The bigger picture - phloem seen through horticultural eyes

Horticulturalists manipulate phloem transport to achieve horticulturally-desirable results. The resulting crop behaviour can help show how phloem works. In apple, the use of dwarfing rootstocks and pruning management has greatly increased fruit sink capacity at the expense of root and shoot sinks. Annual fruit yield can exceed100 t ha –1 and account for 65% of total photosynthate. If photosynthetic capacity exceeds fruit sink capacity, there can be either a feedback reduction in photosynthesis, or accumulation of phloem sugars in the fruit apoplast. In sugarcane, the sink is a linear body encasing the full length of the phloem strands from leaf to root. Lateral unloading must occur along the full length of the phloem. Unloading or loading may occur in the same segment of phloem depending on conditions. In the ornamental lily and daylily, the developing flower bud is a powerful sink, doubling in dry weight each 48 h in the week of development from small bud to opening. Once the flower is open, the phloem changes rapidly from unloading behaviour as the tepals become vigorous exporters. The change in behaviour may be coupled with processes mediating senescence and cell death. Implications to phloem function are discussed.