Moisture stress effects on biomass partitioning in two Sonoran Desert annuals

The relative biomass accumulation rate and allocation to reproduction were quantified for Plantago insularis and Schismus barbatus, growing under four different levels of moisture stress. Analysis of growth by an allometric method indicated that seasonal growth rates were modified by water input, and for all water input treatments Schismus grew more rapidly than Plantago. As the growing season progressed and ambient temperatures rose, incremental water input had an inverse effect upon relative root growth of both species. Relative foliar growth rate of either species was not markedly altered by water input. When reproductive structures were present, their growth rates were higher than those of vegetative structures in both species. Schismus responded to water input by significantly increasing the biomass allocated to reproduction, which increased relative reproductive allocation. The computed reproductive allocation of Plantago is not correlated with water input, although the ultimate reproductive biomasses were correlated with the summation of plant moisture stress during reproduction. Thus, the bar-day formulation is useful in estimating the relative sensitivity of reproduction to drought for native annual species growing in similar habitats with varying levels of moisture stress