Pod photosynthesis and drought adaptation of field grown rape (Brassica napus L.)

In rape (Brassica napus L., cv. Global) seed growth mainly depends on husk CO₂ assimilation. In irrigated plants, the net photosynthetic rate (A_max) was 10–13 μmol CO₂ m^?2 s^?1 in non-maturing pods and correlated with nitrogen content. The stomatal conductance of water vapour (g_H2O) was 0.3 mol m^?2 s^?1 in non-maturing pods. The photosynthetic nitrogen use efficiency (NUE) was 8.3 μmol CO₂g^?1 N s^?1, about one-third of that in leaves. The photosynthetic water use efficiency (WUE; A_maxg_H2O^?1) was similar in pods and leaves. In severely droughted plants, the photosynthetic rate was reduced to 38%. The seed growth rate, however, was not influenced by intermittent periods of water stress, indicating translocation of assimilates to the seeds. The drought resistant character of the pods was due to low specific area, succulence, low stomatal conductance causing a small decrease of ΔΨ day^?1 during soil drying and maintenance of high relative water content during severe drought. A mathematical formulation of the pod water release curve was undertaken. © (1997) Elsevier Science B.V.