Photoinhibition Mechanisms Of Cucumber In Low Temperature And Weak Light

One half of engrafted-cucumber seedlings and self-rooted cucumber seedlings were treated with low temperature (4℃), weak light (100μmol·m^-2·s^-1) and the others with low temperature (4℃) to the roots, room temperature (25℃) and weak light (100μmol·m^-2·s^-1) to the shoots. Physiological indices, such as Chlorophyll fluorescence parameters, 820nm light-absorbed by PSⅠand RuBPCase activity, were measured after 0, 2, 4, 6, 8 hours respectively. The aim was to determine photoinhibition mechanisms of cucumber in low temperature and weak light. The main results are shown as follows:1 The photoinhibition of cucumber in low temperature and weak lightUnder the low temperature and weak light, the fluorescence parameters, except for NPQ which has increased, such as Fv/Fm, Fo,φPSⅡ, ETR, Fv′/Fm′, qP, all gradually decreased with stress time prolonging. These results indicate that photoinhibition existed in the reaction centre of PSⅡduring the treatment process. At the eighth hour of the treatment, some indices of engrafted-cucumber seedlings, such as Fv/Fm,φPSⅡ, ETR, Fv′/Fm′, and qP, were all significantly higher than those self-rooted cucumber seedlings (P<0.05). It suggests that PSⅡreaction centre of engrafted-cucumber seedlings are more tolerant to low temperature and weak light than self-rooted cucumber seedlings.Along with the extension of stress time, the oxidized P700 both in engrafted-cucumbers and self-rooted cucumbers all decreased. This phenomenon indicates that PSⅠwas inhibited by both low temperature and weak light. The higher oxidized P700 activity of engrafted-cucumber seedling shows that PSⅠreaction centre of self-rooted cucumber seedling is more sensitive to low temperature and weak light than engrafted-cucumber seedling.RuBPCase activities both of those two groups of cucumber seedlings decreased with the prolongation of stress time. It indicates that the RuBPCase of cucumber have been inhibited in low temperature and weak light. At the end of stress treatment, significant difference (P<0.01) existed between the RuBPCase activity of engrafted-cucumber seedling and of self-rooted cucumber seedling. This indicates that the RuBPCase of engrafted-cucumber seedling have higher tolerance to low temperature and weak light than that of self-rooted cucumber seedling.Under the treatment with low temperature and weak light, both in engrafted-cucumbers and self-rooted cucumbers, the SOD activities increased gradually at first and then decreased at the eighth hour of the treatment. The higher activity of protective enzymes in engrafted-cucumber seedlings is also one of the reasons why engrafted-cucumber seedlings are more tolerant to low temperature and weak light than self-rooted cucumber seedlings.In conclusion, in low temperature and weak light, the activity of PSⅡreaction centre reduced (Fv/Fm), and electron transport of PSⅡwas blocked (φPSⅡ). However, PSⅡreaction centre has not been destroyed (Fo). During the stress treatment, PSⅠof cucumbers was also inhibited. The decrease of RuBPCase activity shows that photosynthetic dark reaction has been influenced. At the end of stress, Fv/Fm, the oxidized P700, RuBPCase activity of the engrafted-cucumber seedlings decreased by 15.2%, 37.3% and 63.6% respectively compared with the control. It indicates that photosynthetic carbon assimilation may have suffered the most severe inhibition.2 The photoinhibition of cucumber in low temperature to the roots, room temperature and weak light to the shootsUnder the treatment with low temperature to the roots, room temperature and weak light to the shoots, Fv/Fm, Fv′/Fm′and the oxidized P700 of cucumber seedlings did not vary obviously in the first two hours, butφPSⅡ, ETR, qP increased significantly. It indicates that no photoinhibition occurred in cucumber seedlings during early stress time. This is possible that the shoots was in the room temperature and was not influenced by the treatment with low temperature to the roots in the first two hours. But with stress time prolonging, the gradual decreases of indices, such as Fv/Fm,φPSⅡ, ETR, qP, Fv′/Fm′and the oxidized P700, indicate that PSⅡand PSⅠhave been inhibited. This is possible that water-assimilated was restricted by low temperature to the roots and mineral required by photosynthesis could not transport to the shoots in time, these further inhibited the photosynthesis. At the end of stress treatment, Fv/Fm,φPSⅡ, ETR, qP and the oxidized P700 of engrafted-cucumber seedlings were significantly higher than self-rooted cucumber seedlings (P<0.05). It shows that engrafted-cucumber seedlings are more tolerant than self-rooted cucumber seedlings.In conclusion, under the treatment with low temperature to the roots, room temperature and weak light to the shoots, no photoinhibition occurred in cucumber seedlings during early stress time(2h). But with stress time prolonging, PSⅡand PSⅠof cucumber seedlings all have been inhibited. At the end of stress, Fv/Fm, the oxidized P700 of the engrafted-cucumber seedlings decreased by 15.1%, 30% respectively compared with the control. It indicates that PSⅠmay have suffered the more severe inhibition.