Abstract

The present study was conducted to assess the impacts of different water and light intensities on the physiological and the biochemical aspects of cork oak (Quercus suber L.) seedlings from acorns collected from Tabarka (Northern Tunisia). The experiments were divided into two treatments: Water stress (well-watered: 25 to 35%, moderate drought stress: 15 to 20%, and severe drought stress: 7 to 10%) and light intensity stress (full sunlight: 100% of full sunlight, 1,200 μmol m^-2 s^-1, moderate sunlight: 65 to 70% of full sunlight, 800 μmol m^-2 s^-1, low sunlight: 20 to 25% of full sunlight, 300 μmol m^-2 s^-1, and extremely low sunlight: 3 to 4% of full sunlight, 40 μmol m^-2 s^-1). Proline accumulation was higher in the severe drought stress and extremely low sunlight compared with other water and light intensity levels. Total nitrogen concentration (T-N) on a dry mass basis was significantly higher in seedlings grown in the low sunlight and the extremely low sunlight than in the full sunlight and the moderate sunlight. The photosynthetic rate (P_N), the stomatal conductance (Gs), and the transpiration rate (Tr) of the seedlings of Q. suber showed similar tendencies both in the three drought water levels and in the four light intensity levels during treatment periods, while the water use efficiency (WUE) was not significantly different among each treatment. P_N for the full sunlight was lower at a maximum value (≈ 8.2 μmol m^-2 s^-1) compared with the moderate sunlight. On the other hand, the maximum photosynthesis was higher for the moderate sunlight seedlings when compared to the full sunlight seedlings, although the leaf from the full sunlight was light saturated at near 1000 μmol m^-2 s^-1compared with the moderate sunlight. In all treatments, both maximum Rubisco activity and electron transport capacity expressed from the A/Ci response curves (V_cmax and J_max) increased with well-watered (WW) and the full sunlight (FL). Under water treatments, the slopes and explained variances for severe drought stress were considerably lower than those under WW and moderate drought stress (MDS) at 60 and 90 days of treatment. In conclusion, the different water and light intensity levels affected the physiological and the biochemical parameters of Q. suber. The lower assimilation rate was associated with lower stomatal conductance, the nitrogen allocation to photosynthetic functions, maximal Rubisco activity (V_cmax) and electron transport rate (J_max).

Key words: Water, light intensity, relative water content, proline content, total nitrogen content, chlorophyll content, photosynthetic rate, maximum Rubisco activity, maximum electron transport capacity, Quercus suber L.