CONSTANT-LIGHT INJURY OF POTATO - TEMPORAL AND SPATIAL PATTERNS OF CARBON-DIOXIDE ASSIMILATION, STARCH CONTENT, CHLOROPLAST INTEGRITY, AND NECROTIC LESIONS

Expanding leaflets of young 'Kennebec' potato plants (Solanum tuberosu m L.) develop visible necrotic spotting after 8 to 9 days of constant light and constant temperature, but little is known about this disorde r before the appearance of injury, Whole-leaf autoradiography and iodi ne staining of terminal leaflets (5 to 10 mm long at the beginning of the constant-light period) showed a normal pattern of CO2 assimilation and starch content over the entire leaflet surface after 5 days of co nstant light, However, small areas of tissue devoid of CO2 assimilatio n and starch content became apparent on day 6, and these areas expande d to encompass much of the leaflet's medial and basal regions by day 7 , At this stage of leaf development, on day 7, leaflets had attained 5 0% of their final leaflet length and ceased importing photosynthates f rom other leaves, Electron micrographs of chloroplasts from the medial and basal regions of leaflets on day 7 revealed a loss of membrane in tegrity and a senescence-like appearance, At this time, and within the se affected regions, scattered groups of necrotic palisade cells began to appear, These scattered groups soon expanded in size and distribut ion and became apparent as visible necrotic spots on the upper leaflet surface by day 8 or 9, Leaflets on plants grown under constant light but alternating temperatures, an environment known to be noninjurious, did not exhibit visible spotting or tissue devoid of starch content, In addition, none of these injury symptoms developed in 'Denali', a po tato cultivar tolerant of constant light, Despite its occurrence in ex panding leaf tissue, constant-light injury appears to be a senescence- like event that lends to the catastrophic loss of photosynthetic compe tence, starch content, and chloroplast membrane integrity, producing c hlorosis and necrosis of leaves and eventually stunting the plant.