ULTRASTRUCTURE OF CHLOROPLASTS IN LEAVES AND OF PLASTIDS IN ROOT-TIPSOF 2 MAIZE LINES DIFFERING IN CHILLING TOLERANCE

Plants of the chilling sensitive Penjalinan and chilling tolerant Z7 m aize lines were subjected to low temperature 6 degrees C while the con trol plants grew under 24 degrees C for 7 days. Effects of low tempera ture on the structure of both chloroplasts in mesophyll and bundle she ath of the third leaf, and plastids in epidermal and cortical cells at 1 to 2 mm behind the root tip were investigated. In the chilling stre ssed sensitive Penjalinan plants increased size of the mesophyll chlor oplasts with disorganized thylakoids, an increase in both number of pl astoglobuli and area of the membrane free stroma were observed. The on ly structural change induced by 6 degrees C in mesophyll chloroplasts of the tolerant Z7 plants was the appearance of 1 to 2 starch grains i n their stroma. Bundle sheath chloroplasts of the sensitive Penjalinan plants decreased in size, their starch grains disappeared completely, the arrangement of thylakoids was disturbed, and the number of plasto globuli increased. On the contrary, remarkable increase in size due to extreme accumulation of starch grains, as well as less plastoglobuli, occurred in the tolerant Z7 plants. Differences between the sensitive Penjalinan and the tolerant Z7 in structural changes of plastids in t he root cells were found, While starch accumulation was induced by low temperature in Penjalinan, increased frequency of thylakoids and thei r dilation occurred in Z7 plastids. The more severe structural disturb ances and the absence of starch grains in the leaf chloroplasts of chi lling sensitive Penjalinan plants confirms relationships between chill ing tolerance and membrane properties, respiration, photosynthetic, an d assimilate transport activities reported by other authors. The diffe rences in starch accumulation in the root cell plastids might correspo nd with stronger inhibition of the root growth in the sensitive genoty pes.