Responses Of Leaf Stomatal Development To Drought And Effects Of Repeated Drought On Photosynthesis And Drought Resistance Of Maize

Plants grown in arid and semiarid regions often experience more than one period of drought during life cycles. In order to investigate the effects of drought on stomatal development and gas exchange capacity, and crop responses to repeated drought,（1） stomatal density, size, aperture and gas exchange under drought, and（2） plant growth, photosynthesis, antioxidant defense system, and osmotic adjustment under 3-weeks drought, 1-week rewatering and 3-weeks drought again were studied. Results showed that:（1） Independent of soil water conditions, stomatal density and length in trumpeting and filling stages were greater than that in the jointing stage, but stomatal width did not show significant changes in these three stages. Under drought stress, stomatal density increased, but stomatal size decreased. Moreover, under severe water deficit, net photosynthetic rate（P_n）, transpiration rate（T_r）, and stomatal conductance（Gs） decreased significantly. Regression analysis indicated that stomatal density was negatively correlated with T_r and P_n, but positively correlated with instantaneous water use efficiency（WUE_i）.（2） After maize seedlings suffered the first water treatments, under severe water deficit, plant height, total leaf area of individual plant, shoot and root biomass declined significantly, also T_r, G_s, intercellular CO₂ concentration（Ci）, P_n, maximum net photosynthetic rate(A_max), photosynthetic pigments contents, and phosphoenolpyruvate carboxylase（PEPCase） activity, but light compensation point, dark respiration rate and glycolate oxidase（GO） activity increased significantly; O₂^-, H₂O₂, and MDA contents and relative electrolyte leakage increased significantly, also superoxide dismutase（SOD）, catalase（CAT）, ascorbate peroxidase（APX）, monodehydroascorbate reductase（MDHAR）, and glutathione reductase（GR） activities; the contents of free amino acid, proline and soluble sugar which as the key osmotic regulation substances increased significantly. Under medium drought, plant height, leaf area, and shoot biomass decreased significantly, but root biomass did not vary, hence, the ratio of roots to shoots（R/S） increased; moreover, plants did not show significant differences in photosynthetic parameters; medium water deficit caused less damage on membrane system, and only significantly increased MDHAR activity.（3） Rewatering after drought, photosynthetic capacity, these physiological and biochemical indexes except APX activity, and growth rate of plants previously exposed to water deficit could recover to the levels of well-watered plants, but plant height and leaf area did not recover to the levels of control.（4） When maize were subjected to recurrent drought, plants pre-exposed to medium drought showed no significant difference in plant height, biomass, and photosynthetic parameters, but a significant decrease in leaf area, compared to plants only exposed to second medium drought. Plants pre-exposed to severe drought had significantly higher T_r, G_s, C_i, P_n, A_max, apparent quantum yield, photosynthetic pigments contents, PEPCase activity, and GO activity, but significantly lower plant height, leaf area, and biomass, moreover, significantly lower membrane permeability but significantly higher free amino acid than plants without previous exposure. The activities of the key ROS scavenging enzymes of SOD, CAT, APX, MDHAR, and GR enhanced as well under double drought.In conclusion,（1） drought stress could increase stomatal density but decrease stomatal size. The changes of stomatal density were significantly negatively correlated with photosynthesis and transpiration.（2） The first severe drought significantly reduced photosynthetic capacity and maize growth, rewatering could recover photosynthesis and growth rate to the levels of well-watered plants, but could not eliminate the adverse influence of the first drought on growth. The first medium drought could stimulate the growth of maize root system and significantly increased R/S, which can enhance maize drought resistance to subsequent repeated drought, and maintain the total biomass in the control level; the first severe drought could enhance maize drought resistance to subsequent repeated drought by a rapid response of the antioxidant defense system and higher osmotic adjustment ability, but could not compensate for the adverse effect of early drought on plant growth. Hence, in practice, drought hardening should be limited in the level of medium drought, and avoiding severe drought.