Rules Of Source-Sink Exchange Of The Bulb Of Oriental Lily 'Sorbonne'

The bulb of the genus Lilium is composed of scales, roots, shoots, and a basal plate (a compressed stem), among which the former three organs are connected through the basal plate. Scales go through the accumulation, decomposition and re-accumulation of nutrients processes during the growth and development stages, which provides an excellent model to study the source-sink relationship. Oriental hybrid lily'Sorbonne'was used as the subject material to study the series of physiology changes such as the contents of starch, sucrose, soluble sugars, the activities of seven enzymes catalyzing metabolism of sucrose and starch, and the changes of four kinds of endogenous hormones. Besides, the microscopic structure and ultrastructure of scales and basal plate were investigated by optical microscope and transmission electron microscope (TEM). Furthermore, the immunogold electron-microscopy technique was used to determine the subcellular localization ofβ-amylase, one of the key enzymes catalyzing starch breakdown, in scales and basal plate during the growth and development stages. To understand the function of lily bulb especially the relationship of source-sink of lily bulb, the 6(5)-carboxyfluorescein diacetate (CFDA) was applied to study the transport direction of assimilates among the bulb and the aboveground parts. The main results and conclusions are as follows:1 The function of lily bulb is not only serving as source or sink simply to supply or accumulate assimilates during the growth and development. Due to the discrepancy of outer and middle scales serving as source or sink at the same time, the bulb has a state of complex of source and sink between it serving as source and sink merely.2 The lily bulb simply serves as the source to supply nutrients for the growth and development of the shoots after planting to the flower buds developing to 1cm in length. The function transformation of bulb from source to complex of source and sink occurs from the flower buds developing from lcm to 3cm in length. Before anthesis, the bulb has already finished the transformation from a complex of source and sink to sink of the whole plant. After that the bulb only accumulates assimilates for the next growth cycle.3 Starch can be defined as one of the main factors to judge the function of lily bulb as a source or sink during the growth and development stages. Furthermore, the enzymes synthesizing starch, i.e. ADP-glucose pyrophosphorylase (AGPase), soluble starch synthase (SSS) and granule-bound starch synthase (GBSS), especially the latter tow enzymes, also have a close relationship to the sink strength of bulb to accumulate assimilates from the leaves and stem.4 The contents of carbohydrates of basal plate is quite lower than those of scales, but the contents of endogenous hormones in basal plate are higher (ABA and IAA) or close to (GA3 and ZR) those in scales, which means that the basal plate plays an important role in the metabolism among scales and the aboveground parts. As a compressed stem, the basal plate have many vascular bundles distributed among parenchyma cells with a quite complex pattern, which can be seen from the presence of transverse and longitudinal sections of vascular bundles in the same plane. The 6(5)-carboxyfluorescein (CF) observation results show that the transportation direction of assimilate in the phloem of basal plate is bidirectional during the bulb serves as the complex of source and sink.5 The result of immunogold subcellular localization in this study with the polyconalβ-amylase antiserum indicates thatβ-amylase in the scale of lily bulb is mainly distributed in starch granules. The subcellular localization also displays the seasonal changes in quantities ofβ-amylase that essentially increased progressively after planting then reached the maximum at anthesis and dropped to a low density at senescence. The result is in line with the activity changes ofβ-amylase determined by 3, 5-dinitrosalicylic acid method. It also shows thatβ-amylase in the basal plate is mainly localized in starch granules. Besides, there are fewβ-amylase in the sieve plate and P-type plastids as well, indicating that the main function of P-amylase could be preventing the accumulation of starch grains rather than decomposition of starch.6 Assimilates in the scales of lily bulb is unloaded to the phloem parenchyma cells through symplastic pathway, and then transported to the storage parenchyma cells through plasmodesmata between the parenchyma cells, which can be shown by the fact that farther from the phloem higher the density of starch distribution, which forms a concentration gradient of sugar unloading. The exchange of substances and information between the parenchyma cells can not only through plasmodesmata which are symplastic pathways, but also can through cell wall and intercellular spaces which are apoplastic pathways.It is the first study focusing on the source-sink exchange of lily bulb during the growth and development stages. The key conclusion is that between the lily bulb serves as source or sink merely, there is a state of complex as source and sink, which can be used as the key regulation phase for the high quality of lily bulb production.