Use of Physiological Indices as a Screening Technique for Drought Tolerance in OilseedBrassicaSpecies

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 (ψ w ), solute potential (ψ s ), relative water content (RWC), turgor potential (ψ p ), osmotic adjustment (reciprocal of slope b where lnRWC= a-b .lnψ s ), leaf diffusive conductance (K l ), 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 ψ p and K l , and lower canopy temperatures and WLL, than genotypes with lower osmotic adjustment (when decreasing soil water supply caused a lowering of RWC and ψ w ). The former genotypes also extracted more water from deeper soil layers (90–180 cm). The decline in ψ s with decrease in RWC and ψ w was greater for high osmotic adjustment genotypes than for low osmotic adjustment genotypes. Seed yield was positively associated with K l , 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 l ,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.