The spectral irradiance of light penetrating 198 cm of snow was measured with a device employing a photomultiplier tube combined with each of several interference filters. In deep snow, the measured peak in transmission was near 500 nm, but in older, shallower snow the peak shifted to 550 nm. On 2 dates, measurement were made periodically under snow before and after sunrise. There was a spectral shift toward the longer wavelengths with increasing height of the sun above the horizon. Germination of Lactuca sativa cv. Grand Rapids seeds was promoted by a 3—h exposure to the light penetrating 198 cm of snow, and Brassica nigra and Brassica kaber seeds responded similarly to longer exposures under deep snow. Leaf greening, leaf unfolding, and inhibition of shoot elongation were exhibited by Camassia quamash in response to light penetrating the snow, but phototropism was not observed in an experiment designed to detect it.

The daily (24 hour) changes in carbon balance, water loss, and leaf area of whole sorghum plants ( (23), but salinity caused an increase in maintenance respiration (20). Neither salinity nor water stress affected the yield ofgrowth processes, which is the ratio of net carbon gain to gross carbon input after subtracting maintenance losses (20, 23). There are no data on photosynthetic or respiratory rates of plants exposed to both salt stress and water stress.Many plant species adjust osmotically when exposed to either salt in the growth medium (3, 8) or reduced soil moisture (10). Osmotic adjustment is beneficial in that it allows photosynthesis and growth to occur at lower plant water potentials than would otherwise be possible (15). There may well be a metabolic cost of this adjustment; however, the net effect on the daily carbon balance was found to be positive in sorghum plants exposed to water stress (15).It is known that NaCl salinity may be beneficial to the water balance and growth ofhalophytes, particularly under water stress conditions (6,7,14). Salt in the growth medium may provide some benefits even for glycophytes. Two studies ofthe combined effects of salt and water stresses on growth of wheat (Triticum aestivum L.) (21) and maize (Zea mays L.) (22) showed that although salinity reduced the rates of leaf expansion under wellirrigated conditions, it also allowed leaf expansion to continue down to lower leafwater potentials under water stress. Thus, the combined effects of salinity and water stress may be less detrimental to plant growth and carbon gain than the sum of the separate effects of salinity and water stress.In a previous paper we analyzed the water relations and the daily carbon balances of vegetative sorghum plants undergoing osmotic adjustment during a cycle ofwater stress and reirrigation (15). In this paper we extend our studies to include the effects of salinity, both alone and in combination with water stress.Physiological studies have often dealt separately with salt and water stresses, but in the field, salt stress is usually accompanied by water stress. Soil salinity problems occur most often in arid regions where soil moisture deficits are also frequent. Irrigation of crop plants with poor quality irrigation water often results in both salt stress and water stress in the dryer parts ofthe irrigation cycle, yet there is very little published information about the physiological responses of plants to this common condition.Both low soil osmotic potentials (due to dissolved salts) and low soil matric potentials (associated with reduced soil water content) cause lower water potentials in plants. In glycophytes, soil moisture deficit and soil salinity each result in reduced leaf expansion rates and lower photosynthetic rates per unit of leaf area (1,5,18,21). Effects on respiratory rates seem to be more complex, since soil moisture deficits have been shown to reduce both the growth and maintenance components of respiration MATERIAIS AND METHODS Four treatments were studied: (a) ...

The daily (24-hour)

Burrows (1972Burrows ( , 1974 learned later in the growing season. Sexsmith (1969) that winter seed survival of "dormoats" later found, in a greenhouse study, that a (Avena sativa x A.

Irrigated crops experience some water stress between irrigations. If poor quality water is used, the crops are also likely to be salinized. Daily C gains and water losses of salinized and nonsalinized sugarbeet (Beta vulgaris L.), and cowpea (Vigna unguiculata L. Walp.) plants were measured continuously, in whole‐plant assimilation chambers, throughout 2 to 3 weeks of water stress under controlled environment conditions. Equivalent data for sorghum plants [Sorghum bicolor (L.) Moench.] were published previously. Even though the three species showed quite different patterns of response to water stress, mild salinization always had the same effect. It reduced the water loss rate per plant, which allowed the length of the irrigation cycle to be increased, which in turn increased the C gain per cycle and the water use efficiency (C gain per unit of water lost).