The role of the storage carbon of cotyledons in the establishment of seedlings of Hymenaea courbaril under different light conditions.
OBJECTIVE
Hymenaea courbaril (Leguminosae-Caesalpinioideae) is a tree species with wide distribution through all of the Neotropics. It has large seeds (approx. 5 g) with non-photosynthetic storage cotyledons rich (40 %) in a cell wall polysaccharide (xyloglucan) as a carbon reserve. Because it is found in the understorey of tropical forests, it has been considered as a shade-tolerant, late-secondary species. However, the physiological mechanisms involved in seedling establishment, especially regarding the interplay between storage and light intensity, are not understood. In this work, the ecophysiological role of this carbon cotyledon reserve (xyloglucan) is characterized, emphasizing its effects on seedling growth and development during the transition from heterotrophy to autotrophy under different light conditions.


METHODS
Seedlings of H. courbaril were grown in environments with different light intensities, and with or without cotyledons detached before xyloglucan mobilization. Development, growth, photosynthesis and carbon partitioning (dry mass and [14C]sucrose) were analysed in each treatment.


RESULTS
The detachment of cotyledons was not important for seedling survival, but resulted in a strong restriction (50 % less) of shoot growth, which was the main sink for the cotyledon carbon reserves. Carbon restriction promoted an early maturation of the photosynthetic apparatus without changes in the net CO2 fixation per unit area. The reduced surface area of the first leaves in seedlings without cotyledons was evidence of limited growth and development of seedlings in low light conditions (22 micromol m(-2) s(-1) photon flux).


CONCLUSIONS
There is an increase in the importance of storage xyloglucan in cotyledons for H. courbaril seedling development as light intensity decreases, confirming that this polymer plays a key role in the adaptation of this species to establish successfully in the shadowed understorey of the forest.
Santos
Henrique P
HP
EMBRAPA Uva e Vinho. Rua Livramento, 515, Caixa Postal 130, CEP 95700-000, Bento Gonçalves, RS, Brazil.
Buckeridge
Marcos S
MS
D002244:Carbon / Q000378:metabolism*; D018548:Cotyledon / Q000378:metabolism; D004780:Environment, Controlled; D007887:Fabaceae / Q000254:growth & development* / Q000378:metabolism*; D018525:Germination / Q000502:physiology; D005936:Glucans / Q000378:metabolism; D008027:Light; D010788:Photosynthesis; D018520:Plant Shoots / Q000254:growth & development; D036226:Seedlings / Q000254:growth & development* / Q000378:metabolism*; D014990:Xylans / Q000378:metabolism
D005936:Glucans; D014990:Xylans; C029353:xyloglucan; D002244:Carbon