The root of the problem: diverse vulnerability to xylem cavitation found within the root system of wheat plants.

The propagation of xylem embolism throughout the root systems of drought-affected plants remains largely unknown, despite this process being comparatively well characterized in aboveground tissues. We used optical and X-ray imaging to capture xylem embolism propagation across the intact root systems of bread wheat (Triticum aestivum L. 'Krichauff') plants subjected to drying. Patterns in vulnerability to xylem cavitation were examined to investigate whether vulnerability may vary based on root size and placement across the entire root system. Individual plants exhibited similar mean whole root system vulnerabilities to xylem cavitation but showed enormous 6 MPa variation within their component roots (c. 50 roots per plant). Xylem cavitation typically initiated in the smallest, peripheral parts of the root system and moved inwards and upwards towards the root collar last, although this trend was highly variable. This pattern of xylem embolism spread likely results in the sacrifice of replaceable small roots while preserving function in larger, more costly central roots. A distinct pattern of embolism-spread belowground has implications for how we understand the impact of drought in the root system as a critical interface between plant and soil.

Resistant xylem from roots to peduncles sustains reproductive water supply after drought-induced cavitation of wheat leaves.

BACKGROUND AND AIMS: Many annual grasses exhibit drought-avoiding life cycles in which rapid reproduction must be completed before soil water is exhausted. This strategy would seem to require a hydraulic system capable of sustaining reproduction at all costs to the rest of the plant, yet little is known about the whole-plant structure of hydraulic vulnerability in grasses. METHODS: We examine vulnerability to water-stress-induced xylem cavitation in roots, flag leaves, and basal and apical regions of peduncles of wheat (Triticum aestivum L. 'Krichauff') to understand the staged failure of xylem function in severe drought. The functionality of segmented vulnerabilities is tested by conducting rehydration experiments after acute dehydration. KEY RESULTS: We show that water supply to peduncles is more drought resistant than in leaves due to greater xylem cavitation resistance, ensuring a pathway of water can be maintained from the roots to the reproductive tissues even after severe water deficit. Differential rehydration of peduncles compared to leaves following drought confirmed the functionality of xylem supply from roots to seed after water stress sufficient to completely cavitate flag leaf vessels. CONCLUSIONS: These results demonstrate that a proportion of the hydraulic pathway between roots and seeds remains functional under extreme dehydration, suggesting that vulnerability traits in this key grass species reflect its reproductive strategy.

Reproductive water supply is prioritized during drought in tomato.

Reproductive success largely defines the fitness of plant species. Understanding how heat and drought affect plant reproduction is thus key to predicting future plant fitness under rising global temperatures. Recent work suggests reproductive tissues are highly vulnerable to water stress in perennial plants where reproductive sacrifice could preserve plant survival. However, most crop species are annuals where such a strategy would theoretically reduce fitness. We examined the reproductive strategy of tomato (Solanum lycopersicum var. Rheinlands Ruhm) to determine whether water supply to fruits is prioritized above vegetative tissues during drought. Using optical methods, we mapped xylem cavitation and tissue shrinkage in vegetative and reproductive organs during dehydration to determine the priority of water flow under acute water stress. Stems and peduncles of tomato showed significantly greater xylem cavitation resistance than leaves. This maintenance of intact water supply enabled tomato fruit to continue to expand during acute water stress, utilizing xylem water made available by tissue collapse and early cavitation of leaves. Here, tomato plants prioritize water supply to reproductive tissues, maintaining fruit development under drought conditions. These results emphasize the critical role of water transport in shaping life history and suggest a broad relevance of hydraulic prioritization in plant ecology.