Changes in Chloroplast Fine Structure During the Autumnal Senescence of Betula Leaves

During the ageing and senescence of leaves of the birch tree,Betula verrucosa Ehrb., from the fully expanded green stateto the yellow terminal condition, the shape of the chloroplastsalters and their volume decreases to less than one-fifth. Associatedwith these changes is an initial increase in the number of chloroplastlamellae and size of the grana but this is followed by the breakdownof the membrane systems. The chloroplast lipid globules increaseconsiderably in size but show little increase in number. Thestructure of the chloroplast membranes is described in detail.Small particles approximately 8 x 5 nm, become visible on theinner surfaces of the thylakoid membranes during chloroplastbreakdown. The chloroplasts are the first organelles to showsigns of senescence and are the last to remain after the otherorganelles have all broken down. The observations are discussedin relation to the senescence of chloroplasts of herbaceousplants and the development of chromoplasts of fruits.