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Abstract
The global shortage of fresh water is one of the most severe agricultural problems affecting plant growth and crop yield. Therefore, it is important to consider alternative strategies for ameliorating the detrimental effects of water deficit. Previous studies have shown that drought significantly affects tissue expansion, leaf water status and gas exchange mechanisms. Drought tolerance and water use efficiency are often taken loosely as a synonymous, although they are frequently unrelated. Plants exhibiting C4 photosynthesis as Zea mays and Sorghum bicolor are more water-use efficient than those exhibiting C3 photosynthesis.
Aquaporins (AQPs) are water channel proteins of intracellular and plasma membranes that play a crucial role in plant water relations. During drought stress, cell-to-cell water movement through AQPs is believed to play a pivotal role.
We worked on two plants; Zea mays L. which is less tolerant to drought than Sorghum bicolor L. We investigated the response of both plants to drought stress applied under field conditions and controlled conditions by withholding water for 10 d. The plant growth in terms of shoot fresh and dry weights was more severely reduced in maize than in sorghum as a result of drought stress, consistently with reduction of leaf relative water content (RWC). Gas exchange was also more greatly inhibited by drought in maize than in sorghum. As a result, the water use efficiency (WUE) of maize fluctuated according to the time point during the day and in response to drought stress. In contrast, sorghum was able to maintain largely constant WUE during the day in the well-watered plants as well as under drought stress, indicating the sorghum more efficiently controlled its water status in particular water uptake than did maize. Studying the expression of four aquaporin genes (PIP1;5, PIP1;6, PIP2;3 and TIP1;2) revealed that most of the genes responded weakly to drought stress except PIP2;3 which was highly responsive to drought in sorghum but not in maize roots, where it may have supported greater water uptake in sorghum, and thereby maintained higher leaf RWC in sorghum than in maize and hence could account at least in part for the drought tolerance of sorghum as compared to maize.