Apoplastic water flow to cotton leaves and fruits during development

The apoplastic tracer Sulforhodamine G was used to study the maturation of functional, water-conducting xylem vessels into developing cotton fruits and to quantify apoplastic water flow to cotton leaves, bracts and fruits. Segments of cotton branches with one sympodial leaf and subtended fruit were placed in a vial containing a 0.05% (w/v) solution of Sulforhodamine G in distilled water and allowed to transpire freely for 2.5 h, after which the dye was extracted from the different tissues and quantified spectrophotometrically. The value obtained was used as a quantitative indicator of xylem transport to the different organs. When the developing fruit was younger than 23 d post-anthesis, about 85% of the extracted dye was recovered from the sympodial leaf and approximately 10% from the bracts, while the seeds, capsule wall and central column only contained trace amounts. In older segments (>23d post-anthesis), substantial quantities of dye were recovered from the capsule wall (2–10%) and central column (1–2%), but not from the seeds. These results indicate that no significant apoplastic water movement into a developing cotton fruit occurs during the first 3 weeks after anthesis. Therefore, xylem is not the main source of water for young fruits. Cotton fruit diameter increases rapidly during the first 3 weeks of development and we hypothesize that most xylem vessels do not mature until this period of rapid cell enlargement ceases. A theoretical calculation is presented to support the hypothesis that phloem is the major source for the water supply of developing cotton fruits.