A multidisciplinary approach to the study of the plasma membrane of Zea mays pollen during controlled dehydration

A multidisciplinary approach (freeze-fracture, nuclear magnetic resonance, differential scanning calorimetry, isoelectric focusing and fluorochromatic reaction test) has been used to follow the behaviour of Zea mays pollen during dehydration - and to estimate its quality. At anthesis, the water content of maize pollen is 57-58% and the vegetative plasma membrane is continous and well structured with a very low density of intramembraneous particles on the extraplasmic fracture face. Maize pollen grains can withstand the drying process until a water content of 28% is reached, at which point 60-80% of the individuals show a negative reaction in the fluorochromatic test. At this water content, there is no more crystallizable water and thus metabolism decreases, leading to oxidative damage and the formation of gelphase microdomains in the plasma membrane. Consequently, the plasma-membrane permeability is modified. At 15-13% water content, all pollen grains show a negative fluorochromatic reaction, and gel-phase microdomains are more numerous but membranes still have a bilayer structure. Relaxation-time experiments indicate the occurrence of water replacement at the membrane level. Thus, sugar may stabilize the membrane structure at water contents as low as 3%. During the dehydration process, pollen walls act as elastic structures and remain closely applied to the protoplast. The combination of wall deformation and water replacement would permit pollen survival until oxidative damage occurs in the dehydrated grain.