LATITUDINAL VARIATION IN PLANT SIZE AND RELATIVE GROWTH-RATE IN ARABIDOPSIS-THALIANA

Latitude is an important determinant of local environmental conditions that affect plant growth. Forty ecotypes of Arabidopsis thaliana were selected from a wide range of latitudes (from 16 degrees N to 63 degr ees N) to investigate genetic variation in plant size and relative gro wth rate (RGR) along a latitudinal gradient. Plants were grown in a gr eenhouse for 31 days, during which period three consecutive harvests w ere performed. Plants from high latitudes tended to have smaller plant size in terms of seed size, cotyledon width, rosette size, number of rosette leaves, size (leaf area) of the largest leaves, total leaf are a, and total dry weight per plant than those from low latitudes. The m ean (ISE) RGR across ecotypes was 0.229 (+/- 0.0013) day(-1). There wa s, however, significant ecotypic variation, with RGR being negatively correlated with latitude. The two main components of RGR, leaf area ra tio (LAR) and unit leaf rate (ULR), were also correlated with latitude : LAR increased with increasing latitude while ULR decreased with incr easing latitude. It was also found that RGR tended to be negatively co rrelated with LAR, specific leaf area (SLA) and specific root length ( SRL) but to be positively correlated with mean area per leaf (MAL) and ULR. The variation in RGR among ecotypes was relatively small compare d with that in the other traits. RGR may be a conservative trait, whos e variation is constrained by the trade-off between its physiological (i.e. ULR) and morphological(i.e. LAR) components.