Root-shoot growth relations in yemane (Gmelina arborea Roxb.) seedlings as influenced by three levels of water treatment

The results of the study indicated that water treatment could be an effective tool to enhance the morphological development of seedlings during their juvenile stage. Unstressed water treatment accounted for a biggest percentages in average total net growth in basal diameter (1.03 cm), shoot length (74.89 cm) and leaf area (124.87 sq cm) as against the growth obtained by the other two treatments. In terms of shoot biomass, unstressed water treatment acounted for only 136.18 g, which is the smallest, compared to the moderate (182.24 g) and severely stressed (205.84 g) treatments. Of all the root parameters, only the diameter of lateral roots was increased by unstressed water treatment accounting for 1.06 cm slightly lower than the average increment obtained by seedlings in moderate water stressed treatments. Results of analysis indicated that unstressed water treatment generally increased the proportion of aboveground parts while water stressed treatments increased the proportion of belowground root parts. Specifically, severely stressed seedlings obtained the highest average total gain in the number of lateral roots (18.33 pcs), length of lateral roots (20.70 cm), root density (34.50 cc), root:shoot ratio (0.56) and root biomass (62.48 g) as against the values obtained by moderate and unstressed seedlings. The increment in shoot portion of seedlings clearly indicated that water when supplied in sufficient quantity favored the growth and development of aboveground parts with a smaller root system while water stress favored the development of below ground parts. Increments in root parts under water stress can be seen as compensatory growth mechanisms adapted by plants to regulate the adverse effects of water stress. Compensatory growth responses of plant parts were generally mediated by changes in the physiological processes in plants, which are reflected in the evolution of new plant morphologies. Similarly, regression analysis revealed varied correlational strengths and direction between root and shoot parts. What is apparent so far is that water stress treatments applied slowly over a four month period can enhanced seedling quality by inducing favorable morphological changes in both root and shoot morphology. Specifically, hardening treatment induced growth processes included shorter shoot length, more numerous lateral root system, bigger root density, bigger root:shoot ratios and the evolution of a more fibrous root system. These morphological changes have important implication in nursery and plantation management, a more fibrous root system is known to increase the absorptive capacity of roots while seedling with shorter shoot length can be transported with relative ease in plantation areas