Leaf photosynthesis and drought adaptation in field-grown oilseed rape (Brassica napus L)

Photosynthesis and drought adaptation in leaves of field grown rape (Brassica napus L. cv. Global) were investigated in 1992 under temperate climatic conditions in plants grown in lysimeters in a sand and in a loam soil. Light-saturated net photosynthesis (A(max)), leaf conductance to water vapour (g(l)), leaf water potential (psi(l)), leaf osmotic potential at full turgor (psi(pi)(100)), specific leaf area (SLA), spectral reflection index (RT) used as a measure of leaf area, and leaf nitrogen content, were determined in irrigated plants and in plants exposed to soil drying. In the early growth stages before flowering, A(max) was 35-45 mu mol m(-2) s(-1) and g(l) was 1-1.5 mol m(-2) s(-1). Maximum rates of CO2 assimilation greater than 30 mu mol m(-2) s(-1) were obseved for up to 19 days. Stomata partly closed in ageing leaves maintaining a constant C-i/C-a ratio. Both photosynthetic nitrogen use efficiency (NUE; A(max) per unit of nitrogen) and photosynthetic water use efficiency (WUE; A(max)/g(l)) were high compared with efficiencies of stems and husks and of other C-3 plants. In bracts A(max) and g(l) were 10-15 mu mol m(-2) s(-1) and 0.2-0.7 mol m(-2) s(-1), respectively. Both A(max) and g(l) varied linearly with leaf nitrogen content. When soil water was depleted, both psi(pi)(100) and RI decreased relative to controls on both soil types before any significant decrease in psi(l) occurred. On loam with slow soil drying SLA, g(l) and A(max) decreased before any significant decrease in psi(l) occurred. We suggest that these responses might have been triggered by a non-hydraulic signal transmitted from the roots. When water was more depleted, rape maintained positive turgor down to psi(l) of -1.6 MPa. Rape had a high TW/DW ratio (9-11) and a limited ability to adjust osmotically, Delta psi(pi)(100) being at most 0.3-0.4 MPa.