Seasonal root growth of Scots pine seedlings in relation to shoot phenology, carbohydrate status, and nutrient supply

To ascertain whether the growth rhythm of roots differs from that of the shoot, the seasonal pattern of dry mass allocation was determined in 1-year-old Scots pine (Pinus sylvestris L.) seedlings. Gas exchange, chlorophyll fluorescence, and the dynamics of starch and soluble sugars were examined to understand the role of stored carbon and that of current photosynthates in meeting the sink demand of plant organs. In this growth-chamber experiment, hydroponic cultures supplied the seedlings with low (0.25 mM N) or high (2.5 mM N) nutrient level. The climatic conditions in the chamber simulated the weather conditions from May to mid-October in southern Finland. Root growth was most intense at the end of the growing season, at which time shoot growth slowed down. Nutrient level did not affect the growth rhythm of the roots, but the total production of root biomass was favoured by high level of nutrients. The response of root growth to root zone temperature (RZT) was not the same over the growing season, indicating that the sensitivity of root growth to RZT depends on the growth phase of the seedling. The growth rhythm of the roots is probably regulated by several internal and external factors and their interactions, including RZT and availability of photosynthates.