Abstract:

Fine root production and mortality play essential roles in carbon allocation and nutrient cycling in forest ecosystems. Despite the ecological importance of fine root turnover and the tremendous research effort on this subject, our knowledge of fine root mortality is still limited because little is understood on processes of fine root senescence. The most important function of fine roots is nutrient and water acquisition from the soil. Thus, to perform this function, carbohydrates fixed in leaves must be transported belowground to build the root system and support their functions, such as root elongation, nutrient and water uptake, and support microbes in the rhizosphere. More recently, studies on root longevity and root orders have found that first-order roots at the distal end of a root system are thinner in diameter and shorter in life-span than higher order roots. Also, fine root mortality was caused mainly by exhaustion of carbohydrates, suggesting that carbon allocation to fine roots may be an important factor affecting the senescence and mortality of fine roots. The objective of this study was to answer the following two questions: how does senescence of fine roots with different orders responds to reduced C allocation and what is the sequence of senescence from first-order to higher order roots?
This study was conducted in greenhouse on the campus of Northeast Forestry University during growing season from April to October in 2004 using Fraxinus mandshurica seedlings as experimental materials. Three one-year-old F. mandshurica seedlings were planted in each of 30 pots with crop soil in April 2004. At the end of June, two light treatments were carried out. Seedlings in 15 pots were grown under natural sunlight (1 000 μmol·m ^-2·s^-1) used as controls, and the other 15 pots were completely shaded. After treatments for 30, 60 and 120 d, roots in five pots were carefully excavated, washed and separated into root tips, first-order, second-order and third-order roots. Four physiological indices (i.e. root vigor, concentrations of soluble sugars and soluble proteins, and membrane permeability) were analyzed from the root samples of different orders for assessing the degree of fine root senescence.
The results showed that there was reduced C allocation to the fine roots in the shading treatment that induced reduced root vigor, decreased concentrations of soluble sugars and soluble proteins, and increased membrane permeability, compared with controls. After the 60 and 120 d of shading, root vigor reduced 50% and 95%, soluble sugar concentrations decreased about 74% and 73%, and soluble proteins reduced 5% and 30%, respectively, but membrane permeability increased one-fold after 120 d.
These results suggest that reduced C allocation to fine roots caused by leaf shading has great impacts on the physiological functions of fine roots and stimulated root senescence. Root senescence differed significantly among root order: root tips exhibited senescence followed by the first- and second-order roots, and the third-order roots had the least amount of senescence. As the physiological functions of the fine roots changed, coloration of the roots also changed with the root tips being a dark brown, and the first-, second- and third-order roots becoming increasing lighter in color. These results indicat that fine roots senesce in sequence explaining why fine roots of the first-order at the distal ends of a root system have a short life-span and rapid turnover.

Key words: Fine root senescence, Fine root order, Carbon allocation, Fraxinus mandshurica