Effects of Seasonal Variation in Radiation and Temperature on Net Assimilation and Growth Rates in an Arid Climate

The net assimilation rate (E_A), relative growth-rate (R_w), andleaf-area ratio (F_A) were measured for rape (Brassica napus),sunflower (Hetianthus annuus), and maize (Zea mays) at varioustimes of year in an arid climate, using young plants grown widelyspaced on nutrient culture. Multiple regression analysis accountedfor 90–95 per cent of the variation in E_A and R_W in termsof two climatic variables: mean temperature and radiation receipt. E_A rose linearly with radiation in all three species; increasein E_A with temperature was greatest in maize and least (notsignificant) in rape. R_Wrose with radiation and temperature,the latter being the more important variable especially in coolweather; a temperature optimum was shown at 24° C in rape.F_A rose with increase in temperature or decrease in radiation;its variation was due to change in leaf area/leaf weight ratherthan in leaf weight/plant weight. Multiple regression analyses can lead to faulty interpretationif the independent variables are correlated (as are climaticvariables in nature), but conclusions can be checked by controlled-environmentstudies in which climatic factors are not correlated. The presentconclusions are supported by such studies. The regression equations, coupled with average weather records,indicate seasonal cycles of growth parameters. E_A is maximalnear midsummer and minimal near midwinter, following the radiationcycle. Maxima and minima in R_W are about a month later, becauseR_W is affected by the temperature cycle and this lags behindthe radiation cycle. F_A is maximal in autumn and minimal inspring. E_A is highest where radiation receipts near 750 cal cm^–2day^–1 coincide with high temperatures. This combinationoccurs only in clear midsummer weather at low latitudes, andis maintained over long periods only in arid regions. The fact that E_A rose linearly with radiation suggests thatleaf water deficits arising under high radiation had littleeffect on E_A and that saturating levels of light were very high.