Net photosynthesis acclimates to low growth temperature in cotton seedlings by shifting temperature thresholds for photosynthetic component processes and respiration

Abstract

Temperatures between 10 °C and 25 °C limit seedling growth and reversibly inhibit net assimilation (AN) in cold-sensitive plants. Because AN represents multiple component processes operating in a highly concerted manner, quantifying the cold sensitivities of each constituent would provide targets for improving cold tolerance. To address this, Gossypium hirsutum (cotton) seedlings grown under sub-optimal temperatures (20/15 °C) exhibited substantial declines in plant growth, AN, and nearly every thylakoid process assessed, relative to optimal conditions (30/20 °C). In contrast, energy dissipation by photosystem II (PSII) increased under low temperature concomitant with a rise in carotenoid content, hydrogen peroxide production, and ascorbate peroxidase activity. Temperature response experiments defined the temperature needed for 50% reductions in activity for a given process (T50). Respiration (RD) was the most cold-sensitive process followed by thylakoid-dependent processes and gross photosynthesis (AG), whereas AN was the most-cold tolerant process evaluated. AN-Ci experiments revealed that carboxylation was more cold-sensitive than RuBP regeneration. Maximum quantum yield of photosystem II was insensitive to low incubation temperature (Fv/Fm), and T50 could not be estimated. Low temperature-grown plants also exhibited a 1.5 °C increase in T50 for RD, a 2.5 °C decrease in T50 for thylakoid-dependent processes and AG, and a 3.4 °C decrease in T50 for AN. Thus, the current study defines the relative cold sensitivities of the underlying processes driving AN. Additionally, it is concluded that AN acclimates to low growth temperature more than any other process evaluated due to increased cold sensitivity of RD and increased cold tolerance of thylakoid dependent processes and AG.