Effects of environmental factors and endogenous signals on N uptake, N partitioning and taproot vegetative storage protein accumulation in Medicago sativa

Our objectives were to study the regulation of N partitioning within tissue s of non-nodulated alfalfa (Medicago sativa L.) and N storage in taproots a s vegetative storage proteins (VSP) of 15, 19, and 32 kDa and beta -amylase (57 kDa) by environmental (photoperiod, temperature, N availability) and e ndogenous factors (methyl jasmonate). When compared to long-day conditions (LD, 16 h day/8 h night), short-day (SD, 8 h day/16 h night), exposure to l ow temperature (5 degreesC) or application of methyl jasmonate (MeJA, 100 m uM) for 35 d reduced the biomass shoot/ root ratio and modified the source- sink relationships for N. SD and MeJA treatments resulted in partitioning o f N to taproots and a concomitant accumulation of VSPs. In comparison with LD, SD treatment also stimulated beta -amylase gene expression 2.5-fold. Al though low temperature increased the N partitioning to root tissues and the accumulation of soluble proteins in taproot, VSP concentration and beta -a mylase mRNA levels remained low. Increasing N concentration from 1 to 5 mM KNO3 doubled the total dry matter but did not affect the N partitioning wit hin the plant, VSP accumulation, or beta -amylase expression. These results suggested that short photoperiod can result in preferential N allocation t oward taproots with a concomitant induction of VSP accumulation.