Growth and Localized Energy Status in Phosphorus-Stressed Soybean

Abstract

In plants experiencing phosphorus (P) stress, ATP concentrations can be reduced significantly and shoot growth is strongly restricted, raising the possibility that energy availability is responsible for the growth inhibition. Experiments were conducted to investigate the relationship between P deprivation and energy availability in tissues involved in the growth response. Young soybean (Glycine max [L.] Merr. cv. Ransom) plants were deprived of P for 32 days. Leaf initiation and individual leaf expansion were followed along with localized P and ATP concentrations. Tissue analyses revealed preferential distribution of P to the root, which accompanied a decline in the shoot to root dry weight ratio. Even though P concentrations in all shoot tissues dropped sharply, ATP concentrations and energy charge in the shoot meristem region were maintained similar to controls for an extended period when leaf initiation slowed. In the first trifoliolate leaf, ATP and energy charge remained at control levels during the expansion phase, but expansion was inhibited by 50%. Furthermore, ATP levels in root tips were decreased almost 30%, yet growth of the root system was equal to or greater than the control. The absence of a positive correlation between ATP levels and growth responses in the different tissues suggests that energy availability is not a primary factor limiting growth under P stress conditions. The results, along with others from previous experiments, support the notion that a signaling mechanism, as yet unidentified, controls down regulation of cell division in shoot growth regions.