Morphological, growth, and photosynthetic responses of cottonwood hybrid 47-174 (Populus trichocarpa x P. deltoides) to nitrogen fertilization and leaf rust infection

Abstract

The growth, morphological, and photosynthetic responses of cottonwood hybrid 47-174 to nitrogen (N) stress and leaf rust stress and their interaction are summarized herein. High nitrogen (HN) fertilization enhanced growth and photosynthesis as compared to low nitrogen (LN). HN plants developed more and larger leaves, were taller, had greater diameters, and greater dry weights as compared to LN plants. Leaves were developmentally identified by the leaf plastochron index (LPI) system. Subdividing LPIs into 4 structural-functional segments (SFS 1-4) facilitated sampling of the major stages of leaf development of this plant. Indirect measures of leaf nitrogen found higher values in HN than LN plants. However, in both plants leaf nitrogen levels were lowest in SFS 1, the top segment with newly formed leaves, and were high in the three other segments: SFS 2 - newly matured leaves, SFS 3 - fully matured leaves, and SFS 4 - mature, senescening leaves. Both HN and LN plants were susceptible to Melampsora leaf rust infection. Scanning electron microscopy (SEM) of both HN and LN rust infected leaves illustrated the morphological impact of rust infection.

ACi curves (A = photosynthetic assimilation and Ci = internal CO2 concentration) showed that healthy HN leaves had higher A rates compared to LN leaves. In analyzing interaction between N and rust infection, under favorable HN, the middle or SFS 2 segment had higher A rates whereas under LN stress the top SFS 1 had higher A rates. Infected HN leaves had higher A than infected LN leaves in all segments; however, rates were considerably lower than uninfected leaves. A general pattern was noted that HN plants had higher rates of physiological responses than LN plants regardless of whether the leaves were healthy or infected. However, it was important to divide the plants into four SFS segments as this division provided a better picture of how leaf development was influenced by and affected the plant’s response to these two stresses and their interaction.