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.  
 
B def. mustard

B def. canola

Boron deficient canola
(photos courtesy PPI)
 
 B def. alfalfa 
Boron deficient alfalfa
(image by Glen Shewmaker, Univ. of Idaho)
 
B def corn
Boron deficient corn
(image courtesy IPNI)