Fine root anatomy and hydraulic conductance of one-year-old loblolly pine seedlings under different levels of [CO2], fertility, and water availability

Background/Question/Methods

Little is known about the anatomical development of fine roots of different branch orders under changing environmental conditions. In this study, we analyzed the influence of different levels of [CO₂], fertility, and water availability on anatomical features of the first five order roots in one-year-old loblolly pine (Pinus taeda L.) seedlings, and examined hydraulic conductance of fifth-order roots from an anatomical perspective. For one growing season, loblolly pine seedlings were exposed to either ambient or elevated [CO₂] (380 or 700 µl L^-1, C_L or C_H), and watered with an automated irrigation system to maintain 20% (W_L) or 90% (W_H) soil water holding capacity. Seedlings were fertilized twice a week to obtain high (154 mg Excel 15-5-15 Cal Mag water soluble fertilizer for each pot, F_H) and low fertility (15.4 mg Excel 15-5-15 Cal Mag water soluble fertilizer for each pot, F_L). At the end of the growing season, we measured hydraulic conductance in fifth-order roots. After that, we embedded the first five order roots with JB-4 embedding technology and cut the cross sections into 6 μm thick sections with a microtome and examined anatomical characteristics under light microscopy.

Results/Conclusions

First-order roots in all treatment groups had very low stele-to-root diameter ratio, and did not show any secondary xylem development. Seedlings grown under C_H, no matter of the level of fertility and water availability, started to show signs of secondary growth in some second-order roots, although the stele-to-root diameter ratio did not change significantly from that in first-order roots. All of the third-order roots showed secondary growth, and the ratio of stele to root diameter was substantially greater, compared to the first two root orders. Cortex thickness in third-order roots of seedlings grown under W_L was significantly lower than that in seedlings with access to high water availability despite their [CO₂] and fertility status. Seedlings grown under W_H still retained a cortex in some fourth-order roots, but the cortex disappeared in all of the fifth-order roots. Seedlings of C_HW_HN_H have the highest stele-to-root ratio in their fifth-order roots, which resulted in the highest hydraulic conductance compared to other treatment groups. Our results indicate that differences in hydraulic conductance were related to changes in anatomical features. Water availability and [CO₂] had greater effects than fertility on xylem and cortex development in the first five order roots and had a significant influence on the hydraulic conductance in fifth-order roots.