Architectural and physiological characteristics related to the depressed growth of poplars overexpressing xyloglucanase in a field study

Abstract

In a field study, transgenic poplars with lower xyloglucan content showed distinct differences from wild-type plants in having a tubby stem morphology as well as smaller and thicker leaves with a stomatal adjustment dysfunction. Two transgenic lines of Populus alba which overexpressed Aspergillus xyloglucanase (trg300-1, trg300-2) showed inferior growth compared to the wild-type plants (wt) in a field study with two different soil nutrient conditions (fertile and infertile soil areas). In order to elucidate the causes, we examined their aboveground architecture and leaf macronutrients as well as the photosynthetic rate and leaf conductance in day and dark conditions. The transgenic lines in both soil areas had significantly shorter stems and smaller total mass in comparison to wt with the same cross-sectional area of branches or aboveground bodies, describing their smaller, tubby morphology. In addition, the transgenic lines in the fertile area had 16–22 % smaller leaves with similar masses, and larger number of branch tips than wt, which resulted in larger total leaf mass with similar total leaf area in comparison to wt with the same total aboveground mass. With similar stomatal length and density, the dark leaf conductance and minimum leaf conductance were 1.5–3 times and 3–10 times higher, respectively, in the transgenic lines than in wt while the daytime leaf conductance was similar among them. This indicates that the stomata of the transgenic lines are able to open but unable to close completely, possibly leading to greater water loss. Associated with lower xyloglucan content in the transgenic lines, we suggest that xyloglucan plays important roles in establishment of plant architecture as well as stomatal closure, which affect plant growth rate.