Drought and waterlogging are frequent natural disasters in spring and summer. The most direct effect of stress on crops is to inhibit crop growth and reduce biomass [14, 15]. Therefore, biomass is often used as an important measure of crop resistance to stress. In this study, watering at 65% field capacity had the most prominent advantage in terms of the leaf, stem, and root dry matter. Waterlogging alone (W) showed a significant inhibitory effect on the biomass of maize stems, leaves, and roots, indicating that maize at the seedling stage was more resistant to waterlogging stress after one week of water stress hardening (Fig. 1). Therefore, early moderate drought helps to alleviate or improve the effect of subsequent waterlogging on crop biomass, thereby improving crop resistance to subsequent waterlogging. This, in turn, lays a good foundation for promoting crop yield and ensuring economic benefits [16–18]. The canopy-to-root ratio can reflect the sensitivity of crops to stress. The ratio depends on the duration of the stress, and it is considered an adaptation mechanism to stress [19]. Research has revealed that some crop growth characteristics recovered to the control levels in 7 days after a continuous drought followed by waterlogging [20, 21], and another study showed that ABA synthesis induced by water stress is related to the canopy-to-root ratio [19]. Therefore, the canopy-to-root ratio is an-other important measure in studies of crop response to environmental stress. The results of the present study showed that in Phase I, greater water stress had a greater effect on the growth of shoots and leaves above ground and generated a more developed root system; on the seventh day of waterlogging after drought (65%-Wi and 55%-Wi) in Phase II, the canopy-to-root ratio was close to the control level (Fig. 1 (d)). The canopy-to-root ratio reflected the physiological regulation effect of crops under stress of drought followed by waterlogging. The main hazard of waterlogging is the inhibition of root function. The early water stress promoted the growth of the root system and expanded the extent of the root functional zone, which laid a good physiological foundation for the hardening of crop vi-ability under later waterlogging stress and had a buffering and regulating effect on later maintenance of waterlogging damage to the root zone. Increasing dry matter accumulation is the main way to improve the crop harvesting potential [22]. In contrast, soil moisture regulation is an important means to interfere with crop product formation. Excessive irrigation or waterlogging stress will cause obvious changes in crop morphology, which is the most intuitively characterized by the deterioration of leaf photosynthesis and corresponding reduction of transpiration [14]. The present study found that maize transpiration was related to the level of drought and waterlogging. The transpiration of maize un-der drought stress followed by waterlogging was higher than that under waterlogging alone (W). When there was no significant change in stomatal conductance, the daily plant transpiration decreased by 28.41% on the fifth day and by 30.13% on the seventh day of the waterlogging-alone (W) treatment at the seedling stage of maize. The small leaf area under waterlogging alone resulted in a small number of stomata and thereby low transpiration rate. In 65%-Wi, the stomatal conductance gradually decreased on the fourth to fifth day of waterlogging, while at the same time the transpiration gradually reached a peak. The difference in plant transpiration among different treatments under drought stress followed by waterlogging become insignificant starting from day 7 in Phase II, reflecting the hardening of maize to water stress. The physiological stress resistance function manifested itself within 7 days of waterlogging after a drought.
As a stress hormone, ABA plays a very important role in regulating the growth and development of plants, especially in many physiological processes of abiotic stress response (such as high salt, low temperature, and drought) [20]. Studies suggest that a variety of abiotic stresses can cause increased ABA concentrations in plants [19, 23]. Under water stress, the root system is the first part to sense changes in soil moisture. The plant ABA concentration originates from the root system, and its change is, therefore, stimulated by soil water stress [13]. Therefore, the change in plant ABA concentration is usually regulated or corrected through different irrigation methods to optimize crop water use efficiency [12]. In this study, water control followed by waterlogging led to a gradual change in the ABA concentration of maize leaves and roots. Specifically, the ABA concentrations decreased from leaves to the root base, then middle root, and finally root tip. The root system is the source of drought stress signals. Such signals accumulate and spread to the leaves along with the sap flow. Therefore, the leaf becomes the end point of ABA signal accumulation and witnesses the highest ABA concentration. The root system had higher ABA concentrations in 65%-Wi and 55%-Wi than under waterlogging alone (W) on day 7 of waterlogging in Phase II, indicating that drought followed by waterlogging elicits root production of stress signal and changes the physiological properties of the root system. Early water stress awakened the physiological response of maize to the subsequent waterlogging stress. Drought stress had a mediating effect on the physiological functions of maize, and this effect manifested itself in the subsequent stress. Studies on other plants have found that stomatal conductance and transpiration rate of plants are slightly or significantly reduced by drought stress and recover to varying degrees within 15 days of re-hydration [9, 24]. Drought stress inhibits the increase in leaf area. After rehydration starts, the leaf area shows a short and rapid increase, and the dry matter accumulation rate in-creases, resulting in compensatory growth effects [9, 25–28]. In the present study, the ABA concentration of leaves under water stress (55% and 65% field capacity) was significantly higher than that under non-stress treatments (75% field capacity). With the increase in water stress, the ABA concentration of leaves increased, the leaf stomatal conductance and plant transpiration decreased, but the inhibitory effect of water stress on leaf area and dry matter was not significant. On day 1 to 5 of waterlogging, the ABA concentration in the stem between the leaves and the root system increased with increasing water stress. In this period, the leaf stomatal conductance, plant transpiration, and dry matter under 65%-Wi treatment was also recovered and increased, whereas the recoverability of these indexes in 55%-Wi treatment was relatively weak. On day 7 of waterlogging, while the leaf ABA con-centration continued to decrease, the physiological function of the root system was induced and stimulated, resulting in increased root ABA concentration. Consequently, the ABA concentration gradient between leaves and roots decreased, and the ABA concentrations of leaves and roots were relatively balanced. The leaf stomatal conductance and plant transpiration in 65%-W7 and 55% -W7 treatments slowly decreased. The dry matter accumulation rate in 65%-W7 slowed down, and that in 55%-W7 decreased significantly. Rehydration effects occurred within 5 days of waterlogging after moderate drought stress (65% field capacity), and each parameter increased to the compensation point. A more severe water stress (55% field capacity) reduced the compensation effect, which is not conducive to biomass formation. After 7 days of waterlogging following drought stress, the negative effects of waterlogging stress began to appear. Therefore, waterlogging stress after a drought period at the seedling stage of maize should not exceed 7 days.