Low-order fine roots of Picea asperata have different physiological mechanisms in response to seasonal freeze and freeze-thaw of soil.

Seasonal soil freezing (F) and freeze-thaw cycles (FTCs) are common natural phenomena in high latitude or altitude areas of the world, and seriously affect plant physiological processes. However, studies on the effect of soil F and FTCs on fine roots are less common, especially in subalpine coniferous forests of western Sichuan, China. We set up a controlled experiment in growth chambers to explore the effects of F and FTCs on low-order fine roots of Picea asperata and differential responses of first-order roots and the first three root orders (1st, 2nd and 3rd order roots combined as a unit). Soil F and FTCs resulted in serious damage to cell membranes and root vitality of low-order fine roots, accompanied by increased MDA content and O · production. FTCs had a stronger effect than F treatment. In turn, low-order fine roots are the unit that responds to cold stress. These roots had increased unsaturated fatty acid contents, antioxidant enzyme activities, osmolytes and plant hormones contents when acclimation to cold stress. The first-order roots were more sensitive to cold stress than the combined first three root orders for several processes (e.g. antioxidant enzymes, osmolytes and hormones) because of their specific structure and physiological activity. This study explains physiological differences in responses of fine roots of different root orders to seasonal soil freezing, which will improve the understanding of fine root heterogeneity and support agriculture and forest management.