TRANSLOATION AND DISTRIBUTION OF PHOTOASSIMILATES

PHOTOASSIMILATES AND OTHER SMALL ORGANIC SUBSTANCES ARE TRANSLOCATED OVER LONG DISTANCES IN THE PHLOEM

GIRDLING EXPERIMENTS

   1.   M. MALPIGHI REMOVED A RING

      OF BARK CONTAINING PHLOEM

      FROM THE WOOD (THE XYLEM)        OF YOUNG STEMS (GIRDLING)

   2.   WATER AND NUTRIENTS             CONTINUE TO BE TRANSLOCATED

      FROM THE ROOTS TO STEMS

   3.   OVER TIME THE BARK ABOVE          THE GIRDLED AREA SWELLS

      THE SWELLING IS ATTRIBUTED

      TO THE ACCUMULATION OF

      PHOTOASSIMILATE FLOWING

      DOWNWARD.

      THE PHOTOASSIMILATE ALSO

      CONTAINS HORMONES AND            NITROGEN CONTAINING                COMPOUNDS THAT STIMULATE       CELL DIVISION AND GROWTH

PROBLEMS WITH ANALYSIS OF PHLOEM

CONTENTS

   1.   PHLOEM CELLS ARE LIVING           CELLS SO THEIR OWN CONTENTS      WOULD BE PRESENT ALONG WITH    THE PHOTOASSIMILATE

   2.   SOME PLANTS PHLOEM CONTENTS

      GEL ON EXPOSURE TO AIR

   3.   THE PHLOEM CONTENTS MAY BE

      CONTAMINATED BY OTHER NON-

      PHLOEM CELLS CONTENTS

THE PROBLEM OF CONTENT CONTAMINATION WAS SOLVED BY ENTOMOLOGISTS.

   1.   APHIDS FEED ON SOME PLANTS

      BY INSERTING THEIR STYLUS

      DIRECTLY INTO A SIEVE TUBE

      ELEMENT

   2.   THE APHID IS ALLOWED TO

      FEED FOR A WHILE AND THEN

      IT IS ANESTHETIZED WITH

      CO₂

   3.   ONCE ANESTHETIZED THE            STYLUS IS CUT OFF, LEAVING

      IT STUCK IN THE SIEVE TUBE

      ELEMENT AND OOZING PHLOEM

      CELL EXUDATE

   4.   THE EXUDATE IS COLLECTED

      AND ANALYZED

   5.   THIS ALSO DEMONSTRATES THAT

      THE PHLOEM CONTENTS ARE

      UNDER PRESSURE

THE PHLOEM CONTENTS CAN ALSO BE

DETERMINED BY EXPOSING THE PHOTOSYNTHESIZING LEAVES TO ¹⁴CO₂

   1.   BEET LEAVES WERE EXPOSED TO

      RADIOACTIVE CARBON DIOXIDE

      FOR 10 MINUTES

   2.   THE LEAVES ARE EXCISED AND

      THE EXUDATE FROZEN

   3.   CROSS SECTIONS OF THE            PETIOLE ARE MADE AND PLACED

      ON XRAY FILM

   4.   IMAGES ON THE XRAY FILM          SHOW THE RADIOACTIVITY TO          BE IN THE PHLOEM

COMPOSITION OF PHLOEM EXUDATE

ORGANICS: (IN A CASTOR BEAN)

   SUCROSE       80-106 mg/l

   PROTEIN       1.45-2.20 mg/l

   AMINO ACIDS    5.2 mg/l

   MALIC ACID     2.0-3.2 mg/l    

INORGANICS:

   ANIONS        20-30 meq/l

   CATIONS       100-125 meq/l

THE PROTEIN FOUND IS P-PROTEIN

   1.   P-PROTEIN IS SHORT FOR            PHLOEM PROTEIN

   2.   CAN BE SEEN USING A LIGHT

      MICROSCOPE

   3.   ORIGINALLY THOUGHT TO BE          CARBOHYDRATE AND CALLED           “SLIME”

   4.   MAY BUILD UP AND FORM A

      “SLIME PLUG” ON THE SIEVE

      PLATE

THE PREDOMINENT SUGAR IS SUCROSE

OTHER SUGARS ARE:

   RAFFINOSE

   STACHYOSE

   VERBASCOSE

   MANNITOL

   SORBITOL

WHY IS SUCROSE THE TRANSLOCATOR OF CHOICE?

   SUCROSE IS NONREDUCING SO WILL

   NOT REACT WITH MANY THINGS

   THE BOND BETWEEN GLUCOSE AND

   FRUCTOSE IS RELATIVELY STABLE

PHLOEM LOADING AND UNLOADING

PHLOEM LOADING = MOVEMENT OF

PHOTOASSIMILATE FROM THE MESOPHYLL CELLS (SOUORCE) INTO THE PHLOEM

PHLOEM UNLOADING = MOVEMENT FROM THE SIEVE ELEMENT INTO THE SINK

PHLOEM LOADING:

1.   PHOTOASSIMILATE MOVED FROM

   THE MESOPHYLL CELL INTO A

   MINOR VEIN CALLED THE SIEVE

   ELEMENT-COMPANION CELL

   COMPLEX (SE-CC)

2.   MOVEMENT FROM MESOPHYLL TO THE

   SE-CC MAY BE:

   a.   APOPLASTIC

   b.   SYMPLASTIC  

   THIS MOVEMENT IS AGAINST A    CONCENTRATION GRADIENT SO IT    MUST OCCUR BY ACTIVE TRANSPORT

PHLOEM UNLOADING:

   1.   MAY OCCUR SYMPLASTICALLY

      OR APOPLASTICALLY

      a.   APOPLASTICALLY

         1.   SUCROSE BROKEN DOWN

            INTO GLUCOSE AND

            FRUCTOSE BY ACID

            INVERTASE

         2.   GLUCOSE AND FRUCTOSE

            TAKEN UP BY THE CELL

         3.   GLUCOSE AND FRUCTOSE

            REASSEMBLED INTO

            SUCROSE AND                        TRANSPORTED INTO THE

            VACUOLE