为了探求玉米(Zea mays)光合作用和生长对重复干旱的响应机制, 采用盆栽试验, 分别测定了不同程度土壤干旱处理3周时、随后复水1周时以及再次不同程度干旱处理3周时玉米幼苗光合参数和生长的变化。第一次土壤干旱处理后, 重度干旱处理显著降低玉米株高、单株总叶面积、地上部分及根系生物量以及叶片的蒸腾速率(T_r)、气孔导度(G_s)、胞间CO₂浓度(C_i)、净光合速率(P_n)和最大净光合速率(A_max), 但显著提高光补偿点和暗呼吸速率; 中度干旱处理同样显著降低玉米株高、叶面积和地上部分生物量, 但对根系生物量无影响, 因而根冠比增大, 对上述光合参数的负效应也不具有显著性。复水可使前期经受中度和重度干旱处理的玉米植株的光合能力和生长速率恢复到正常水分条件下生长的植株的水平, 但株高和叶面积没有恢复到对照水平。当玉米再次经受水分亏缺处理时, 与只遭受第二次中度或重度干旱处理的植株相比, 经历过前期中度干旱处理的植株的株高、生物量和光合参数没有显著变化, 但叶面积显著下降; 经历过前期重度干旱处理植株的T_r、G_s、C_i、P_n、A_max和表观量子效率显著升高, 而株高、叶面积和生物量显著降低。综上所述, 第一次重度干旱处理显著降低玉米叶片的光合能力和生长, 复水可使光合能力和生长速率恢复到正常水分条件下生长植株的水平, 但不能消除前期干旱对生长产生的不利影响。前期中度干旱可以刺激玉米根系的生长和显著提高根冠比, 有利于提高对二次干旱的抵抗能力, 并使总的生物量保持在对照水平, 而前期重度干旱处理虽然在光合作用上能提高植株对二次干旱的抵御能力, 但不能弥补前期干旱处理对生长的不利影响。因此, 在生产实践中, 如果进行抗旱锻炼, 应限制在中度干旱水平, 避免重度干旱。

Aims Our objective was to investigate the responses of maize photosynthesis and growth to repeated drought.Methods Maize seedlings were exposed to different soil water deficit for three weeks, then rewatering for one week, and again to different water deficit for three weeks, to examine the effects of repeated drought on photosynthesis and growth.Important findings After the first water deficit treatments, under severe drought, plant height, total leaf area of individual plant, shoot and root biomass declined significantly, also transpiration rate (T_r), stomatal conductance (G_s), intercellular CO₂ concentration (C_i), net photosynthetic rate (P_n), maximum net photosynthetic rate (A_max), but light compensation point and dark respiration rate 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. After rewatering, photosynthesis 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 the control. 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, and, apparent quantum yield but significantly lower plant height, leaf area, and biomass than plants without previous exposure. These results indicated that 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 in the aspect of photosynthesis, 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.