Nonami H, Boyer J S
Department of Soil and Crop Sciences, Texas A&M University, College Station, Texas 77843.
Plant Physiol. 1987 Mar;83(3):596-601. doi: 10.1104/pp.83.3.596.
We developed a new method to measure the solute concentration in the apoplast of stem tissue involving pressurizing the roots of intact seedlings (Glycine max [L.] Merr. or Pisum sativum L.), collecting a small amount of exudate from the surface of the stem under saturating humidities, and determining the osmotic potential of the solution with a micro-osmometer capable of measuring small volumes (0.5 microliter). In the elongating region, the apoplast concentrations were very low (equivalent to osmotic potentials of -0.03 to -0.04 megapascal) and negligible compared to the water potential of the apoplast (-0.15 to -0.30 megapascal) measured directly by isopiestic psychrometry in intact plants. Most of the apoplast water potential consisted of a negative pressure that could be measured with a pressure chamber (-0.15 to -0.28 megapascal). Tests showed that earlier methods involving infiltration of intercellular spaces or pressurizing cut segments caused solute to be released to the apoplast and resulted in spuriously high concentrations. These results indicate that, although a small amount of solute is present in the apoplast, the major component is a tension that is part of a growth-induced gradient in water potential in the enlarging tissue. The gradient originates from the extension of the cell walls, which prevents turgor from reaching its maximum and creates a growth-induced water potential that causes water to move from the xylem at a rate that satisfies the rate of enlargement. The magnitude of the gradient implies that growing tissue contains a large resistance to water movement.
我们开发了一种新方法来测量茎组织质外体中的溶质浓度，该方法包括对完整幼苗（大豆[Glycine max (L.) Merr.]或豌豆[Pisum sativum L.]）的根部施加压力，在饱和湿度下从茎表面收集少量渗出液，并用能够测量小体积（0.5微升）的微渗透压计测定溶液的渗透势。在伸长区，质外体浓度非常低（相当于渗透势为-0.03至-0.04兆帕），与通过等压湿度计直接在完整植物中测量的质外体水势（-0.15至-0.30兆帕）相比可忽略不计。大部分质外体水势由负压组成，可用压力室测量（-0.15至-0.28兆帕）。测试表明，早期涉及细胞间隙渗透或对切段施加压力的方法会导致溶质释放到质外体中，从而产生虚假的高浓度。这些结果表明，尽管质外体中存在少量溶质，但主要成分是一种张力，它是正在生长的组织中由生长诱导的水势梯度的一部分。该梯度源于细胞壁的伸展，这阻止了膨压达到最大值，并产生了由生长诱导的水势，导致水以满足伸长速率的速度从木质部移动。梯度的大小意味着生长中的组织对水分移动具有很大的阻力。
