Severe Preharvest Drought Elevates Respiration and Storage Rot in Postharvest Sugarbeet Roots

Sugarbeets are largely produced without irrigation, making
drought stress inevitable when rainfall is insufficient.
Whether drought stress impacts root storage, however, is
currently unknown. Research was conducted to determine
the effect of preharvest water stress on postharvest sugarbeet
root respiration rate and susceptibility to storage rots
as these traits are the primary determinants for sucrose loss
and quality deterioration. Greenhouse-grown plants were
subjected to four levels of water deficit by discontinuing
watering for 0, 7, 14 or 21 days prior to harvest. Plants receiving
water-restrictive treatments displayed physiological
stress by leaf epinasty, reductions in net photosynthetic
rate and leaf relative water content and increases in leaf
temperature, whereas the water content of roots harvested
from these plants progressively decreased with the severity
of the preharvest water-deficit treatment. Harvested roots
from all watering treatments were stored at 10°C and 95%
relative humidity for up to 12 weeks and evaluated for respiration
rate and susceptibility to storage rot. Root respiration
rate during storage was inversely related to root water
content at harvest by second-order equations, such that respiration
was not significantly affected by minor reductions
in root water content but increased exponentially for roots
obtained from severely drought-stressed plants with water
contents at harvest of ≤75%. Similarly, roots with water
contents ≤75% had elevated levels of electrolyte leakage,
a measure of cellular membrane damage, and were more
susceptible to dehydration and fungal infection during
storage. In separate experiments, roots harvested from water-
stressed plants were inoculated with Botrytis cinerea or
Penicillium vulpinum, two causal agents for storage rots.
In these experiments, preharvest water stress quantitatively
increased root rot and qualitatively altered symptoms
of their infection. Overall, these results demonstrate that
severe preharvest drought stress is likely to significantly
increase sugarbeet root storage losses caused by root respiration
and storage rots and that storage losses are likely
to accelerate with time in storage. However, mild-to-moderate
drought conditions prior to harvest are expected to
have no or minimal effect on storage losses from root respiration
or storage rots.