# Adaptive Mechanisms of White-Flowered Alfalfa (Medicago sativa L.) in High-Altitude Cold and Saline–Alkali Environments

## TL;DR

White-flowered alfalfa on the Qinghai–Tibet Plateau adapts to harsh conditions by reallocating resources from growth to defense mechanisms.

## Contribution

The study reveals that white-flowered alfalfa exhibits a coordinated adaptive syndrome involving metabolic and hormonal changes.

## Key findings

- White-flowered alfalfa shows reduced flower size and chlorophyll but increased jasmonic acid and antioxidant defenses.
- Phenylpropanoid pathway reprogramming suppresses anthocyanin biosynthesis, redirecting resources to stress-related compounds.
- The white-flower trait is part of a broader adaptive strategy involving hormone signaling and metabolic shifts.

## Abstract

This study explored why some alfalfa plants on the Qinghai–Tibet Plateau have white flowers instead of the usual purple. We found that the white-flowered plants have made a trade-off: they invest less energy in flower size and growth, and more in defending themselves against the harsh, salty environment. Specifically, these plants have boosted their chemical defenses, including antioxidant enzymes and protective compounds. By analyzing the plants’ genes and metabolites, we discovered that the cellular pathway responsible for purple flower color was dialed down, which redirected the plant’s resources towards producing these defensive chemicals instead. This shows that the white flower is not just a color change, but a marker of a major shift in the plant’s survival strategy. This knowledge could be very useful for developing new alfalfa varieties that are better equipped to handle stressful growing conditions.

White-flowered alfalfa (Medicago sativa L.) persisting in Qinghai–Tibet Plateau’s saline–alkali habitats provides a unique model to explore floral color polymorphism-linked ecological adaptation. We systematically compared phenotypic, physiological, transcriptomic, and metabolomic profiles of white-flowered (WF) and purple-flowered (PF) alfalfa under high-altitude cold/saline–alkali field conditions (three biological replicates; Student’s t-test). WF showed a significant growth-defense trade-off: flower size and chlorophyll a content decreased by 18.9% and 46.0%, with reduced gibberellin levels, while jasmonic acid (36.3%), proline (51.5%), antioxidant enzyme activities, total flavonoids (17.7%), and condensed tannins (18.2%) were significantly increased (p < 0.001). Multi-omics analysis revealed phenylpropanoid pathway reprogramming (suppressed anthocyanin biosynthesis, precursor accumulation) and coordinated hormone signaling (jasmonic acid activation, salicylic acid inhibition). Our findings confirm the white-flower trait is not an isolated mutation. It is a key component of a coordinated adaptive syndrome, mediated by metabolic reprogramming and hormonal crosstalk. These results provide theoretical and technical support for breeding stress-resistant alfalfa varieties suitable for marginal land cultivation.