Lehrstuhl Pflanzenphysiologie, Universität Bayreuth, Postfach 101251, W-8580, Bayreuth, Germany.
Planta. 1992 Jul;187(4):460-74. doi: 10.1007/BF00199964.
Ricinus communis cv. Carmencita seedlings with their cotyledons incubated in sucrose solution and their hypocotyls cut to induce exudation of phloem sap, constitute a system of sucrose fluxes into and out of the cotyledons. This system was characterized with respect to quasi-steady-state conditions of sucrose uptake and export and then used to investigate the pathways of sucrose during phloem loading. The redistribution of (14)C-labelled internal sucrose between the three "compartments", cotyledons (mesophyll), exudate (sieve tubes) and incubation medium (cell-wall space), was measured in the presence or absence of external nonlabelled sucrose. It was found that mesophyll-derived labelled and external sucrose compete at uptake sites in the apoplasm. On the basis of the specific radioactivity of sucrose which reflects the proportionate intermixture of mesophyll-derived and external sucrose in the three "compartments", it was determined that about 50% of the sucrose exported is loaded directly from the apoplasm, while the other half takes the route via the mesophyll. It was confirmed that mesophyll-derived sucrose is released into the apoplasm, so that the existence of an indirect apoplasmic loading pathway is established. Calculations depending on the concentration gradients of labelled and non-labelled sucrose in the cell-wall space are presented to quantify tentatively the proportions of direct and indirect apoplasmic as well as symplasmic loading.
蓖麻 cv. Carmencita 实生苗的子叶在蔗糖溶液中孵育，下胚轴被切割以诱导韧皮部汁液的渗出，构成了蔗糖进入和离开子叶的通量系统。该系统在蔗糖摄取和输出的准稳态条件下进行了特征描述，然后用于研究韧皮部装载过程中蔗糖的途径。在存在或不存在外部非标记蔗糖的情况下，测量了（14）C 标记的内部蔗糖在三个“隔室”（子叶（叶肉）、渗出物（筛管）和孵育介质（细胞壁空间））之间的重新分布。发现叶肉衍生的标记和外部蔗糖在质外体的摄取部位竞争。基于反映叶肉衍生和外部蔗糖在三个“隔室”中混合比例的蔗糖比放射性，可以确定大约 50%的输出蔗糖是直接从质外体装载的，而另一半则通过叶肉进行。证实了叶肉衍生的蔗糖被释放到质外体中，从而建立了间接质外体装载途径的存在。根据细胞壁空间中标记和非标记蔗糖的浓度梯度进行的计算，初步量化了直接和间接质外体以及共质体装载的比例。
