Intra-/inter-specific competition is the key external factor influencing plant growth, physiologies and functions [3, 27]. The few studies that have explored links plant growth traits and competition have shown the relationships were complex, as a few of plant hormones involved in plant response to competition [3, 7, 9, 11, 14, 18]. Growing space for shoot and roots plays an important role in interception of radiation and absorption of water and nutrients, respectively. These authors usually planted the single plant and competing plants in pots with same volume, the difference in growing space was overlooked. Unlike to the competition literatures, the effect of pot size on plant response was explicitly accounted for in our experiment. Our results shown that the effects of pot size on plant growth and hormone synthesis were dependent on soil water availability. Soil drying stimulates ABA formation in roots, translocation to leaves, then reduction in stomatal opening and plant growth . Moreover, soil drying promotes soil compaction and in-crease in plant ethylene production . Our results also demonstrated that soil drying-induced compaction increased foliar ethylene evolution in both the big and small pot. The severe drought (12 days after deficit irrigation) significantly decreased ABA and ethylene synthesis in the small pot compared with the big pot. Moreover, as the physical properties were different between the compost we used in this experiment and soil, soil drying-induced compaction in soil maybe more significant. Therefore, to compare soil drying-induced influences on plant physiology and phenotype in different volumes, soil compaction and soil water availability need to be considered.
Plant hormone ABA and ethylene are known to regulate stomatal opening in response to competition from a neighbor [3, 7, 18]. With growing WT tomato and NR or FL tomato in one pot, we also concluded that ABA and ethylene involved in plant response to intraspecific competition. The decreased in stomatal conductance, leaf area and transpiration, which was induced by intraspecific competition, was accompanied by an increased in plant hormone concentrations. Vysotskaya et al.  found no significant difference in ABA concentration in xylem sap between single tomato and competing tomato. Whereas the elevated foliar ABA concentration in competing plants was measured in our experiment, which was in accord with earlier conclusions [18, 29]. Vysotskaya et al.  indicated that the competing from neighbors increased ABA concentration in lettuce shoot. Kurepin et al.  attributed the increased ABA concentration in Helianthus annuus leaves to shade light-reduced R/FR ratio. Intraspecific competition increased ABA concentration in tomato in our experiment, which contrasted with the lettuce data of Vysotskaya et al. . This may be explained by different species, soil and environments.
Both ABA and ethylene are known to regulate stomatal opening in response to reduced water availability [28, 30–31]. As soil dries, the competition decreased gs by elevating ABA and ethylene accumulations, whereas, under severe drought, the competition cannot reduce gs even increasing in these plant hormones accumulations. During the early stages of soil drought before hydraulic signals were produced, plant hormones dominated plant in response to stress . Under severe drought, chemical signals became less important when LWP declines and leaves wilt . The results indicated that the competition-induced decline in gs may be covered by the severe soil-drying-induced decrease in gs, as hydraulic signals most probably dominate (Fig. 2a).
Except for severe, competition regulated plant stomatal closure mainly through non-hydraulic signals [11–12]. Ethylene is an important phytohormone of sensing competing neighbors and determining plant responses to neighbors [7, 13–14]. Under well-watered condition, the absence of canopy competition cannot significantly reduce the stomatal opening (Table 3). Ethylene and the red: far-red light ratio (R:FR) are most important above-ground signals of plant neighbor detection [13, 33]. Canopy separation minimized the plant neighbor sensing by ethylene and R: FR. Although root exudates can serve as a belowground neighbor detection signal , belowground neighbor detection most prob-ably occur through reduction of local soil water and nutrients . As presence of above-ground sensing signals in the competing pot with root separation, the competition-induced influences on plant growth occurred observably.
Neighbors can be detected by several sensing signals in aboveground and below-ground through neighbor-induced changes in resource availability. The absence of canopy competition significantly decreased ABA accumulation in roots and then stomatal conductance, which contrasted with the result under full irrigation, indicating the below-ground neighbor detection signals were enhanced by soil drought. Although ethylene evolution was increased by drought, elevated ethylene synthesis did not significantly affect stomatal opening. Under some circumstances, ethylene can modulate stomatal responses to a given ABA concentration [17, 36–37].
Vysotskaya et al. [7, 18] suggested that several plant hormones, such as ABA, ethylene, auxin and cytokinins, involved in plant growth response to competition from neighbors, while the interactive mechanism of multi-hormones regulating plant response to competition still was unclear. Further experiments are necessary to learn more about interaction between competition and defense responses. Moreover, only a few of competition experiments were conducted under natural field conditions. Therefore, better understanding the multi-hormones mediated plant-plant interactions could help to optimize plant density and to understand plant behaviors in natural environment.