#67 - May 2023 - Garden Stuff - © Sandy Lang - slang@xtra.co.nz
CUTICLES & STOMATAMay/June: Late autumn/early winter. The short, cold, damp, wintery days are upon us. Time to tidy, plan, prune and plant (cold and short-day tolerant leaf veggies and broad beans). If you want a new perennial, buy it early to get the best specimen – not other’s rejects.Wet chemistry: Life started in the sea. Today, the ‘chemistry of life’ still goes on in water - inside every microbe, plant and animal. So: How did plants escape the watery womb of the sea, to invade the land where the soil is much less watery and the air is dangerously dry? Water potential: Let me introduce a new term ‘water potential’ (WP). It’s a measure of the availability of water and has units of pressure ‘Pascals’ (Pa). Because 1 Pa is very small, we usually measure pressures in thousands of them (kPa) or in millions (MPa). Some examples... Cold tap: The WP in a kitchen tap is about 250,000 Pa (0.25 MPa). About the same as the air pressure in a car tyre.Pure water: The availability of pure water at the surface of a lake is exactly 0.0 MPa (by definition). Wherever else we look, the availability of water is usually less than 0.0 MPa, so WP is usually negative. Soil WP: This varies between 0.0 MPa (waterlogged) and -1.5 MPa (plants wilt, some die).Air WP: If you thought dry soil was dry, get this... At 100% relative humidity (RH) air WP is 0.0 MPa; at 99 RH it is -1.4 MPa; at 90% RH it is -14.2 MPa; at 50% RH it is -93.5 MPa and at 10% RH it is -300 MPa. The air is rarely as humid as 99% RH and often falls to below 30% RH.But: How can plants survive on land when their life chemistry can function only in water? Answer: Every outer surface is covered by a super-waterproof, super-thin (0.0002 cm [a polythene bag is 0.005 cm]) waxy cuticle. (Google plant cuticle) But: How can a plant take in CO₂ (for photosynthesis) and liberate O₂ (photosynthesis biproduct) through such a highly impermeable cuticle? Answer: Plants have tiny valves in their skin (Google stomata). These open and shut depending on time of day and how much water the plant gets from the soil. They take about 10 min to open or shut. _______________________
CUTICLES & STOMATAMay/June: Late autumn/early winter. The short, cold, damp, wintery days are upon us. Time to tidy, plan, prune and plant (cold and short-day tolerant leaf veggies and broad beans). If you want a new perennial, buy it early to get the best specimen – not other’s rejects.Wet chemistry: Life started in the sea. Today, the ‘chemistry of life’ still goes on in water - inside every microbe, plant and animal. So: How did plants escape the watery womb of the sea, to invade the land where the soil is much less watery and the air is dangerously dry? Water potential: Let me introduce a new term ‘water potential’ (WP). It’s a measure of the availability of water and has units of pressure ‘Pascals’ (Pa). Because 1 Pa is very small, we usually measure pressures in thousands of them (kPa) or in millions (MPa). Some examples... Cold tap: The WP in a kitchen tap is about 250,000 Pa (0.25 MPa). About the same as the air pressure in a car tyre.Pure water: The availability of pure water at the surface of a lake is exactly 0.0 MPa (by definition). Wherever else we look, the availability of water is usually less than 0.0 MPa, so WP is usually negative. Soil WP: This varies between 0.0 MPa (waterlogged) and -1.5 MPa (plants wilt, some die).Air WP: If you thought dry soil was dry, get this... At 100% relative humidity (RH) air WP is 0.0 MPa; at 99 RH it is -1.4 MPa; at 90% RH it is -14.2 MPa; at 50% RH it is -93.5 MPa and at 10% RH it is -300 MPa. The air is rarely as humid as 99% RH and often falls to below 30% RH.But: How can plants survive on land when their life chemistry can function only in water? Answer: Every outer surface is covered by a super-waterproof, super-thin (0.0002 cm [a polythene bag is 0.005 cm]) waxy cuticle. (Google plant cuticle) But: How can a plant take in CO₂ (for photosynthesis) and liberate O₂ (photosynthesis biproduct) through such a highly impermeable cuticle? Answer: Plants have tiny valves in their skin (Google stomata). These open and shut depending on time of day and how much water the plant gets from the soil. They take about 10 min to open or shut. _______________________