Determination of nitrogen status in maize senescing leaves

Chlorophyll meters have been successfully used in assessing leaf greenness and nitrogen (N) status for several crops, but for plants with large leaves such as maize (Zea mays L.) within-leaf blade variations in meter readings may contribute an additional error in determining leaf N status. Within-leaf variation is highest during the senescence period when chlorophyll meters can be an important selection tool for identifying genotypes with the stay-green as opposed to early-senescent trait. Two hundred forty leaves were sampled from an ongoing field experiment involving two contrasting hybrids ('Pride 5', an early senescing hybrid, and 'Pioneer 3902', a stay-green hybrid) grown at three N fertilizer rates (0, 100, 200 kg N ha(-1)) to determine the better measurement position for rapid, non-destructive field assessment of leaf N status. Each leaf blade was divided into two equal length segments: the proximal segment adjacent to the stalk and the distal (or tip) segment. Paired SPAD-502 meter readings and leaf samples for laboratory N concentration determination were obtained from each segment. Total leaf N concentration was better estimated by SPAD-502 meter readings on leaf distal segments at 2 wk (r=0.91, P<0.01) and 5 wk after silking (r-0.65, P<0.01) than by meter readings on proximal leaf segments. Our results indicate that as corn leaves senesced, total leafN concentration was better estimated by distal leaf segment samples than by proximal segment samples. Furthermore, senescence was evident on distal leaf segment samples before proximal leaf segments and grain yield was positively correlated (r values ranged from 0.80 to 0.92) with distal leaf N levels from 2 wk to 5 wk after silking.