Protein turnover: possible contributing factor in determining drought tolerance in Nicotiana tabacum L.

Drought stress-induced changes in the water-soluble protein concentration and total number of sulphydryl (-SH) groups were monitored under controlled environmental conditions in four tobacco cultivars (Nicotiana tabacum L.) with different drought tolerances. The drought stress ranged from light (-0.52 MPa) to severe (-2.5 MPa). A statistically significant decline in both water-soluble protein concentration and total number of -SH groups already occurred at a leaf water potential (ψL of −0.77 MPa. The initial rate of decline in both the concentration of water-soluble protein and the number of -SH groups, as well as the end-values reached by both these parameters, differed significantly between the drought-tolerant (GS46 and ELSOMA) and drought-sensitive (TL33 and CDL28) cultivars. In contrast with the fast initial decline observed in GS46 and ELSOMA at light stress levels, which stabilized and occurred more gradually as the drought stress intensified, the concentration of water-soluble protein and the number of -SH groups declined at a slower but continuous rate in TL33 and CDL28 as ψL decreased. At a ψL of −2.51 MPa, the number of -SH groups present in GS46 and ELSOMA respectively, was 63.6% and 65.2%, and that of TL33 and CDL28 was 39.9% and 46.8% of their respective control values. Furthermore, the concentration of water-soluble protein of ELSOMA was still 75.5% and that of GS46, 65.4 % of their controls, even at a ψ of −2.51 MPa, which contrasts with the corresponding values of 46.6% and 55.3% for TL33 and CDL28, respectively. The steep initial decline in the water-soluble protein concentration of ELSOMA and GS46 is explained on the basis of the concept of protein turnover. The known difference in drought tolerance between the four tobacco cultivars was best explained by a combination of the -SH/-SS hypothesis and the concept of protein turnover.