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

The relationships between leaf nutrient content, leaf age, and within-canop y light exposure were studied in 10-11-year-old Macadamia integrifolia cvv. 660, 781, and 344 at Alstonville (28 degrees 59'S, 149 degreesE), New Sout h Wales, during autumn and spring 1996. Quantum point sensors were placed a t 16 positions in the canopy to give mean 24-hourly photosynthetic photon f lux density (PFD) readings, which ranged from 13 to 540 mu mol/m(2).sec. At each of these positions, the youngest terminal leaf (YTL), the youngest fu lly 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 distri ct, 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 densit y 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 fact or of 1.3 to 17.9. At Alstonville, leaf parameters [N%, P%, specific leaf weight (SLW), N amou nt per unit leaf area (N area), and P area] increased (P < 0.001) with incr easing PFD. Using regression analyses, the maximum R-2 was 0.59. Age affect ed (P < 0.05) leaf parameters: for N%, N area, and SLW, HOL > FEL = YTL; an d 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 g ave the highest R values (-0.60 and -0.40 at low canopy positions), and nei ther was a suitable replacement for percentage ground cover as a leaf-based shading indicator. The slope of the regression line (regression coefficien t) between a leaf parameter and tree height for each macadamia site was det ermined. The regression coefficient for N area gave the best correlation wi th percentage ground cover (R-2 = 0.55, P < 0.01) and may be useful as a le af-based shading indicator. At position 1, HOL N concentration ranged from 1.3% to 1.8% and P concentra tion 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 th e 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 o f detrimental effects of high leaf N on yield were not supported by the cur rent study. Modification of the current diagnostic leaf sampling protocol i s 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 nutr itional status of macadamia orchards.