Research Plant Physiologist USDA ARS Raleigh, North Carolina
Soybeans are known to have limited cold acclimation potential. The photosynthetic capacity of soybean is decreased in the cold; however, the exact timing of this decrease has yet to be investigated. Chlorophyll a fluorescence was measured every five minutes for the first hour of cold exposure under stead-state illumination and every hour for seven hours with dark-acclimation preceding measurements. PSII maximum efficiency (FvI/FMI) began to decrease at 15 minutes, with significant decrease beginning at 25 minutes. PSII operating efficiency (FqI/FMI) and electron transport rates also decreased at 15 minutes with significance at 25 minutes. Additionally, chlorophyll a fluorescence transient curve showed that electron flow through photosystem II began to slow after 15 minutes further demonstrating that electron transport through PSII was depressed after a very short period of cold exposure. Under dark adapted conditions, the maximum efficiency of PSII (FV/FM) was not significantly impacted by cold until six hours post exposure and electron transport was not significantly impacted until seven hours. When combined with the steady state illuminated results, these data indicate that the photodamage noted in operational photosynthesis is transient and can be alleviated until at least six hours have passed. Interestingly, during cold conditions soybean seedling leaves exhibit nastic movements. This leaf movement begins within the first five to fifty minutes in a light-dependent manner which aligns well with the steady-state photosynthesis rates that began to decrease at 15 minutes with significance at 25 minutes post cold exposure. Additionally, these movements do not recover the next morning as sleep movements do. Since dark-adapted photosynthesis showed no permanent damage before six hours of cold treatment, the lack of recovery overnight is consistent with the timing of permanent PSII damage.