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
D. O. Huett, B. J. Gogel, N. M. Meyers, C. A. McConchie, L. M. McFadyen and S. C. Morris
Australian Journal of Agricultural Research
52(4) 513 - 522
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.Keywords: photosynthetic, photon flux density, leaf age, canopy, shading.
Full text doi:10.1071/AR00066
© CSIRO 2001