Mäkelä Pirjo, McLaughlin John E, Boyer John S
College of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes, DE 19958, USA.
Ann Bot. 2005 Oct;96(5):939-49. doi: 10.1093/aob/mci246. Epub 2005 Aug 12.
Shade or inadequate water can inhibit photosynthesis and limit the development of maize (Zea mays) ovaries around the time of pollination, potentially reducing the number of kernels at harvest. This study investigated whether the decreased photosynthesis diminished only the sugar supply or also altered the transport path to the ovaries.
Photosynthesis and water potentials (Psiw) were measured in the leaves while dry matter delivery was monitored in the ovaries. Ovary glucose, starch and acid invertase activities were measured in situ. Stems were fed xylem-mobile safranin or phloem-mobile carboxyfluorescein (CF), and the dye transport to the ovaries was determined.
Under normal conditions, the ovaries gained in dry mass, and starch accumulated in the pedicel and ovary wall. Glucose accumulated in the pedicel, apparently in the apoplast where insoluble (cell-wall-bound) acid invertase acted on the arriving sucrose. A glucose gradient developed from pedicel to nucellus. Safranin moved in the xylem and did not reach the ovary, but CF moved in the phloem and arrived at the ovary. CF also spread into the pedicel but unlike glucose it did not enter the nucellus. Low Psiw or shade decreased leaf photosynthesis, ovary dry mass accumulation, invertase activities, pedicel glucose, starch accumulation and CF delivery. Removal of these treatments reversed the effects.
The success of CF in tracing the general path and rate of carbohydrate transport gave visual evidence that phloem transport to the ovary decreased at low Psiw or in the shade but otherwise remained functional. The decreases indicated that losses in carbohydrate delivery are central features of failed ovary development under these conditions. The selectivity of transport into the nucellus resembled the situation later when embryo and endosperm are present and selective uptake occurs from the apoplast.
遮荫或水分不足会抑制光合作用，并在授粉期左右限制玉米（Zea mays）雌穗的发育，可能减少收获时的籽粒数量。本研究调查了光合作用下降是否仅减少了糖分供应，还是也改变了向雌穗的运输途径。
在叶片中测量光合作用和水势（Ψw），同时监测雌穗中的干物质输送。原位测量雌穗中的葡萄糖、淀粉和酸性转化酶活性。给茎部注入木质部可移动的番红或韧皮部可移动的羧基荧光素（CF），并测定染料向雌穗的运输情况。
在正常条件下，雌穗干重增加，淀粉在花梗和子房壁中积累。葡萄糖在花梗中积累，显然是在质外体中，不溶性（细胞壁结合）酸性转化酶作用于到达的蔗糖。从花梗到珠心形成了葡萄糖梯度。番红在木质部中移动，未到达雌穗，但CF在韧皮部中移动并到达雌穗。CF也扩散到花梗中，但与葡萄糖不同，它没有进入珠心。低Ψw或遮荫会降低叶片光合作用、雌穗干物质积累、转化酶活性、花梗葡萄糖、淀粉积累和CF输送。去除这些处理后，效果逆转。
CF成功追踪碳水化合物运输的总体途径和速率，直观地证明了在低Ψw或遮荫条件下，向雌穗的韧皮部运输减少，但在其他情况下仍保持功能。这些减少表明，在这些条件下，碳水化合物输送的损失是雌穗发育失败的核心特征。向珠心运输的选择性类似于后来存在胚和胚乳且从质外体进行选择性吸收时的情况。
