Boron deficient plants have chlorotic young leaves and death of the main growing point. Leaves may develop dark brown, irregular lesions. Whitish-yellow spots may also form at the base of the leaves. Leaves may become thickened, distorted and curled. Stems may be stunted and flower buds may fail to form or be misshapen.  
 
Mustard leaf with yellow and orange spots between veins from B deficiency

Canola plants with yellow and orange spots on lower leaves from B deficiency

Boron deficient canola
(photos courtesy PPI)
 
 Alfalfa leaves with browning from B deficiency 
Boron deficient alfalfa
(image by Glen Shewmaker, Univ. of Idaho)
 
Corn leaves with distorted growth near tips from B deficiency
Boron deficient corn
(image courtesy IPNI)