The Growth-Inhibitory Effect of Increased Planting Density Can Be Reduced by Abscisic Acid-Degrading Bacteria

Abstract

High-density planting can increase crop productivity per unit area of cultivated land. However, the application of this technology is limited by the inhibition of plant growth in the presence of neighbors, which is not only due to their competition for resources but is also caused by growth regulators. Specifically, the abscisic acid (ABA) accumulated in plants under increased density of planting has been shown to inhibit their growth. The goal of the present study was to test the hypothesis that bacteria capable of degrading ABA can reduce the growth inhibitory effect of competition among plants by reducing concentration of this hormone in plants and their environment. Lettuce plants were grown both individually and three per pot; the rhizosphere was inoculated with a strain of Pseudomonas plecoglossicida 2.4-D capable of degrading ABA. Plant growth was recorded in parallel with immunoassaying ABA concentration in the pots and plants. The presence of neighbors indeed inhibited the growth of non-inoculated lettuce plants. Bacterial inoculation positively affected the growth of grouped plants, reducing the negative effects of competition. The bacteria-induced increase in the mass of competing plants was greater than that in the single ones. ABA concentration was increased by the presence of neighbors both in soil and plant shoots associated with the inhibition of plant growth, but accumulation of this hormone as well as inhibition of the growth of grouped plants was prevented by bacteria. The results confirm the role of ABA in the response of plants to the presence of competitors as well as the possibility of reducing the negative effect of competition on plant productivity with the help of bacteria capable of degrading this hormone.

Keywords: Lactuca sativa L.; abscisic acid; growth; plant competition; plant growth-promoting rhizobacteria (PGPR); planting density.

Figure 1

Dynamics of water content of sandy soil in pots containing individual plants or groups (three per pot), expressed as a % of total water capacity, measured immediately before watering. The rhizosphere was inoculated with bacteria (10⁶ and 10⁸) one day after planting. n = 12.

Figure 2

Nitrogen index (NBI) measured after 5 (A) and 13 (B) days of growing lettuce plants individually or in groups of three. The rhizosphere was inoculated with bacteria (10⁶ and 10⁸) one day after planting. Means ± SE are presented. Statistically different means (n = 40) are indicated by different letters (p < 0.05, ANOVA, Duncan’s test).

Figure 3

Shoot (A,C) and root (B,D) fresh weight measured after 5 (A,B) and 13 (C,D) days of growing lettuce plants individually or in groups of three. The rhizosphere was inoculated with bacteria (10⁶ and 10⁸) one day after planting. Means ± SE are presented. Statistically different means (n = 12) are indicated by different letters (p < 0.05, ANOVA, Duncan’s test).

Figure 4

Leaf area (A,C) and root-to-shoot fresh weight ratio (B,D) measured after 5 (A,B) and 13 (C,D) days of growing lettuce plants individually or in groups of three. The rhizosphere was inoculated with bacteria (10⁶ and 10⁸) one day after planting. Statistically different means (n = 12) are indicated by different letters (p < 0.05, ANOVA, Duncan’s test).

Figure 5

Chlorophyll content (µg/cm²) measured after 5 (A) and 13 (B) days of growing lettuce plants individually or in groups of three. The rhizosphere was inoculated with bacteria (10⁶ and 10⁸) one day after planting. Statistically different means (n = 40) are indicated by different letters (p < 0.05, ANOVA, Duncan’s test).

Figure 6

Chromatographic profiles of ABA ((5.58 ± 0.03) min release time) in the control (without bacteria) (A) and in the culture media of P. plecoglossicida 2.4-D (B). C—dynamics of ABA content in the control medium and in the medium with bacteria (C), n = 9.

Figure 7

ABA content per g fresh weight of shoots (A) and roots (B) and per pot with sand soil (C), measured after 13 days of growing lettuce plants individually or in groups of three. The rhizosphere was inoculated with bacteria (10⁶ and 10⁸) one day after planting. Statistically different means (n = 12) are indicated by different letters (p < 0.05, ANOVA, Duncan’s test).