Stomatal and Metabolic Regulation of Photosynthetic Responses to Drought Stress and Re-Watering in Kentucky Bluegrass.

Photosynthesis inhibition induced by drought stress may be due to stomatal and/or metabolic limitation. The objective of this study was to investigate changes of photosynthetic components associated with stomatal and metabolic limitations during drought stress and recovery in Kentuchy bluegrass (Poa pratensis L.). Two cultivars differing in drought stress, ‘Midnight’ (tolerant) and ‘Brilliant’ (sensitive), were subjected to drought stress for 15 days, and then allowed to recover by re-watering for 10 days in growth chambers. Net photosynthetic rate (P_n), stomatal conductance (g_s) and transpiration rate (Tr) decreased, with more rapid decline in ‘Brilliant’ during 15 d of drought period. Fv/Fm, Rubisco activity and activation state declined during drought stress, but were higher in ‘Midnight’ than in ‘Brilliant’ within 15 d of treatment. A/C_i curves analysis showed that stomatal limitations (Ls %) and non-stomatal limitations (Lns %) on photosynthesis, CO₂ compensation point (CP) increased with drought stress progression, but to a great extent for ‘Brilliant’. Maximum CO₂ assimilation (A_max) and carboxylation efficiency (CE) declined, but ‘Midnight’ had significantly higher levels of both A_max and CE than that of ‘Brilliant’ at moderate to severe drought stress. After re-watering, RWC restored to about 90% of the control levels in both cultivars, while P_n, g_s, Tr and Fv/Fm recovered only 38-80% of the control levels, but the rate of recovery was faster in ‘Midnight’ than in ‘Brilliant’ during 10 d of re-watering period. Ls, Lns and CP declined, and A_max, CE increased after re-watering, with more rapid change in all parameters in ‘Midnight’ than in ‘Brilliant’. These results indicated that drought tolerance of Kentucky bluegrass was associated with the maintenance of higher P_n and A_max, which could be attributed to more active Rubisco activase, higher CE, and lower stomatal limitation. All these factors may also contribute to the superior recuperative ability in Kentucky bluegrass.