PETIOLE SAP NITRATE IS BETTER THAN TOTAL NITROGEN IN DRIED LEAF FOR INDICATING NITROGEN STATUS AND YIELD RESPONSIVENESS OF CAPSICUM IN SUBTROPICAL AUSTRALIA

This study was conducted to assess the usefulness of petiole sap nitra te and total nitrogen (N) in dried leaf for determining N status and y ield response in capsicum (Capsicum annuum L.) grown with plastic mulc h and trickle irrigation in subtropical Australia. Five rates of N (0, 70, 140, 210, 280 kg/ha) were applied in factorial combination with 2 rates of potassium (K: 0, 200 kg/ha) in randomised block experiments to capsicum cv. Bell Tower grown at Bundaberg Research Station in spri ng 1990 and autumn 1991. Critical nutrient ranges for nitrate concentr ation in petiole sap and for total N concentration in dried youngest m ature leaf blades plus petioles (YMB + P) were derived at different st ages of crop development (bud development, ED; first anthesis, FA; 80% flowering, F; fruit set, FS). Sap nitrate was about 5 times more sens itive to changes in N application than total N. Petiole sap nitrate ac counted for a greater amount of the variation in marketable fruit yiel d (quadratic square root relationships, 0.45 less than or equal to R(2 ) less than or equal to 0.83) than total N concentration in dried YMB + P (linear relationships, 0.29 less than or equal to R(2) less than o r equal to 0.74). Simple linear regressions indicated a stronger relat ionship between applied N and petiole sap nitrate concentration than t otal N concentration in dried YMB + P (range in R(2) values among 8 sa mpling events: 0.71-0.91 for petiole sap nitrate, 0.35-0.78 for YMB P total N). For the fertiliser application strategy, 60% of N was appl ied pre-fruitset and 40% after. Sap nitrate concentrations associated with 95 and 100% of maximum marketable fruit yield increased from ED ( 5010-6000 mg/L spring, 4980-5280 mg/L autumn) to FA (6220-7065 mg/L sp ring, 5550-6000 mg/L autumn). After FA, the range progressively decrea sed to 1640-2800 and 520-1220 mg/L at FS, for spring and autumn, respe ctively. It was concluded that petiole sap nitrate was a better indica tor of plant N status and yield response than total N concentration in dried YMB + P for capsicum in subtropical Australia. A critical petio le sap K concentration (corresponding with maximum yield and at which no yield response to K addition was measured) of >4800 mg/L was propos ed by correlating sap K with yield responses.