The Physiology of Flowering Plants - KHAM PHA MOI

FRUIT DEVELOPMENT 309AUXINSGIBBERELLINSCYTOKININSFertilization1.5 cm 2cm Mature1.0 cmgreen0.8 cmCelldivisionCell expansionPhytosterolsBreaker Turning OrangeRipeningCarotenoidsRedfirmRedripeFig: 11:21 Tomato (Lycopersiconesculentum) fruit development. Asthe fruits develop they undergoperiods of cell division and cellexpansion, followed by ripening.Fruits attain their full size at the‘mature green’ stage but are unripe.The onset of ripening coincides withthe first sign of colour development(‘breaker’ stage). The auxin,cytokinin and gibberellic acidcontent of tomato fruit is greatestduring the early stages ofdevelopment as the fruit increasesin size. Abscisic acid increases laterduring fruit growth whilst ethyleneis associated with ripening.Redrawn from Gillaspy et al.(1993).ABSCISIC ACIDETHYLENEor pericarp (composed of the endocarp, mesocarp and exocarp) coveredby a thin cuticle. The septa of the carpels divide the fruit intolocules and the seeds are attached to an axial placenta. The majorityof the cells which make up the carpel are large, vacuolated andcontain chloroplasts. These cells are photosynthetically active andexpress many photosynthetic genes. Unlike a leaf, the fruit is also astrong sink for carbohydrate, provided as sucrose by the rest of theplant. A concentration gradient is maintained into the fruit as a resultof sucrose degradation, principally by the enzyme sucrose synthase(which, in spite of its name, generally acts to break down sucroseunder physiological conditions, rather than to catalyse its synthesis).The final stage of fruit development is ripening. In species such asgrape and cherry this is a gradual process following earlier stages offruit development. However, many other species, including tomato,apple and avocado, exhibit an abrupt increase in respiration at theonset of ripening – the climacteric. Many physiological changesoccur in the fruit as it ripens. In tomato the chlorophyll content ofthe fruit declines whilst the carotenoid content increases, giving thefruit a red colouration. The chlorophyll-containing chloroplasts areconverted into carotenoid-containing chromoplasts and photosyntheticcapacity is lost. Sucrose is converted into glucose and fructoseby the enzyme invertase, leading to an increase in sweetness andpalatability. The fruit softens as enzymes are synthesized whichdegrade the pectin of the middle lamella, loosening the connectionsbetween cells. Again, many cell wall remodelling enzymes areexpressed which alter wall plasticity.