Evaluating the leaf chlorophyll meter as a tool for nitrogen management in sugarcane

Abstract

Nitrogen is an important constituent of chlorophyll, the compound that gives plants their green colour and the compound that absorbs radiation energy as the basis of plant growth. When nitrogen is in short supply, the chlorophyll content of leaves is reduced and in extreme cases, the leaves take on a yellow (or chlorotic) colour. There is an upper limit to leaf "greenness" and leaf nitrogen can continue to rise even when chlorophyll has reached a "plateau". These simple concepts are illustrated in the figure below. This project was aimed at evaluating a portable leaf chlorophyll meter as a tool in nitrogen management of sugarcane. As chlorophyll is a critical component of the photosynthetic system, reductions in leaf chlorophyll may result in lower sugarcane growth rates and this may impact on final sugar yields. In addition, leaf nitrogen is expensive and time consuming to determine on an extensive basis. The notion was that the chlorophyll meter could provide a far simpler alternative to leaf analysis for the monitoring of nitrogen status of sugarcane crops. The project consisted of measurements of leaf nitrogen and chlorophyll on sugarcane crops grown under a range of nitrogen regimes at locations in the Herbert, Bundaberg and northern NSW regions. Leaves at various levels in the canopy were measured and in the majority of instances samples of the bulk canopy were taken. Samples of the sap from the leaf mid-ribs were collected at the same time and analysed for nitrate using a rapid field test. In all 450 leaf N - chlorophyll meter comparisons were collected at various occasions over the November 1993 to June 1994 period.