Ultrastructural studies on the stability of chloroplasts in attached leaves of spinach (Spinacia oleracea L.) subjected to high temperature conditions
The literature relating to ultrastructural studies of chloro-plast development in the leaves of angiosperm plants up to chloroplast senescence is reviewed, and attention is also given to the effects of high temperature on chloroplast ultrastructure and components. Ultrastructural changes are observed in the developing plastids of the primary leaves of spinach (Spinacia oleracea L.) under 14 h light/ 10 h dark conditions at normal growing temperature (22°C) and at high temperature (30°C) and also in complete darkness at 22°C.Under these three conditions plastid development proceeds from amylo-plasts with the continuous formation of small segments of lamella which at later stages unite and develop grana. Under normal temperature conditions only an association is observed between the plastid and endoplasmic reticulum at very early stages of plastid development and amoeboid plastids are seen at an intermediate stage of plastid development. At high temperature the chloroplasts become swollen and some of them burst, liberating their contents free in the cell cytoplasm. In the dark the plastids develop into etioplasts having prolamellar bodies with typical paracrystalline structure. Swelling of the plastid lamellae is observed at both high temperature and in complete darkness. Plastid division is not seen under high temperature conditions. Ultrastructural changes of the leaf chloroplasts during their growth and senescence is studied in detail under normal growing temperature of the plant (22°C) and also under higher temperature conditions (25°C, 30° C, and 35°C) starting with 4-weeks old plants. At normal growing temperature during the growth of the young chloro-plasts to maturity, both the size and number of grana show a continuous increase along with the corresponding increase in the chlorophyll content and leaf fresh weight. When the leaf begins to show senescence the chloroplasts show a change in shape and a reduction in size. The terminal ends of the thylakoids first show club-shaped swellingsand this is followed by the breaking of the lamellae at various places resulting in the development of very variable irregularly-shaped stroma lamellae with characteristic ring or hook-shaped or looped lamellae. This extensive development of the abnormal lamellar system with such peculiar arrangements is hitherto unknown in the senescing chloroplasts of higher plants. Osmiophilic globules also show a great increase both in number and size attaining largest size in chloroplasts of the completely senesced leaf tissue. Under high temperature conditions all these changes are enhanced and brought about earlier as the temperature is increased. The effects of high temperature on chloroplast ultrastructure are further exploited by keeping the plants at higher temperatures for successively longer time intervals. The ultrastruetural changes are therefore investigated in newly expanded leaves on plants kept at the 4 temperature conditions. Two conclusions have been drawn from this investigation: (i) the higher the temperature to which the plant is subjected, the quicker the structural changes and abnormalities which the newly differentiated chloroplast shows and (ii) the lower the temperature to which the plant is subjected, the longer the time it requires for showing plastid structural changes and abnormalities. The structural changes shown by the newly formed chloroplasts includes the change in shape, reduction in size of the chloroplast, reduction in size of the interconnecting stroma lamellae, disappearance of starch grains and appearance of a few vesicles, increase in number and size of osmiophilic globules, club-shaped swellings at the terminal ends of the thylakoids and breaking of lamellae. All these structural changes of the chloroplasts are followed by the swelling of the chloroplasts which later burst at particular points thus liberating the contents free in the cell cytoplasm. The transformation of the chloroplasts into etioplasts in complete darkness has also been investigated in the attached leaves under normal growing temperature of the plant and also under high temperatureconditions (30°C). Prolamellar body formation with typical para-crystalline structure is observed under normal temperature conditions after 128 hours of darkness. At 30°C the prolamellar body formation was not observed but instead several large vesicles are produced.