INTERCELLULAR SPACE VARIATION AMONG AIR-CURED NICOTIANA-TABACUM-L GENOTYPES AND ITS RELATION TO THEIR WATER-USE EFFICIENCY

The possible causal relationship between the percentage intercellular spaces in the leaf palisade parenchyma and water use efficiency (WUE) was investigated in related Nicotiana tabacum L. genotypes grown under controlled environmental conditions. Experimental observations includ ed the initial quantification of the percentage intercellular spaces, and the measurement of gas exchange by infra-red gas analysis prior to , during, and after recovery from drought stress. Statistically signif icant differences in the percentage intercellular spaces per surface a rea existed between the respective genotypes. The percentage intercell ular spaces correlated positively with the pre-stress photosynthetic a nd transpiration rates, as well as with the WUE of drought-stressed pl ants, but negatively with the WUE of recovered plants and the rate of stomatal conductance during and after drought stress. It is concluded that in air-cured tobacco, high pre-stress photosynthetic rates are ac companied by high transpiration rates in genotypes with a high percent age of intercellular spaces. This discourages use of the former in dro ught tolerance selection programmes, as used in the past, while select ion on the basis of a smaller percentage intercellular spaces would se em to be a more valid screening parameter. This contention is supporte d by data which indicate that the genotypes with a high percentage int ercellular spaces maintain a higher degree of WUE when drought-stresse d by sharply decreasing their stomatal conductance, but, contrary to t he genotypes with a lower percentage intercellular spaces, they are un able to reachieve or exceed their pre-stress WUE when rewatered.