Efficacy of treatments for delaying senescence of wheat leaves. II. Senescence and grain yield under field conditions

Longevity of leaves directly influences grain yield of wheat (Triticum aestivum L.), but is greatly diminished by environmental stresses. The objective of these studies was to identify treatments that might delay senescence of wheat leaves and increase grain yield. 'Nesma' wheat was grown on a heavy clay soil at Meknes, Morocco, for two seasons, the first characterized by mild stress and the second by more severe stress from inadequate precipitation and high temperature. Three planting dates, two seeding rates, and foliar applications of NH4NO3 and KH2PO4 in randomized complete block designs were tested for efficacy in delaying senescence of leaves as measured by proteolytic activity, chlorophyll and protein concentrations, and grain yield. Advancing the planting date from early December, the optimum time, to November in the first year and delaying planting until January in the second year reduced grain yields in both seasons. Increasing the seeding rate from 200 to 400 kernels m-2 increased grain yields only in the second year, and foliar applications of 10 kg N as NH4NO3 ha-1 were ineffective in both years. Foliar applications of 5 to 10 kg ha-1 KH2PO4, in contrast, often increased grain yields by 1 Mg ha-1 or more. Senescence processes were enhanced by late planting because of high temperatures during maturation and by dense seeding because of interplant competition. Treatments that increased grain yields usually slowed senescence processes. Applying dilute solutions of KH2PO4 was most effective and, after further testing, might have merit for wheat in regions where drought and high-temperature environmental stresses occur