Shoot temperatures and thermal tolerances for succulent species of Haworthia and Lithops
Abstract. Temperatures of small succulent plants, such as species in the genera Haworthia and Lithops, are highly influenced by temperatures of the surrounding soil. Indeed, the minimum and the maximum temperatures of the upper leaf epidermis of Haworthia retusa. H. turgida. Lithops leslei, and L. turbiniformis were generally within 1°C of the accompanying soil surface temperatures. An energybudget model closely predicted such soil-to-plant temperature differences as well as the effect of the greater convective exchange for the protruding Haworthia species compared with the Lithops species, which were flush with the soil surface. Although a lower shortwave absorptance would reduce maximum shoot temperatures, the shortwave absorptances of all four species were similar to those of the soil in their respective native habitats in South Africa. Tolerances of the four species to low and to high temperatures at three different day/night air temperatures (15°C/5°C, 30°C/20°C, and 45°C/35°C) were analysed using cellular accumulation of a vital stain, neutral red. Chlorenchyma cells were slightly more tolerant of extreme temperatures than were cells of the water-storage parenchyma. In this regard, H. retusa survived low and high temperatures that killed the water-storage parenchyma but not the chlorenchyma. Acclimation to low temperatures and to high temperatures, which was exhibited by all four species, led to estimated tolerances to 1 h at −16°C and 68°C. Although the low temperature tolerance is not particularly noteworthy, very few vascular plants are reportedly able to tolerate such high temperatures.