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_e), leaf water potential (Ψ_e), leaf osmotic potential at full turgor (Ψ_π100), specific leaf area (SLA), spectral reflection index (RI) 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 μmol m^-2 s^-1 and ge was 1-1.5 mol m^-2 s^-1. Maximum rates of CO₂ assimilation greater than 30 μmol 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_e) were high compared with efficiencies of stems and husks and of other C₃ plants. In bracts A_max and ge were 10-15 μmol m^-2 s^-1 and 0.2-0.7 mol mol m^-2 s^-1, respectively. Both A_max and g_e varied linearly with leaf nitrogen content.

When soil water was depleted, both Ψ_π100 and RI decreased relative to controls on both soil types before any significant decrease in Ψ_π occurred. On loam with slow soil drying SLA, g_e and A_max decreased before any significant decrease in Ψ_e 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 Ψ_e of -1.6 MPa. Rape had a high TW/DW ratio (9-11) and a 6 limited ability to adjust osmotically, ΔΨ_e100 being at most 0.3-0.4 MPa. Keywords: age effects; canola; osmotic adjustment; photosynthetic nitrogen use efficiency; photosynthetic water use efficiency; pressure-volume curves

Full text doi:10.1071/PP9960631

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