Gradients of seed photosynthesis and its role for oxygen balancing

Seeds are generally viewed in the context of plant reproduction and the supply of food and feed, but only seldom as a site of photosynthesis. However, the seeds of many plant species are green, at least during their early development, which raises the issue of the significance of this greening for seed development. Here we describe the two contrasting modes of photosynthesis in the developing seed. The dicotyledonous pea seed has a green embryo, while the monocotyledonous barley caryopsis has a chlorenchymatic layer surrounding its non-green endosperm (storage organ). We have employed pulse-amplitude-modulated fluorescence and oxygen-sensitive microsensors to localize and describe gradient distributions of photosynthetic activity across the seed/caryopsis, and have discussed its role in maintaining the endogenous O₂ balance. We also report the lack of photosynthetic activity in the stay-green embryo axis of the sacred lotus (Nelumbo nucifera) seed following imbibition. The observations are discussed with respect to in vivo light supply and contrasted with the characteristics of leaf photosynthesis.