Ethylene insensitivity does not increase leaf area or relative growth rate in Arabidopsis, Nicotiana tabacum, and Petunia x hybrida

Abstract

The plant hormone ethylene plays a role in various growth related processes. In this detailed study of the vegetative growth of Arabidopsis, Nicotiana tabacum, and Petunia x hybrida plants, we show that ethylene insensitivity does not result in an increased total leaf area or relative growth rate (RGR) under optimal growth conditions. When grown in semiclosed containers, leaf area of ethylene-insensitive plants was larger compared to the wild type. This effect was caused by a buildup of ethylene inside these containers, which inhibited the growth of wild-type plants. Ethylene-insensitive Arabidopsis and N. tabacum plants had a lower biomass, which was mainly the result of a smaller seed mass. RGR of vegetative plants was not affected by ethylene insensitivity, but the underlying components of RGR differed; specific leaf area (leaf area per unit leaf mass) was higher, and unit leaf rate (growth rate per unit leaf area) was lower. The latter was a result of a slower rate of photosynthesis per unit leaf area in the ethylene-insensitive plants.

Figure 1.

Comparison of total leaf areas of ethylene-sensitive and -insensitive genotypes. A, Arabidopsis wild-type and ethylene-insensitive etr1-1 mutants grown for 2 weeks in sealed petri dishes (n = 40), in petri dishes containing Ethysorb (n = 40), and on hydroponics in the open (n = 6). Mean values ± se. ^***, significant difference between ethylene-sensitive and -insensitive plants (P < 0.001). Ethylene concentrations inside the petri dishes were: wild type, 0.08 μL L⁻¹; etr1-1, 0.70 μL L⁻¹; wild type + Ethysorb, 0.02 μL L⁻¹; and etr1-1 + Ethysorb, 0.03 μL L⁻¹. B, Total leaf area after 2 weeks (Arabidopsis, triangles) or 4 weeks (N. tabacum, squares; P. hybrida, circles) of growth on hydroponics. The dotted line represents the 1:1 ratio. ^*, significant difference between ethylene-sensitive and -insensitive plants (P < 0.05). Mean values ± se (n = 18).

Figure 2.

Ethylene-sensitive (left) and -insensitive (right) Arabidopsis (top), N. tabacum (middle), and P. hybrida (bottom) plants after 14 d of growth on soil.

Figure 3.

Total dry mass of ethylene-insensitive Arabidopsis, N. tabacum, and P. hybrida relative to the ethylene-sensitive controls (represented by the dotted line). The first time point is at the seed stage, the second time point is a seedling with two leaves, and the third and fourth time points correspond with the two harvests taken for the growth analysis. In Arabidopsis and N. tabacum, the differences in mass in adult plants are predominantly caused by a difference in seed mass.

Figure 4.

Effect of ethylene insensitivity on the RGR of plants growing on hydroponics for 2 weeks (Arabidopsis, triangles) or 4 weeks (N. tabacum, squares; P. hybrida, circles). The dotted line represents the 1:1 ratio. Mean values ± se (n = 18).

Figure 5.

The relationship between leaf number and total plant dry mass of Arabidopsis (triangles), N. tabacum (squares), and P. hybrida (circles). Closed symbols and continuous lines represent the ethylene-insensitive genotypes, whereas open symbols and dotted lines represent the ethylene-sensitive plants. Mean values ± se (n = 18) are shown.

Figure 6.

Effect of ethylene insensitivity on leaf area per unit leaf mass (SLA; A), leaf mass per plant mass (LMF; B), and root mass per plant mass (RMF; C) of Arabidopsis (triangles), N. tabacum (squares), and P. hybrida (circles). Values are plotted against whole-plant dry mass so that plants of similar size and stage of development can be compared. Closed symbols and continuous lines represent the ethylene-insensitive genotypes, and open symbols and dotted lines represent the ethylene-sensitive plants. ^***, significant difference in the last harvest with P < 0.001; ^*, significant difference with P < 0.05. Mean values ± se (n = 18) are shown.

Figure 7.

Effect of ethylene insensitivity on growth rate per unit area (ULR; A). Symbols and lines are explained in Figure 4. ^* represents a significant difference with P < 0.05. Mean values ± se are given, n = 18. B, Components of the ULR expressed as percentage from ethylene-sensitive plants (100%): photosynthesis (PS_A), fraction of daily fixed carbon that is incorporated (FCI), and carbon concentration ([C]). n = 8.

Figure 8.

Effect of ethylene insensitivity on organic nitrogen content (N_ORG; A) and the relationship between ULR and SLA (B). The relationship between SLA and ULR or N_ORG suggests a trade-off resulting in a constant RGR. Symbols are explained in Figure 4. Mean values ± se are given (^*, P < 0.05).