Plant parts doing unexpected things: Part 1 (or, Giving transpiration a little boost…)

All botanists (plant biologists/plant scientists/phytologists…) worthy of the name should be able to state the important roles played by various plant parts. Stems, for example, support the leaves(!), help to conduct water, photosynthates and other solutes to other plant parts, engage in some photosynthesis (primarily when young), and store such materials as starch. Well, so much for the commonplace quartet of functions. As botanists of an enquiring and sufficiently sceptical nature we probably also know that such a list is never complete, and plants can usually be found that defy convention and engage in practices additional to the received wisdom of general texts. So, welcome news that Or Sperling et al. have discovered the phenomenon of ‘transpirational-boosting’* in the date palm. Famously, the date palm (Phoenix dactylifera) grows in desert-like areas of northern Africa and the Middle East. Deserts are defined primarily in terms of low rainfall; consequently, water is a limiting resource to plant growth in that challenging environment. Yet palms are substantial arborescent monocots that can grow to 30 m tall and whose ecological dominance is sustained with uniquely high rates of transpiration. How is this possible in such water-limited regions? Recognising that the high transpiration rates cannot be sustained by soil water supply alone, Sperling et al. examined the reservoir of water within the palm’s stem. Using a combination of heat dissipation, gravimetric sampling and time domain reflectometry (you’ll need to read the open-access paper for the details of these techniques!), they determined that date palms substantially rely on the exploitation and recharge of the stem reservoir in their water budget; stem-located water contributes approximately 25 % of the daily transpiration rate. The date palm stem holds around 1 m³ of water and transpirational losses are recharged by more than 50 litres each night, which, the team argue, is sufficient to maintain daily reuse throughout the growing season. Although irrigated palms were specifically investigated, that still leaves 75 % of the water usage to be supplied externally. Whilst this column is not the place to engage in debate on contentious topics, such as globally growing demands (yes, unintentional pun noted…) on, and concerns over, future availability of fresh water, and not overlooking the issue of salinisation of soil that may accompany such anthropogenic irrigation practices, it is worth just raising the question of how long one can continue to engage in, or justify, human appropriation of water in this way. However, given the socio-economic importance, etc., of date palms – whose genome has been sequenced by Ibrahim S. Al-Mssallem et al. – perhaps there is a suitable case for ‘engineering’ of this magnificent monocot to enhance the contribution of transpirational-boosting**? Or, if we turn this discovery around, what about the rest of the approximately 351,999 other angiosperm species that haven’t been so examined? Might not more of them have evolved this T-B*** mechanism? And, if so, are estimates of future water demand by crops and other plants in need of revision? Botany, not afraid to tackle the big issues of the day (and tomorrow…)! Regardless, date palm is maybe another plant to add to the more conventional ones of cacti and euphorbs as examples of ‘stem succulents’.

* The ever-helpful Mr P Cuttings has kindly given this newly discovered phenomenon its suitably catchy name for the benefit of all plant science textbook writers (etc.) – Ed.

** No inverted commas this time – that must mean that this newly coined term is gaining acceptance by the community at large… – Ed.

*** And, having now been reduced to an initialism (as distinct from the frequently mis-applied term acronym), this phrase seems to be here to stay(?) – Ed.