RELATIONSHIP BETWEEN LEAF GAS-EXCHANGE CHARACTERISTICS AND PRODUCTIVITY OF POTATO CLONES GROWN AT DIFFERENT TEMPERATURES

The rate of dark respiration (Rd) and net photosynthesis (Pn) at vario us leaf temperatures was examined in three potato clones (Solanum tube rosum L.) differing in heat tolerance. Plants were grown at low (25/12 C, day/night) and high (35/25 C) greenhouse air temperatures for five weeks, beginning two weeks after tuberization. Gas exchange character istics were measured by manometric and infrared gas analyzer technique s. Respiration:photosynthesis ratios were calculated as indicators of leaf carbon balance. High greenhouse temperature reduced whole plant a nd tuber growth rate of all clones, however, the reduction was highest in the cultivar Russet Burbank (heat sensitive). Gas exchange charact eristics did not explain differences in heat tolerance. The heat toler ant cultivar Desiree had Rd similar to Russet Burbank, while the clone DTO-28, also heat tolerant, had lower Rd of mature leaves than Russet Burbank or Desiree. However, all clones had similar Rd of immature le aves. There was no apparent relationship between heat tolerance and Pn for the three clones. DTO-28 had lower respiration:photosynthesis rat ios of immature and mature leaves than Russet Burbank 4 weeks after th e start of the high temperature treatment. Desiree had respiration:pho tosynthesis ratios as high as Russet Burbank. At different sampling ti mes, Rd increased in a linear and curvilinear manner with increasing l eaf temperature up to 40 C. Heat tolerant and sensitive clones had sim ilar rates of increase in Rd with increasing leaf temperature. Simulta neous measurement of Rd and Pn did not help explain differences in hea t tolerance among clones. However, determination of respiration:photos ynthesis ratios may help explain the physiological basis for heat tole rance of some clones.