Leaf Photosynthesis and Drought Adaptation in Field-Grown Oilseed Rape (Brassica napus L.)

Abstract

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.