#69 - July 2023 - Garden Stuff © Sandy Lang - slang@xtra.co.nzC3, C4 & CAMJuly/August: Mid/late-winter. Days longer and warmer. Finish winter jobs. Prune only when dry.WUE: Last month, I talked about plant ‘water use efficiency’. The amount of sugar (energy food) produced by photosynthesis, per the amount of water lost through the stomata. Hydrophytes (plants adapted to wet environments) have lower WUEs than mesophytes (plants adapted to drier environments), than xerophytes (plants adapted to very dry environments). Plants use many tricks to raise their WUEs to cope with dry/very dry environments (Google xerophytes) but two are worth special mention. When plants have applied all the usual tricks but still need more, they mess around with photosynthesis itself. C3 Plants: About 85% of all plants have so-called C3 photosynthesis. Photosynthesis is a complex process (Google photosynthesis). Sufficient to say it involves two sets of reactions: (a) light-independent ones and (b) light-dependent ones. These C3 plants waste some of their light energy due to photorespiration because they carry out these two sets of reactions in the same cells and they also waste water by carrying out these two sets of reactions at the same time. Two strategies improve things: (1) do these in different cells and (2) do them at different times.C4 Plants (different cells): About 3% of all plants have C4 photosynthesis. (Google C4 photosynthesis). The C4 plants have a special ‘Kranz’ leaf anatomy that lets them reduce energy-wasting photorespiration by carrying out the light-independent reactions in their ‘bundle-sheath’ cells and the light-dependent ones in their ‘mesophyll’ cells. This spatial separation makes C4 photosynthesis about 50% more efficient than C3 photosynthesis, especially under harsh conditions – hot, limited water. Some important C4 plants are maize, sugarcane and paspalum.CAM Plants (different times): And when even C4 photosynthesis is not enough, there’s crassulacean acid metabolism (Google CAM plants). About 8% of all plants have CAM photosynthesis. These have the highest WUEs of all and they inhabit the hottest, driest environments. They achieve this by carrying out the two sets of photosynthetic reactions at different times. They open their stomata at night (cooler, more humid, less water loss), take in CO₂ and store it. Then they close their stomata all day (hot, dry) and photosynthesise their stored CO₂ to make sugars. This way CAM plants have very high WUEs. Some important CAM plants are cacti, succulents, pineapples and 40% of orchids.___________________________________