#68 - June 2023 - Garden Stuff © Sandy Lang - slang@xtra.co.nz
WATER USE EFFICIENCYJune/July: Early/mid-winter. Tidy, plan, plant, prune and mulch (conserves water, builds soil, supresses weeds). Stomata: Last month I said how the ‘chemistry of life’ goes on only in water, but land and air are very dry. So, while land plants are ‘wet inside’, to stop them drying out they must be covered by a super-thin, super-waterproof membrane, the cuticle. But the cuticle also stops the plant taking in CO₂, so it stops photosynthesis - the process by which plants make their energy food (sugar) from CO₂ and sunlight. But the cuticle is perforated by millions (~10,000 /cm²) of tiny valves, stomata, which allow it to take in CO₂ and photosynthesise (good) but they also allow water vapour to escape (bad).Control: The great thing about stomata is they can open and close to control the uptake of CO₂ and the loss of water, so as to achieve a best compromise. So, stomata close at night. There’s no photosynthesis in the dark, so why waste water by opening at night? Stomata also do some extra tricks like closing in the hot part of the day (when the air is driest) but open earlier and later (when it’s more humid).WUE: So, stomata maximise the amount of photosynthesis, while minimising the amount of water loss. This optimisation gives rise to the idea of a plant’s water use efficiency (WUE) - the amount of photosynthesis (grams of sugar) associated with a certain amount of water loss (kilograms of water) – so the units of WUE are g/kg. Because land plants have evolved to optimise WUE in different environments, some achieve higher WUEs than others. Most WUEs fall in the range 0.5 to 1 g/kg (examples are legumes 0.40, oilseeds 0.70 and clover 0.88 g/kg) but other WUEs are a lot higher (the cereals 2.4, some grasses 2.7 and some cacti 6.17 g/kg).In your garden: There are four ways WUE is relevant to you: (1) High-WUE plants make better use of the same amount of rain than low-WUE ones, so grow better with limited rainfall or in poor soil; (2) Many weeds are high-WUE - a reason they grow so well; (3) If you supplement limited/erratic rainfall by watering your garden, you will get a proportional increase in growth (don’t over water); and (4) Mulching increases growth because it helps the soil store more water.___________________________________