ABSTRACT
Four groundnut (Arachis hypogaea L.) varieties (SAMNUT 21, SAMNUT 22, SMANUT 23 and SAMNUT 24) were grown in the Botanical garden, Department of Biological Sciences, Ahmadu Bello University, Zaria to determine the effects of defoliation on the growth and yield of the varieties. The experiments were laid out in a completely randomized design with five treatments (defoliation) levels. The plants were subjected to 5 levels of defoliation: 0 (no defoliation), 25, 50, 75 and 100% at 5 weeks after planting (WAP). The plants were sampled at 4, 7 and 10 WAP for assessment of growth parameters and at harvest the yield parameters were determined. The results of this study revealed that, 75% defoliation increased plant height in most cases. The control and 25% defoliation were found to increase root nodules, shoot and root dry matter, and shoot and root relative growth rate (RGR) in most cases. The 75 and 100% defoliation levels were found to significantly reduce vegetative parameters in groundnut varieties except plant height. The control and 50% defoliation had the highest flower production but flower production decreased with increase in defoliation with 100% defoliation level being the lowest in most cases. Total pods, number of matured pods per plant, haulms weight, pod dry weight and seed weight decreased with increase in defoliation but the number of immatured pods per plant increased with increase in defoliation level. The 25 and 50% defoliation levels were found to increase pod yield and haulms weight respectively but the 100% defoliation level produced the lowest yield in most cases. The data showed that varieties SAMNUT 22 and SAMNUT 21 showed higher values in vegetative parameters and yield parameters than the other varieties. Also, SAMNUT 23 showed the lowest values in both vegetative and yield parameters. In conclusion, the impact of defoliation on both vegetative and yield varies among varieties. The 25 and 50% defoliation increased vegetative and yield parameters in most cases but the 75 and 100% defoliation levels adversely reduced plant development and yield.

CHAPTER ONE

1.0      INTRODUCTION

1.1 Origin and Classification of Groundnut
Groundnut (Arachis hypogea L.) belongs to Family: Fabaceae, Sub-Family: Papilionaceae, Genus: Arachis and Species: hypogaea. It originated from Latin America and was introduced into West Africa by Portuguese traders in the 16th century (Shankarappa et al., 2003; Olawale et al., 2014). The origin of this crop dates back to 350 BC (Hommones, 1994). Hommones, (1994) also reported that, the first probable domestication of groundnut took place in the valley of the Panama and Paraguay River System in the Grain Chaco area of South America and then move to the North America through slave trade.

1.2 Botanical Description

Reports showed that the cultivated groundnuts are divided into two large botanical groups:

„Virginia‟ and „Spanish-Valencia‟, on the basis of branching pattern “alternate” and “sequential”. They stated that these cultivars are group within the two branching pattern considered as subspecies. In the „Virginia‟ group, the main stem does not have reproductive axes. Alternating pairs of vegetative and reproductive axes are borne on the laterals branches and on other branches. In the „Spanish- Valencia‟ group, reproductive branches are borne in a series on successive nodes of the cotyledonary and other lateral branches, on which the first branch is always reproductive. Reproductive branches are also borne directly on the main axis at higher nodes (Prasad et al., 2010).

1.3 Requirements for Groundnut Growth
Groundnut is essentially a tropical plant and requires a long and warm growing season. The favourable climate for groundnut is a well-distributed rainfall of at least 500mm during the crop-growing season, and with abundance of sunshine and relatively warm temperature.