Effect of phosphorus availability on basal root shallowness in common bean

Root gravitropism may be an important element of plant response to phosphor us availability because it determines root foraging in fertile topsoil hori zons, and thereby phosphorus acquisition. In this study we seek to test thi s hypothesis in both two dimensional paper growth pouch and three-dimension al solid media of sand and soil cultures. Five common bean ( Phaseolus vulg aris L.) genotypes with contrasting adaptation to low phosphorus availabili ty were evaluated in growth pouches over 6 days of growth, and in sand cult ure and soil culture over 4 weeks of growth. In all three media, phosphorus availability regulated the gravitropic response of basal roots in a genoty pe-dependent manner. In pouches, sand, and soil, the phosphorus-inefficient genotype DOR 364 had deeper roots with phosphorus stress, whereas the phos phorus-efficient genotype G19833 responded to phosphorus stress by producin g shallower roots. Genotypes were most responsive to phosphorus stress in s and culture, where relative root allocation to the 0-3- and 3-6-cm horizons increased 50% with phosphorus stress, and varied 300% (3-6 cm) to 500% (0- 3 cm) among genotypes. Our results indicate that (1) phosphorus availabilit y regulates root gravitropic growth in both paper and solid media, (2) resp onses observed in young seedlings continue throughout vegetative growth, (3 ) the response of root gravitropism to phosphorus availability varies among genotypes, and (4) genotypic adaptation to low phosphorus availability is correlated with the ability to allocate roots to shallow soil horizons unde r phosphorus stress.