Quote:Light interception can be increased by reducing the distance between the rows,
increasing the height of the trees and orienting the rows wherever possible in a N-S direction.

Light, Canopies,Fruit and Dollars

John W. Palmer
HortResearch
Nelson Research Center
Motueka, New Zealand

Presented at the 42ndAnnual IDFTA Conference, February 20-24, 1999, Hamilton, Ontario,
Canada.

INTRODUCTION
All plant life depends upon the conversion of light energy into chemical energy via the process
of photosynthesis.Fruitgrowers, like all farmers, are dependent on this process to convert the
light energy falling on their orchard into the large, juicy, attractive fruit we know as the
cultivated apple.There is, however, little indication of large cultivar or rootstock effects on the
light saturated photosynthetic rate per unit leaf area.In contrast there are major effects of
cultivar and rootstock on leaf area per tree.Therefore the seasonal carbon dioxide uptake by an
apple tree would be expected to be related to the leaf area and its disposition in relation to the
light and its duration.Several researchers have shown a close relationship between intercepted
light and dry matter production from a range of crops, including apples.Intercepted light is the
light which is captured by the plant, largely by the leaves.In an apple orchard the majority of the
light which is not intercepted falls on the orchard floor and grass alleyway.Dry matter
production includes shoots, leaves, fruit and roots.As there is little sale for apple leaves, we will
concentrate on the influence of light on the fruit component, not forgetting that the leaves, shoots
and roots play a vital role in producing the fruit.

YIELD AND LIGHT
A number of researchers in Europe and the USA have shown a close relationship between light
interception and yield of apples. The amount of light intercepted by an apple orchard depends
upon two characteristics:1) the light environment, the amount and angular distribution of the
sunlight and 2) the canopy characteristics, the row spacing, orientation, leaf area and the three
dimensional arrangement of that leaf area in the orchard.

The light environment is dependent on both the latitude, as this determines the incoming solar
radiation above the atmosphere, and the cloudiness of the particular location.The influence of
latitude on incoming solar radiation is given in Figure 1.Maximum solar radiation receipt
occurs in the latitude band of 25-30o; regions closer to the equator are characterized by high
rainfall with its associated clouds, while higher latitude locations have greatly reduced solar
radiation receipt due to clouds and the short days during the winter.When comparisons of solar
radiation are made over the peak 4 months, differences in radiation receipt between latitudes of
30 to 55oare reduced due to the long day length at higher latitudes during the summer.
Nevertheless, receipt of photosynthetically active radiation (PAR) increases by 0.03 GJ m-2per
degree of latitude between 55 and 35o.Making some simple assumptions, Wagenmakers (1991)
calculated that this could be equivalent to 2.5 t/ha (1.1 ton/acre) per degree latitude.Within a
latitude band, continental climates typically have clear skies in the summer while more maritime

climates are characterized by cloudier skies.The time from the last spring frost to the first
autumn frost determines not only the particular period of the year when crop growth can occur,
but also fixes the seasonal pattern of incoming light.(This can obviously be slightly extended in
the spring with the use of frost protection.)

In terms of the natural environment, the location of New Zealand is ideal for fruit growing, as it
enjoys a long growing season with high solar radiation, adequate winter chill but not excessively
high temperatures during the growing season.The latter ensures both adequate color formation
on partly red cultivars and reduced respiration rates and little reduction of net carbon dioxide
uptake by the tree during the day due to high temperatures.

Once the grower has settled on his particular location, there is little he can do to alter the
incoming light environment other than reduce it with hail netting.In contrast, the canopy
characteristics are under the grower’scontrol.He can choose the shape, size and characteristics
of the canopy and therefore how much of the available light the orchard intercepts.The ratio of
leaf area per tree to area allocated per tree (leaf area index; LAI) has the major influence on light
interception.Mature modern apple orchards typically have LAIs in the middle of the growing
season within the range 1.5 to 2.5 (Jackson, 1980).The three dimensional arrangement of the
leaf area can modify the light interception, particularly at high LAIs.Light interception
obviously can be increased by spreading the orchard canopy as uniformly as possible over the
land; unfortunately this prevents access to the trees for the cultural operations of spraying,
pruning and picking.Orchard design therefore has to work within this major physical constraint.
The inevitable row structure of the canopy means that light interception can be increased by
reducing the distance between the rows, increasing the height of the trees and orienting the rows
wherever possible in a N-S direction.

Although the relationship between total dry matter production and light interception is often very
tight, the relationship between yield of fruit and light interception shows a greater variability
(Figure 2).This is to be expected as the partitioning of carbohydrate into fruit depends upon
crop load and the light distribution within the canopy.Obviously following a spring frost event
or with a strongly biennial cultivar in the off year, crop load can be dramatically reduced with
little change in light interception.In the absence of such adverse events, crop load is regularly
altered by the grower’sthinning practices—total yield of fruit per tree is reduced in order to
increase average fruit size.Therefore, care must be exercised in comparing the relationship
between light interception and yield between different orchard systems, particularly where
comparisons are made within one season and therefore subject to possible confounding of system
with crop load.On the other hand, some of the variation seen in comparisons of light
interception and yield genuinely reflect differences in system.Crop load can be altered by the
light distribution within the tree; heavy internal shading can reduce flower bud production, fruit
set and fruit size.Excellent work has been done by Wünsche et al. (1996) in elucidating the
importance of light incidence on spur leaves in different systems.

Even greater care must be used when yield and light interception data are compared across
locations or regions as, in this case, location can influence fruit growth and yield via temperature.
Temperature can have a major effect on the rate of plant processes, particularly those involving
cell division and respiration.Early season temperature during the cell division stage can have a
large effect on the final fruit size.

Nevertheless, light interception forms a much more useful basis for comparing trees at different
spacings or systems than tree volume or trunk cross-sectional area.If two systems achieve a
similar light interception yet differ in yield, it indicates that one system is able to partition more
dry matter into fruit than the other, via a better distribution of light within the canopy or via a
rootstock effect.Although light interception is frequently recorded in the middle of the season
after shoot growth has stopped, comparisons of systems may require measurements throughout
the season as the seasonal pattern of light interception may be influenced by system where
systems vary greatly in canopy form, e.g., light interception may increase more rapidly in spring
on the distributed canopies of a Y trellis than the more concentrated canopy of a central leader
tree.

LIGHT AND FRUIT QUALITY
There has been much detailed work over the last 30 years on the influence of light on fruit
quality, particularly by Jackson and Palmer in the UK, Ferree, Rom, Barritt and Robinson in the
USA and Tustin and Warrington in New Zealand.Consistently across these environments and
across cultivars, shading has been shown to dramatically reduce fruit quality.Table 1
summarizes the effects of shading on reproductive and fruit characteristics and Table 2 the
effects of shading on leaf characteristics.Interestingly these effects are common across a whole
range of perennial fruit species—apples, pears, citrus, peaches, cherries, raspberries (Palmer,
1989).

It is very apparent therefore that reduced light can have serious effects upon the production and
external and internal fruit quality of apple trees.Fruit color is often the most sensitive indicator
of shading with striped or partially red cultivars and, as such, gives a good indicator of likely
differences in internal fruit quality.For fully red cultivars that color even in deep shade the
absence of skin color differences can lead to false assumptions about internal fruit quality.
Similarly background color has been used successfully in some cultivars as an indicator of
maturity—a fully red skin can completely hide differences in background color.

As the effects of shade on fruit and leaf characteristics are so clear, the grower does not
necessarily need sophisticated light-measuring equipment to determine whether the canopy has
excess internal shading.The fruit quality and leaf characteristics in different regions of the tree
can give a clear indication.

ORCHARDSYSTEMS
An orchard system is a combination of management practices relating to rootstock, pruning, tree
training, tree spacing and the associated farm machinery.Biologically, however, a system can be
understood to manipulate, first, the light interception and distribution and, second, the
partitioning of carbohydrate into fruit.To be successful the system must work within the
physical constraints of access and the biological constraints of the effect of shading on fruit
quality.Horticulturists, however, are great manipulators of the plant kingdom and fruitgrowers
are no exception.The emphasis over the last few years has been on manipulation via genetics of
the scion and rootstock and via pruning and training.Spur types, particularly of Red Delicious,
proved invaluable in the USA, while the Dutch led the way with detailed tree training on M.9 in
the slender spindle and its variants.The flirtation with plant growth regulators (PGRs) in the
1970s and ’80sdid not result in major sustained use as a tool for canopy manipulation in apple
production.There has, however, been a small resurgence of activity in the PGR area of late,