Growth evaluation during the vegetative phase of dicotyledonous weeds and under phosphorus deficiency

Abstract

With the objective of searching physiological plant traits associated to phosphorus (P) deficiency, growth parameters were evaluated during the early vegetative phase of development in two dicotyledonous wild species with different growth habits, but coexisting in the same environment. Ruellia tuberosa L. and Euphorbia heterophylla L. were grown from seeds collected from nearby unfertilized areas. Sand culture experiments were conducted under greenhouse conditions using 950 mL pots which were irrigated every second day with 100 mL of either the nutrient solution containing 1.0 or 0.01 mM P (KH₂PO₄) or with water. In a preliminary experiment, seedling development was studied in order to determine the time for seedling establishment to produce plants of appropriate size for the P experiments. The time was 17 days for Euphorbia and 24 for Ruellia plants. After seedling establishment, the duration of the P experiments was fixed at 3 weeks within the exponential growth phase of each species. Three plants from each P treatment were harvested every two days for leaf area and biomass determinations. Relative growth rates (RGR) and relative leaf area expansion rates (RLAER) were examined as physiological indicators of plant performance and mean and maximum rates calculated by fitting a growth function to the natural logarithms of dry weight and leaf areas for the whole experimental period. Large differences in RGR and RLAER were observed between the high and low P plants of each species, as well as in shoot to root ratios and total root length. It was found that P deficiency primarily affected growth by reducing total leaf area and root length and that RGR, RLAER, and shoot to root ratios were adequate physiological indicators of P deficiency, therefore, they may be useful tools if used in screening procedures. The implications of the small size of the seeds are discussed as related to nutrient concentration in the dry seed and seedling growth.