The Effect of Water Deficit On the Seasonal Pattern of Taproot Assimilates for 4 Contrasting Alfalfa Cultivars in Tasmania, Australia.

Taproot assimilates are a major driver of regrowth following defoliation and over wintering in alfalfa (Medicago sativa).  However, little information is available comparing seasonal changes in the taproot assimilate pools of contrasting alfalfa cultivars exposed to variations in availability of water. 

Four alfalfa cultivars were grown irrigated or dryland over the 2007/08 season at Elliott, Tasmania (41°4’48”S, 145°46’12” E, 1200mm annual rainfall, Humic Eutrodox soil).  Cultivars were chosen to represent a wide range of fall dormancy (FD) and consisted of DuPuits (FD 3), Grasslands Kaituna (FD 4.5), SARDI-7 (FD 7) and SARDI-10 (FD 10).  The concentrations of taproot soluble carbohydrates, starch, amino acids and proteins as well as the protein pool composition were monitored at each defoliation for the length of the growing season. 

The cultivar effect on seasonal changes in taproot carbohydrate pools was different under each water regime.  Under irrigated conditions taproot starch declined in SARDI 7 and SARDI 10 over the growing season while taproot starch concentration remained stable for all cultivars under dryland conditions.  Under dryland conditions taproot soluble sugar concentration only changed through the season for DuPuits, where it increased from summer to winter.  In contrast, taproot soluble sugar concentrations increased in all cultivars from mid-summer to winter under irrigation.  Taproot soluble protein concentration remained stable for all cultivars under dryland conditions.  Under irrigated conditions during summer, taproot soluble protein concentrations remained stable for all cultivars apart from Grasslands Kaituna.  By early fall however, taproot soluble protein concentrations decreased in all cultivars before re-accumulating in winter.  The proportion of taproot vegetative storage proteins (VSP) under dryland conditions increased over the summer, while with irrigation, taproot VSP decreased throughout the summer before re-accumulating during autumn.