Partitioning of 14C-photosynthate among Competing Sinks during Flowering and Early Fruiting in Snap Bean

Photoassimilate distribution was evaluated in bush snap beans ( Phaseolus vulgaris L. ‘Oregon 1604’) field-grown at low and high plant densities (33 and 54 plants·m −2 ) and irrigated when soil water potential reached −0.06 MPa (high) or −0.25 MPa (low irrigation). A single trifoliolate leaf per plant, either at a lower (node 2 or 3) or upper (node 5) canopy position, was briefly dosed with 14 CO 2 and plants harvested after 27 hr. This treatment occurred on 3 dates (25, 27, and 31 July), corresponding to the days when flowers at each of the 3 raceme nodes of the terminal inflorescence sequentially reached 1 to 2 days post-anthesis. 14 C activity was measured in shoot parts, including individual flowers and pods. Partitioning was calculated as the percentage of distribution of 14 C exported from the exposed leaf. Although irrigation-density effects on 14 C-partitioning were slight, the percentage of 14 C-assimilate translocated from the dosed leaf to its nearby inflorescence increased sharply at each successive sampling date. On 25 July, flowers at each of the 3 raceme nodes of the terminal inflorescence had only 2–4% of the 14 C activity, whereas on 31 July those at the proximal 2 positions had 34% compared to 2% at the distal raceme node. Flowers and pods at distal raceme node positions received small percentages of exported 14 C and tended to abscise, while those at proximal raceme nodes sequestered a much higher fraction of 14 C and developed mature pods. Sink intensity (the percentage of 14 C/dry weight) of flowers at a raceme node was lowest near anthesis, peaked 3 to 4 days after anthesis, and then declined. Maximum sink intensity of basal flowers in an axillary inflorescence lower in the canopy was twice as high as in comparable flowers of the terminal raceme. The reduced accumulation of photoassimilate and declining sink intensity in organs at distal raceme nodes may contribute to their increased rate of abscission.