Soybean shoot growth during phosphorus deprivation

Decreases in shoot growth and development are the earliest and most apparent changes when plants become P deficient and are often the most pronounced. Decreases in shoot growth and concomitantly sustained root growth lead to a reduced shoot to root ratio. The cause for the differential response in the shoot and root is unknown. Alterations in the processes of growth, cell expansion and cell division are undoubtedly central to the changes. Previous research has indicated that ATP concentrations can be reduced significantly during P stress, raising the possibility that energy availability is responsible for the growth inhibition. In addition, restrictions in nitrogen and water movement from the root to the shoot have been suggested as possible controlling factors of shoot growth during phosphorus stress. The results indicate that expansion of leaves under P-stress was limited by the number of cell divisions, which would imply control of cell division by a common regulatory factor within the leaf canopy. The absence of a positive relationship between ATP levels and growth responses in the different tissues suggests that energy capacity is not a primary factor limiting growth under P stress conditions. Down regulation of water flow g−1 of root was coordinated with the reduction in total leaf area expansion and reduced development of vascular tissues in hypocotyls. The inhibition of N translocation to the shoot was highly correlated with the inhibited leaf area (r2 = 0.90). Consideration of shoot development changes, however, led to the conclusion that N transport did not control leaf expansion, but was coordinated with it.