The Effects of Low/High Light Regimes on the Stomatal Density of Soybean and Sunflower Plants

Hypothesis

P nutritional status of soyabean and sunflower plants can affect stomatal density by promoting production rate of stomata per epidermal cell.

Results

Leaf area and shoot dry matter weight

Effects of soil P and soil water

ANOVA demonstrates that each of the two growth parameters was significantly increased by increases in P (Fig. 2). The increase in soil water also significantly increased the two growth parameters. Significant interactions between P and soil water on both growth parameters indicate that growth promotion as a result of P was greater under MSW and HSW conditions than under LSW.

Effects of soil P and atmospheric CO2

ANOVA demonstrates that each of the two growth parameters was significantly increased by increases in P (Fig. 2). The increase in CO2 significantly increased shoot dry matter weight but had no significant effect on leaf area. Significant interaction between P and CO2 was found on leaf area but not on shoot dry matter weight. (Yoshie 370-374)

Stomatal density and stomatal index

Figure 3 shows stomatal images of soyabean and sunflower plants observed in both experiments (for the effects of soil P and soil water experiment, only the images under HSW are shown). Stomatal densities and indices are shown in Figs 4 and 5, respectively.

Effects of soil P and soil water

ANOVA demonstrates that the two main effects (P and water) on stomatal density were significant for both adaxial and abaxial leaf surfaces (Fig. 4). Looking at the mean values for each main effect, either higher P (HP, 133 ± 6; MP, 121 ± 6; LP, 97 ± 3; and 0P, 88 ± 7 for the adaxial surface; and HP, 339 ± 12; MP, 286 ± 12; LP, 261 ± 14 and 0P, 233 ± 14 for the abaxial surface), or lower soil water (HSW, 108 ± 8; MSW, 102 ± 5; and LSW, 119 ± 6 for the adaxial surface; and HSW, 206 ± 17; MSW, 264 ± 13; and LSW, 309 ± 11 for the abaxial surface) increased stomatal density. However, such a generalization is dangerous because the interaction between these was also significant on both surfaces. Indeed, the increase of stomatal density under the LSW condition was distinct under 0P, LP and MP conditions, but was reversed under HP conditions on both leaf surfaces.

The effect of P on stomatal index was significant, particularly for the abaxial surface (P < 0.001), but also (very weakly) for the adaxial surface (P = 0.083) (Fig. 5). Higher P resulted in greater stomatal index. However, the effect of soil water on stomatal index was not significant on either leaf surface. An interaction between P and soil water was significant on the abaxial surface. Thus, the increase of stomatal index as a result of P was greater under the HSW condition than under MSW and LSW on the abaxial surface. The significant increases in both stomatal density and index as a result of higher P indicate that P promotes stomatal formation. Contrasting with the effect of P, the significant increase of stomatal density as a result of lower soil water should be caused by reduction of epidermal cell expansion, as soil water has no significant effect on stomatal index.

Effects of soil P ...