Response mechanisms of Annual bluegrass (Poa annua) to cold, drought, combined stresses and recovery in morphology, photosynthesis, physiology and microstructure

Drought and cold stresses co-occur in nature, and significantly limited agricultural productivity in northwest China. Their effects on plant photosynthesis, physiology and biochemistry, and microstructure have been extensively studied, but their combined stress mechanisms remain unclear. Therefore, growth chamber experiments were conducted using the Annual bluegrass (Poa annua) with the 'HZ' and 'ZQ' germplasms, our goal was to clarify the responses of leaves to cold (C), drought (D), and combined (D&C) stresses in the morphology, photosynthesis, physiology and biochemistry, and microstructure. The results showed that three stress types significantly suppressed growth, reduced photosynthetic pigments, photosynthetic capacity, and photosynthetic enzyme activities, with 'ZQ' being more sensitive than 'HZ'. C stress (12/36 h) improved chlorophyll fluorescence parameters in 'HZ' (except for photochemical quenching coefficient), while D and D&C stresses diminished these parameters of both germplasms. Under single and combined stresses, leaf thickness and cuticle thickness generally increased then decreased with stress duration, except for 'ZQ' under D stress. D stress increased reactive oxygen species, relative conductivity, malondialdehyde content and stomatal density, while inhibited stomatal size. Conversely, C stress exacerbated the negative effects of drought on these traits. The D&C stress enhanced antioxidant enzyme activity and proline content, which were similar to the responses to D stress, except for 'ZQ' under D&C stress. After 24 h of stress relief, the recoveries degree of most traits in plant under D and D&C stresses were similar, with 'HZ' recovering more than 'ZQ'. This study indicated drought played a dominant role in combined stress. However, some unique responses could not be inferred from the superimposed effects of some single stresses.