Agriculture Reference
In-Depth Information
) and also reductions in stomatal density (number cm
−
)
and chlorophyll (
effects (Table
.
gcm
−
) with increasing shade (Asada and Ogasawara,
µ
).
Barden(
)showedthatthespecificleaf weightwasincreasedbyexposure
to full greenhouse sun in comparison to artificial shade both during and after
the period of leaf expansion. Shading of spurs may result in the spur and
bourse shoot leaves (see Figure
) emerging from them in the following
year having lower specific leaf weights (Tustin
et al.
,
.
,p.
), although Jackson
and Palmer (
) found that shading whole trees did not result in lower
specificleafweightinthefollowingyear,possiblybecausethepreviouslyshaded
trees had less vigorous shoot growth and hence better current-season light
exposure.
Shade also influences leaf chloroplast structure. The thickness of grana
within chloroplasts of mature apple leaves increases with depth in the palisade
layer and with external shade. Transferring trees from light to shade and
vice
versa
indicated that grana thickness increased with increasing shade but did not
decrease with increased light intensity (Skene,
). In general chloroplasts
with thick grana have low photosynthetic electron transport capacity, and light
saturation of photosynthesis occurs at relatively low light intensities. They also,
however, have low respiration rates and so enable efficient use of low levels of
light.
The mechanism by which 'sun' or 'shade' leaf characteristics are induced by
relative light exposure during leaf expansion is not clear. One hypothesis is that
the leaves which transpire most receive a greater proportion of the nutrients
and hormones that are transported upwards in the transpiration stream in
the xylem (Flore and Lakso,
). Evidence to support this comes from the
facts that leaf nutrient concentration per unit area is generally much higher
the greater the light intensity (Table
), that many of the characteristics of
'sun' leaves can be induced by cytokinin treatment of low-light leaves, and
that shading mature leaves on apple shoots resulted in the developing leaves
attaining more pronounced 'sun-type' characteristics (Flore and Lakso,
.
).
Beakbane (
) found that treatment with gibberellic acid resulted in changes
in the structure of the palisade mesophyll and the spongy mesophyll of apple
leaf disks to give a 'shade-type' leaf structure.
Leaf production and growth
The first leaves to emerge are the primary leaves on spurs (Forshey
et al.
,
; Lakso,
). Hansen (
) reported usually
or
leaves per bud in
Denmark, Barritt
et al.
(
primary leaves per fruiting spur
of 'Oregon Spur Delicious' in Washington State, USA, by late April. Most
) an average of
.
