No 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.

Plants of Hordeum vulgare cv. Clipper were grown in gravel culture in solutions containing 0, 100 or 175 mol m-3 NaCl from the seedling stage until anthesis. When salt was first applied, leaf 4 was elongating within the sheath, and leaf 7 had yet to appear on the growing apex. These two leaves were sampled most intensively. Relative to the controls, salinity reduced the final area of all leaves progressively with higher node of insertion, and in the 175 mol m-3 NaCl treatment the reduction was more than twice that in 100 mol m-3 NaCl. Because leaves were at very different stages of development when salt was applied, the progressive reduction inferred there was no evidence of a particular sensitivity to salinity at different stages of leaf expansion. The peak CO2 exchange rate (CER), which occurred shortly after ligule emergence of each leaf was reduced far more in leaf 4 than in higher positions. Indeed, CER averaged throughout the leaf's life, was actually increased by salinity in upper leaves which initiated and expanded during the salt treatment. However, the reduction in leaf area was not offset by this increase in CER. The integration of these adjustments in area per leaf and CER resulted in a reduction in carbon fixation per leaf of about 30 and 60%, by the 100 and 175 mol m-3 treatments respectively for all leaves. There was no obvious relation between CER and Na+ and Cl- concentrations in leaf 7, and between CER and Na+ concentrations in leaf 4. Regression analysis showed that chloride and sodium ions in the leaves accounted for 54% of the statistical variation in CER in leaf 4 but for an insignificant proportion of the variation in leaf 7. As leaf 7 is presumably no different from leaf 4 in its ionic response, we conclude that the high leaf concentrations of Na+ and Cl- did not directly affect the rate of photosynthesis.

Well watered mini-crops of sunflower were grown either in summer or winter in glasshouses maintained under five temperature regimes and a 16 h photoperiod. A field crop was grown concurrently with the summer glasshouse study.Summer radiation (25 5 MJ day-') increased the size and/or number of many of the variables measured compared with winter radiation (9.5 MJ day-'). However, there was interaction between light and temperature upon phenological development, plant height, leaf number and harvest index. Seed production declined at temperatures above 18/13"C in summer and above 24/19"C in winter radiation, but fatty acid composition of the seed oil changed progressively with increasing temperature and was unaffected by radiation.Leaf area per plant increased faster under summer than winter radiation and in almost all temperature regimes reached considerably higher final values which resulted in a greater percentage of the incident radiation being intercepted. Temperature, though affecting the growth patterns and final areas of individual leaves in the canopies, did not alter the relationship between leaf area index and radiation interception. The light extinction coefficient changed with leaf area index and differed between summer and winter.Biomass per plant at maturity (B, g) was best related to radiation interception up to anthesis (I, MJ m-?), such that B = -234+541ogJ, r2 = 0.91, but seed number (S) was correlated similarly with radiation interception and with the number of degree days (D) accumulated between floral initiation and anthesis (S = 1137+0~0051-0.7620, R2 = 0.90). Yield (Y, g per plant) was dependent on seed number, mean temperature ( T ) and radiation intercepted between anthesis and maturity, and the leaf area present at anthesis. However, over 97% of the variation in yield could be accounted for by the temperature and radiation factors in the manner Y = 39,07+0,0471-1 .26T. Harvest index and yield were not correlated for the cultivar examined.

A large body of literature on the responses of yield and other agronomic traits to planting patterns in rainfed sunflower (Helianthus annuus L.) is reviewed. Extensive studies on the effect of plant density on yield have given inconsistent results which are still not well explained by other agronomic traits. The effects of row spacing on yield and other traits seems to depend greatly on the environment where the trial is conducted and on the cultivar used. Information on the effect of row orientation is scarce and needs to be further studied over a large range of latitudes with the assistance of simulating models.