ASSESSING THE DROUGHT-RESISTANCE ADAPTIVE ADVANTAGE OF SOME ANATOMICAL AND PHYSIOLOGICAL FEATURES IN NICOTIANA-TABACUM

As part of an extensive research programme that attempts to explain an d quantify the known difference in drought tolerance of four Nicotiana tabacum L. cultivars, some aspects that may contribute towards the av oidance and tolerance of drought stress were investigated to evaluate overall resistance. The results presented emphasize the adaptive signi ficance of effective leaf movements in determining drought tolerance, by reducing or preventing damage to the photosynthetic system caused b y photoinhibition and direct heat damage. Leaf movement seemed to be a chieved with greater efficiency by the drought-tolerant cultivars GS46 and Elsoma. Drought stress specifically influenced the carotenoid com position. A strong quantitative correlation existed between the format ion of zeaxanthin in the xanthophyll cycle and the type of fluorescenc e quenching, which is indicative of nonradiative energy dissipation. T he latter occurred to a lesser extent in the drought-tolerant cultivar s GS46 and Elsoma. This phenomenon also indicated that the rate consta nt for nonradiative energy dissipation in tobacco remains relatively s mall in relation to the rate constant for fluorescence during drought stress. Furthermore, irrespective of their drought tolerance, it would seem that tobacco plants have a capability towards starch overproduct ion, though this was less pronounced in the drought-sensitive cultivar s TL33 and CDL28. Owing to anatomical differences, resistance to water flow varied between the respective cultivars, as did the percentage i ntercellular spaces, both of which correlated positively with the slow er decrease in water use efficiency and faster recovery upon rehydrati on of the drought-tolerant cultivars.