Functioning of Conducting Elements of Phloem and Xylem in the Stalk of the Developing Fruit of Lupinus albus L

Abstract

The economy of water and dry matter in fruits of white lupin (L. albus L.) was studied by measuring transpiration, CO² exchange and weight changes over an 8-week growth period. These data were combined with analyses of upper stem tracheal (xylem) sap and fruit tip phloem sap to estimate rates of import of dry matter and water through xylem and phloem. Assuming unidirectional mass flow to operate, phloem carried 97 % of the dry matter and 27 % of the water entering the fruit, and xylem supplied the remainder of the water, 30 % of the minerals and 10 % of the amino compounds.

The structure of fruit stalks of various age was examined by light microscopy. Cross-sectional areas occupied by different tissues were measured, and numbers and mean transverse areas of conducting elements estimated. Sieve tubes comprised 0.66-1.13% of the stalk's cross-sectional area, vessels plus tracheids 0.27 - 0.50%. Secondary xylem consisted mainly of fibres. Sieve tubes occupied 18 - 27% of the phloem cross-sectional area. The fully grown stalk showed approximately three times as many sieve tubes in transverse section as vessels plus tracheids.

Transfer of dry matter and water through the fruit stalk was highest during the exponential phase of fruit growth. Peak specific mass transfer in sieve tubes was 3.95 g dry matter per square centimetre of sieve tube per hour, in tracheids plus vessels 0.28 g cm² h^-1. Maximum flow velocity in sieve tubes was 22 cm h^-1, in tracheids plus vessels, 147 cm h^-1. Values for specific mass transfer and flow velocities were several times less in stalks of very young or near-mature fruits.