Chemical Priming with Brassinosteroids to Mitigate Responses of Avocado (<i>Persea americana</i>) Trees to Flooding Stress

Abstract

Priming involves the exposure of plants to a mild stress to increase tolerance to a more intense stress in the future. Chemical priming with brassinosteroids reduces the negative effects of flooding on the physiology and survival of some plant species. Avocado trees are very susceptible to flooding, and flooding susceptibility is related to the rootstock, which is often derived from seeds. In this study, the effects of chemical priming with two brassinosteroids, 28-homobrassinolide or 24-epibrassinolide, to improve the tolerance of avocado (Persea americana Mill.) trees to short-term flooding was assessed in two separate experiments with seedling trees of avocado cultivars Monroe (Experiment 1) and Reed (Experiment 2). In each experiment, trees were treated with a soil drench of 28-homobrassinolide, 24-epibrassinolide, or deionized water as a control. Trees in each chemical treatment were divided into two flooding treatments: flooded by submerging potted trees in water to above the soil surface or nonflooded. After the flooding treatments had begun, we measured the leaf gas exchange (net CO2 assimilation (A), stomatal conductance (gs), transpiration (Tr) and water use efficiency (WUE)) and the maximal potential quantum efficiency of photosystem II (the ratio of variable to maximum chlorophyll fluorescence; Fv/Fm) daily during the flooding period as indicators of plant stress. After the trees had been unflooded, leaf gas exchange and Fv/Fm were measured periodically during the recovery period. In both experiments, flooding reduced leaf gas exchange and Fv/Fm of trees in the chemical control and 28-homobrassinolide treatments beginning 2–3 days after flooding began until the end of the experiment. However, there was a negligible effect of flooding on leaf gas exchange or Fv/Fm of trees treated with 24-epibrassinolide. In both experiments, a greater percentage of the flooded plants treated with 24-epibrassinolide survived compared to plants in the two other chemical treatments. The results of this study indicate that pretreatment (priming) with 24-epibrassinolide as a soil drench prior to flooding reduces flooding stress of avocado trees.

Keywords

• avocado
• <i>Persea americana</i>
• flooding
• chemical priming
• brassinosteroids
• leaf gas exchange