The effect of phosphorus availability on the carbon economy of contrastingcommon bean (Phaseolus vulgaris L.) genotypes

A common response to low phosphorus availability is increased relative biom ass allocation to roots. The resulting increase in root:shoot ratio presuma bly enhances phosphorus acquisition, but may also reduce growth rates by di verting carbon to the production of heterotrophic rather than photosyntheti c tissues. To assess the importance of increased carbon allocation to roots for the adaptation of plants to low P availability, carbon budgets were co nstructed for four common bean genotypes with contrasting adaptation to low phosphorus availability in the field ('phosphorus efficiency'). Solid-phas e-buffered silica sand provided low (1 muM), medium (10 muM), and high (30 muM) phosphorus availability. Compared to the high phosphorus treatment, pl ant growth was reduced by 20% by medium phosphorus availability and by more than 90% by low phosphorus availability. Low phosphorus plants utilized a significantly larger fraction of their daytime net carbon assimilation on r oot respiration (c, 40%) compared to medium and high phosphorus plants (c, 20%), No significant difference was found among genotypes in this respect. Genotypes also had similar rates of P absorption per unit root weight and p lant growth per unit of P absorbed, However, P-efficient genotypes allocate d a larger fraction of their biomass to root growth, especially under low P conditions. Efficient genotypes had lower rates of root respiration than i nefficient genotypes, which enabled them to maintain greater root biomass a llocation than inefficient genotypes without increasing overall root carbon costs.