Majeda Roely
General Commission for Scientific Agricultural Research-
DeirEzzor Research Center
Damascus University, Faculty of Agriculture
2008

Abstract

A field experiment was conducted at Mreiyeh Research Station in Deir Ezzor during two growing seasons (2005, 2006) to evaluate the response of four maize genotypes (two varieties: Gouta1, Gouta82, and two hybrids: Basel1 and Basel2) for water stress conditions applied during different phonological stages of the crop life cycle by the complete cease of irrigation.
The experiment was established according to the randomized complete block design (RCBD) with three replications each treatment.
Exposure of maize plants to water deficit during the vegetative growth stage led to a substantial decline in the plant height of all the investigated corn genotypes, but water stress did not affect plant height during the flowering and maturity growth stages due to the completion of vegetative growth and stem elongation. Plant height was significantly higher in the plants of the hybrid Basel2, followed by the variety Gouta82 during the two growing seasons compared with the other two genotypes under investigation.
The leaf area index (LAI) was significantly higher in the variety Gouta82 (2.00), and the hybrid Basel2 (1.78) without significant differences between them compared with the other two genotypes. It has been found that the leaf area index was significantly higher under well-irrigated conditions (1.73) compared to water-stressed conditions, especially during the vegetative growth stage (1.55). This might be attributed to the reduction of the leaf cell elongation as a consequence of the decline of turgor potential (¨p) inside the cells. The increase of leaf area index was positively correlated with the plant assimilation rate ( A ), and dry matter accumulation.
The two genotypes, Gouta82 and Basel2 were capable of maintaining significantly higher leaf relative water content (LRWC) (73.71, 71.28% respectively) compared with the other two genotypes. The relative water content and leaf area index were significantly higher during the first growing season (2005) compared with the following second growing season (2006), because the maximum mean temperature was lower, and the relative humidity (RH%) was higher during the former compared with the latter, which limited the rate of growth  and increased the loss of water by evapo-transpiration. Positive significant correlation occurred between RWC and LAI (r = 0.78*), indicating the importance of water availability to maintain turgor potential required for cell expansion of the aerial plant parts.
The genotype capacity of sustaining the relative water content and leaf area index significantly correlated with the production capacity (grain yield) (r = 0.67*, 0.71* respectively).
The two genotypes, Gouta82 and Basel2 were more capable of maintaining the cytoplasmic membrane stability (CMS), where the solute leakage through the plasma membranes was  significantly higher in the two drought-susceptible genotypes, Basel1 and Gouta1 (51.66, 50.18 respectively) compared with the other two drought-tolerant genotypes (Basel2 and Gouta82).
There was a negative correlation between the relative water content and the ratio of solute leakages indicating the relevance of water availability for maintaining of membrane integrity.
Water stress conditions, particularly during the anthesis adversely affected on all the yield related components, but there was a genetic variation in the response of the studied genotypes. It has been found that the number of rows per head was highly determined with the head diameter rather than the head length, where the head diameter was significantly higher in the two corn varieties, Gouta1 and Gouta82, which formed significantly higher number of rows per head compared with the other two hybrids. The number of rows per head was significantly higher during the first growing season.
Drought during the vegetative and anthesis periods caused an increase in the tasseling-silking interval, which badly affected the number of grains per row of all studied genotypes.
The 100-kernel weight was significantly higher in the hybrid Basel2 followed by the variety Gouta82 (29.17, 29.11 g respectively), while it was the least in the single hybrid Basel1 (27.95 g). The 100-kernel weight was significantly higher in the non-water-stressed plants, and in the genotypes which maintained significantly higher leaf area index, indicating the significance of synthesizing of higher amount of dry matter, and the availability of water for the translocation of photo-assimilates from source into sink, enhancing the grain filling and the individual grain weight as a consequence. The mean grain yield was significantly higher in the hybrid Basel2 and the variety Gouta82 (857.6, 843.2 kg/dunum-1 respectively) compared with the other two genotypes. The grain yield was significantly lower in the plants exposed to water stress during the flowering stage for both the seasons (648.7 kg/ dunum-1) compared with the other growth stages.
In general, drought caused an estimated reduction of 23.78% in the grain yield during different growth stages. Drought stress caused a substantial decrease of 26.11% in the leaf area ratio (LAR) compared with the control, and the net assimilation rate (NAR) declined by 39.85%, due to the reduction of assimilation rate, and increase of respiration rate. The relative growth rate (RGR) declined by 48.58% under water deficit conditions. Generally, there was no positive
correlation between all the growth parameters and the grain yield or their components. Relative growth rate positively correlated with the net assimilation rate (r = 0.71*), and the leaf area index (r = 0.88*), indicating the relevance of increasing of leaf area index, and improving the plant canopy architecture to substantially increase corn grain yield.