Leaf nitrogen and phosphorus levels in macadamias in response to canopy position and light exposure, their potential as leaf-based shading indicators, and implications for diagnostic leaf sampling protocols
Authors: Huett D. O. , Gogel B. J. , Meyers N. M. , McConchie C. A. , McFadyen L. M. , Morris S. C. 
Published: September 2002

The relationships between leaf nutrient content, leaf age, and within-canopy
light exposure were studied in 10–11-year-old
Macadamia integrifolia cvv. 660, 781, and 344 at
Alstonville (28˚59′S, 149˚E), New South Wales, during autumn
and spring 1996. Quantum point sensors were placed at 16 positions in the
canopy to give mean 24-hourly photosynthetic photon flux density (PFD)
readings, which ranged from 13 to 540 mol/m2.sec. At
each of these positions, the youngest terminal leaf (YTL), the youngest fully
expanded leaf (FEL) from a current flush, and a 6–7-month-old hardened
off leaf (HOL) were sampled. In 1997, at 12 sites in the Alstonville district,
leaves of cv. 344 were sampled (FEL and HOL) at 5 equidistant positions from
the bottom, a height of 1.2 m (position 1), to the top (position 5), on the
N–NE side of trees in late spring. The sites varied in canopy density
from 50% to 95% ground cover, and PFD from the bottom shaded
position to the top exposed position in the canopy across all sites increased
by a factor of 1.3 to 17.9.
At Alstonville, leaf parameters [N%, P%, specific leaf
weight (SLW), N amount per unit leaf area (N area), and P area] increased
(P < 0.001) with increasing PFD. Using regression
analyses, the maximum R2 was 0.59.
Age affected (P < 0.05) leaf parameters: for
N%, N area, and SLW, HOL > FEL = YTL; and for P% and P
area, YTL = FEL > HOL. Cultivar did not affect
(P > 0.05) N%, N area or SLW; for P%
and P area, cv. 660 > 781 > 344 (P < 0.05).
At the Alstonville district sites, leaf parameters increased with PFD
(P < 0.05). At each tree sampling position there was
a weak negative correlation (P < 0.05) between the
leaf parameters and percentage ground cover across all sites, which declined
with height (and PFD). Nitrogen area and P area gave the highest
R values (–0.60 and –0.40 at low canopy
positions), and neither was a suitable replacement for percentage ground cover
as a leaf-based shading indicator. The slope of the regression line
(regression coefficient) between a leaf parameter and tree height for each
macadamia site was determined. The regression coefficient for N area gave the
best correlation with percentage ground cover
(R2 = 0.55,
P < 0.01) and may be useful as a leaf-based shading
indicator.
At position 1, HOL N concentration ranged from 1.3% to 1.8% and
P concentration from 0.06% to 0.11% across all sites. At each of
the 5 tree positions, the N parameters were very poorly correlated with kernel
yield, and for the HOL P parameters, there was a weak negative correlation
(R = –0.521 to –0.673,
P < 0.05) at tree positions 1 and 2 with kernel
yield.
Current recommendations to reduce macadamia leaf N concentrations because of
detrimental effects of high leaf N on yield were not supported by the current
study. Modification of the current diagnostic leaf sampling protocol is
recommended to avoid the reduction in leaf N and P concentrations through
shading and the cultivar effects on P concentration. We conclude that the
current diagnostic leaf N and P standards cannot reliably diagnose the
nutritional status of macadamia orchards.