Effects Of Shading On Fine Roots Senescence Of Fraxinus Mandshurica Seedlings

Fine root senescence was an important question on the study of fine root longevity and turnover. The physiological metabolism, enzyme activity and DNA fragment of fine roots were studied for understanding principle of fine root senescence of Manchurian ash (Fraxinus mandshurica) seedlings under the stress of shading treatment in greenhouse, and some parameters were applied for assessing degrees of fine root senescence. The objective of this study was to explain how C allocation affected on fine roots senescence and their senescence sequence. The main conclusions of investigation as follows:1 The results showed that reduced C allocation to fine roots, the root vigor, soluble sugar, and soluble proteins decreased with increasing root order, and MDA and membrane penetration increased, comparing with control. Those suggested that fine root senescence in physiological view was affected by C allocation and had a sequence from root tips, first order to the third order roots.2 When C allocation to fine roots was stopped, activities of protective enzymes, such as SOD, POD, and CAT, were changed with increased root order. The data under shading treatment indicated that fine root could keep the balance between production and scavenging of active oxygen by heightening activities of the enzymes at the early treatment. In contrast to the early treatment time, activities of the enzymes, such as SOD, POD, and CAT, were broken and disproported in middle or long time C stress. These enzyme systems changed which suggested that the balance of production and scavenging systems of active oxygen were broken, and fine root senesced and died off. These results also indicated that protective enzymes were correlated with root order, and fine root senescence, at protective enzyme level, had a sequence from root tips, first order to the third order roots.3 The most important indicator of fine root senescence was DNA fragmentation in latter shading treatment, DNA fragmentation was found in cell of root tip and the first order root although DNA fragmentation pattern becoming smear bands in agarose gel. In the second and third order roots, DNA had not been observed any fragmentation on electrophoresis map. However, the beginning of fine root senescence had been proved in root tip, and subsequently the first order roots in this experiment.These results suggested that reduced C allocation to fine roots caused by leaf shading had great impacts on physiological functions of fine roots and stimulated senescence. With the different root branching order, difference of root senescence was larger significantly. Root tips firstly exhibited senescence, and subsequently the first and second-orders, the third-order roots had the slight senescence comparing with root tip and the first two orders in this experiment. These results indicated that fine roots had the senescence sequence which explained why fine roots of the first-order at the distal ends of a root system had a short life-span and rapid turnover.