USE OF PETIOLAR SAP NITRATE FOR ASSESSING NITROGEN STATUS OF IRRIGATED BRUSSELS SPROUT CROPS

ABSTRACT

Four field experiments were carried out during 1992/93 (sites 1 and 2) and 1993/94 (sites 3 and 4) to assess the effects of nitrogen (N), at rates up to 600 kg N ha⁻¹ on sap nitrate concentrations in petioles of the youngest fully expanded leaves (P-YFEL) of Brussels sprouts (Brassica oleracea L. var. gemmifera). The experiments were located in commercial plantings in the Mount Lofty Ranges of South Australia. Plant samples were collected at 2–4 week intervals from 4 to 28 weeks after the plants were transplanted. Temporal variation and the effect on sap nitrate concentration of sampling leaves next in age to the index leaf (YFEL−1 to YFEL+2), were also studied. Nitrate concentrations in the sap of P-YFEL showed poor sensitivity to variations in side-dressed N fertilizer and the magnitude of the changes varied between sites and sampling time. For example, at 12 weeks after transplanting, sap nitrate concentrations varied by less than 2,000 mg L⁻¹ over the range of N applied. Nitrate concentrations in petioles also varied with the age of the leaf sampled. Nitrate concentrations in petioles sampled at 4, 6, 8, and 10 weeks after transplanting increased with leaf age. Linear and quadratic models were used to study the relationships between sap nitrate concentration in P-YFEL sampled during the period 4 to 24 weeks after transplanting and yield response to applied N. Significant relationships were only found at site 2, 8 weeks after transplanting, R²=0.98; site 3 at 18 weeks, R²=0.87 and site 4 at 8 and 10 weeks, R²=0.93 and 0.87, respectively. Based on lack of sensitivity (as indicated by the narrow range in petiolar sap nitrate concentrations in response to variations in N supply), temporal variation (changes in petiolar sap nitrate concentrations during the season) and the lack of consistent relationships between sap nitrate concentrations and yield response, we concluded that sap nitrate concentration is not a useful indicator of plant N status and yield response of Brussels sprouts.

ACKNOWLEDGMENTS

We thank Kate Frost, Janice Cecil, Belinda Rawnsley, and Louise Chvyl for capable assistance with field and laboratory work; officers of the Chemistry Division for laboratory nutrient analyses and Michael Rettke for comments on the manuscript. The Horticultural Research and Development Corporation and the Brussels Sprout Section of the South Australian Farmers Federation are also thanked for their financial support and encouragement. We extend special thanks to those growers on whose land and crops these experiments were conducted.