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

Abstract

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.