Sources of Variation in Specific Leaf Area in Wheat Grown at High Temperature

Abstract

In the belief that specific leaf area (SLA, cm2 g1 dry weight) may be correlated with plant growth rate under certain conditions, and thus may be used in selecting genotypes for high growth rates, studies were commenced to examine the main sources of variation in SLA. In most studies, wheat plants were grown in controlled environments with high mean temperature and vapour pressure deficit and with varying radiation regimes. All environmental variables followed cosine patterns which in amplitude mimicked field conditions. Specific leaf area varied depending on: (1) location along a leaf: it decreased from tip to base by some 35%; (2) leaf age: SLA declined between leaf appearance and 10-12 days later then, after a plateau of varying length, it rose again; (3) leaf insertion: SLA of the flag leaf was the most stable with leaf age; (4) plant density: in canopies the final rise in SLA with leaf age occurred earlier than in spaced plants; (5) radiation: SLA declined at approximately 4 cm2 g-1 for each mol quanta m-2 day-1 increase in radiation; (6) genotype: SLA differed among genotypes and the ranking was unchanged by changes in radiation. In two genotypes studied in detail, higher SLA equated with faster leaf area production and larger leaves, but not with chlorophyll content per unit area, stornatal frequency or net CO2 exchange rate per unit area (CER), though it was noted that both SLA and CER changed inversely with leaf aging after the leaf was fully expanded. The importance of SLA in active (positive and negative) and passive roles in plant growth is discussed.