Impacts of short-term drastic temperature drop in autumn on PSⅡ activity in grape leaves

Abstract: 【Objective】Frosts happening in spring and autumn, namely the“late frost”and the“earlyfrost”respectively, are a disaster to growing crops. It happens irregularly and lasts for a short period (onenight). However, once occurring, it causes irretrievable damage on organs such as leaf, flower even fruit,which lead to great crop and economic losses. It is thus of vital importance to have an insight about themechanism of frost damage. Most studies of frost focus on the“late frost”in spring and few focus on the“early frost”. Therefore, little is known about the effects of frost in autumn on plants. Leaf is the major sen⁃sitive organ to the“early frost”. The basic function of leaf is photosynthesis, and it is necessary to studythe effect of the“early frost”on photosynthesis. The important issue is to explore the impact of short-termlow temperature in autumn on PSⅡ photochemical activity in grape leaves and determine the threshold oflow temperature that grape leaves can endure.【Methods】Healthy green leaves of‘Merlot’(Vitis vinifera）were selected on October 4-5, 2015. Five plots in the vineyard were selected randomly to collect healthyand uniform green leaves. 6 functional leaves were collected at dusk from both sides of the canopy at theheight of 1 m in each plot, generating a total of 30 leaves collected. The average temperature was about15 ℃ during leaf collection. Leaves were packed with wet filter paper to prevent desiccation and trans⁃ferred to laboratory in 30 min. After cleaning with water, the leaves were stacked up and leaf disks with0.8 cm in diameter were cut out avoiding the vein. The leaf disks were placed in an aluminum foil box (15cm×10 cm), and a piece of wet filter paper was placed on the surface to prevent dehydration. The alumi⁃num foil box were floated in a cold alcohol bath with the temperature cooled down from 15 ℃ to the targettemperatures (4，0，-4 and -8 ℃) at a rate about 0.4 ℃·min-1; each target temperature was hold for 10min. 30 leaves were sampled after cooling and fast chlorophyll fluorescence and reflectance spectrumwere monitored and analyzed.【Results】Inhibition in photochemical activity of PSⅡ was induced by theseshort-term low temperatures.F v/F m,F v/F 0 andPI abs showed a declining trend with the decrease of tempera⁃ture. The lower the temperature was, the greater drops inF v/F m,F v/F 0 andPI abs occurred. When the tempera⁃ture dropped to -4 ℃ and maintained at this temperature for 10 min,F v/F m,F v/F 0 andPI abs decreased signif⁃icantly by 10.99%, 32.10% and 45.31%, respectively. Light use efficiency of leaf was decreased by shorttermlow temperatures. With the decrease in temperature, trapped energy flux per CSTR 0/CS 0, the elec⁃tron transport flux per CS andPRI also showed a decreasing trend, with significant differences occurringat 0, -4 and 4 ℃ respectively from the control. Injury in the donor side of PSⅡ occurred with the sharptemperature drops. It is accepted that the injury in the donor side of PSⅡ occurred when“K”point ap⁃peared in OJIP curve.W k was a parameter to show the relative degree of injury in the donor side. Standard⁃ization was carried out using the data from O-J to show the relative change in fast fluorescence. It wasshown that“K”point appeared 10 min after temperature dropped from 15 ℃ to 4 ℃. This was an evi⁃dence to prove that injury in the donor side happened. As the temperature dropped further, phiE 0,S m andΨ 0 showed a decreasing trend. However,V j showed a trend of increase, indicating thatQ A was restoreddeeply which led to an accumulation ofQ A-, which blocked the ability of electron transport of PSⅡ.【Con⁃lusion】Grape leaves are sensitive to drastic temperature drop in autumn. PSⅡ photochemical activity ismaintained when temperature is no lower than 4 ℃. A comprehensive decrease in PSⅡ photochemical activity happens when temperature drops to -4 ℃ in a short period.