately, for each day of the experiment. On each hour,
RGR and flows data were averaged as reported in Fig. 2 and hours
were used as time step in the analysis.
2.3. Water relations
The daily patterns of leaf, stem and fruit pressure potentials in
the xylem were monitored at 34 DAFB at about 0400, 0900, 1200,
1500, 1800 and 2400 HR using a Scholander pressure chamber.
With this technique, the pressure potential recorded on leaf and
stem can be assumed to be equal to the water potential as the
B. Morandi et al. / Scientia Horticulturae 127 (2011) 347–352
349
Fig. 1. Means (±SE) of daily fruit RGR (a), phloem (b), xylem (c) and transpiration (d) flows to/from control (white bars) and shaded (grey bars) fruit. Each bar represents
the mean of 4 replicates. The grey area represents the period of shading. Treatments were compared using a repeated ANOVA analysis. Stars indicate a statistical difference
between treatments (P < 0.05).
Fig. 2. Diurnal courses of fruit RGR (a, b, c, d) and transpiration (e, f, g, h), xylem (i, j, k, l), and phloem (m, n, o, p) flow rates (mg g−1 min−1 ) for control (continuous line) and
shaded (dashed lines) fruit at 28 (a, e, i, m), 31 (b, f, j, n), 34 (c, g, k, o) and 37 (d, h, l, p) DAFB: two days before and one, four and seven days after the beginning of shading,
respectively. Each line represents the mean of 4 replicates. Treatments were compared using a repeated ANOVA analysis. Stars indicate a statistical difference in the hourly
means between the two treatments with P < 0.05.
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B. Morandi et al. / Scientia Horticulturae 127 (2011) 347–352
Fig. 3. Daily courses of air VPD (kPa) in control conditions.
concentration of the xylem sap is almost zero. On the contrary,
fruit pressure potential may not coincide with its water potential,
as the osmotic concentration in the fruit apoplast may be significantly high (Matthews and Shackel, 2005). For these reasons in this
paper “water potential” is used for leaf and stem while “pressure
potential” is adopted for the parameter measured on fruit.
Leaf water potential was measured on 4 shoot leaves, covered
by aluminum foil just before excision (Turner and Long, 1980),
using a Scholander (Soilmosture Equipment Corp., Santa Barbara,
U.S.A.) pressure chamber. The water potential of the stem was measured on four leaves, placed on fruit-bearing shoots, covered with
aluminum foil at least 90 min prior to measurement to allow equilibration with the stem. Fruit pressure potential was measured on
4 fruit close to the leaves used for stem determinations. At each
recording time, means (±SE) were computed for leaf, stem and fruit
water potential.
3. Results
3.1. Environmental conditions
The period considered was characterized by sunny conditions,
except at 36 DAFB, when some rain occurred. During the rest of the
experiment daily average VPDs ranged from 0.75 to 0.91 kPa with
maxima between 1.6 and 2.2 kPa (Fig. 3).
3.2. Shading effects on daily fruit RGR, xylem, phloem and
transpiration flows
Over the whole experiment shading had a negative effect on
both fruit RGR and phloem flow, but no effect on daily fruit transpiration and on xylem flow. Within a single day of measurement
however, shading showed a significant effect on all the parameters
considered (Table 1).
Before shading was applied (28 and 29 DAFB) no differences
appeared in fruit daily RGR, or in the amounts of xylem, phloem
and transpiration flows received and lost by the fruit. Similarly, no
differences in fruit RGR, and in vascular and transpiration flows
were detected in the same day when the treatment was applied
(30 DAFB) (Fig. 1).
The effect of shading on fruit RGR became apparent 2 days after
treatment application, when shaded fruit grew at about half the
rates showed by control fruit. The negative effect of shading on
fruit RGR progressively increased throughout the experiment, with
Table 1
P-Values resulting from the repeated ANOVA performed on fruit daily RGR, transpiration, xylem and phloem flows. The analysis considers the whole duration of the
experiment (28–37 DAFB), ns: not significant.
Daily values (gFW g−1 d−1 )
Trt
Time
Trt × time
RGR
Transpiration
Xylem
Phloem
0.003
ns
ns
0.0004
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
ns
ns
0.0012
a reduction of about 85% compared to controls, 6–7 days after shading (36–37 DAFB) (Fig. 1a).
Phloem flow was affected by shading even before fruit RGR,
as a significant reduction of this parameter was already evident
1 day after treatment imposition (31 DAFB), with a 44% lower flow
in shaded than control fruit. Similarly to RGR, the negative effect of
shading on phloem flow increased throughout the experiment, up
to 85% and 68% reductions, 6 and 7 days after treatment application, respectively (Fig. 1b). Xylem and transpiration flows showed
a decreasing trend throughout the experiment (Fig. 1c, d) but neither one was significant (Fig. 1c, d) (Table 1), with the exception
of xylem flow at 34 DAFB (4 days after shading imposition) which
was 45% lower in treated fruit (Fig. 1c).
3.3. Shading effects on the daily patterns of fruit RGR, xylem,
phloem and transpiration flows
On the two days before (28 and 29 DAFB) (Fig. 2a, e, i, m)
(Table 2), and on the day of shading imposition (30 DAFB) (Table 2),
fruit RGR, phloem, xylem and transpiration flows showed similar
daily patterns, without differences between treatments (Table 2).
On this “pre-treatment” period, fruit RGRs maintained low values
during the morning, then they increased until reaching maxima in
the mid-afternoon. In the evening they decreased again to values
around 0.05–0.1 g g−1 min−1 which were maintained throughout
the night (Fig. 2a). Phloem flows followed similar patterns with low
values during the night and the morning hours and an afternoon
peak in correspondence with maxima in fruit RGRs (Fig. 2m). Transpiration rates increased after dawn until reaching maxima in the
mid-day hours and decreased again in the evening (Fig. 2e). Xylem
flow showed daily patterns that were symmetric to those shown
by transpiration (Fig. 2i).
Hourly differences in fruit RGR appeared from the first day and
persisted until the end of the experiment (37 DAFB). Throughout
this period, the RGR of shaded fruit was significantly reduced during
the late afternoon and the night, the times when this parameter maintained the highest daily values in control conditions. This
afternoon peak was progressively reduced in shaded fruit until low
and constant RGR values were attained at the end of the experiment (Fig. 2b–d). In a similar way hourly differences in the phloem
appeared 31 DAFB, with a reduced flow to shaded fruit, especially
during late afternoon (Fig. 2n). These differences in phloem flow
remained until shading removal (Fig. 2o, p). Xylem flow maintained
similar hourly rates between treatments until 34 DAFB, when a
reduction in the afternoon values appeared in shaded fruit (Fig. 2k).
Afternoon and night reductions of xylem flow to shaded fruit continued until the end of the experiment, being evident also at 35
(data not shown) and 37 DAFB (Fig. 2l). Despite the higher afternoon
and night xylem flow, control fruit received very low xylem sap
during the midday hours (Fig. 2l). Xylem backflow from these fruit
was also recorded at this time of day at 35 and 36 DAFB (data not
shown). Despite these hourly differences, the amount of xylem sap
received was not statistically different between treatments, in any
of the days monitored, with the exception of 34 DAFB (Table 2). Fruit
transpiration rate was not affected by shading on any of the days
monitored (Table 2), despite some differences between treatments
occasionally appeared during the day (Fig. 2e–h).
3.4. Water relations
Both in control and shaded trees, the water potential of leaf and
stem and the pressure potential in fruit xylem vessels decreased
after dawn, showed the most negative values during the mid-day
hours, increased during the afternoon and reached the highest daily
values during the evening. Shading affected whole tree water relations during most of the day with higher values in shaded trees.
B. Morandi et al. / Scientia Horticulturae 127 (2011) 347–352
351
Table 2
P-Values resulting from the repeated ANOVA performed on fruit RGR, transpiration,
xylem and phloem flows for each day of measurement. On each date, the analysis
considers data collected on the whole 24 h.
DAFB
Trt
Time
Trt × time
28 (pre-shading)
RGR
Transpiration
Xylem
Phloem
ns
ns
ns
ns
<0.0001
<0.0001
<0.0001
<0.0001
ns
ns
ns
ns
29 (pre-shading)
RGR
Transpiration
Xylem
Phloem
ns
ns
ns
ns
<0.0001
<0.0001
<0.0001
<0.0001
0.0003
ns
ns
<0.0001
30 (Shading imposition)
RGR
Transpiration
Xylem
Phloem
ns
ns
ns
ns
<0.0001
<0.0001
<0.0001
<0.0001
0.0002
ns
<0.0001
<0.0001
31 (1 day after shading)
RGR
Transpiration
Xylem
Phloem
ns
ns
ns
0.0044
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
ns
ns
ns
32 (2 days after shading)
RGR
Transpiration
Xylem
Phloem
0.0033
ns
ns
0.0009
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
ns
ns
ns
33 (3 days after shading)
RGR
Transpiration
Xylem
Phloem
ns
ns
ns
ns
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
ns
0.0029
ns
34 (4 days after shading)
RGR
Transpiration
Xylem
Phloem
0.0011
ns
0.0379
0.0067
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
ns
ns
ns
35 (5 days after shading)
RGR
Transpiration
Xylem
Phloem
0.0024
ns
ns
0.0006
<0.0001
<0.0001
<0.0001
ns
<0.0001
ns
<0.0001
0.0104
36 (6 days after shading)
RGR
Transpiration
Xylem
Phloem
0.0040
ns
ns
0.0065
<0.0001
<0.0001
<0.0001
0.0002
0.0001
0.0148
<0.0001
0.0449
37 (7 days after shading)
RGR
Transpiration
Xylem
Phloem
0.0008
ns
ns
0.0178
<0.0001
<0.0001
<0.0001
0.0025
<0.0001
<0.0001
<0.0001
0.0045
These differences were enhanced during the mid-day hours when
control fruit showed minima in all the pressure potentials monitored. At this time, leaf water status was the parameter most
affected by shading, with values that were about 1MPa higher than
controls (Fig. 4).
4. Discussion
The application of shading in apple gradually reduced fruit daily
growth rate (Fig. 1a) and progressively modified its daily patterns
during the experiment (Fig. 2a–d), as reported also by Zibordi et al.
(2009). This effect must be attributed to the simultaneous decrease
in the daily amount of phloem sap received by the fruit in shading
conditions (Fig. 1b), rather than to variations in the daily amounts
Fig. 4. Daily patterns of leaf (close triangles) and stem (close circles) water potentials, and fruit (open squares) pressure potential in the xylem (±SE) measured at
34 DAFB in control (a) and shaded (b) apple trees. Each point represents the mean
of 4 replicates.
of water imported and lost by xylem and transpiration flows. In
fact, with the exception of the fourth day after shading (34 DAFB),
no differences between treatments were found for transpiration
and xylem flows, during the whole experimental period (Fig. 1c, d)
(Table 1).
The progressive reduction in daily fruit transpiration shown by
both treatments during the experiment (Fig. 1d) (Table 1) must
be attributed to the physiological decrease of apple fruit surface
conductance (Jones and Higgs, 1982) rather than to changes in the
weather conditions, which were variable but did not show a clear
trend during the period considered (Fig. 3). As decreasing amounts
of water were lost by transpiration, lower amounts of water were
simultaneously retrieved via xylem flow, which, for both treatments, showed a significant decreasing trend during the period
considered (Table 1).
Despite same day-to-day variability in the VPD, the lack of differences on daily fruit water exchanges (transpiration and xylem
flows) between treatments suggests that fruit ability to promote
translocation towards itself was not directly modified by the microclimate changes that might have occurred under the shading net.
Rather, the decrease in fruit growth rates and the consequent fruit
drop reported by Zibordi et al. (2009) in response to shading should
be mainly attributed to effects on the source organs, in particular
to the decrease in leaf photosynthesis. In their work, Zibordi et al.
(2009) showed that shading trees 30 DAFB resulted in a 90% reduction in whole canopy assimilation, which lasted until the removal
of shading and was followed by fruit drop comparable to that of
the chemically thinned controls. In addition to this, the reduction
of the carbon pool available for phloem loading may further lower
the hydrostatic pressure generated at the source end of the phloem
pathway, and decrease the driving force for phloem translocation
towards the fruit (Münch, 1930; Lalonde et al., 2003). These events,
both occurring at source level, may thus be the main responsible
for the reduction of the phloem flow to shaded fruit.
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B. Morandi et al. / Scientia Horticulturae 127 (2011) 347–352
As phloem flow to treated fruit decreased during the experiment, we might hypothesize that reduced amounts of solutes
were progressively unloaded to these fruit. This would lead to
gradually lower the osmotic concentration in the fruit apoplast
and to increase the pressure potentials in the fruit xylem vessels
(Matthews and Shackel, 2005). This rise in the xylem pressure
potential of the fruit is confirmed by data recorded at 34 DAFB
(Fig. 4). The difference in the fruit water status could be responsible
for the afternoon and night decreases in the xylem flow to shaded
fruit, which were recorded on the same and on the following days
(Fig. 2k, l). However, despite in this period control fruit received
higher amount of xylem sap during the afternoon and the night,
their xylem import was very low during the midday hours. At this
time of the day, some xylem backflows from fruit to leaves were
also recorded on days 35 and 36 (data not shown) due to the much
more negative water potentials experienced by control leaves. Low
or negative xylem flows in the midday hours further decrease control fruit pressure potential, so that, when later in the afternoon
control leaves close their stomata (Giuliani et al., 1997) and rise
their water potential, xylem flow can easily reach control fruit. The
strong increase in the water potentials of shaded leaves may be
mainly due to the negative effect of shading on stomatal conductance, whose regulation is strictly dependent on light irradiance
(Jarvis, 1976; Nobel, 2005).
As xylem import to shaded fruit was lower than controls during
the afternoon and the night but higher during the midday hours,
no statistical differences in the daily amounts (Fig. 1c) and in the
pattern (Table 2) of this flow appeared between treatments until
the end of the experiment.
From the first days of shading, differences in fruit RGR between
treatments occurred mainly during the afternoon and the night
due to the higher phloem flows recorded in control fruit at this
time of day. The much higher photosynthetic activity carried out
by control trees during the previous hours (Zibordi et al., 2009)
should increase carbon availability in the afternoon thus enhancing phloem differences between treatments at this time of day
(Fig. 2m–p). However, while whole canopy assimilation drops
immediately after the application of shading (Zibordi et al., 2009)
the treatment effect on phloem flow is delayed, being low in the
first days of the experiment and increasing with time (Fig. 1b). This
points to a role for tree carbon reserves in the effort to sustain
the growth of the reproductive sinks under carbon deficit conditions. The remobilization of carbon reserves is likely to have an
impact on the relationship between timing of shading and intensity of fruit drop and adds a further variable to be accounted for
in the management of shading-for thinning. In fact, the amount
of reserves available may depend on many factors, like size of the
woody organs and tree conditions and assimilation performances
during the previous season.
5. Conclusions
This study shows how early season shading in apple affects fruit
growth indirectly, by reducing the amount of assimilates fixed by
canopy photosynthesis, and not by directly modifying fruit sink
strength through changes in fruit micro-environmental conditions.
Phloem is the first flow to be affected by reduced assimilate availability and it plays the major role in determining the daily decrease
in fruit growth rate. In addition to this, the progressive reduction in
phloem flow causes a delayed, secondary effect on the xylem flow,
which is lowered at specific times during the day; i.e. when xylem
flows to control fruit are at their highest in the day, thanks to their
low pressure potentials.
These findings would suggest that the length of shading necessary to reach the desired thinning level might be simply determined
based on the determination of missed carbon assimilation under
shading conditions. However, the whole process is probably complicated by the remobilization of carbon reserves, which delays
the effects of shading both on fruit growth and on fruit final drop.
This mechanism has not been deeply investigated yet and needs
to be considered when studying the use of shading for thinning in
apple.
Acknowledgements
This work was supported by the ISAFRUIT Project, which is
funded by the European Commission under the Thematic Priority 5
– Food Quality and Safety of the 6th Framework Programme of RTD
(Contract No. FP6-FOOD–CT-2006-016279). The views and opinions expressed in this publication are purely those of the authors
and may not, in any circumstances, be regarded as stating an official
position of the European Commission.
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