A model for the inhibition of bud outgrowth by a growing stem. A The growth of stem intemodes in auxin or strigolactone deficient mutant plants or plants grown at high light intensity or low planting density is suppressed. A short intemode is not strong sink for sucrose. Therefore, excess sucrose exported from photosynthetic leaves to the stem overflow into axillary buds and induces bud outgrowth.
B Intrinsic factors such as auxin and strigolactones and environmental factors such as shade promote stem intemode elongation in the main shoot. Elongated intemode, which is a strong sink, inhibits bud outgrowth indirectly by limiting sugar supply to axillary bud.
The plant source—sink relations is regulated by intrinsic and environmental factors making shoot branching a complex trait that cannot be predicted easily without considering the growth and developmental status of the whole plant and prevailing environmental conditions. Reappraisal of the source—sink status in shoot branching mutants and wild-types and systematic study of the effect of source—sink status of the main shoot on dormancy and outgrowth of axillary buds might advance our knowledge of the physiological basis of apical dominance and shoot branching in plants.
Future studies should accurately determine the sink or source status of an organ being manipulated. For example, the cotyledons in pea contribute to seed germination.
The nutrient reserve and biomass of the cotyledons are exhausted within the first 10 days after sowing, during which the plant transitions from heterotrophic to autotrophic growth Hanley et al. Experiments involving cotyledon removal or defoliation of young newly formed or old non-photosynthetic leaves assuming that they are source of nutrients or photoassimilates might lead to incorrect conclusions.
Besides their role in shoot branching, sugars are also important in many other aspects of plant growth and development including phase transitions from juvenile to adult and from vegetative to flowering Wahl et al. Therefore, when investigating plant growth and development, sugar demand and supply should be taken into consideration. The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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