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Figure 1. 

The effect of natural drought stress on the relative soil moisture content of different apple rootstocks over the duration of 2, 4, 6, 8, and 10 d. Error bars refer to the average value ± SD from three biological replicates. Different letters indicate a significant difference at p < 0.05.

Figure 2. 

Morphological changes of leaves among eight genotypes of apple rootstocks. Samples were taken when the relative soil water content reached 65%–75% (mild drought stress), 45%–55% (moderate drought stress), and 25%–35% (severe drought stress); the scale bar = 10 cm. From left to right, the same rootstock represents three states of stress. The anatomical structure of different rootstock leaves under severe drought stress (relative soil water content reached 25%–35%); the scale bar = 250 µm.

Figure 3. 

The effect of different levels of drought on the relative water content (RWC) of leaves on different apple rootstocks. Samples were harvested when the relative soil water content reached 65%–75% (mild drought stress), 45%–55% (moderate drought stress), and 25%–35% (severe drought stress). Error bars refer to the average value ± SD from three biological replicates. Different letters indicate a significant difference at p < 0.05.

Figure 4. 

The effect of different levels of drought on the photosynthetic parameters, such as photosynthetic rate (Pn), transpiration rate (Tr), intercellular CO₂ concentration (Ci), and stomatal conductance (Gs), in different apple rootstocks. Samples were harvested when the relative soil water content reached 65%–75% (mild drought stress), 45%–55% (moderate drought stress), and 25%–35% (severe drought stress). Error bars refer to the average value ± SD from three biological replicates. Different letters indicate a significant difference at p < 0.05.

Figure 5. 

The effect of different levels of drought on the contents of photosynthetic pigments (chlorophyll and carotenoid) in different apple rootstocks. Samples were harvested when the relative soil water content reached 65%–75% (mild drought stress), 45%–55% (moderate drought stress), and 25%–35% (severe drought stress). Error bars refer to the average value ± SD from three biological replicates. Different letters indicate a significant difference at p < 0.05.

Figure 6. 

The effect of different levels of drought on the chlorophyll fluorescence (photochemical efficiency of PSII reaction centre, ${\text Φ} $PSII and non-photochemical quenching coefficient, NPQ) in leaves of different apple rootstocks. Samples were harvested when the relative soil water content reached 65%–75% (mild drought stress), 45%–55% (moderate drought stress), and 25%–35% (severe drought stress). Error bars refer to the average value ± SD from three biological replicates. Different letters indicate a significant difference at p < 0.05.

Figure 7. 

The effect of different levels of drought on the water use efficiency (WUE) of different apple rootstocks. Samples were harvested when the relative soil water content reached 65%–75% (mild drought stress), 45%–55% (moderate drought stress), and 25%–35% (severe drought stress). Error bars refer to the average value ± SD from three biological replicates. Different letters indicate a significant difference at p < 0.05.

Figure 8. 

The effect of different levels of drought on non-structural carbohydrates (NSC) and reducing sugar content in leaves of different apple rootstocks. Samples were harvested when the relative soil water content reached 65%–75% (mild drought stress), 45%–55% (moderate drought stress), and 25%–35% (severe drought stress). Error bars refer to the average value ± SD from three biological replicates. Different letters indicate a significant difference at p < 0.05.

Figure 9. 

The effect of different levels of drought on the relative electrical conductivity and root vigor of different apple rootstocks. Samples were harvested when the relative soil water content reached 65%–75% (mild drought stress), 45%–55% (moderate drought stress), and 25%–35% (severe drought stress). Error bars refer to the average value ± SD from three biological replicates. Different letters indicate a significant difference at p < 0.05.