Phenotypic characterization of drought responses in red clover (Trifolium pratense L.)

Introduction: Red clover (Trifolium pratense) is a protein-rich, short-lived perennial
forage crop that can achieve high yields, but suffers increasingly from drought in
different cultivation areas. Breeding for increased adaptation to drought is becoming
essential, but at this stage it is unclear which traits breeders should target to
phenotype responses to drought that allow them to identify the most promising
red clover genotypes. In this study, we assessed how prolonged periods of drought
affected plant growth in field conditions, and which traits could be used to
distinguish better adapted plant material.
Methods: A diverse panel of 395 red clover accessions was evaluated during two
growing seasons. We simulated 6-to-8-week drought periods during two
consecutive summers, using mobile rain-out shelters, while an irrigated control
field was established in an adjacent parcel. Plant growth was monitored throughout
both growing seasons using multiple flights with a drone equipped with RGB and
thermal sensors. At various observationmoments throughout both growing seasons,
wemeasured canopy cover (CC) and canopy height (CH). The cropwater stress index
(CWSI) was determined at two moments, during or shortly after the drought event.

Results: Manual and UAV-derived measurements for CH were well correlated,indicating that UAV-derivedmeasurements can be reliably used in red clover. In both
years, CC, CH and CWSI were affected by drought, with measurable growth
reductions by the end of the drought periods, and during the recovery phase. We
found that the end of the drought treatment and the recovery phase of
approximately 20 days after drought were suitable periods to phenotype drought
responses and to distinguish among genotypes.

Discussion: Multifactorial analysis of accession responses revealed interactions ofthe maturity type with drought responses, which suggests the presence of two
independent strategies in red clover: ‘drought tolerance’ and ‘drought recovery’. We
further found that a large proportion of the accessions able to perform well under
well-watered conditions were also the ones that were less affected by drought. The
results of this investigation are interpreted in view of the development of breeding for
adaptation to drought in red clover.