When melon seedlings (Cucumis melo L. Ximiya No. 1) were cultured in a growth chamber with about 150 micro mol m(-2) s(-1) photon flux density, the leaf photosynthetic ability reduced dramatically as leaf position decreased from the top. The application of 5-aminolevulinic acid (ALA) solutions significantly increased the net photosynthetic rate (P(n)) as well as apparent quantum yield (AQY), carboxylation efficiency (CE) and stomata conductance (G(s)). After irrigation with 10 ml of ALA solution (10 mg l(-1) or 100 mg l(-1)) per container filled with approximately 250 g clean sand for 3 days, the leaf P(n) was about 40-200% higher than that of controls, and AQY, CE and G(s) increased 21-271%, 55-210% and 60-335%, respectively. Furthermore, ALA treatments increased leaf chlorophyll content and soluble sugar levels, as well as the rate of dark respiration, but decreased the rate of respiration under light. On the other hand, after melon seedlings that had been cultured in the chamber suffered chilling at 8 degrees C for 4 h and then recovered at 25-30 degrees C for 2 and 20 h, the P(n) of the water-irrigated plants was only 12-18% and 37-47%, respectively, compared with the initial P(n) before chilling treatment. If the seedlings underwent the same treatment but with ALA (10 mg l(-1)), the respective P(n) was 22-38% and 76-101%, compared with that of the control before chilling stress. If chilling was prolonged for 6 h, the ALA-pre-treated plants only showed a few symptoms in the leaf margins whereas all water-irrigated plants died, which suggested that ALA presumably promoted chilling tolerance of the plants under low light.

The objectives of the present study were to compare the dynamic of the growth and photosynthetic characteristics of cotton varieties contrasting in waterlogging (WL) tolerance when subjected to hypoxia stress. The growth of the WL-sensitive genotypes was notably inhibited by WL, mainly as a result of a significant reduction in the net photosynthesis (PN) after two days of hypoxia treatment; in the tolerant varieties, no significant changes in PN were observed until 8 d after hypoxia onset. The intercellular CO2 concentration and maximal photochemical efficiency of PSII significantly declined, and the nonphotochemical quenching increased in the sensitive varieties. Infrared thermography showed that a low stomatal conductance resulted in an increased leaf temperature under hypoxia stress. Spectral image analysis suggested that the pigment content and water content rapidly decreased in the leaves of the sensitive varieties. It is concluded that maintaining stomatal opening through the interaction of ethylene and abscisic acid may be an important strategy to improve waterlogging tolerance in cotton.Additional key words: chlorophyll fluorescence; Gossypium hirsutum; hyperspectral image; infrared thermal image. Abbreviations: Ci -intercellular CO2 concentration; E -transpiration rate; Fv/Fm -optimal/maximal quantum yield of PSII; gs -stomatal conductance; NDWI -normalized difference water index; NPQ -nonphotochemical quenching coefficient; PN -net photosynthetic rate; PSSRa -pigment ratio index a; qp -photochemical quenching coefficient. Acknowledgement: We appreciate the contribution of C. Li and Y.W. Nong on experimental execution. We thank to The National Natural Science Fund (No. 31471496) for financial support.

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