Use of physiological indices as a screening technique for drought tolerance in oilseed Brassica species

Plants of 24 genotypes of four Brassica species (B. campestris, B. carinata, B. juncea, B. napus) were grown on stored soil water in a sandy loam soil under field conditions. Water use was recorded throughout the growing season. Leaf water potential (psi(w)), solute potential (psi(s)), relative water content (RWC), turgor potential (psi(p)), osmotic adjustment (reciprocal of slope b where 1n RWC = a-b.1n psi(s)), leaf diffusive conductance (K-1), the difference between canopy and air temperature (T-c-T-a) and water loss from excised leaves (WLL) were measured at siliqua formation. Genotypes with higher osmotic adjustment maintained higher psi(p) and K-1, and lower canopy temperatures and WLL, than genotypes with lower osmotic adjustment (when decreasing soil water supply caused a lowering of RWC and psi(w)). The former genotypes also extracted more water from deeper soil layers (90-80 cm). The decline in psi(s) with decrease in RWC and psi(w) was greater for high osmotic adjustment genotypes than for low osmotic adjustment genotypes. Seed yield was positively associated with K-1, 1/b and the degree of leaf cooling, and negatively with WLL. However, since T-c-T-a and WLL are easier to measure than 1/b and K-1, T-c-T-a and WLL measured around mid-day at the siliqua formation stage could be used to screen large numbers of germplasm lines of Brassica species for drought tolerance under field conditions. (C) 1998 Annals of Botany Company.