changes in cell wall during fruit ripening, Lecture notes of Agronomy

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CHANGES IN

CELLWALL

DURING FRUIT

RIPENING

Anu Ann Augustine 2017-12-

FRUIT RIPENING

 final phase of fruit development  (^) Includes the softening of fleshy fruits  (^) Qualitative transformation of a mature fruit  The fruit is said to be ripe when it attains its full flavour and aroma (watada et al ., 1984).  (^) Ripening is the sequence of changes in colour, flavor, texture which leads to a state at which fruits are acceptable for consumption

HORTICULTURAL AND COMMERCIAL VIEWPOINT

 POSITIVE  (^) Imparts flavour, colour  Increase nutritional quality  fibre content and composition  (^) Vitamins and antioxidants  (^) Texture – consumer acceptance  NEGATIVE  (^) Susceptibilty to pests and diseases  Reduced storage life  Transport and storage losses  (^) Physical damage during handling

MAJOR CHANGES IN RIPENING

5  Cell wall changes  (^) Starch - starch to sugar conversion  Organic acids - decrease  (^) Pigments – Chl destruction, synthesis of carotenoids, anthocyanin  Flavouring compounds  (^) Ascorbic acid - decrease  Phenolics – decrease  (^) Amino acids decrease (exception banana) and proteins increase  (^) Respiration  Ethylene

FRUIT CELL WALL



Primary cell wall:



relatively thin and weak



composed of rigid cellulose microfibrils



held together by networks of matrix glycans

(hemicelluloses) and pectins and small amounts of

structural proteins and phenolics

MIDDLE LAMELLA

 Middle lamella – pectin rich – HGA mostly  (^) Glueing of neighbouring cells  (^) Intercellular adhesion

1. HGA becomes more negative charged – by removal of methyl ester groups by PME (Pectin Methyl Esterase) – leaves carboxyl groups Ca ions forms bond to form calcium- pectate gel Provides wall strength and intercellular bonding in ripe fruit

CHANGES IN CELLWALL

 During Fruit ripening,  (^) cell wall polysaccharides are modified by enzymes secreted from symplast  changes- species specific : cultivar -specific

 Dissolution of primary cell wall  (^) Disruption of middle lamella  solubilisation, depolymerization of polysaccharides of cellwall  rearrangement of the associations  (^) as a result of combined action of by cell modifying enzymes  (^) both on pectic and hemicellulosic fragments

HYDROLYTIC EZYMES

 Polygalacturonase (PG)  (^) Pectin Methyl Esterase (PME)  (^) Pectate Lyase (PL)  Rhamnogalacturonase (RG)  Cellulase  β- galactosidase (Tuker, 1993) No single cell wall enzyme alone account for the textural changes

LOOSENING OF CELLWALL MEDIATED BY

EXPANSINS

 Expansin :  Proteins with no hydrolytic activity  (^) Enhance accessibility of wall polymers to enzyme action  (^) Accelerate wall hydrolysis  Disrupt H- bonds between cellulose microfibrils and xyloglucans

DEPOLYMERISATION OF MATRIX GLYCAN

(HEMICELLULOSES)

 Common feature of fruit ripening in most of the fruits  (^) Exception : apples  According to softening behavior, 2 types

1. Soften greatly : Tomato
2. Soften moderately : Pear, Cranberry

PECTIN MODIFICATIONS

 SOLUBILIZATION – Universal  (^) DEPOLYMERIZATION – occurs additionally in some fruits