Low Light Stress Down-Regulated Rubisco Gene Expression and Photosynthetic Capacity During Cucumber （Cucumis sativus L.） Leaf Development

Low light stress is one of the most important factors affecting photosynthesis and growth in winter production of cucumber （Cucumis sativus L.） in solar greenhouses in northern China. Here, two genotypes of cucumber （Deltastar and Jinyan 2） are used to determine the effect of low light stress on Rubisco expression and photosynthesis of leaves from emergence to senescence. During leaf development, the net photosynthetic rate （PN）, stomatal conductance （gs）, Rubisco initial activity and activation state, transcript levels of rbcL and rbcS, and the abundance of rbcL and rbcS DNA in these two genotypes increase rapidly to reach maximum in 10-20 d, and then decrease gradually. Meanwhile, the actual photosystem II efficiency （OpSll） of cucumber leaves slowly increased in the early leaf developing stages, but it declined quickly in leaf senescent stages, accompanied by an increased non-photochemical quenching （NPQ）. Moreover, PN, gs, initial Rubisco activity, and abundance of protein, mRNA and DNA of Rubisco subunits of leaves grown under 100 μmol m^-2 s^-1 are lower, and require more time to reach their maxima than those grown under 600 μmol m^-2 s^-1 during leaf development. All these results suggest that lower photosynthetic capacity of cucumber leaves from emergence to senescence under low light stress is probably due to down-regulated Rubisco gene expression in transcript and protein levels, and decreased initial and total activity as well as activation state of Rubisco. Deltastar performs better than Jinyan 2 under low light stress.