High Root Temperatures: A Buried Threat to Plant Growth

Abstract

Growing plants in containerized systems can result in high root temperatures (HRT) as containers, media, and roots above the ground are exposed to air and sunlight, commonly experiencing temperatures over 50C. Damage caused by HRT and associated consequences for growth are not well characterized amongst herbaceous plants. The research in this thesis evaluated how HRT impacted physiological and morphological responses of eight tomato (Solanum lycopersicum) varieties characterized as ‘heat-tolerant’ or ‘sensitive’ based upon aboveground traits. The first pair of experiments quantified respiration rates and electrolyte leakage of excised whole root masses in response to acute HRT exposure between 48 and 62C. Root respiration rates increased from 21.6 µmol hr-1 g-1 at 48C to 26.9 µmol hr-1 g-1 at 51C, and then decreased to approximately 0 µmol hr-1 g-1 at 57C. Varieties did not differ in responses to root temperature. Root temperature and variety interacted to impact proportional electrolyte leakage, which increased across varieties between 50 and 54C. Results of these experiments suggested that critical physical and metabolic damage occurs to tomato roots at >50C. For the second pair of experiments, morphological and photosynthetic responses of two tomato varieties previously characterized as heat-tolerant (‘Solar Fire’) or -sensitive (‘Amana Orange’) were assessed. Plants were grown at root temperatures ranging from 25 to 60C for 8 h-1 d-1 over 10 d, and differences in morphology were noted. Plant height and leaf size decreased as temperature increased. Shoot and root fresh and dry mass gain decreased when RT increased from 35 to 50C. ‘Solar Fire’ and ‘Amana Orange’ did not differ in fresh and dry mass gain responses or percent reduction in shoot and root mass gain. Root masses of ‘Solar Fire’ and ‘Amana Orange’ were also heated to 55C for 260 min in the afternoon of one day and plants were evaluated for changes in leaf photosynthetic rate and stomatal conductance the following four days. Photosynthetic rate and stomatal conductance decreased after one 55C RT exposure for 4 d compared to plants maintained at 25C. ‘Solar Fire’ and ‘Amana Orange’ differed in percent reduction in stomatal conductance. The results suggested diurnal, short-term HRT negatively impacted growth and photosynthesis regardless of reported above-ground heat tolerance, and that even one supraoptimal HRT event could reduce photosynthetic activity for days. Lastly, five root-associated fungi and bacteria (Azospirillum brasiliense, Bacillus amyloliquifaciens, Curvularia protuberata, Glomus intraradices, and Trichoderma harzianum), thought to confer increased resistance to biotic and abiotic stresses, were explored for their potential to alleviate HRT effects on tomato growth. ‘Amana Orange’ seedlings were inoculated with the before-mentioned microbes and exposed to root temperatures between 35 (control) and 55C (HRT) for 8 h-1 d-1 over a 10 d period. Plant height and shoot, root, and total plant fresh and dry mass decreased as root temperature increased from 35 to 50C. Dry mass gain of roots and shoots did not differ between un-inoculated and inoculated plants, but some differences were observed between inoculant species. The results suggested HRT have detrimental effects on above- and below-ground tomato growth and inoculation with the before-mentioned organisms did not alleviate those negative effects.