Nocturnal stachyose metabolism in leaf tissues of Xerosicyos danguyi H. Humb.

Leaf discs obtained from mature leaves of Xerosicyos danguyi were found to contain appreciable levels of stachyose throughout an 8-h nocturnal period during which this plant performs Crassulacean acid metabolism (CAM). In contrast, in mesophyll tissues obtained from paradermal sections of these same leaf discs and which were devoid of vascular tissues, stachyose pools were rapidly depleted during the nocturnal phase. The pattern of this depletion followed closely the depletion pattern observed for starch, indicating that mesophyll stachyose was possibly involved in nocturnal CAM processes and was not necessarily being used for export. Pulse-labelling of intact X. danguyi leaves prior to excision of leaf discs and mesophyll samples also indicated that, while labelled stachyose had turned-over completely in the mesophyll tissues by the end of the nocturnal period, substantial levels of labelled stachyose were always recovered from the leaf discs from which these mesophyll samples were derived. The data indicate the existence of two separate pools of stachyose in the X. danguyi leaf, one a mesophyll pool which turns over rapidly at night and which may be involved to a small extent in nocturnal CAM processes, and the other a pool associated with and possibly synthesized by the vascular tissues and which presumably represents the phloem-transport pool.