Impact of drought on the diurnal variation of photosynthesis and environmental factors in Phyllostachys edulis leaves of different ages

This study aims to clarify the response of the diurnal variation of photosynthesis during bamboo shoot stage to drought for different ages of moso bamboo, and to provide reference for water management for moso bamboo under climate change.

The “ceiling method” was used to simulate throughfall exclusion for the wild bamboo forest. One-year-old, three-year-old, and five-year-old moso bamboo plants served as experimental subjects. The Li-6400 portable photosynthesis meter was used to measure the daily changes of gas exchange parameters and correlation analysis were conducted to explore the influence of environmental factors on the photosynthetic capacity of bamboo under different drought treatments.

1) Significant differences existed in the diurnal variations of gas exchange parameters among control and drought-treated bamboo leaves during the shoot-bearing stage, with these variations influenced by bamboo age. Net photosynthetic rate (P_n) exhibited diverse diurnal patterns: D-3a treatment showed a double-peak pattern, demonstrating the typical photosynthetic ‘midday break’ phenomenon; CK-1a, CK-3a, CK-5a, and D-5a treatments exhibited a single-peak pattern; while D-1a treatment displayed a “U”-shaped curve, peaking at 18:00; Diurnal variations in transpiration rate (T_r) also exhibited treatment specificity: CK-1a showed a single peak (peak at 14:00), D-1a displayed a “W”-shaped pattern (peak at 18:00), while CK-3a, CK-5a, D-3a, and D-5a all exhibited double peaks. Drought delayed the first peak by 4 hours, while the second peak occurred at 16:00 in all cases. Regarding stomatal conductance (G_s), the control bamboo of all ages exhibited a single peak pattern (evening peak at 16:00). The D-5a treatment showed a gradual increase throughout the day (peak delayed to 18:00), while the D-1a and D-3a treatments exhibited a “W”-shaped pattern (peak at 18:00). The diurnal variation of leaf vapor pressure deficit (V_pdL) exhibited a single-peak pattern (peak at 14:00) in CK-1a, CK-3a, D-1a, and D-3a treatments. CK-5a also showed a single peak but delayed to 16:00, while D-5a displayed a double-peak pattern. Water use efficiency (W_UE) exhibited diverse diurnal patterns: D-5a exhibited a single-peak pattern; CK-1a and D-3a showed a double-peak pattern; CK-3a exhibited a gradual decline; CK-5a showed a gradual decrease; and D-1a exhibited a “V”-shaped pattern; 2) Correlation analysis indicated that among the control Phyllostachys edulis parameters, negative correlations were more prevalent, whereas positive correlations were more common under drought conditions. Under drought, P_n and G_s showed positive correlations across all age groups of Phyllostachys edulis, with the correlation being significant (P<0.05) only in 5-year-old bamboo. The CK-3a and CK-5a treatments exhibited weak positive correlations; 3) Path analysis revealed that the primary direct factors influencing P_n in the D-1a, D-3a, and D-5a treatments were C_i, T_r, and V_pdL, respectively. Both D-1a and D-5a exhibited a relatively strong direct influence on P_n through G_s.

The daily peak value of P_n in bamboo of different ages under drought is 4-6 hours earlier than that of different ages of bamboo under the control. After 14:00, P_n, G_s, and W_UE values of all-aged Phyllostachys edulis under drought were higher than those of the control. The second peak of G_s in bamboo of different ages under drought was delayed by 2 hours compared to the control, and drought increased the W_UE of bamboo after 14:00. The correlation between G_s and T_r under drought condition shows a U-shaped regulation with increasing age, first decreasing and then increasing, reflecting the shift in regulatory strategies between G_s and T_r during bamboo shoot stage under drought conditions.