Abstract: Water stress is one of the main driving forces to affect the structure and function of ecosystem in arid and desert regions. The objective of this study was to analyze the relationship between aboveground and belowground biomass of two desert plant species at their growth stages under water stress. The processes of biomass accumulation and allocation of Malcolmia africana and Ceratocarpus arenarius at their growth stages were researched by pot experiment. Three water supply treatments, i.e. the water-control treatment (CK), moderate water stress (LW1) and severe water stress (LW2), were applied in the experiment. The results showed that whether there was water stress or not, the accumulation of total biomass was fitted well by Sigmoidal growth model. The effect of water stress on total biomass accumulation was not obvious at early growth stages, but was significantly reduced at late stages. The proportions of leaves in total biomass of M. africana and C. arenarius were decreased by 8.9%-10.6% and 3.1%-3.4% respectively with the plant growth every 10 days, which were almost equal to the increased proportion of stems. Water stress could significantly increase root biomass, but the total biomass accumulation was still dominated by the increase of aboveground biomass. There were four visible stages of rootshoot ratio change during the whole growth stage of these two plant species.

Key words: water stress, root-shoot ratio, desert region, Malcolmia africana, Ceratocarpus arenarius